MODELING THE AQUEOUS CHEMISTRY OF

Transkript

1 MODELING THE AQUEOUS CHEMISTRY OF URANIUM ON A NATIONAL LEVEL IN SWEDEN Åsa Löv August 2012 TRITA-LWR Degree Project 12:32 ISSN X LWR-EX-12:32

2 Åsa Löv TRITA LWR Degree Project 12:32 Åsa Löv 2012 Degree Project for the master program Environmental Engineering and Sustainable Infrastructure Research Group Environmental Geochemistry Department of Land and Water Resources Engineering Royal Institute of Technology (KTH) SE STOCKHOLM, Sweden Reference to this publication should be written as: Löv, Å (2012) Modeling the aqueous chemistry of uranium On a national level in Sweden TRITA LWR Degree Project 12:32 ii

3 Modeling the aqueous chemistry of uranium On a national level in Sweden SUMMARY IN SWEDISH I dagens samhälle är rent grundvatten en av de mest essentiella naturresurserna ett land kan ha. Således är det mycket angeläget att säkerställa att grundvattnet är inom ramarna för de nationella och internationella kraven. I Sverige förlitar sig 1.2 miljoner personer på privata brunnar, varav personer har borrade brunnar (SGU, 2008). Enligt Sveriges lagstiftning är det upp till varje hushåll att säkerställa att grundvattnet som pumpas i brunnen håller bra kvalitet. Karlsson (2010) visade dock att de flesta husägarna är ovetande om riskerna med förhöjda halter av ämnen som uran, arsenik samt andra metaller. Sveriges Geologiska Undersökning (SGU) har ansvar för miljömålet Grundvatten av god kvalitet. Sedan dess har ett delmål formulerats; år 2020 ska dricksvatten från privata brunnar möta de svenska riktlinjerna. Strålsäkerhetsmyndigheten (SSM) har ansvaret för miljömålet Säker strålmiljö. SGU och SSM inledde år 2001 ett sexårigt samarbete med målet att kartlägga den rumsliga, och till viss utsträckning tidmässiga, variationerna av radioaktiva ämnen och metaller i privata brunnar. Vattenprover togs från brunnar runtom i Sverige och en kemisk analys utfördes. Resultatet från undersökningen visade på att 17 % av privatborrade brunnar överstiger Sveriges riktvärde för uran (15 μg/l), och 2 % av brunnarna hade koncentrationer som översteg 100 μg/l. Den senare koncentrationen indikerar att konsumenten kommer att inmundiga en stråldos om 0.1 msv/år. WHOs riktvärde var 15 μg/l fram till juli 2011, då det ändrades till 30 μg/l. Riktvärdet för uran är baserat på den kemiska toxiciteten och strålningsrisken för uran, men där den kemiska toxiciteten är den främsta riskfaktorn (WHO, 2005; Ek et. al 2008; Prat et al. 2009). Det finns ett flertal tekniker på marknaden för avlägsnande av uran från grundvatten: jonbyte; anjoner och katjoner, omvänd osmos, aktivt kol, elektrodialys och koagulering. En förutsättning för att vissa utav dessa tekniker ska vara effektiva krävs kunskap om vilket komplex uran är bundet i, teknikerna bygger på ofta på att laddningen på ämnet som önskas avlägsnas är känt (Socialstyrelsen 2006). I detta examensarbete utfördes en simulering av urans förekomstformer i bergborrade privatbrunnar för data från SGU och SSM med hjälp av det kemiska jämviktsprogrammet Visual Minteq 3.0. Denna modellering, till skillnad från tidigare modeller för urans grundvattenkemi, inkluderade löst organiskt kol (DOC) samt element som tidigare inte ansetts vara essentiella ligander till uran. DOC visade sig vara en mycket viktig faktor, även i mycket låga koncentrationer, och särskilt vid låga ph-värden. Kalcium-karbonat-komplexen är det dominerande komplexet i alkaliska vatten. Dock påvisas det att de organiska komplexen inte är helt försumbara heller i alkaliska vatten, även under högt CO 2-tryck samt höga halter av karbonater, vilket tidigare har antagits. Det har tidigare av forskare diskuterats att radioaktiva ämnen och metaller är naturligt förekommande i berggrunden, dock varierar koncentrationen mycket beroende på berggrundstyp (Bucher et al. 2008; Ek et. al 2008; Prat et al. 2009). För att få förståelse om hur urankoncentrationen varierar rumsligt i Sverige samt hur bildandet av de olika urankomplexen är korrelerade till en viss typ av berggrund utfördes en rumslig analys i ArcGIS. Resultatet påvisade att berggrunden inte är en styrande faktor av vilket urankomplex som kommer att bildas. Det fanns således mycket liten korrelation mellan berggrundstypen och vilket urankomplex som dominerade. Orsaken till detta ligger troligtvis i den felmarginal koordinaterna har inom ett relativt litet område, Siljansringen, där ett flertal olika berggrunder samexisterar på en liten yta. iii

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5 Modeling the aqueous chemistry of uranium On a national level in Sweden ACKNOWLEDGEMENT I would like to thank my advisor Ann-Catrine Norrström for suggesting this topic and for the enthusiastic guidance and feedback trough out the work of the thesis. I am also very grateful for the input from Jon Petter Gustafsson. I would also like to thank the Swedish Geological Survey and the Swedish Radiation Safety Authority for providing the chemical data, and once again the Swedish Geological Survey for the geological data. I would also like to thank my family, boyfriend and friends for all the support thoughout my years as a student, and who has always believed in me. Stockholm, August 2012 Åsa Löv v

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7 Modeling the aqueous chemistry of uranium On a national level in Sweden ABBREVIATIONS Bq: Becquerel; Radioactive decay per second (SI unit) DOC : Dissolved Organic Carbon DOM : Dissolved Organic Matter LOAEL: Lowest Adverse Effect Level TDS: Total Dissolved Solids SGU: Swedish Geological Survey SSM: Swedish Radiation Safety Authority (S)EPA: (Swedish) Environmental Protection Agency pco 2 pressure: partial pressure of CO 2 ppm: Parts Per Million = 1 mg/kg WHO: World Health Organization vii

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9 Modeling the aqueous chemistry of uranium On a national level in Sweden TABLE OF CONTENT Summary in Swedish... iii Acknowledgement... v Abbreviations... vii Table of content... ix Abstract Introduction Aims and objectives Background Constructing a well Legislation Technique Uranium isotopes and U(VI) chemistry Uranium isotopes Uranium (VI) chemistry Groundwater and geology Groundwater in the bedrock Uranium and geology Uranium in the bedrock groundwater Toxicity of uranium Guideline value for uranium Uranium removal from groundwater Previous uranium speciation modelling Methods Literature study Speciation modelling Assumptions made in this model Statistics Sensitivity analysis Spatial correlation using ArcGIS Sampling by SGU Results Seasonal variation of uranium concentration and ph Uranium concentration in relation with ph and alkalinity Uranium concentration and depth of well Uranium speciation correlation Dominating uranium species The uranyl ion and ph Uranyl Carbonates Uranyl hydroxides U-DOM and ph Uranyl fluoride and ph Uranyl silicic acid and ph Cation-, anions- and neutral uranyl complexes and ph Sensitivity analysis The effect of introducing DOC The effect of temperature The effect of pressure ix

10 Åsa Löv TRITA LWR Degree Project 12: ArcGIS correlation between uranium concentration, species and bedrock Correlation of uranium concentration and bedrock Correlation of ph and bedrock Sampling density in a specific area, Siljansringen Discussion Speciation modeling Seasonal variation of parameters Uranium speciation Sensitivity of model Spatial analysis Uncertainties and drawbacks Future work Conclusions References Other references Appendix I. Default formation constants in VISUAL Minteq I Appendix II. Correlation between specie formed and concentration of element and physical parameters... II Appendix III. Result from correlation analysis.... VII x

11 Modeling the aqueous chemistry of uranium On a national level in Sweden ABSTRACT In Sweden 1.2 million people rely on private wells; of these use drilled wells. Elevated concentrations of radionuclides in the drinking water have been a long-term problem in Sweden. The well owners are not aware of the risk of elevated concentrations of radionuclides, nor do they have knowledge concerning if their particular well has elevated concentrations. In the survey initiated in 2001 by the Swedish Geological Survey (SGU) and Swedish Radiation Safety Authority (SSM) it was discovered that 17 % of the private drilled wells exceeded the Swedish guideline value of uranium; which is 15 μg/l, and 2 % had a uranium concentration beyond 100 μg/l. There are several methods available to reduce the uranium content in drinking water, though many of these methods are based on the charge of the uranium species. Knowing the speciation of uranium will rationalize the treatment for uranium, thus justifies the goal of this study, which is; to model the chemistry of uranium in private drilled wells, using Visual MINTEQ 3.0 and attempt to find a correlation of the bedrock source of water, using ArcGIS It was discovered that the U-DOM species predominates in the acidic waters (ph < 7) and Ca-carbonates in the alkaline waters (ph > 7). Hydroxides are also a major species formed; these complexes predominate around ph 7. It was also affirmed that the physical parameters such as ph, pco 2 and the introduction of DOM, affects the outcome of the model noticeably. The ph value is the only parameter which has a correlation to all the species formed. The other factor acknowledged to influence it is the pco 2 applied. The outcome of the model can be regarded as reliable, and is in agreement with what previous studies has shown. No correlation could be interpreted between the results and the bedrock source nor was any spatial trend observed. Keywords: Uranium species; Uranium in groundwater; Private drilled wells; Visual MINTEQ 1. INTRODUCTION In the contemporary society clean groundwater is one of the most important natural resources a country can have. Thus it is important to ensure that the quality of the groundwater is within the national and international standards. In Sweden 1.2 million people rely on private wells; whereof people have drilled wells (SGU, 2008). In Sweden it is up to every individual well owner to assure that the water they use is of good quality, but according to Karlsson (2010) most of the house owners are not aware of the risks of elevated concentration of elements like uranium, arsenic and other metals. In 2001, the Swedish Geological Survey (SGU) and Swedish Radiation Safety Authority initiated in 2001 a six-year cooperation with the aim of mapping the spatial, and to some extent the temporal, variation of radioactive elements and metals in private wells. It was discovered that 17 % of the private drilled wells exceeded the guideline value of uranium; which is 15 μg/l, and 2 % had a uranium concentration beyond 100 μg/l. The latter concentration indicates that the consumer of this water will receive a radiation dosage of 0.1 msv/year. The guideline value of 15 μg/l is based on both the radiation aspect of uranium and the toxicity, where the toxicity is the main issue when consuming uranium rich water (WHO, 2005; Ek et. al 2008; Prat et al. 2009). In the survey carried out by SGU and SSM water samples were mainly taken from drilled wells and dug wells; this report will only focus on the drilled wells. The drilled wells are located in crystalline and sedimentary bedrock, and receive the water from water bearing fractures in the bedrock (Ek et. al 2008). Radioactive elements and metals are naturally occurring in the bedrock but the concentration is highly varying depending on the bedrock, thus the groundwater chemistry in the bedrock is highly correlated with the bedrock type (Ek et. al 2008; Bucher et al. 2008; Prat et al. 2009) Aims and objectives The goal of this report is to study the spatial distribution of uranium speciation in private drilled wells in Sweden. There has been a consensus that carbonate-uranyl-complexes are the dominant species since most groundwater in the bedrock is assumed to be slightly alkaline. However there is reason to question this consensus and a modelling exercise aimed at 1

