Uranium in natural waters and the environment: distribution, speciation and impact

The concentrations of U in natural waters are usually low, being typically less than 4 μg/L in river water, around 3.3 μg/L in open seawater, and usually less than 5 μg/L in groundwater. Higher concentrations can occur in both surface water and groundwater and the range spans some six orders of magnitude, with extremes in the mg/L range. However, such extremes in surface water are rare and linked to localized mineralization or evaporation in alkaline lakes. High concentrations in groundwater, substantially above the WHO provisional guideline value for U in drinking water of 30 μg/L, are associated most strongly with (i) granitic and felsic volcanic aquifers, (ii) continental sandstone aquifers especially in alluvial plains and (iii) areas of U mineralization. High-U groundwater provinces are more common in arid and semi-arid terrains where evaporation is an additional factor involved in concentrating U and other solutes. Examples of granitic and felsic volcanic terrains with documented high U concentrations include several parts of peninsular India, eastern USA, Canada, South Korea, southern Finland, Norway, Switzerland and Burundi. Examples of continental sandstone aquifers include the alluvial plains of the Indo-Gangetic Basin of India and Pakistan, the Central Valley, High Plains, Carson Desert, Española Basin and Edwards-Trinity aquifers of the USA, Datong Basin, China, parts of Iraq and the loess of the Chaco-Pampean Plain, Argentina. Many of these plains host eroded deposits of granitic and felsic volcanic precursors which likely act as primary sources of U. Numerous examples exist of groundwater impacted by U mineralization, often accompanied by mining, including locations in USA, Australia, Brazil, Canada, Portugal, China, Egypt and Germany. These may host high to extreme concentrations of U but are typically of localized extent. The overarching mechanisms of U mobilization in water are now well-established and depend broadly on redox conditions, pH and solute chemistry, which are shaped by the geological conditions outlined above. Uranium is recognized to be mobile in its oxic, U(VI) state, at neutral to alkaline pH (7–9) and is aided by the formation of stable U–CO3(±Ca, Mg) complexes. In such oxic and alkaline conditions, U commonly covaries with other similarly controlled anions and oxyanions such as F, As, V and Mo. Uranium is also mobile at acidic pH (2–4), principally as the uranyl cation UO22+. Mobility in U mineralized areas may therefore occur in neutral to alkaline conditions or in conditions with acid drainage, depending on the local occurrence and capacity for pH buffering by carbonate minerals. In groundwater, mobilization has also been observed in mildly (Mn-) reducing conditions. Uranium is immobile in more strongly (Fe-, SO4-) reducing conditions as it is reduced to U(IV) and is either precipitated as a crystalline or ‘non-crystalline’ form of UO2 or is sorbed to mineral surfaces. A more detailed understanding of U chemistry in the natural environment is challenging because of the large number of complexes formed, the strong binding to oxides and humic substances and their interactions, including ternary oxide-humic-U interactions. Improved quantification of these interactions will require updating of the commonly-used speciation software and databases to include the most recent developments in surface complexation models. Also, given their important role in maintaining low U concentrations in many natural waters, the nature and solubility of the amorphous or non-crystalline forms of UO2 that result from microbial reduction of U(VI) need improved quantification. Even where high-U groundwater exists, percentage exceedances of the WHO guideline value are variable and often small. More rigorous testing programmes to establish usable sources are therefore warranted in such vulnerable aquifers. As drinking-water regulation for U is a relatively recent introduction in many countries (e.g. the European Union), testing is not yet routine or established and data are still relatively limited. Acquisition of more data will establish whether analogous aquifers elsewhere in the world have similar patterns of aqueous U distribution. In the high-U groundwater regions that have been recognized so far, the general absence of evidence for clinical health symptoms is a positive finding and tempers the scale of public health concern, though it also highlights a need for continued investigation

Water resource implications of the proposed Greenwood Community Forest

The implications for both water use and water quality of the proposed Greenwood Community Forest (Nottinghamshire) are examined. Of the 44000 ha in the designated area, 2700 ha are already afforested and under the proposals up to a further 10000 ha could become afforested. This would change the forest cover of the Sherwood Sandstone outcrop from its present 10% to 25-35%. This is a significant change in land use and will have implications for the quantity and quality of recharge to the underlying aquifer

