Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso

This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L⁻¹, they have a large range from <0.5 to 1630 μg L⁻¹. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L⁻¹. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O₂ concentrations <2 mg L⁻¹. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.     

Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin,Inner Mongolia

Observed As concentrations in groundwater from boreholes and wells in the Huhhot Basin of Inner Mongolia, northern China, range between <1 μg l⁻¹ and 1480 μg l⁻¹. The aquifers are composed of Quaternary (largely Holocene) lacustrine and fluvial sediments. High concentrations are found in groundwater from both shallow and deep boreholes as well as from some dug wells (well depths ranging between <10 m and 400 m). Populations from the affected areas experience a number of As-related health problems, the most notable of which are skin lesions (keratosis, melanosis, skin cancer) but with internal cancers (lung and bladder cancer) also having been reported. In both the shallow and deep aquifers, groundwaters evolve down the flow gradient from oxidising conditions along the basin margins to reducing conditions in the low-lying central part of the basin. High As concentrations occur in anaerobic groundwaters from this low-lying area and are associated with moderately high dissolved Fe as well as high Mn, NH₄, dissolved organic C (DOC), HCO₃ and P concentrations. Many of the deep groundwaters have particularly enriched DOC concentrations (up to 30 mg l⁻¹) and are often brown as a result of the high concentrations of organic acid. In the reducing groundwaters, inorganic As(III) constitutes typically more than 60% of the total dissolved As. The highest As concentrations tend to be found in groundwater with low SO₄ concentrations and indicate that As mobilisation occurs under strongly reducing conditions, where SO₄ reduction has been an active process. High concentrations of Fe, Mn, NH₄, HCO₃ and P are a common feature of reducing high-As groundwater provinces (e.g. Bangladesh, West Bengal). High concentrations of organic acid (humic, fulvic acid) are not a universal feature of such aquifers, but have been found in groundwaters from Taiwan and Hungary for example. The observed range of total As concentrations in sediments is 3–29 mg kg⁻¹ (n=12) and the concentrations correlate positively with total Fe. Up to 30% of the As is oxalate-extractable and taken to be associated largely with Fe oxides. The release of As into solution under the reducing conditions is believed to be by desorption coupled with reductive dissolution of the Fe oxide minerals. The association of dissolved As with constituents such as HCO₃, DOC and P may be a coincidence related to the prevalent reducing conditions and slow groundwater flow, but they may also be directly involved because of their competition with As for binding sites on the Fe oxides. The Huhhot groundwaters also have some high concentrations of dissolved U (up to 53 μg l⁻¹) and F⁻ (up to 6.8 mg l⁻¹). In contrast to As, U occurs predominantly under the more oxidising conditions along the basin margins. Fluoride occurs dominantly in the shallow groundwaters which have Na and HCO₃ as the dominant ions. The combination of slow flow of groundwater and the young age of the aquifer sediments are also considered potentially important causes of the high dissolved As concentrations observed as the sediments are likely to contain newly-formed and reactive minerals and have not been well flushed since burial.     

Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)₃⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)₆⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.     

Chromium oxidation by manganese (hydr)oxides in a California aquifer

There are increasing concerns with elevated levels of Cr(VI) in the environment because it is a strong oxidant, corrosive, and carcinogenic. The concerns extend to the presence of Cr(VI) in many aquifers in California and elsewhere, where relatively high levels have been attributed to both industrial pollution and natural processes. The authors have, therefore, determined if natural redox processes contribute to the presence of high Cr(VI) concentrations (6–36 μg L⁻¹) in an aquifer in central California relative to non-detectable concentrations (<0.1 μg L⁻¹) in an adjacent aquifer. Specifically, the distribution and the redox speciation of dissolved (<0.45 μm) Cr have been compared with those of particulate Mn and Fe oxy-hydroxides in sediments, using X-ray absorption spectroscopy at the Mn and Fe L-edges. The analyses show a correlation between the presence of dissolved Cr(VI) and Mn (hydr)oxide minerals, which are the only common, naturally occurring minerals known to oxidize Cr(III) in laboratory experiments. This covariance substantiates the results of those experiments and previous field studies that indicate natural oxidation mechanisms might account for the relatively high levels of Cr(VI) in the study site, as well as for elevated concentrations in other aquifers with similar biogeochemical conditions.     