12 Åsa Löv TRITA LWR Degree Project 12:32 which speciation might occur in the Swedish groundwater is much needed since it has never been done to a national extent before. In Sweden there are drilled wells which provide water that exceeds the national standards (15 µg/l) by 89 times, this gives this study a good reason to be conducted. To get further understanding of the speciation, a correlation between influencing parameters and a correlation of uranium speciation to the bedrock, has been carried out. The bedrock is the main contributor of dissolved ions in the groundwater, thus the major influence on the groundwater chemistry. Parameters such as ph, redox, atmospheric pressure state, temperature and the presence of dissolved organic carbon; DOC are important influences on the speciation of uranium. The main questions to be answered are: a. What are the predominating uranium species in private drilled wells in Sweden? b. Which is/are the predominating factor(s) governing the uranium speciation? c. How is the uranium species distributed spatially in Sweden? d. Is there a correlation between the formation of uranium species and the bedrock the well is located in? 2. BACKGROUND There are many factors to be considered when performing and evaluating the result of a modelling exercise. This section will give a background of various aspects from constructing a well to the groundwater chemistry of uranium. Moreover results from previous modelling of the speciation of uranium in groundwater will be presented Constructing a well During the past ten years approximately to wells have been drilled in Sweden annually. When constructing a well there are regulations which states that the SGU has to be informed and the well-protocol, which has been published by SGU, has to be filled out. This information is then transferred to a well archive that is used when mapping the groundwater and hydrogeology, including when consulting property owners, well-borers, fitters and other consultants. The well archive is also important when performing groundwater prospecting and when a groundwater problem is discovered; having information about all the wells in the area simplifies the work of locating the source of the problem and finding a solution to the problem (SGU, 2008) Legislation To decrease the risks which are aligned with drilling of a well there are strict rules of how it should be carried out. The company performing well drilling should be a certified company which guarantees that the company is educated within the area, and the person performing the drilling, the well-borer, should be a certified professional. This is an important set of tools towards a safe water quality, since all interference with the ground and the bedrock could be a threat to the groundwater quality (SGU, 2008) Technique Drilling a well is a delicate task since a small mistake could have severe consequences for the consumer of the water. The first step when planning a well is to choose an appropriate location. The well should be located upstream areas which are at risk to be contaminated. The groundwater typically follows the terrain hence, the well should be located in elevated areas, where the risk of contaminated water is not as pronounced. The distance to the contamination Fig 1. Outline of a drilled well (SGU, 2008). 2

13 Modeling the aqueous chemistry of uranium On a national level in Sweden source depends on the geological conditions in the area, such as; the infiltration capacity and the depth to the groundwater level including the slant of the groundwater surface, and the nature of the contaminant (SGU, 2008). When a suitable location has been establish, the construction of the well can be initiated. The bedrock is usually covered by a soil layer which has to be drilled through before starting the drilling into the actual bedrock (Fig. 1). When the hole through the soil layer is constructed a casing tube, usually made of steel, is placed. The space between the casing tube and the bedrock must be sealed carefully to ensure that soil and shallow groundwater cannot enter the well. The second step is the drilling in the bedrock; the drilling is proceeded until the right amount of water is discovered. The amount of water which can be extracted from a well depends on the geological conditions. A newly constructed well usually provide about l/h, and a well located in sedimentary bedrock could have the capability of providing l/h. Usually 100 l/h is enough for a normal household (SGU, 2008) Uranium isotopes and U(VI) chemistry Uranium is a naturally occurring metallic element with the atomic number 92 and an atomic weight of g/mol. It is silver-grey and belongs to the actinide group (Smedley et al. 2006). Uranium commonly exists in a hexavalent state (VI), but can occur in valence states of (II), (III), (IV) and (V) as well, depending on redox conditions of the surrounding environment. The hexavalent state is usually related with the uranyl ion (UO 2 2+) which is the most common form of uranium in the environment (WHO, 2005) Uranium isotopes There are three naturally occurring uranium isotopes; 238 U, 235 U, 234 U, which usually are abundant in a mixture in the nature (Table 1). This ratio can though change slightly in nature due to natural nuclear- and chemical processes; this results in fractionation (Smedley et al. 2006). All three isotopes decay with both alpha and gamma emission (WHO 2005; Ek et. al 2008). Uranium is though a weak radioactive element compared to other radioactive elements (Smedley et al. 2006) and it has a long half life (Smedley et al. 2006; Ek et al. 2008). It is the 235U isotope which is used as fuel in nuclear plants. Table 1. The activity and weight in 1gram of naturally occurring uranium (Modified from Ek et. al 2008 and Smedley et al. 2006). Nuclide Alpha activity (kbq/g) Weight (mg/g) Half-life (years) 238 U million 235 U million 234 U The occurrence of this isotope is though naturally very limited, it is only about 0.72 % of the uranium in the nature which is 235 U (Marshak, 2005; Ek et. al 2008). Terms which are commonly used when the ratio of the 235 U isotope has changed are; enriched and depleted uranium. The former refers to uranium mixture which has an increased amount of 235 U compared to the natural occurring uranium, and the latter to a mixture which has a lower amount of 235 U, compared to naturally occurring uranium (ATSDR, 2011) Uranium (VI) chemistry The most abundant uranium ion is the hexavalent ion. It forms the uranyl ion: UO 2 2+ which in turn will form complexes with ligands and organic matter in natural waters (Vaaramaa, 2003). Hence only the occurrence of speciation with uranyl will be discussed further. Fig 2. Species distribution of the dioxouranium(vi) hydroxide system at 25 C in the range 4.5 < ph < 10. The precipitation of solid phases is suppressed (Grenthe et al. 2004). 3

14 Åsa Löv TRITA LWR Degree Project 12:32 Uranium (VI) hydroxide complexes In oxidizing waters U(VI) is transported in the ion uranyl; UO 2 2+, which can form complexes with other ligands. The complex which is formed is highly dependent on the ph and the composition of the ions in the water. In water where only uranium is present, uranyl-hydroxides will predominate at all ph values (Fig. 2) and depending on the uranium concentration, polynuclear species will be the predominating species (Langmuir, 1978; Vaaramaa, 2003; Grenthe et al. 2004). At ph > 10.5 the positively charged uranyl-hydroxides will predominate (Swedish Health Authorities, 2006). Uranium (VI) carbonate complexes The most common ligand for the uranyl ion in natural waters is carbonate and it will predominate in all alkaline waters. The carbonate complexes decreases the adsorption ability of uranium, thus increases the mobility (Langmuir, 1978; Vaaramaa, 2003). Vaaramaa (2003) states that in the uranyl-hydroxide-carbonate system the UO 2CO 3 0, UO 2(CO 3) 2 2 and UO 2(CO 3) 3 4 will predominate in the ph range of 6-8, and the uranyl di- and tri-carbonate complexes at ph above ph 7.5, in comparison to the Swedish Health Authorities who claims the following speciation schedule in their report from 2006 (Table 2). Uranium(VI) organic complexes Uranyl organic specices (U-DOM) can only exist at ph values up to between 6 and 7, whilst carbonate speciation predominates in alkaline water (Vaaramaa, 2003). In acidic waters, U-DOM might even be the dominating species (Gustafsson et al. 2009). It has also been discovered that uranyl has a higher affinity to humic acid compared to fulvic acid (Langmuir, 1997). The hexavalent uranium ion can bind to humic acids at ph as low as 2. Fulvic acid will not form a complex with uranium at such a low ph (Li et al. 1980). Uranium(VI) in calcium carbonate, fluoride, phosphate, silicic acid and sulfate complexes Fluoride, phosphate and sulfate complexes can only be formed under acidic conditions, and calcium-carbonate-uranyl species can be formed in alkaline conditions. Phosphate and sulfate complexes mainly occur close to mining areas, where the uranium concentrations are very high. In most groundwater, uranyl mainly forms complexes with fluoride in acidic waters (Langmuir, 1978; Vaaramaa, 2003). Silicic acid forms the relatively weak complex; UO 2H 3SiO 4, which can exist at ph 6 (Langmuir, 1978). Table 2. Dominating species of uranium in groundwater (pco2 = 10-2, atm, CT, U = 2.38 mg/l, 25 C). ph <5 UO 2 2+ Predominating species Predominating ion charge Divalent cation UO 2CO 0 3 Neutral molecule UO 2(CO 3) 2 2 >7.6 UO 2(CO 3) 3 4 Divalent anion Tetravalent anion 2.3. Groundwater and geology Groundwater exists in all kinds of geological media; eskers, soil, and bedrock. The quality of the groundwater is strongly correlated with the chemistry of the surrounding media, the porosity, the permeability and the amount and the size of the fractures in the bedrock. Groundwater is the zone in the subsurface where all pores are water saturated. The rain infiltrates through the ground and trickle though the surface layer into the pores in the soil and the fractures in the bedrock until it reach the groundwater zone (Hiscock 2005; SGU, 2008). There are four different kinds of groundwater conditions; unconfined, concealed, confined and artesian (Hiscock, 2005). In an unconfined aquifer the level of the groundwater surface is where the water and the air meet in the pores or fractures, changes seasonally though out the year. During the late summer and early fall the groundwater level will be lower due to the increased uptake of water by plants, and sometimes just before the snowmelt in early spring (SGU, 2008). The confined aquifer is trapped between two layers; thus it is held under pressure. When the layer is penetrated the water level in the tube will rise to the groundwater level. The difference between a confined and an artesian is that when concealing layer is penetrated the water will rise to the ground surface or under increased pressure above the ground surface (Hiscock, 2005) Groundwater in the bedrock The most common bedrock type in Sweden are crystalline bedrock types; such as granites and gneisses. These are dense bedrock types and the water occurs in the fractures of the bulk rock. The amount of water in crystalline bedrock is thus controlled by the secondary porosity, the abundance of fractures and how the fractures are interconnected. The secondary porosity is created after the formation of the geological formation, in comparison to the primary porosity which is the natural porosity which is 4

15 Modeling the aqueous chemistry of uranium On a national level in Sweden developed during the formation of the geological body. In some parts of Sweden, sedimentary bedrock is the predominant bedrock. The sedimentary bedrock consists of either sandstone or limestone, which both are porous. The sedimentary bedrock can consist of both primary and secondary porosity; this results in that the bulk rock can hold groundwater, in addition to the fractures in the rock (Hiscock, 2005; SGU, 2008). Sedimentary bedrock is though much more easily weathered compared to crystalline bedrock which might result in a higher amount of dissolved ions and TDS in the groundwater. The groundwater quality is also interconnected with how deep in the ground the groundwater exits; generally it is known that the deeper the water is extracted from, the better the quality. Long residence time might though decrease the water quality, since the amount of dissolved ions will increase due to increased interaction time with the bedrock. The quality of the groundwater is also affected by human actions; such as waste dumps and salt from salting the roads. The damage produced by humans is though easier to prevent than the natural ones (SGU, 2008) Uranium and geology In Sweden there are four main areas which have high concentrations of uranium. One of them is the Upper Cambrian and Lower Odrovician sediments in southern Sweden. It stretches along the Caledonian mountain range, where the uranium occurs in alum shales. An area in northern Sweden just south of the polar circle, Arjeplog- Arvidsjaur-Sorsele, also contains high concentrations of uranium. This area contains more than 20 occurrences of uranium which are discordant, of vein- or impregnation type. The third area is in central Sweden, just north of Östersund, also here in discordant formations. The fourth province is located in northern Sweden, near Åsele (OECD, 2005). Bedrock types which commonly contain elevated concentrations of uranium are silicon rich types such as; granite, pegmatite and syenite. Sedimentary bedrock usually has lower concentrations of uranium, except some uranium rich veins. Alum shale is an exception, but wells are rarely located in areas where alum shale in present, since the overall water quality is general low (Birke et al. 2010). The characteristics of a uranium deposit associated with a granitic rock are; low CaO and MgO and high total alkalinity compared to average granitic rocks (Zielinski et al. 1991). The most common uranium-bearing minerals are: uraninite (UO 2), pitchblende (U 3O 8), coffenite ((USiO 4) 1-x(OH 4x)), autunite (Ca(UO 2) 2(PO 4) H 2O) and uranophane (Ca(UO 2) 2SiO 3(OH) 2*5(H 2O). These minerals are relatively rare and restricted to uranium mineralized zones. It is estimated that approximately 5 % of all identified minerals contain uranium (Smedley et al. 2006). Uranyl minerals are soluble at ph 5-8.5, and this is also when the adsorption of minerals is the strongest. Langmuir et al. (1978) states the solubility is dominated by the sorption, when controlling the uranium mobility in the water. The concentration of uranium is though not very high throughout the rock. Uranium tends to concentrate in cracks where hot water circulates though a pluton and dissolves the uranium and later precipitates it along cracks with other elements (Marshak, 2005). Uranium can also concentrate when water percolates though uranium-rich sedimentary bedrock. The uranium dissolves and later precipitates in another location (Marshak, 2005; Smedley et al. 2006). The uranium concentration in different types of rocks can vary greatly (Table 3). The valence state of uranium in sedimentary rocks is U(IV) and is believed to originate from granites which have been eroded. There is also a correlation between higher uranium concentrations and high concentrations of organic material in sedimentary bedrock (Andréasson, 2006; Birke et al. 2010). The bedrock has formed under reducing conditions which has favoured the stability of U(IV) and the strong binding to organic material (Birke et al. 2010). Phosphate ores has been discovered to be uranium rich; concentrations of mg/kg uranium usually occur in phosphate ores (Ragheb et al. 2010). Table 3. Variations of U content in different bedrock types (Modified from Ek et. al 2008). Source Average of the world 3 U (mg/kg) Intrusive basic bedrock Granites 2-6 Granites, Uranium rich 8-40 Lime stone Sand stone Clay slate 1-10 Alum shale U-ore of high quality