Geochemical properties of aquifers and other geological formations in the UK

The intrinsic geochemical characteristics of geological for-mations have a considerable influence in controlling solute and pollutant transport behaviour during groundwater flow through the shallow geosphere. The interactions between solute or pollutants and the surface geochemistry of the rock matrix will often determine both the extent and speed of solute transport in the saturated and unsaturated zones. Consequently, understanding these processes is of critical importance for a range of environmental management re-quirements, such as landfill leachate monitoring or contam-inated land evaluation, including requirements related to statutory obligations for ensuring good groundwater status under the EU Water Framework Directive. Risk assessment and management approaches frequent-ly make use of numerical geochemical modelling to predict contaminant transport. These models necessarily require parameterization of the geochemical properties of the geo-logical formations involved and the predictions which can be obtained are inevitably only as good as the quality of the data which are used. However, the natural variation in li-thologies and extensive spatial heterogeneities of the UK rock formations result in considerable variability of the most important geochemical properties. Identifying or ob-taining good relevant data for calculations can be difficult; new laboratory measurements can be expensive and time-consuming, published data are relatively sparse and existing data from previous site investigations are often held com-mercially and are difficult to get hold of. This study, supported by the British Geological Survey and the Environment Agency, presents the first comprehen-sive national compilation of geochemical properties data of relevance to geochemical modelling. An assessment has been made of existing available primary data. Relatively few data are available, but those which are have been col-lated. To underpin this the project has undertaken an exten-sive programme of new experimental measurements on the geochemical properties of samples from geological for-mations across the country. Initially attention was focussed on England and Wales, but this was later expanded to in-clude data and samples from Scotland. Over 600 new sam-ples have been included, providing by far the largest high-quality internally-consistent dataset currently available for these parameters. The geochemical properties addressed are those consid-ered to be of greatest significance for the purposes of mod-elling and risk assessment, namely: • cation exchange capacity (CEC); • fractions of organic and inorganic carbon (fOC, fIC); • extractable (readily soluble) element contents of iron (Fe), manganese (Mn) and sulphur (S); • whole-rock and clay-fraction mineralogy. It is intended that the Geochemical Properties Manual rep-resented by this report and database should provide a relia-ble reference resource for practitioners carrying out site in-vestigations in the future. Whilst site-specific parameter measurements will always provide the greatest confidence, this manual will provide a benchmark of what is known and what can reasonably be expected for the geochemical prop-erties of given types of geological formations. In this re-spect the manual is related to the manuals of physical prop-erties of major and minor aquifers produced by BGS previ-ously. However, for geochemical properties date have been included for any geological formations, not only aquifers, as aquitards and aquicludes also play an important role in constraining transport behaviour. Chapters 1 and 2 of the report introduce the data compi-lation, structure and presentation. Chapters 3 and 4 provide a brief overview of the principles of geochemical modelling and of the use of geochemical data in geochemical model-ling. Chapters 5-8 then provide the bulk of the report, cata-loguing the available data by geology for a selection of the key geochemical parameters relevant to numerical model-ling. The data are presented as numerical and graphical sta-tistical summaries to try to assist the user in finding the most suitable parameter values to use in their own circum-stances. This report (OR/12/090) supersedes an earlier version (CR/06/216N) which is now withdrawn. It contains some corrections, updated lithostratigraphical classifications and additional data added to the database up to the end of De-cember 2012

Arsenic and selenium

Arsenic (As) and selenium (Se) have become increasingly important in environmental geochemistry because of their significance to human health. Their concentrations vary markedly in the environment, partly in relation to geology and partly as a result of human activity. Some of the contamination evident today probably dates back to the first settled civilizations that used metals. This chapter outlines the main effects of arsenic and selenium on human and animal health, their abundance and distribution in the environment, sampling and analysis, and the main factors controlling their speciation and cycling. Such information should help to identify aquifers, water resources, and soils at risk from high concentrations of arsenic and selenium, and areas of selenium deficiency. Human activity has had, and is likely to continue to have, a major role in releasing arsenic and selenium from the geosphere and in perturbing the natural distribution of these and other elements over the Earth’s surface

Fundamental aspects of metal speciation and transport in metal-contaminated soils and aquifers (FAMEST). Second Annual Report.