Groundwater flow in an arsenic-contaminated aquifer,Mekong Delta,Cambodia

To advance understanding of hydrological influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50 km² area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer (K ≈ 10⁻⁴ ms⁻¹) and overlying clay aquitard (K ≈ 10⁻⁸ ms⁻¹) were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04–0.4 ma⁻¹ downward through the clay and 1–13 ma⁻¹ horizontally within the sand aquifer, producing aquifer residence times on the order 100–1000 a. The results of numerical modeling support this conceptual model of the flow system and, when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2–10 m of fine-grained material) are an important source of As to the underlying aquifer.     

Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of Shahai in the Hetao basin,Inner Mongolia

Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5–20 m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720 μg/L, with 71% of samples exceeding the WHO drinking water guideline value (10 μg/L). Fluoride concentrations range between 0.30 and 2.57 mg/L. There is no significant correlation between As and F⁻ concentrations. Greater As concentrations were found with increasing well depth. However, F⁻ concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations (<10 μg/L) are found in groundwater at depths less than 10 m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10 m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O₂ into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10 m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow–brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca²⁺ concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F⁻ adsorption–desorption are likely controlling the concentration of F⁻ in groundwater.     

Organic carbon mobilization in a Bangladesh aquifer explained by seasonal monsoon-driven storativity changes

Currently, the most widely accepted hypothesis to explain high As concentrations in Bangladesh groundwaters is that dissolved organic C (DOC) reduces solid Fe (hydr)oxides and mobilizes sorbed arsenate. The nature of the DOC and its release mechanism are still controversial. Based on weekly to biweekly sampling over the course of one monsoon cycle at six monitoring wells of different depths, it is proposed that storativity changes drive natural DOC release from clay–peat layers to the adjacent aquifers. With a decrease in hydraulic heads during the dry season, total mineralization and DOC concentrations increased. With the onset of the rainy season and an increase in hydraulic heads, release of clay–peat derived components stopped and vertical water displacement due to groundwater recharge from rainwater occurred, causing aquifer flushing and a decrease in total mineralization and DOC concentrations. Total As and DOC concentrations correlated over depth. However, at the depth of maximum concentrations, the As peak was observed during the rainy season. At present, the reason for this inverse seasonal trend between As and DOC is unclear. Higher mineralization or DOC concentrations could lead to increased As sorption or the increased arsenite release is a time-lag abiotic or microbial response to the DOC peak. The vulnerability of the Pleistocene aquifer towards increased As concentrations was found to be much higher than previously assumed. Though sorption capacities were determined to be higher than in the Holocene aquifer, probably due to intact Fe (hydr)oxides, long-term continuous As input from overlying clay and peat layers by the proposed seasonal storativity changes has led to increased aqueous As concentrations of 85 μg/L, considerably higher than drinking water standards. Until now, aquifer and especially aquitard and aquiclude hydraulics have not been considered sufficiently when attempting to explain As mobilization in Bangladesh.     

Naturally occurring arsenic: Mobilization at a landfill in Maine and implications for remediation

Elevated levels of dissolved arsenic (∼300 μg L⁻¹) have been detected beneath and in groundwater plumes extending away from a closed landfill in southern Maine. This study sought to determine the source of arsenic to the aquifer, the processes responsible for arsenic mobilization, and to evaluate the effectiveness of remediation efforts that have occurred at this site. The As appears to originate in the natural (glacial) aquifer solids, which contain ∼5 mg kg⁻¹ As on a dry weight basis. This conclusion is supported by the relatively uniform distribution of As in sediment samples, results of laboratory batch incubation experiments, and comparisons with groundwaters in nearby wetlands, which also have high levels of dissolved As that do not appear to originate within the landfill. The As is mobilized in the subsurface by strongly reducing conditions beneath the landfill and in nearby wetlands. In the aquifer beneath the landfill, the average oxidation–reduction potential (ORP) is −95 mV (Eh + 105 mV), and these reducing conditions were primarily induced by landfill leachate. Remediation efforts at this site have included installation of a low permeability clay cap; groundwater extraction, oxidation, and re-injection; and subsurface oxidation by injection of magnesium peroxide. The natural source of arsenic within the aquifer solids, coupled with widespread reducing conditions, has severely limited the effectiveness of these interventions on groundwater arsenic concentrations.     