16 Åsa Löv TRITA LWR Degree Project 12:32 In oxidizing conditions uranium has been confirmed to be relatively soluble and, in reducing conditions the opposite. In oxidizing environments uranium forms the uranyl ion, discussed earlier, which forms complexes with common groundwater components like carbonate, sulfate, phosphate and fluoride, and as a result it becomes mobile. In reducing environments uranium will precipitate and become immobile. This could result in a redistribution of uranium depending on different redox states though out the rock outcrop or fracture. Oxidizing conditions increase the mobility of uranium and, the oxidizing conditions decrease with increasing depth due to consumption of oxygen by organic and inorganic reactions along the flow paths. As a result uranium will mineralize deeper down in the fracture zones (Zielinski et al. 1991; Drake et al. 2009). Oxidation of U(IV) to U(VI) results in leaching of the bulk uranium content. In reducing conditions the isotope 234 U is more abundant in the oxidizing state U(VI) than 238 U, and is more easily leached than 238 U. In outcrops, or close to fractures, the analytical equilibrium of the alpha activity of isotope ratio of 234 U/ 238 U = 1.0 usually deviates. If the isotope ratio measured is < 1.0, this indicates an enhanced leaching of the lighter isotope 234 U; which is the typical scenario. When studying the leaching of all U isotopes, the ratio 230Th/ 238 U and 230Th/ 234 U is used; as Thorium (Th) is relatively insoluble. Evidence of intense weathering will give a ratio of > 1.0, and enrichment of U a ratio of < 1.0. The ratio of 234 U/ 238 U is slowly readjusted over time, due to decay or growth of 234U. The larger the ratio is the more weathering the rock has undergone (Zielinski 1991, Drake et al. 2009). The disequilibrium decreases with depth due to more stagnant conditions (Fig. 3). The redox zone has both annual- and short-term variations (Drake et al. 2009). The chemical composition in the crystalline rock changes with depth in fractures. When Ca 2+ and SO 4 2- are released by weathering of the rock due to water-rock interaction, the dissolved oxygen is decreased and the water becomes reducing. Mineral coatings on fracture walls is a result of redox-processes changing as an effect of the water going from oxidizing near the surface to reducing further downwards. The minerals are dissolved in the oxidizing areas and precipitated in the reducing areas, forming fracture coatings (Bucher et al. 2008). This area is called the redox front and can be detected by analysing the fracture mineralogy and geochemistry (Drake et al. 2009). Drake et al. (2009) states that the redox front generally occurs in the top meters, this can be seen on the distribution of the redox sensitive minerals; goethite and pyrite. The goethite is predominating in the oxidizing environment and, pyrite in the reducing. Looking at the Fig 3. Outline of how uranium redox conditions and isotope fractionation change though out a fracture (Drake et al. 2009). 6

17 Modeling the aqueous chemistry of uranium On a national level in Sweden uranium it can be observed that in the top 20 meters uranium is mainly removed due to the oxidizing conditions or has experienced complex removal. In the range of meters both deposition and removal is indicated, and below 55 meters uranium is deposited. This is also correlated with the scattered deposition of goethite in the reducing depths, down to approximately 80 meters. Signs of uranium removal meters down furthermore correlate with fractures with elevated transmissivity. This is an indicator of that fractures with elevated transmissivity is required to transport oxygenated groundwater to deeper than 30 meters (Drake et al. 2009). Elevated transmissivity could also be an indicator of large water-volume to rocksurface-area ratios, which could dilute the uranium concentration. Elevated transmissivity might also be an indicator of higher groundwater velocities (Lawrence et al. 1990). Elevated water velocity results in shorter water-rock interaction time, which could result in decreased weathering. In sedimentary bedrocks it is easier to distinguish where the redox front is located; there is usually a colour change where the front is situated (Drake et al. 2009) Uranium in the bedrock groundwater Uranium in the groundwater originates from weathering of the bedrock, or from anthropogenic sources such as uranium mining (Prat et al. 2009). What species of uranium is dominating in the groundwater, and the concentration of uranium, is influenced by several parameters (Bucher et al. 2008). Elevated concentrations of uranium in groundwater are correlated with low ph, low adsorption capacity, and presence of uranium ores (Birke et al. 2010). As mentioned the source rock is one of the main parameters and the vicinity to water to the uranium bearing rocks or minerals. The ph and the redox state of the water have great influence on what species uranium will form (Bucher et al. 2008). Equilibrium with one of the most common uranium bearing minerals, uraninite, limits the concentration of uranium in the groundwater to 0.06 µg/l in anoxic conditions (Smedley et al. 2006). Uranium exists in three oxidations states in groundwater depending on redox conditions; U(IV), U(V) and U(VI) (Langmuir, 1997). The insolubility of uraninite and coffenite at normal groundwater ph (4-8) makes uranium practically immobile in reducing waters (Fig. 4) (Langmuir 1978; Smedly et al. 2006). pe is a measure of the redox state, low pe values indicates reducing conditions and Fig o C, pco 2 = atm. (Smedley et al. 2006). elevated pe values indicate oxidizing conditions. The graph thus gives an indication of which species of uranium can be expected under various ph and redox conditions. Moreover at which redox conditions and ph uranium can be expected in solid state. Common elements in the groundwater such as carbonate, phosphates, fluorite, sulfate, silicate, calcium, potassium and vanadate are all important complex forming ions for uranium, though carbonates and calcium are known to be the predominating ones (Zielinski et al. 1991; Bucher et al. 2008). The presence of highly sorptive materials such as; organic matter, ferric, manganese, and titanium oxy-hyroxides and clay affects the concentration of mobile uranium (Zielinski et al. 1991; Smedley et al. 2006; Bucher et al. 2008; Jokelainen et al 2010). Uranium is adsorbed to clay minerals very efficiently as a result of the large contact surface of the clay mineral. The sorption is most efficient in the near neutral ph range (Smedley et al. 2006) and in waters with high alkalinity the adsorption of uranium to iron hydroxides decreases (Ek et al. 2008). The degree of hydraulic isolation, meaning the degree of mixture of fresh surface water and subsurface water, and the seasonal variability of the water, such as temperature, are also important factors for concentration or speciation (Langmuir 1978). The temperature being fairly low during most of the year in Sweden decreases the weathering rate (Ek et al. 2008). It has also been stated by Aastrup et al. (1995) that there are temporal variations in the groundwater chemistry. The temporal; seasonal, variation decreases with increasing depth (Bucher et al. 2008). 7

18 Åsa Löv TRITA LWR Degree Project 12:32 The chemistry of the groundwater is highly dependent on the physical and chemical interactions in the media that the water travels through. When the water infiltrates through the soil layer interactions such as weathering of the soil and ion exchange will take place. Easily weathered soils, e.g. lime rich soils, will increase the concentration of carbonates, calcium and magnesium. In areas with increased temperatures the weathering is more abundant. Since the most common bedrock type in Sweden is granite, which is not easily weathered bedrock, this consequently leads to a soil layer which will be hard to weather; additionally the temperatures are comparatively low though out most of the year, this is also reflected in the groundwater temperatures in Sweden. These factors together result in a low concentration of total dissolved solids (TDS) (Ek et al. 2009). The oxygen concentration and TDS are negatively correlated. The total TDS is lower in oxygenated water, than in reduced water. The groundwater chemistry is thus largely controlled by recharge water with low TDS; water containing primary minerals of the rock matrix which are not in equilibrium, thus striving to reach equilibrium state. Where the permeability is high, hence the advection is increased, the water travels faster and the contact time with the rock is shorter, accordingly the amount of TDS is most likely to be higher in water with lower advection (Bucher et al. 2008). Furthermore, the groundwater in Sweden should have relatively low TDS, thus elevated oxygen levels; resulting in an average of higher mobility of U(VI) Toxicity of uranium Earlier in this report it was stated that it is not the radioactive properties of uranium which are of major concern for humans, but the chemical toxicity of uranium. An intake of 100 µg/l throughout a year is needed to reach the radiation guideline value; 0.1 msv/year (Swedish Health Authorities, 2006). Most studies covering exposure to uranium and the effects have been carried out on rats (Kurttio et al. 2002; Svensson et al. 2005; Prat et al, 2009). These studies has clearly revealed that uranium is nephrotoxic; toxic to kidneys. Surveys which have studied the effect of uranium in drinking water from drilled wells have establish that very high dosages of uranium (10-25 mg/kg body weight) can cause acute renal failure and, irreversible damage to the kidneys (Kurttio et al. 2002). Zamora et al. (1998) studied a population who were exposed to a much lower dosage of uranium ( µg/kg body weight), and it was concluded that there is an increase of uranium in the urine, however that this will not necessarily result in kidney failure or evidently illness, nonetheless that long term consumtion of uranium will interfere with the functionality of the kidneys. Kuritto et al. (2002) delineates that the renal effects caused by uranium is not correlated with the duration of exposure, hence the short term exposure which could affect the kidneys. This means that the effect of uranium is presumably not accumulative. Only a small fraction of the ingested uranium is accumulated in the body; a minimum of 98 % ingested in soluble form is discharged with faeces. When the uranium reaches the blood stream it forms a complex with bicarbonate, proteins and red blood cells (Svensson et al. 2005). However, 66 % of the uranium which reaches the blood exits the body within 24 hours though the urine. The remaining fraction is adsorbed in the kidneys (12-15 %), bones (10-15 %) and the soft tissue (Prat et al. 2009). The uranium does not stay in these organs indefinitely; 50 % of the uranium leaves the kidneys in 2-6 days and half of the uranium in the bones after 11 days (ATSDR, 2011). The Swedish National Food Administration though claims that the average half-life of uranium in the body is a half a year, up to a year, and the half-life in the skeleton is significantly longer (Svensson, 2005) and WHO (2005) states that the half life in the kidneys is 15 days. Out of the uranium entering the skeleton, about 85 % replaces the calcium in the bones, which could result in osteoporosis (Svensson et al. 2005). The ability of the body to adsorb uranium depends on the diet of the person who is exposed. A person whom is indulging uranium rich water on a fasting stomach is believed to adsorb a higher percentage of the uranium, or a person who has iron deficiency will absorb more uranium in the intestinal canal (Svensson, 2005). It has moreover been discovered that uranium can accumulate in the brain, and cause signs of depression and agitation. The accumulation is not uniform though out the brain, and the effect is dose dependent (Lestaevel et al. 2005). It is apparent that there are uncertainties in the matter of the toxicity of uranium; both when discussing if the uranium accumulates or not, and if it does, how much it accumulates. The specie of uranium has been claimed to be of major importance of the toxicity of uranium. Ca 2UO 2(CO 3) (aq) and CaUO 2(CO 3) 2- is nontoxic and non-bioavailable, whereas UO 2(CO 3) 3 4-8