The aims of the FAMEST project are to apply the latest geochemical methods and models to practical problems of metal toxicity and pollution at metal-contaminated sites. Studies are taking place at sites in the UK, France, Switzerland and the Netherlands. The study areas include polluted sites near old metal smelters, an agricultural field heavily contaminated by sewage sludge some 20 years ago, and an experimental site where different amounts of copper had been added in a controlled way some years ago. One aim is to be able to predict from a minimum number of basic measurements of the affected soils or aquifer materials the present-day pore water concentrations and the speciation of the metals within this pore water. This gives a good idea of the potential toxicity of the water. Once this and the rate of water movement are known, it should also be possible to determine the transport of metals through the affected soils and hence estimate the persistence of the metals in the soils and their potential impact on local water bodies. Key targets being addressed in the F AMEST project are: • to derive a generic set of proton and metal ion interaction parameters for the binding of metal ions to natural organic matter, specifically to fulvic and humic acids and to use these data for the modelling the binding of metals to the organic component of soils and soil solutions (UK); • to develop a set of procedures to characterize metal-contaminated soils using chemical extraction and spectroscopic techniques (EXAFS, EPR) and to use this information to predict the metal concentrations in pore water (soil solution) and its variation with depth (France); • to develop methods for measuring and characterising the transport of metals through soils. In particular, testing various multicomponent transport models for predicting the results of transport experiments with contaminated and control soils (Switzerland); • to develop a method for measuring 'free' (i.e. not complexed by organic matter) metal ion concentrations in soils and solutions using a novel Donnan membrane technique (the Netherlands). The project is designed to cover field, laboratory and modelling studies in about equal measure. In particular, it is hoped that the considerable accumulated experience of the project team in modelling chemical speciation in laboratory systems can be applied to the 'real world'. We hope that the results of this work will be transferred to the wider world in terms of revised working procedures and improved computer models. These can then be incorporated by others into future risk assessments

Geostatistical analysis of arsenic concentration in groundwater in Bangladesh using disjunctive kriging

The National Hydrochemical Survey of Bangladesh sampled the water from 3,534 tube wells for arsenic throughout most of Bangladesh. It showed that 27% of the shallow tube wells (less than 150 m deep) and 1% of the deep tube wells (more than 150 m deep) exceeded the Bangladesh standard for arsenic in drinking water (50 µg L–1). Statistical analyses revealed the main characteristics of the arsenic distribution. Concentrations ranged from less than the detection limit (0.5 µg L–1), to as much as 1,600 µg L–1, though with a very skewed distribution, and with spatial dependence extending to some 180 km. Disjunctive kriging was used to estimate concentrations of arsenic in the shallow ground water and to map the probability that the national limit for arsenic in drinking water was exceeded for most of the country (the Chittagong Hill Tracts and the southern coastal region were excluded). A clear regional pattern was identified, with large probabilities in the south of the country and small probabilities in much of the north including the Pleistocene Tracts. Using these probabilities, it was estimated that approximately 35 million people are exposed to arsenic concentrations in groundwater exceeding 50 µg L–1 and 57 million people are exposed to concentrations exceeding 10 µg L–1 (the WHO guideline value)

Baseline Scotland : groundwater chemistry of southern Scotland

This report describes the baseline groundwater chemistry of bedrock aquifers in southern Scotland, from the Scotland-England border to the Southern Upland Fault, with the exception of the highly productive Permian aquifers, which are treated separately. Four main aquifer groups are considered: sedimentary and metasedimentary rocks of both Ordovician and Silurian age; Devonian sedimentary rocks; and Carboniferous sedimentary rocks. There are also small outcrops of igneous rocks, mainly granite and lavas

A review of the source, behaviour and distribution of arsenic in natural waters

The range of As concentrations found in natural waters is large, ranging from less than 0.5 μg l−1 to more than 5000 μg l−1. Typical concentrations in freshwater are less than 10 μg l−1 and frequently less than 1 μg l−1. Rarely, much higher concentrations are found, particularly in groundwater. In such areas, more than 10% of wells may be ‘affected’ (defined as those exceeding 50 μg l−1) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concentrations is greatest in the Bengal Basin with more than 40 million people drinking water containing ‘excessive’ As. These large-scale ‘natural’ As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localised scale, in areas of mining activity and where oxidation of sulphide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1–20 mg kg−1. There appear to be two distinct ‘triggers’ that can lead to the release of As on a large scale. The first is the development of high pH (>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As (especially As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, especially Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO4 concentrations are small (typically 1 mg l−1 or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concentrations in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concentrations of dissolved Fe