Concentrations and speciation of arsenic along a groundwater flow-path in the Upper Floridan aquifer, Florida, USA

Arsenic (As) concentrations and speciation were determined in groundwaters along a flow-path in the Upper Floridan aquifer (UFA) to investigate the biogeochemical “evolution“ of As in this relatively pristine aquifer. Dissolved inorganic As species were separated in the field using anion-exchange chromatography and subsequently analyzed by inductively coupled plasma mass spectrometry. Total As concentrations are higher in the recharge area groundwaters compared to down-gradient portions of UFA. Redox conditions vary from relatively oxic to anoxic along the flow-path. Mobilization of As species in UFA groundwaters is influenced by ferric iron reduction and subsequent dissolution, sulfate reduction, and probable pyrite precipitation that are inferred from the data to occur along distinct regions of the flow-path. In general, the distribution of As species are consistent with equilibrium thermodynamics, such that arsenate dominates in more oxidizing waters near the recharge area, and arsenite predominates in the progressively reducing groundwaters beyond the recharge area.     

Hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from La Pampa,Argentina

Groundwaters from Quaternary loess aquifers in northern La Pampa Province of central Argentina have significant quality problems due to high concentrations of potentially harmful elements such as As, F, NO₃-N, B, Mo, Se and U and high salinity. The extent of the problems is not well-defined, but is believed to cover large parts of the Argentine Chaco-Pampean Plain, over an area of perhaps 10⁶ km². Groundwaters from La Pampa have a very large range of chemical compositions and spatial variability is considerable over distances of a few km. Dissolved As spans over 4 orders of magnitude (<4–5300 μg l⁻¹) and concentrations of F have a range of 0.03–29 mg l⁻¹, B of 0.5–14 mg l^−l, V of 0.02–5.4 mg l⁻¹, NO₃–N of <0.2–140 mg l⁻¹, Mo of 2.7–990 μg l⁻¹ and U of 6.2–250 μg l⁻¹. Of the groundwaters investigated, 95% exceed 10 μg As l⁻¹ (the WHO guideline value) and 73% exceed 50 μg As l⁻¹ (the Argentine national standard). In addition, 83% exceed the WHO guideline value for F (1.5 mg l⁻¹), 99% for B (0.5 mg l⁻¹), 47% for NO₃-N (11.3 mg l⁻¹), 39% for Mo (70 μg l⁻¹), 32% for Se (10 μg l⁻¹) and 100% for U (2 μg l⁻¹). Total dissolved solids range between 730 and 11400 mg l⁻¹, the high values resulting mainly from evaporation under ambient semi-arid climatic conditions. The groundwaters are universally oxidising with high dissolved-O₂ concentrations. Groundwater pHs are neutral to alkaline (7.0–8.7). Arsenic is present in solution predominantly as As(V). Groundwater As correlates positively with pH, alkalinity (HCO₃), F and V. Weaker correlations are also observed with B, Mo, U and Be. Desorption of these elements from metal oxides, especially Fe and Mn oxides under the high-pH conditions is considered an important control on their mobilisation. Mutual competition between these elements for sorption sites on oxide minerals may also have enhanced their mobility. Weathering of primary silicate minerals and accessory minerals such as apatite in the loess and incorporated volcanic ash may also have contributed a proportion of the dissolved As and other trace elements. Concentrations of As and other anions and oxyanions appear to be particularly high in groundwaters close to low-lying depressions which act as localised groundwater-discharge zones. Concentrations up to 7500 μg l⁻¹ were found in saturated-zone porewaters extracted from a cored borehole adjacent to one such depression. Concentrations are also relatively high where groundwater is abstracted from close to the water table, presumably because this zone is a location of more active weathering reactions. The development of groundwaters with high pH and alkalinity results from silicate and carbonate reactions, facilitated by the arid climatic conditions. These factors, together with the young age of the loess sediments and slow groundwater flow have enabled the accumulation of the high concentrations of As and other elements in solution without significant opportunity for flushing of the aquifer to enable their removal.     