19 Modeling the aqueous chemistry of uranium On a national level in Sweden is cytotoxic, meaning that it is toxic to cells. It is nevertheless not known if the calcium-uraniumcarbonate complex could be dissociated in the stomach due to the low ph in the gastric acid (Prat et al. 2009). Accordingly, the calciumuranium-carbonate complex might be toxic nonetheless Guideline value for uranium The guideline value for uranium in Sweden is 15 µg/l, which is the same guideline value that WHO suggested in year 2005 (WHO did change their guideline value in July 2011 to 30 µg/l), and it is based on the toxic effects of uranium (Svensson, 2005; WHO, 2005). This value denotes that if drinking water has a higher concentration than 15 µg/l, mitigation measures should be implemented. The value of 15 µg/l is based on the result of the most extensive animal study which was carried out on rats by Gilman in He studied the Lowest Adverse Effect Level (LOAEL) in the liver, thyroid gland and the kidneys, when rats ingested uranium rich drinking water during a period of 91 days. The LOAEL value obtained was 60 µg/kg body weight per day. The safety factor that WHO has adapted to the result to the guideline value is 100, resulting in; 0.6 µg/kg body weight per day. This is to compensate for differences in amount of intake though drinking, solid food and body weight; the original value was divided by 10 for the extrapolation between species, and again by 10 due to the variation within the species. For humans a bodyweight of 60 kg and a daily intake of water of 2 liter were assumed. 80 % of the ingested uranium is assumed to be through drinking water. This results in 36 µg of uranium per person per day, whereof ~30 µg is allowed to be ingested through drinking water The assumed amount of water consumed per day per person is two liters, which finally gives the guideline value of 15 µg/l (WHO, 2005). In Sweden only 39.4 % of the population drink more than one liter of tap water daily (Maxe, 2007), which indicates that most people probably consume less than two liter of water daily. From the radiation aspect, drinking water with 15 µg/l only increases the natural background radiation 2-4 msv with a few percentage units, and will thus not affect the carcinogenic risk (Svensson, 2005) Uranium removal from groundwater There are several methods on the market for the removal of uranium from the groundwater. A requirement for these techniques to be efficient is to know the speciation of the uranium in the groundwater which is extracted, since the technique in one or another way depends on the change of the uranium species (Swedish Health Authorities, 2006). As mentioned earlier the charge of the most common speciation of uranium in groundwater changes rapidly depending ph, thus it is very important to be aware of the water conditions in each well. Since the speciation of uranium is highly dependent on the local groundwater chemistry the technique chosen must be specified for a certain well. Some of the most common once are; ion-exchange; anion and cation, reversed osmosis, activated carbon, electro dialysis, lime softening and coagulation. Many of these methods are expensive and high-technological, nonetheless they are very efficient. Coagulation techniques have been proved to have a removal efficiency of % and ion exchange approximately % (Smedley et al. 2006; Swedish Health Authorities 2006) Previous uranium speciation modelling The modelling performed by Langmuir (1978) was one of the first uranium speciation modelling and it revealed that in waters with typical concentrations of fluoride, chloride, sulfate and phosphate and a ph below 3-4; U(IV) fluoride complexes predominates (Fig. 5). Under oxidized conditions, in ph below 5, uranyl-fluoride and the uranyl ion predominates. Langmuir (1978) also state that in alkaline waters uranyl-carbonate complexes predominates. Later modelling performed by Langmuir (1997) also indicates that the carbonate speciation is of great Fig 5. Result from Langmuir (1978) when performing one of the first uranium speciation modelling. 9

20 Åsa Löv TRITA LWR Degree Project 12:32 Fig 6. Langmuir (1997) states that the carbonate speciation is of great importance at ph > 6. importance in groundwater, where carbonate species can be observed at acidic ph values (Fig. 6). The latter modelling which has been performed agrees to a large extent with the result which Langmuir arrived at almost 35 years ago. Prat et al. (2009) preformed a modelling exercise and discovered that interactions with calcium are definitely the most important and that fluoride also is an important speciation agent. He discovered nevertheless that uranyl speciation with magnesium and strontium can be neglected. In waters with ph 8-9 and high HCO 3 - concentrations, HCO 3 - dissociates to CO 3 - and H +, and CO 3 - is expected to be the most important ligand for uranyl. In waters with HCO 3 - concentration of mm and alkaline ph, hydrolysis of U(VI) can be neglected. The total organic concentration (TOC) was discovered to be low; 10ppm, which corresponds to less than 1 mm, representing a low concentration in natural waters (Prat et al 2009). The transportation of uranium with humic colloids can at ph greater than 7, and when there is a substantial amount of carbonate present, be neglected (Ranville et al. 2007). In Prat s et al. (2009) study the presence of organic matter was neglected in the species analysis of uranium, since the samples all had a ph > 7.9. Gustafsson et al. (2008) also states that dissolved organic matter (DOM) is a very important complex agent in groundwater with low ph, and that this complex might even be the predominating one in the lower ph range. He also agrees with Prat et al. (2009) and Ranville et al. (2007) that at ph > 7, and at elevated pco 2 in waters with elevated concentrations of carbonates; which is common conditions for groundwater, DOM can be neglected, and the uranium-carbonate complex will be predominating. Calcium has a major influence on the speciation of U(VI) and consequently also on the sorption and mobility of U(VI) in aquifers (Prat et al. 2009). The complex Ca 2- UO 2(CO 3) (aq) influences the speciation of uranium significantly in the ph rage 6-10 in calciumand uranium rich waters (Bernhard et al. 2001). Gammons et al. (2003) discovered when performing a uranium speciation modeling exercise on sampled mine wastewater that under very acidic conditions (ph < 2.5) uranyl sulfates together with uranyl ion by itself will be the absolute predominating species, reaching fractions between % and 27 % respectively. 3. METHODS This study is based on a data set provided by SGU and SSM, and the goal was to perform a uranium speciation modelling based on the local groundwater chemical conditions of every sample. To do this it is important to understand which parameters are important to regard and to be clear of what constants to use. The methodlogy is divided into four subdivisions; literature study, species modelling in Visual MINTEQ 3.0, a statistics analysis using SPSS 20.0 and Excel and, the result of the modelling exercise correlated with the bedrock using ArcMap 10 and plotted with SPSS Literature study Most of the literature used was found at The Royal Institute of Technology s library website, which links to several databases such as Web of Science, which was frequently used. The literature study resulted in the Background section which is a good basis for what it is important to consider when performing the modelling, what could be expected and, when evaluating the model as well as a foundation for the discussion of the result Speciation modelling Chemical speciation is used to investigate the distribution of an element between chemical species in a system. The speciation of an element decides the fate of the compound and its toxicity and transport mechanisms. This in addition to the complexity of deciding the speciation using analytical chemistry makes chemical modeling tools very powerful. When all parameters and elements have been stated in the model the model will use the equilibrium constants and calculate the speciation of all elements entered in the model, with reference to the physical parameters entered as well. Physical parameters include temperature and pressure and additional 10

21 Modeling the aqueous chemistry of uranium On a national level in Sweden model specified parameters. The equilibrium constants are default in the program and are originally determined experimentally (VanBriesen et al. 2010). When performing a model there are several parameters which should be considered when evaluating the outcome of the model. There are four major uncertainties which should be taken into consideration; Decision rule uncertainty, model uncertainty, parameter uncertainty and parameter variability. Model uncertainty also includes the decision of which elements shall be included (VanBriesen et al. 2010). These uncertainties will be discussed in the later in the report Assumptions made in this model To narrow down the amount of samples from the 1042 samples to a comprehensible number some requirements for the samples to be included were set up. This was also important to do so that the outcome of the model cannot be blamed on the elements selected to be regarded. The samples chosen to be included in the model was required to have a minimum uranium concentration of 15 µg/l and measurement data of all elements available. It was also required that the water samples were raw water, thus the water was untreated. The final number of water samples came to be 76. This number is a comprehensible amount, but still large enough to make a statistical analysis. As a result of the focus on water samples with high concentration it has been assumed that all the samples are at oxidizing conditions. Comparing the concentrations of iron, manganese and sulphate to SEPA (2000) classification values for oxidizing/reducing groundwater types this assumption can be established to be true. Uranium concentration is not likely to be as high in reduced waters due to the high Kd value for uranium in such environment. It is due to former statement also assumed that there is no precipitation, solid form, of uranium in the water. To ensure that the competition between ions where not neglected, all elements which were provided in the dataset was used during the modelling. The temperature was set to 5 o C, thus this is the average groundwater temperature in Sweden (SEPA 1995). The pco 2 pressure was set to 038 atm, which is the atmospheric pressure. 038 atm was chosen since the speciation that the consumer ingests is what is of interest in this study, not necessarily where the water originates. The ionic strength was set to be calculated for each sample, since this is a model showing the existing conditions in the environment. The formation constants used in the modelling is the default in Visual MINTEQ 3.0, which originates from Guillamont et al. (2003) and, Dong et al. (2006) for the two calciumuranyl-carbonate complexes (Appendix I). Some of the samples have an excess of anions. This makes it possible to calculate the amount of DOC in the samples. But since this cannot be done for all the samples, the modelling was proceeded by introducing 1 mg/l DOC for all the samples. This is a low amount of DOC, which can be expected in waters which originate from these depths (Lindquist et al. 2010). Prat et al. (2009) measures a TOC concentration of 10 mg/l and announces this to be low concentration. An even lower concentration was chosen, since for the samples which it would have been possible to calculate the amount of DOC for, would have ended up with concentration at approximately 1 mg/l. The Stockholm Humic Model (SHM) was used for the speciation with DOM. It is a modern and advanced model and is considered to be a state-of-the-art model (Visual MINTEQ 3.0 a brief tutorial). The model was set to assume that 70 % of the DOM was fulvic acid, this value has been used earlier by Gustafsson et al. (2009) when performing groundwater chemistry modeling Statistics The correlation between the different parameters was examined. Using SPSS 20.0 it was possible to evaluate if the correlation was significant. The level of the correlation coefficient Pearson s r was set to > 0.5, to be regarded as significant. SPSS also calculated the significance of the correlation, p. The result will present the 0.05 and the 0.01 level, 0.01 having the most significant correlation. Excel and SPSS were used to create graphs Sensitivity analysis A sensitivity analysis was performed in an attempt to explore how the different physical and chemical parameters, which were chosen, affect the outcome of the model. The addition of DOC was tested, and how the speciation changes in the absence of organic matter. The affect of the temperature was tested as well, since this is something that varies seasonally, and could possibly affect the water chemistry thus the speciation of uranium. In the model a pco 2 of 038 atm was applied. By multiplying this by eight, the partial pressure at approximately 70 meters of depth is reached. This is the aver- 11