Screening for long-term trends in groundwater nitrate monitoring data

A large body of UK groundwater nitrate data has been analysed by linear regression to define past trends and estimate future concentrations. Robust regression was used. The datasets showed too many irregularities to justify more traditional time-series approaches such as ARIMA-type methods. Tests were included for lack of linearity, outliers, seasonality and a break in the trend (by piecewise linear regression). Of the series analysed, 21% showed a significant improvement in the overall fit when a break was included. Half of these indicated an increase in trend with time. Significant seasonality was found in about one-third of the series, with the largest nitrate concentrations usually found during winter months. Inclusion of nearby water-level data as an additional explanatory variable successfully accounted for much of this seasonality. Based on 309 datasets from 191 distinct sites, nitrate concentrations were found to be rising at an average of 0.34 mg NO3 l–1 a–1. In 2000, 34% of the sites analysed exceeded the 50 mg l–1 EU drinking water standard. If present trends continue, 41% could exceed the standard by 2015. We explored an alternative to the previously proposed Water Framework Directive aggregation approach for estimating trends in whole groundwater bodies (the ‘Grath’ approach: spatially average then find the trend). We first determined the trends for single boreholes and then spatially averaged these. This approach preserves information about the spatial distribution of trends within the water body and is less sensitive to ‘missing data’

Nitrate trends in groundwater

Some 450 groundwater nitrate datasets from England were examined and, where the data were suitable, trends determined. Although the datasets were not randomly selected from all possible boreholes in England, they represent a fairly broad cross-section of such boreholes. They covered a wide range of aquifers. Many of these were from working public supply wells, and the selection may therefore exclude high nitrate sources which have already been taken out of supply. Datasets from observation wells were also included, but many of these had much less available data. More than one third of the datasets were rejected for being too short (span of less than 5 years or fewer than 20 observations), too irregular or too variable. Time series in which there were obviously highly nonlinear trends were also excluded. Trends were determined by linear regression. Tests were included for the lack of linearity, the presence of outliers, for seasonality and for possible breaks in the trend including reversals of trend. After exclusion of data where trend fitting was unsatisfactory, 309 datasets were finally selected from 191 different sites. For multi-borehole sites, median values were used to obtain the summary statistics. For these 191 sites groundwater nitrate concentrations were found to be rising at an average of 0.34 mg NO3 L-1 year-1. Average trends were greatest in the Lincolnshire Limestone aquifer (0.96 mg NO3 L-1 year-1) and lowest in the Magnesian Limestone aquifer (0.18 mg NO3 L-1 year-1). Average trends for the Chalk and Triassic sandstone aquifers were 0.38 mg NO3 L-1 year-1 and 0.44 mg NO3 L-1 year-1, respectively. An assessment of seasonality in nitrate concentrations was also made by including a term for the month of sampling in the regression model. Significant (p<0.05) seasonality was found in about one third of the series tested. This showed higher concentrations during the winter months. Breaks in a linear trend were detected by fitting a piecewise linear regression to the data with automatic detection of the break point. 21% of the time series analysed showed a significant improvement in the overall fit when such a break was included. 10.5% of these indicated an increase in trend with time and 10.5% a decrease. The best-fitting model was used to estimate the nitrate concentration on 1 January 2000 and January 2015 for all sites. For 2000, this showed that nitrate concentrations in the major aquifers on this date were broadly similar apart from the Magnesian Limestone. The highest concentrations were in the Oolitic limestone (50 mg NO3 L-1) and the lowest in the Magnesian Limestone (8.2 mg NO3 L-1). The Chalk and the Triassic sandstone had average concentrations of 42 mg NO3 L-1 and 46.3 mg NO3 L-1 respectively. The average of all sites was 37.8 mg NO3 L-1. The highest nitrate concentrations were found in areas around the Wash, the Chalk of south Yorkshire/East Anglia, and the Permo-Triassic Sandstone of Yorkshire/Nottinghamshire. By 2015 the average concentration will have increased to 43.6 mg NO3 L-1. The highest concentrations are predicted to be in the Lower Greensand (58.8 mg NO3 L-1) and the lowest in the Magnesian Limestone (12.3 mg NO3 L-1). The Chalk and the Triassic sandstone will have average concentrations of 50.5 mg NO3 L-1 and 52.6 mg NO3 L-1 respectively. In 2000, 34% of sites exceeded the 50 mg/L. It is estimated that if present trends continue, 41% of groundwater sources could exceed the 50 mg/L standard by 2015