22 Åsa Löv TRITA LWR Degree Project 12:32 age depth of the wells; accordingly an understanding of the difference by doing a uranium speciation modeling at the point of origin and one at atmospheric pressure will be achieved. The five samples were chosen so that the whole range of ph values which are represented in the samples modeled, from 6.3 to 8, would be represented. ph was chosen since it has been proven to be the most important factor for uranium speciation Spatial correlation using ArcGIS A correlation between uranium concentration, speciation and bedrock type was performed with ArcGIS 10. SGU s geological database (regional and local geological maps) was used for the correlation and coordinates for each well where provided with the dataset from SGU and SSM. This study was an attempt to explain the speciation of uranium in correlation with the bedrock type. Furthermore to investigate if this could be a factor to consider when deciding what kind of treatment is needed to reduce the uranium concentrations in the drinking water. The boxplots were created in SPSS Sampling by SGU The sampling carried out by SGU and SSM is explained in their report Ek et al. (2008). Before sampling the water has been flushed for minutes, depending on depth of well, size of the pressure vessel and installations. Conductivity, temperature and ph was measured in field, the temperature is not confirmed anywhere. All samples used in this model were unfiltered; some of the water was sampled from the well and some from the tap. Major ions were measured according to standard methods and the uranium concentration was measured at SGUs laboratory with ICP-MS. The total alpha activity from natural uranium was also measured with LSC (Liquid Scintillation Counting); this can be recalculated to µg/l. Ek et al. (2008) states that the latter method could result in a measurement error. The lowest detection limit for uranium was 0.04 Bq/l, corresponding to 1.6 µg/l. 4. RESULTS Parameters were tested for correlation and the significance of the correlation. The significant correlations (p < 0.01) will be presented below and all of the correlation results can be viewed in Appendix II. Parameters which were tested but which did not show a correlation with speciation within the significance level was; alkalinity and, different concentrations of speciation elements such as fluoride and silicon concentration, nor ionic strength. There was no correlation between uranium concentration and any other constituent or element in the water samples. Neither did the depth of the well affect the outcome of the modeling exercise either Seasonal variation of uranium concentration and ph Seasonal variation of the uranium concentration and ph though out the year was plotted. The uranium concentration was plotted to understand if the amount of uranium changes seasonally, thus if the same extent of treatment is needed though out the year (Fig. 7). ph is the strongest factor for uranium speciation and seasonal variations were plotted as an attempt to investigate if there could be a seasonal variation of the uranium speciation. Fig 7. To the left; uranium concentration variations thoughout the year. To the right; ph variation thoughout the year. 12

23 Modeling the aqueous chemistry of uranium On a national level in Sweden Fig 8. The dominating uranium species formed in the model and ph for the different samples. The uranium concentration measured appears to have some kind of seasonal variations, thus the peak concentrations occur during autumn. The ph of the water does not seem to change attributable to season. It should be noticed that the sampling has only occurred from April to December, neither has any samples been taken in May Uranium concentration in relation with ph and alkalinity In this correlation all samples provided by SGU were included, not only the ones included in the modeling exercise. This was done to confirm that the samples used in the modeling exercise had similar water chemistry as most of the samples, and that the samples used in the model actually represents the average Swedish bedrock groundwater. The only difference is that the samples used in the model all had a uranium concentration which exceeds the guideline value in Sweden. The uranium concentration does not have a correlation with ph in the water samples analyzed. Most of the samples have a concentration around 1-40 µg/l and are concentrated at ph levels around 7.5. The samples diverging from these values are occurs randomly in the graph. The uranium concentration is neither ph dependent or dependent on alkalinity. There is no correlation between the uranium concentration in the water samples and alkalinity. Most samples have an alkalinity < 400 mg HCO 3/l, but the samples with extreme alkalinity do not have exceptionally high or low concentration of uranium. It is also evident that the samples chosen for the modeling exercise represents the rest of the samples well regarding alkalinity Uranium concentration and depth of well The depth of the well does not have a correlation with the uranium concentration and, is just above the significance level. There was no correlation discovered between the speciation of uranium and the depth of the well Uranium speciation correlation The complexes that made up more than 99 % of the species in the sample will be presented below. These complexes were; uranyl hydroxides, carbonates, calcium carbonates, DOM-complexes, fluorides, phosphates and silicon complexes. The results will be presented as percentages of a certain compound in the sample and not the concentration. This is because the interest lies within knowing what kind of species predominates in different waters with high uranium concentrations Dominating uranium species Plotting the result from the modeling exercise it was apparent that ph is the factor which has most influence over which uranium species is formed. This was revealed to be true regarding all water types sampled in this study. 13

24 Åsa Löv TRITA LWR Degree Project 12:32 Fig 9. Correlation between formation of the three different uranyl carbonates and ph. It is obvious that the ph and the formation of U-DOM are closely correlated (Fig. 8), just as for the carbonates and Ca-carbonates. U-DOM is predominating in the lower ph ranges and, the Ca-carbonate complexes in the alkaline waters. The formation of hydroxide complexes also has a clear correlation with ph. As for the formation of uranyl silicic acid and fluorides, these species are not at dominating but do have a negative correlation with ph. The samples with the more acidic water have the highest fraction of uranyl silicate complexes. These samples do not have exceptionally high concentrations of silicon, ( mg/l), some of these samples have low alkalinity ( mg HCO 3 -/l). The same samples also have the highest fraction of fluoride complexes, and the same pattern applies. This verifies that ph is the absolute most important factor when evaluating the speciation of uranium The uranyl ion and ph There is an obvious trend between ph and the stability of the uranyl ion, r = The fraction of the ion is though low at all ph-values. The uranyl ion is favored by the formation of complexes with ligands Uranyl Carbonates The uranyl carbonate specie is one of the dominating species at and around ph 7 (Fig 9). Uranyl Carbonates and ph The formation of uranyl carbonates has a correlation with ph; r = The carbonates tend to be most stable at and around ph 7, and descend in fraction toward both acidic and alkaline water. The dominating carbonate uranyl species is the uncharged UO 2CO 3 (aq). SPSS did not calculate a significant correlation for UO 2(CO 3) 2 2- r = , although a trend can be suspected when looking at the plot. The correlation for the two other complexes is very strong with ph; UO 2CO 3 r = and UO 2(CO 3) 4 3- r = Ca-carbonates-pH Ca-carbonate complex formation is clearly the predominating specie. At ph > 7.5, nearly all uranium exists in a Ca-carbonate complex. The significant positive correlation was established to be r = Ca-carbonate and Ca concentration A slight correlation between the formation of Ca 2UO 2(CO 3) 3 and the calcium concentration was discovered (r = 0.490), the correlation between the formation of CaUO 2(CO 3) 3-2 is not significant, (r = ) neither is the correlation between the formation of total Ca-carbonates and the calcium concentration significant (r = 0.332). From mg/l calcium the fraction of calcium carbonate uranyl is fairly variable. 14

25 Modeling the aqueous chemistry of uranium On a national level in Sweden Fig 10. The relationship between the formation of Ca 2UO 2(CO 3) 3 and Ca-concentration and ph. The groundwater sample with the highest fraction of Ca 2UO 2(CO 3) 3 also has the most elevated concentration of calcium and elevated alkalinity ( mg HCO 3 -/l) and uranium concentration (88.24 µg/l). When plotting both the calcium concentration and the ph in relation to the formation of Ca 2UO 2(CO 3) 3 two groups are visualized (Fig. 10). The formation of Ca 2UO 2(CO 3) 3 is more likely to be lower than 30 % if ph is less than approximately 7.2, and the formation is likely to be larger than 40 % if the ph is more than approximately 7.2. There is no correlation between ph and the calcium concentration, within this ph range Uranyl hydroxides Uranyl hydroxides are one of the dominating species at and around ph 7. A significant correlation between the formation of hydroxides and ph was discovered (r = -0,845). The hydroxides are dominating at ph 7 and less, and the fraction of hydroxides is descending towards more alkaline water. The dominating hydroxide is the UO 2OH + at ph < 7 and at ph > 7 UO 2(OH) 2 is predominate (Fig. 11). Examining the correlation for the hydroxide complexes separately it is observed that both UO 2OH + have significant level of correlation; r = , and UO 2(OH) 2 also has a strong correlation (r = -0,776). Fig 11. Correlation between the individual uranyl hydroxide complexes and ph. 15

26 Åsa Löv TRITA LWR Degree Project 12:32 Fig 12. Significant correlation between ph and the formation of U- DOM complexes U-DOM and ph A significant negative correlation (r = ) was discovered between ph and the formation U-DOM complexes (Fig. 12). The acidic conditions have a large range of different fractions of U-DOM, but in the water with ph 8, the fraction of U-DOM can be considered as 0%. The more alkaline the groundwater is the lower the fraction of U-DOM complexes is formed. The fraction of U-DOM varies greatly for ph 7.3 and even though a low concentration was applied to the model, fractions up to 35 % can the observed. For ph 7.4 most of the samples have low fractions of U-DOM, with only a couple of samples with fractions close to 10 % Uranyl fluoride and ph Uranyl fluoride complexes exists in acidic water (ph < 7), and a significant correlation was calculated; r = Organic material is a stronger competitor for the speciation though; accordingly DOM complexes will predominate under acidic conditions if organic material is present. The water samples having the highest fractions of fluoride complexes all have fluoride concentrations above, or close to, the Swedish guideline value 1.5 mg/l (Swedish National Food Administration, 2011). The concentration of fluoride in the water sample did not have any significant correlation with the fraction of fluoride speciation with uranyl, this is probably because of the affinity of uranium to form a complex with DOM. Fig 13. There is a clear trend of the formation of anions, cations and uncharged complexes and ph. 16

27 Modeling the aqueous chemistry of uranium On a national level in Sweden Fig 14. The effect of introducing 1 mg/l DOC at different ph Uranyl silicic acid and ph A strong negative correlation was discovered between ph and the formation of silicic acid uranyl complexes, r = The fraction decreases with increasing alkalinity Cation-, anions- and neutral uranyl complexes and ph The correlation for anions and cation regarding ph is significant, strong correlation for anions and relatively strong correlation for cations, r = and r = , respectively (Fig. 13). For the two different categories all anions and cations, including all the species making up the 99%, has been added together, respectively. The anions have a positive correlation with ph, the fraction of anions increase with increasing ph, and the pattern follows a steady increase. The cations have a significant negative correlation with ph. The linear correlation of neutral complexes is not as strong as for the charged ions but it is significant, r = 0.563, and the trend is clear. Fig 15. The temperature (5-25 o C) does not have a substantial effect on the speciation of uranium. 17

28 Åsa Löv TRITA LWR Degree Project 12:32 Fig 16. The effect of elevated pco 2 and assuming possible formation of calcite Sensitivity analysis The sensitivity analysis confirms the effect of the defined parameters. The defined parameters tested are the introduction of 1 mg/l DOC, temperature and pressure The effect of introducing DOC The effect of introducing DOC is apparent in the lower ph range (Fig. 14). The DOM-speciation is the one of the predominating species and about 30 % of the uranium complexes will be in an organic complex at ph 6.7. It is also, as predicted, clear that in the absence of organic matter, fluoride- and silicic acid speciation including the formation of UO 2OH + become more important. The samples with a ph > 7 are not affected significantly by the introduction of organic matter. The Ca-carbonate species are the predominating, showing that carbonates have a higher affinity to uranium than organic matter, fluoride and silicon, in alkaline waters The effect of temperature The temperature does not have a substantial effect on the speciation of uranium (Fig. 15), when comparing temperatures which exists in the Swedish environment. The difference can only be observed in the alkaline samples where the UO 2CO 3 (aq) is predominating in the higher temperature The effect of pressure Elevated fractions of carbonates were discovered in the acidic samples when the pco 2 increased by 8 * atm pressure; this is en effect of the carbonate system in water (Fig. 16). When increasing the pressure the saturation limit for calcite is reached (Gustafsson et al. 2009). When modeling the samples with higher pressure possible formation of calcite was assumed. The speciation of uranyl and DOM is lower at the higher pressure but still the second most abundant species. In the alkaline samples the difference is not as pronounced, Ca-carbonates are predominate. The formation of UO 2(CO 3) 4 3- becomes more pronounced when increasing the pressure. When not assuming possible formation of calcite the fraction of UO 2(CO 3) 4 3- was negligible, and it was not plotted in the result ArcGIS correlation between uranium concentration, species and bedrock Spatial distribution of uranyl complexes in examined wells throughout Sweden. A correlation to the bedrock type the well was situated in was performed using ArcMap 10. For some of the wells there was no geological data; the total number of wells which there was geologic data available, was 68 (Table 4). The most abundant bedrock type where a drilled well was located was granite. The classification is according to SGU classification in the geological data file provided. Metamorph includes both felsic metavulvanite and gneiss. The peak uranium concentrations occur in sedimentary bedrock types. 18

29 Modeling the aqueous chemistry of uranium On a national level in Sweden Table 4. Number of samples in each bedrock type in the correlation analysis with ArcGIS. Bedrock type Granite 38 Granodiorite 12 Metamorphic 9 Siltstone to sandstone 7 Limestone 2 Total number of samples 68 Number of samples Moreover no clear trend was discovered in this study, thus only the correlation of uranum concentration and bedrock, and correlation between ph and bedrock will be presented. The outcome of the correlation for the uranium species is found in Appendix III Correlation of uranium concentration and bedrock The siltstone and limestone contains the most elevated concentrations of uranium. The median value is similar for all the remaining bedrock types. The highest concentration of uranium exists in the Siljansringen area. Most samples are within the µg/l range (Appendix III) Correlation of ph and bedrock Granite has both the most alkaline water and the most acidic. The water from the sedimentary bedrock only has a ph exceeding 7. The ph varies considerably within the geographical area of Siljansringen (Appendix III) Sampling density in a specific area, Siljansringen The sampling points in the Siljansringen area are located close to each other (Fig. 17). This should be regarded when evaluating the results from the spatial correlation, as this can be the source of error to the spatial correlation between bedrock and uranium speciation. 5. DISCUSSION Fig 17. Sampling points in the Siljansringen area. When modeling, there are many parameters that could give a misleading outcome. It is very important to understand that a model is only an attempt to mirror the reality. Uranium has been modeled before but never, as far as I know, to this extent; with this amount of samples and this national extent. Uranium has the ability to form many different species and is a very complex element which is of concern in many different areas in our society. In this study a speciation modeling was performed in an attempt to understand which uranium speciation which will predominate in private drilled wells in Sweden. The water chemistry of the samples was provided by SGU and SSM. But some parameters which are of great importance for the outcome of the modeling had to be estimated, in an attempt to assimilate the environment were the water is extracted, such as; pco 2, temperature, concentration of DOC. The modeling was performed to get a better understanding of which parameters govern the concentration and the speciation of uranium, the spatial differences and correlation with bedrock Speciation modeling The model was set to run at 5 o C, since this is the average groundwater temperature in Sweden. The assumed pco 2 was set to 0,00038 atm, which is the atmospheric pressure. This pressure was chosen since this is the pressure the water will be under when the consumer consumes it. The speciation of uranium will not be the same at much higher pco 2 deeper in the bedrock. The correlation to the bedrock is still valid since the contribution from the bedrock only is the concentration of the ions water and not the speciation. It was possible to calculate the amount of DOC for approximately half of the 19

30 Åsa Löv TRITA LWR Degree Project 12:32 samples. Since it was not possible to perform this for all of the samples a low concentration of DOC was introduced. For the samples where it was possible to calculate the DOC concentration, the result came to be just over 1 mg/l. If the DOC is calculated only for some samples it gives the modelling an inconsistency in the model which would not have been accepted. Indirectly it would also have been assumed that the samples with an excess of cations would have larger measurement error, than the samples with an excess of anions. Previous researchers (Ranville et al. 2007; Gustafsson et al. 2009; Prat et al. 2009) have claimed that if the ph of the water is > 7.9 the speciation with DOM can be neglected. Since the highest ph value of the water samples in this study was 8, DOM should be introduced. Only a low concentration was applied since most of the water samples originnate from deep bedrock fractures, and by only applying a low concentration it is also possible to detect how dominating the DOM speciation is. Thus it should be remembered that if the actual composition of DOM deviates from 70 % fulvic acid, the result might turn out different, hence humic acid is a much stronger complexion agent than fulvic acid. The formation constants, which originate from Guillamont et al (2003) and, Dong et al. (2006) for the speciation were kept in Visual MINTEQ 3.0. These were the same constants which has been used earlier when Gustafsson et al. (2009) modelled the speciation of uranium, thus can be considered as reliable constants Seasonal variation of parameters Seasonal variations in the groundwater flow could indicate differences in advection which could affect the amount of TDS in the water. As explained earlier, the composition of the water chemistry is a very important factor and this is partly governed by factors like advection. The water could be assumed to have higher velocity during spring and the autumn, due to snowmelt and rain, thus the concentration of uranium was expected to be lower during these seasons, as a result of dilution. This was not the outcome when plotting seasonal variations. The opposite can be divined, the peak concentrations only occur during autumn. This could plausibly be explained by the elevated amount of water which could lead to increased weathering of the bedrock, hence increased amount of dissolved ions. This seasonal variation contradicts what Ek et al. (2008) discovered in their study of seasonal variation; where they did not find any significant seasonal variation in uranium concentration. The ph does not confirm a seasonal variation. It can therefore from this study be assumed that the speciation of uranium does not change though out the year, since ph is the most governing factor for speciation of uranium. The samples only represent late spring, summer, autumn and late autumn; no samples were taken during winter nor in May. This should be regarded as a drawback when evaluating the seasonal variances. Bucher et al. (2008) discusses that the seasonal variation decreases with increasing depth. If the wells would have been shallower, there might have been a clearer trend Uranium speciation As mentioned in the background SGU (2008) claims that the groundwater quality in correlated with the depth of the well. From the result of this modeling exercise this cannot be proven. There is no correlation with the concentration of uranium and depth or, with the speciation of uranium and depth. This is interesting since Drake et al. (2009) discusses how the redox chemistry changes at meters depth, but it is not evident in the result of this study. It could be imagined that the samples from a deeper well would contain a lower concentration of uranium, because of the reducing conditions. Thus all wells which have high concentrations of uranyl fluoride are meters deep, which does not support this thesis. Uranium is very redox sensitive and in the model oxidizing conditions were assumed, since the interest for this study lies within the speciation of uranium at the point of consumption. Different redox states should though be considered when modeling uranium at the origin of the water. The speciation of uranyl carbonates does in general correlate well with what previous research has published, but there are some differences. The results show that UO 2CO 3 0 predominates up to ph 7.5 and at ph levels above that UO 2(CO 3) 2 2 and UO 2(CO 3) 3 4 will predominate, whilst the Swedish Health Authority states that UO 2CO 0 3 only exists at ph 5-6.5, UO 2(CO 3) 2 2 at ph and at higher ph ranges UO 2(CO 3) 3 4 will predominate. As mentioned earlier in the report, OU 2(CO 3) 3 4- is cytotoxic, thus it is shown to be the dominating species in all water samples with ph > The difference of 1 unit in the ph range, thus assuming the wrong charge of the ion, could result in inefficient treatment. The occurrence of calcium carbonate uranyl agrees well with what is suggested by Bernhard 20

31 Modeling the aqueous chemistry of uranium On a national level in Sweden et al. (2001) Gustafsson et al. (2009) and Prat et al. (2009). The alkalinity and Ca 2+ concentration are in Prat s et al. (2009) samples comparable to the alkalinity and the Ca 2+ concentration used in this study. Gustafsson et al. (2009) does not state which concentration of Ca 2+ was used in the model, only that it was calculated from charge balance. Bernhard et al. (2001) though only considers elements relevant to formation of Ca 2UO 2(CO 3) 3 (aq). This proves that the general water chemistry is not very important at ph > 8, when there are natural calcium concentrations available. The grouping in the scatter plot visualizing the correlation of calcium concentration, ph and the formation of Ca 2UO 2(CO 3) 3 (aq) is worth noticing. There seems to be a gap between the fractions of 30 to 40 %, where the ph of 7.2 seems to be the dividing factor, whilst the concentration of calcium does not seem to have major influence. It is interesting how the calcium carbonate uranyl specie is as predominate in study as the result shows, when the calcite precipitates at ph = 8.04 at pco 2 = 7.6*10-4 atm, and at 7.39 at elevated pco 2 (Gustafsson et al. 2009). The water samples analyzed in this model all have ph 8, thus it is not obvious that Ca-carbonates would be the dominating specie. Nevertheless, because of a weak correlation of the formation of Ca 2UO 2(CO 3) 3 it could be stated that when the concentration of Ca 2+ is elevated there is a plausibility of formation of calcium carbonate uranyl. Since this is supposeable a non-toxic species, and there is a slight correlation between the concentration of calcium and fraction of calcium carbonate uranyl fraction, calcium could be added to alkaline waters to reduce the toxicity of uranium. The research regarding the toxicity of calcium carbonate uranyl should be elaborated before such a technique is implemented. It is interesting to see that the formation of carbonate species is not significantly correlated with the alkalinity of the water, but only the ph and to some extent the concentration of calcium. The hydroxides together have a very strong correlation with ph. The peak fractions exist between ph The result correlates well with Prat et al. (2009); that hydrolysis can be neglected in alkaline water. Langmuir (1978) and Grenthe et al. (2004) states that UO 2(OH) 2 predominates at neutral ph, in this study UO 2OH + predominate < 6.8, and then UO 2(OH) < ph < 7.5. There is no correlation between the concentration of uranium and the formation of uranyl hydroxides, which is contradictive to what previously has been revealed by both Langmuir (1978) and Grenthe et al. (2004). The organic complexes with uranium are immensely predominating in the lower ph range. It can though be observed that the fraction of U- DOM can vary greatly in ph just above 7, but at ph 8 the fraction of U-DOM can be neglected, regardless of the water chemistry. As can be observed in the U-DOM graph is that there are fractions of up to 25 % of organic complexes up to ph values of 7.3. This is not interrelated with the assumption that Ranville et al. (2007), Gustafsson et al, (2009) and Prat et al. (2009) made; that alkaline waters with ph > 7, organic complexes can be neglected. Since the DOM complexes are predominate it can be suggested that a small amount should be introduced when modeling uranium, even though the ph is > 7, only to ensure that the DOM species can be ignored as an important complex. I do not agree with Ranville et al. (2007), Gustafsson et al (2009) and Prat et al. (2009) that the formation of organic complexes can be neglected at ph > 7, at elevated pco 2. The bicarbonate concentrations used in the model are comparable to the concentration Prat et al. (2009) used. He denotes these concentrations to be elevated. This proves that even though the carbonate concentrations are elevated, the formation of U-DOM cannot be neglected in alkaline waters. The pressure applied in the sensitivity analysis reflects 70 meters depth, if a lower pressure would have been applied, the fraction of organic complexes would be higher. The formation of uranyl fluorides reaches up to a fraction of 18 %, and has a significant correlation with ph, but not with the fluoride concentration in the sample. It should though be mentioned that the samples with the highest fluoride concentration all have concentration close to or above the Swedish guideline value. The fluoride concentration used in the model is higher than the concentration Langmuir (1978) used; this should not have a major influence since there is no strong correlation between the formation of uranyl fluoride complexes and fluoride concentration. The formation of uranyl silicic acid has a significant correlation with ph. The formation of silicic acid uranyl complexes does not have a correlation with silicon concentration. The formation of phosphate uranyl was neglible in this study. Because of the correlation of uranium formations with phosphates 21

32 Åsa Löv TRITA LWR Degree Project 12:32 in phosphate ores and autunite, formation of uranyl species with phosphates was expected. The fraction of these species would probably have been elevated if DOM would have been neglected, which is proven in the sensitivity analysis when excluding DOM from the model. The result for the speciation with silicic acid is interrelated with Langmuir (1978), who states that uranyl can complex with silicic acid at ph 6. The result in this model shows that independently from the water type, the peak of silicic acid complexes are just above ph 6 and, the fraction decreases with increasing ph. When analyzing the anion-, cation and neutral complexes fractions in the samples it is apparent that it is ph dependent. No other parameter, physical or chemical, has a significant correlation r 0.5, other than ph. The anions predominate in the alkaline waters because all the cations are more strongly absorbed in alkaline water, and the cations predominate in the lower ph range since the anions are more strongly adsorbed to the surfaces in acidic water (Gustafsson et al. 2007). The neutral complexes will predominate at ph 7.2. This can be referred back to basic water chemistry where anions tend to predominate in acidic water and cations in alkaline water. It is observable that the neutral uranyl carbonate- and hydroxide species are dominating in the order of neutral ph. It can easily be stated that by measuring the ph, the dominant charge can be predicted, and treatment method can be chosen Sensitivity of model The sensitivity analysis of the model gives further understanding of which physical parameters that are important when performing uranium groundwater modelling. Five samples with different ph (between ph 6.3-8) were chosen in a way that these would represent all of the samples in the model. The samples were chosen accordingly to ph, since this has been proven to be the most important factor for speciation of uranium. As previous models have shown, when DOM is not present fluoride and silicon become more important speciation agents at lower ph. The alkaline waters were not affected by the introduction of DOM. The temperature did not affect the speciation significantly, thus there should not be any seasonal variations in the speciation of uranium. Langmuir et al. (1978) states that the temperature is one important parameter for the speciation of uranium, according to the sensitivity test; this cannot be proven. Langmuir et al. (1978) proves that when increasing the temperature to 100 o C under atmospheric pressure the importance of carbonate complexes decreases and the hydroxyl complexes predominates. These conditions do not exist in the natural environment, thus it has not been tested in this study. As expected when increasing the pco 2, the formation of Ca-carbonate uranyl increases significantly at ph < 7. In the samples with ph < 7 fraction of UO 2CO 3 (aq) increases significantly at elevated pressure. This can be explained by the carbonate system in water. When increasing the pco 2 the relationship between CO 2 (g) and H 2CO 3 will work towards equilibrium, which favor the formation of bicarbonate and in the extension will favor the formation of carbonates. Langmuir (1978) uses slightly higher pressure, atm, which is a larger pressure than both Gustafsson et al. (2009) and what been used in this study. Prat et al. (2009) does not declare what pco 2 was used; this is considered a foremost drawback in that article. If assuming possible formation of calcite when increasing the pressure in the sensitivity test, the speciation does not change significantly for most species, besides for UO 2CO Since this is believed to be a cytotoxic specie, it should be noted. The result of the sensitivity model regarding pressure is an attempt to illustrate how the chemistry of the water changes when pumping it from the deep bedrock to the ground surface. As a result of the sensitivity analysis it is obvious that it is important to understand the physical parameters in the environment, not only the chemical composition, and what the purpose of the study is Spatial analysis The bedrock groundwater containing most uranium was sandstone to limestone. Uranium mostly occurs in veins in sedimentary bedrock (Birke et al. 2010), in comparison to igneous bedrock where the uranium exists distributed throughout the bedrock. The spatial distribution of uranium concentration including the ph and the predominating species is discovered to be somewhat homogeneously distributed. There are no clusters revealed and obvious difference can be seen in the studied area. The correlation between bedrock and speciation of uranium is not always evident. The ph correlations show that the sedimentary bedrock types have fairly stable ph values compared to the igneous where the ph fluctuates greatly. The explanation probably lies in the increased amount of carbonates in the sedimentary bedrock, especially limestone. The same pattern can 22

33 Modeling the aqueous chemistry of uranium On a national level in Sweden be observed for the formation of organic speciation. The limestone has very low fractions of organic species meanwhile the fraction in igneous and metamorphic bedrock fluctuates greatly. The carbonate species which were expected to predominate in limestone is confirmed. The igneous bedrock thus holds high fractions of carbonates as well, and has very high fluctuations in the fraction. Generally it can be stated that this model indicates that the bedrock type does not influence greatly on the formation on the uranyl speciation. The fraction of a species can vary greatly with in a geographical area. Hence an assumption of the speciation according to bedrock type cannot be assumed with confidence. The result is contradictive to previous statements for scientists (Ek et. al 2008, Bucher et al. 2008, Prat et al. 2009). An explanation to the outcome of this correlation would be the complexity of the geology in Siljansringen. An accuracy error in the geology data set from SGU, will lead to an error in the correlation. The geology in this area is very mixed, with limestone features within granitic rock, due to the meteor collision. This is a possible explanation for the indifference between the bedrock types. Fractures might be interconnected between the bedrock types, which also will affect the result when performing a correlation analysis with the bedrock. Ek et al. (2008) performed a minor study in Siljansringen area and states that it is plausible that it is only the wells located in parent rock which contains very high concentrations and that the wells located in the sedimentary bedrock contains lower levels of uranium. This is something that should be verified with further studies. Since the distribution of samples between the different bedrock types is skewed, it is not possible to make a reliable statistical analysis of the result. This is something which needs to be considered when drawing conclusions from the result of the spatial analysis. Meanwhile the restriction that the only samples which will be regarded in the study are samples with elevated uranium concentrations, this restriction affected the outcome as well. There were no samples which were extracted from the areas mentioned having the highest uranium concentrations, section Uranium and geology. The natural explanation for this is that drilled wells are not usually located in areas where it is known that the groundwater quality might not be within the national standards Uncertainties and drawbacks The data set provided by SGU and SSM included 1044 samples and the sampling was carried out between 2001 and This amount of samples and the long time period could easily be a source of several measurement errors. Different sampling procedures and handling of samples could affect the result, such as ph which can be interpreted wrong if measured for too short or too long. Thus if something went wrong in the sampling or handling of the sample, the result of the model might turn out misleading. When choosing which samples to include in the model it was important that they all had measurement results for all elements analysed. This was to exclude any competition between elements, and a better understanding of which elements affects the speciation of uranium would be understood well. A drawback is that the samples chosen are in many cases clustered to certain areas. The area containing most samples with are thoroughly analysed is Siljansringen, this area is very well investigated, thus accurate data is provided. This shows the importance of having a standardization of sampling, some analysis has been more accurately performed and some are less accurate. All of the chemical compounds included in the model do not have significance for forming complexes with uranium. I chose to include them for the reason of competition amongst the elements. A compound which might not be likely to form a complex with uranium might have higher affinity to form a complex with another element, and forms a complex with that element instead. Hence as a consequence of competition another element will form a complex with uranium. If only the compounds which are likely to form a complex with uranium would have been included, it might have resulted in a misleading outcome. Since this is a model of the environmental state, this is essential to acquire an accurate an outcome. The uncertainty of the parameters is discussed under 5.4 Sensitivity of the model. The correlation level was set to r = 0.5, and the significance to p = This is commonly considered to be a high correlation and significance level. By setting Pearsons r at 0.5 principally assures that the conclusions drawn from the statistical analysis will not commit any conclusions. The drawback is of course the opposite that some relations that might be relevant were omitted. I chose to ensure my conclusions by 23

34 Åsa Löv TRITA LWR Degree Project 12:32 choosing a high level of correlation and significance. It is important to not only look at the number calculated by SPSS, but to also look at the scatter plots. Some relations did not turn out linearly significant, but does have an apparent correlation nonetheless. The result of the model correlates well with what has been discovered concerning the uranium chemistry in previous research and modeling. This makes the result reliable and it can be assumed that the result mirrors the natural environment well, thus the result can be used when deciding what kind of treatment which would most efficiently reduce the amount of uranium Future work Next step in this study would be to make sure that all well users are aware of how they can mitigate their uranium problem. Some wells have very high uranium concentrations and mitigations measures should be taken instantaneously. When deciding what kind of treatment which should be implemented, investigating the ph will with much confidence reveal what type and charge of uranyl complex which can be expected to predominate, thus what charge the complex will have. This is a cheap way of getting an idea of what kind of treatment should be applied. To decrease the dissolved uranium concentration, anoxic conditions could be created. The uranium would then precipitate and not be available for humans in their drinking water. Creating an anoxic environment requires advanced technology, and reduces the quality of the water (SEPA, 2000). For continued research of the groundwater chemistry of uranium new water samples should be extracted from Mora, the Siljansringen area, where it was shown that the speciation differed largely within a relatively small geographical area. Chemical analysis for metals and anions should be conducted; in addition to a speciation modeling to achieve further understanding of which parameters mainly governs the uranium groundwater chemistry. Geophysical measurements would be beneficiary in an attempt to localize the fractures in the bedrock. Isotope analysis of uranium is an excellent way of investigating the origin of the uranium. Natural fractionation alters the 234 U/ 238 U ratio, and nuclear emission from anthropogenic activities can cause deviation in the 235 U/ 238 U ratio (Brennecka et al. 2010). If the source of uranium is anthropogenic it is easier to implement mitigation measures, than if the source is natural. Isotope studies are widely used within the geochemical society and is a well-known technique. The correlation between bedrock type and uranium concentration including the speciation of uranium showed that the assumption many scientist has made, that granite is the main source of uranium might be misleading, thus it should be examined more closely. This knowledge could later be used in risk analysis, which is beneficiary when planning the infrastructure. The knowledge of how the toxicity of uranium affects the human body; both when discussing the accumulation of uranium, and if it does, how much it accumulates, is tentative. There are vast uncertainties and more research is needed within the area. In view of that and in order to increase the public health, more research is needed within the area. 6. CONCLUSIONS From this study it is evident that ph is the most important parameter when modeling uranium speciation. This facilitates when deciding upon treatment method. When performing the model it is important to consider DOM speciation of uranium, it is the dominating species at ph < 7, even at low concentrations, thus should be considered in all models so that no speciation of uranium is omitted. To assume that the dominating uranium species would be a Ca-carbonate species is not always accurate. Ca-carbonate species are one of the dominating ones, but other species should be noted as well. Nonetheless the outcome of this model can be regarded as reliable, thus it agrees with many of the previous findings regarding the aqueous chemistry of uranium. Modeling the chemistry of uranium is a delicate task, where small changes of parameters affect the outcome of the model vastly. Ergo, further studies regarding the behavior of uranium in groundwater is needed, and a veryfication of the model is of great importance. Furthermore it should be mentioned that this model is the first one not only regarding the elements which are important speciation agents for uranium, but which also considers the competition between all elements in the water. 24

35 Modeling the aqueous chemistry of uranium On a national level in Sweden REFERENCES Aastrup, M., Thunholm, B., Johnson, J., Bertills, U., Berntell, A. (1995) Grundvattnets kemi i Sverige. Naturvårdsverkets Rapport p. Andréasson, P-G. (2006) Geobiosfären en introduktion. Studentlitteratur, Lund. 604p. ATSDR (2011) Draft Toxicological profile for uranium. Agency for Toxic Substances and Disease Registry. 528p. Bernhard, G., Geipel, T., Reich, V., Brendler, V., Amayri, S., Nitsche, H. (2001) Uranyl (VI) carbonate complex formation: Validation of the Ca 2UO 2(CO 3) (aq) species. Radiochim. Acta. 89: Birke, M., Rauch, U., Lorenz, H., Kringel, R. (2010) Distribution of uranium in German bottled and tap water. Journal of Geochemical Exploration. 107: Brennecka, G. A., Borg, L. E., Hutcheon, I. D., Sharp, M. A., Anbar A. D. (2010) Natural variations in uranium isotope ratios of uranium ore concentrates: Understanding the 238 U/ 235 U fractionation mechanism. Earth and Planetary Science Letters. 291: Bucher, K., Zhu, Y., Stober, I. (2008) Groundwater in fractured crystalline rocks, the Clara mine, Black forest (Germany). Int J Earth Sci (GeolRundusch). 98: Dong, W., Brooks, S.C. (2006) Determination of the formation constants of ternarycomplexes of uranyl and carbonate with alkaline earth metals (Mg 2+, Ca 2+, Sr 2+,and Ba 2+ ) using anion exchange method. Environ. Sci. Technol. 40: Drake, H., Tullborg, E-L., Mackenzie, A. B. (2009) Detecting the near-surface redox front in crystalline bedrock using fracture mineral distribution, geochemistry and U-series disequilibrium. Applied Geochemistry. 24: Ek, B-M., Thunholm, B., Östergren, I., Falk, R., Mjönes, L. (2008) Naturligt radioaktiva ämnen, arsenik och andra metaller i dricksvatten från enskilda brunnar. SSI Rapport 2008: p. Gammons, C. H., Wood, S. A,. James P. J., James P. M. (2003) Geochemistry of the rare-earth elements and uranium in the acidic Berkeley Pit lake, Butte, Montana. Chemical Geology. 198: Grenthe, I., Fuger, J., Konings, R. J. M., Lemire, R. J., Muller, A. B., Nguyen-Trung, C., Wanner, H. (2004) Chemical thermodynamics of Uranium. Nuclear Energy Agency, Organisation For Economic Co-Operation And Development. 715p. Guillamount, R., Fanghänel, T., Fuger, J., Grenthe, I., Neck, V., Palmer, D., Rand, M.H. (2003) Update on the chemical chermodynamics of uranium, neptunium, plutonium, americium and technetium. Elsevier, Amsterdam. 970p. Gustafsson, J. P., Jacks, G., Simonson, M., Nilsson, I. (2007) Soil and water chemistry. Department of Land and water resources engineering, Royal Institute of Technology. Stockholm. 168p. Gustafsson, J. P., Dässman, E., Bäckström, M. (2009) Towards a constistent geochemical model for prediction of uranium (VI) removal from groundwater by ferrihydrite. Applied Geochemistry. 24: Hiscock, K. M. (2005) Hydrogeology principles and practice. Blackwell Publishing. 389p. Jokelainen, L., Markovaara-Koivisto, M., Read, D., Lindberg, A., Siitari-Kauppi, M., Hellmuth, K-H. (2010) Understanding uranium behavior at the Askola uranium mineralization. Radiochim. Acta. 98: Karlsson, C. (2010) Radionuclides in drinking water a survey regarding mitigation measures. TRITA- LWR Degree project p. Kurttio, P., Auvinen, A., Salonen, L., Saha, H., Pekkanen, J., Mäkeläinen, I., Väisänen, S. B., Penttilä, I. M., Komulainen, H. (2002) Renal effects of uranium in drinking water. Environmental Health Perspectives. 110: Langmuir, D. (1978) Uranium solution-mineral equlibria at low temperatures with applications to sedimentary ore deposits. Geochimica et Cosmochimica Acta. 42: Langmuir, D. (1997) Aqueous Environmental Chemistry. Prentice-Hall, Inc. New Jersey. 600p. Lawrence, E., Poeter, E., Wanty, R. (1990) Geohydrologic, geochemical, and geologic controls on the occurrence of radon in groundwater near Conifer, Colorado, USA. Journal of Hydrology. 127:

36 Åsa Löv TRITA LWR Degree Project 12:32 Lestaevel, P., Houpert, P., Bussy, C., Dhieux, B., Gourmelon, P., Paquet, F. (2005) The brain is a target organ after acute exposure to depleted uranium. Toxicology. 212: Li, W. C., Victor, D. M., Chakrabarti C. L. (1980) Effect of ph and Uranium Concentration on Interaction of Uranium(V1) and Uranium(1V) with Organic Ligands in Aqueous Solutions. Anal. Chem. 52: Lindquist, A., Nilsson, K. (2010) Site investigation SFR, Hydrochemical characterisation of groundwater in borehole KFR105, results from five investigated borehole sections. Geosigma AB and SKB. 66p. Marshak, S. (2005) Earth Portrait of a planet. Second edition. Norton & Company, New York. 748p. Maxe, L. (2007) Enskild vattenförsörjning kunskapsunderlag inför uppföljning av ett nytt delmål. SGU- Rapport 2007:10. 67p. OECD, IAEA (2005) Uranium 2005: Resources, Production and Demand. A joint report by the OECD Nuclear Agency and the International Atomic Energy Agency IAEA. 383p. Prat, O., Vercouter, T., Ansoborlo, E., Fichet, P., Perret, P., Kurttio, P., Salonen, L. (2009) Uranium speciation in drinking water from drilled wells in southern Finland and its potential links to health effects. Environmental Science and Technology. 34: Ragheb, M., Khasawneh, M. (2010) Uranium fuel as byproduct of phosphate fertilizer production. Proceedings of the 1 st International nuclear and renewable energy conference (INREC10), Amman, Jordan, March p. Ranville, J. F., Hendry, M. J., Reszat, T. N., Xie, Q., Honeyman, B. D. (2007) Quantifying uranium speciation by groundwater dissolved organic carbon using asymmetrical flow field-flow fractionation. J. Contam. Hydrol: 91: SEPA (1995) Värmepumpar & närmiljön. Swedish Environmental Protection Agency. 62p. SEPA (2000) Environmental Quality Criteria Groundwater. Swedish Environmental Protection Agency, Report p. SGU (2008) Att borra brunn för energi och vatten en vägledning. Swedish Geological Survey. 40p. Smedley, P. L., Smith, B., Abesser, C., Lapworth, D. (2006) Uranium occurrence and behaviour in British groundwater groundwater systems & water quality. Programme Commissioned Report Cr/06/050n. British Geological Survey. 50p. Swedish Health Authorities (2006) Dricksvattenrening med avseende på uran. Socialstyrelsen, Sweden. no p. Svensson, K., Darnerud, P. O., Skerfving, S. (2005) A risk assessment of uranium in drinking water. National Food Administration, Rapport Uppsala. 35p. Svensson, K. (2005) Uran i dricksvatten kan påverka njurarna. Vår Föda. 5: Vaaramaa, K. (2003) Physico-chemical forms of natural radionuclides in drilled well waters and their removal by ion exchange. Academic dissertation, University of Helsinki, Finland. 62p. VanBriesen, J. M., Small, M., Weber, C., Wilson, J. (2010) Modeling of Pollutants in Complex Environmental Systems. ILM Publications, a trading division of international Labmate limited. 2: 512p. Visual MINTEQ 3.0 a brief tutorial. 5p. WHO (2005) Uranium in drinking water. World Health Organization. 15p. Zamora, M. L., Tracy, B. L., Zielinski, J. M., Meyerhof, D. P., Mossf, M. A. (1998) Chronic ingestion of uranium in drinking water: A study of kidney bioeffects in humans. Toxicological Sciences. 43: Zielinski, R. A., Burruss, R. C. (1991) Petrogenesis and geological history of a uranium source rock: a case study in northeaster Washington, USA. Applied Geochemistry. 6: Other references Swedish National Food Administration (2011) web: attenkvalitet/fluorid/ updated: , accessed on

37 Modeling the aqueous chemistry of uranium On a national level in Sweden APPENDIX I. DEFAULT FORMATION CONSTANTS IN VISUAL MINTEQ page Table 1A. Default formation contants Reaction logk at 25 o C, I = 0 ΔH r (kj/mol) UO H 2O UO 2OH + + H UO H 2O UO 2(OH) H UO H 2O UO 2(OH) H UO H 2O (UO 2) 2(OH) H UO H 2O (UO 2) 3(OH) H UO H 2O (UO 2) 3(OH) H UO H 2O (UO 2) 3(OH) H UO H 2O (UO 2) 4(OH) H UO CO 3 2- UO 2CO 3 0 UO CO 3 2- UO 2(CO 3) 2 2- UO CO 3 2- UO 2(CO 3) 3 4- Ca 2+ + UO CO 3 2- CaUO 2(CO 3) 3 2-2Ca 2+ + UO CO 3 2- Ca 2UO 2(CO 3) UO F - UO 2F + (aq) UO F - UO 2F 2 (aq) UO F - UO 2F 3 - (aq) UO SO 4 2- UO 2SO UO PO 4 3- UO 2PO UO PO 4 3- UO 2HPO UO H 4SiO 4 UO 2H 3SiO H I

38 UO2+2 UO2OH+ UO2 (OH)3- UO2(OH)2 (aq) (UO2)3 (OH)5+ (UO2)4 (OH)7+ UO2CO3 (aq) UO2 (CO3)2-2 UO2 (CO3)3-4 Ca2UO2 (CO3)3 (aq) CaUO2 (CO3)3-2 UO2F+ Ionic Strength mol/l Sig 0,27 0,117 0,03 0,017 0,262 0,327 0, ,147 0,000 0,873 0,122 r -0,176-0,262-0,258-0,283-0,135-0,118-0,092-0,250-0,175 0,416-0,019-0,185 Phosphat e mg/l Sig r Silicon mg/l Sig r Uranium µg/l Sig r Fluoride mg/l Sig r Calcium mg/l Sig r Alkalinity mg HCO3/l Sig r depth of well (m) Sig r sampling date Sig r Sig ph r Åsa Löv TRITA LWR Degree Project 12:32 2 pages APPENDIX II. CORRELATION BETWEEN SPECIE FORMED AND CONCENTRATION OF ELEMENT AND PHYSICAL PARAMETERS. Table 2A. Correlation and significance of correlation. Correlation is significant at the 0.05 level (2-tailed). Correlation is significant at the 0.01 level (2-tailed). II

39 Modeling the aqueous chemistry of uranium On a national level in Sweden III Table 3A. Correlation and significance of correlation. ph r Sig sampling date r Sig depth of well (m) r Sig Alkalinity mg HCO3/l r Sig Calcium mg/l r Sig Fluoride mg/l r.249 * Sig Uraniu m µg/l r Sig Silicon mg/l r Sig Phosphat e mg/l r *.260 * * Sig Ionic Strengt h mol/l r -0,197-0,033-0,073-0,271-0,159-0,288-0,321 0,300-0,191-0,126-0,073-0,264 0,444 Sig 0,103 0,281 0,547 0,020 0,186 0,015 0,006 0,011 0,111 0,295 0,545 0,026 0,000 UO2F2 (aq) UO2SO4 (aq) UO2 HPO4(aq) UO2H3SiO4+ U-DOM TOT hydroxides TOT Carbonates TOT Ca-carbonates TOT Fluorides TOT Phosphates TOT Anions TOT Cations TOT Uncharged

40

41 Modeling the aqueous chemistry of uranium On a national level in Sweden 5 pages APPENDIX III. RESULT FROM CORRELATION ANALYSIS. Fig 1A. Uranium concentration in relation to bedrock type. Fig 2A. ph in relation to bedrock type and spatial distribution of ph. VII

42 Åsa Löv TRITA LWR Degree Project 12:32 Fig 3A. U-DOM fraction in relation to bedrock type. Fig 4A. Hydroxide complex fraction in relation to bedrock type VIII

43 Modeling the aqueous chemistry of uranium On a national level in Sweden Fig 5A. Carbonate complex fraction in relation to bedrock type Fig 6A. Ca-carbonate fraction in relation to bedrock type. IX