Geochemistry of shallow ground water in coastal plain environments in the southeastern United States: implications for aquifer susceptibility

Ground-water chemistry data from coastal plain environments have been examined to determine the geochemical conditions and processes that occur in these areas and assess their implications for aquifer susceptibility. Two distinct geochemical environments were studied to represent a range of conditions: an inner coastal plain setting having more well-drained soils and lower organic carbon (C) content and an outer coastal plain environment that has more poorly drained soils and high organic C content. Higher concentrations of most major ions and dissolved inorganic and organic C in the outer coastal plain setting indicate a greater degree of mineral dissolution and organic matter oxidation. Accordingly, outer coastal plain waters are more reducing than inner coastal plain waters. Low dissolved oxygen (O₂) and nitrate (NO₃⁻) concentrations and high iron (Fe) concentrations indicate that ferric iron (Fe (III)) is an important electron acceptor in this setting, while dissolved O₂ is the most common terminal electron acceptor in the inner coastal plain setting.The presence of a wide range of redox conditions in the shallow aquifer system examined here underscores the importance of providing a detailed geochemical characterization of ground water when assessing the intrinsic susceptibility of coastal plain settings. The greater prevalence of aerobic conditions in the inner coastal plain setting makes this region more susceptible to contamination by constituents that are more stable under these conditions and is consistent with the significantly (p<0.05) higher concentrations of NO₃⁻ found in this setting. Herbicides and their transformation products were frequently detected (36% of wells sampled), however concentrations were typically low (<0.1 μg/L). Shallow water table depths often found in coastal plain settings may result in an increased risk of the detection of pesticides (e.g., alachlor) that degrade rapidly in the unsaturated zone.     

Arsenic release from chlorine-promoted alteration of a sulfide cement horizon: Evidence from batch studies on the St. Peter Sandstone,Wisconsin, USA

Elevated As concentrations have been measured in wells in the St. Peter Sandstone aquifer of eastern Wisconsin, USA. The primary source is As-bearing sulfide minerals (pyrite and marcasite) within the aquifer. There is concern that well disinfection by chlorination may facilitate As release to groundwater by increasing the rate and extent of sulfide oxidation. The objective of this study was to examine the abiotic processes that mobilize As from the aquifer solids during controlled exposure to chlorinated solutions. Thin sections made from sulfidic aquifer material were characterized by quantitative electron probe micro-analysis before and after 24 h exposure to solutions of different Cl₂ concentrations. Batch experiments using crushed aquifer solids were also conducted to examine changes in solution chemistry over 24 h. Results of the combined experiments indicate that Cl₂ addition affects As release and uptake in two ways. First, Cl₂ increases oxidation of sulfide minerals, releasing more As from the mineral structure. Chlorine addition also increases the rate of Fe(II) oxidation and subsequent hydrous ferric oxide (HFO) precipitation, allowing for increased uptake of As onto the mineral surface. Although HFOs can act as sinks for As, they can release As if biogeochemical conditions (e.g. redox, pH) change. These results have implications not only for disinfection of drinking water wells in the study area, but also suggest that introduction of oxidants may adversely affect water quality during aquifer storage and recovery programs in aquifers containing As-bearing minerals.     

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.     

Geochemical response of acid groundwater to neutralization by alkaline recharge

Solute transport and chemical neutralization (pH 3 to 7) within a shallow heterogeneous aquifer producing acid mine drainage (AMD) are examined at an abandoned surface coal mine in West Virginia. The aquifer is undergoing partial neutralization by mixing with alkalinity from a leaking sludge disposal pond, extending in preferential zones controlled by aquifer heterogeneity. Hydraulic heads interpolated from wells indicate leakage from a central alkaline (pH 7.1, 0.72 meq/L alkalinity) sludge pond is a principal source of recharge. Chemically-conservative sodium, added to AMD during treatment and leaked into the aquifer with the sludge, develops a dispersion plume over a restricted portion of the aquifer that correlates with pH, hydraulic head, and dissolved metals distributions. Concentrations of aluminum, iron, sulfate and acidity display higher concentrations downgradient from the pond as sludge alkalinity is consumed along flow paths. Before reaching springs, most dissolved iron is oxidized and hydrolyzed, likely precipitating in the aquifer as a ferric hydroxide or hydroxysulfate phase. The spatial pattern of iron and aluminum concentrations suggests accelerated oxidation caused by gas transport along the outer slopes of the spoil. Dissolved aluminum concentrations increase with total acidity, suggesting that dissolution of silicate minerals results from acidity released by iron hydrolysis. Neutralization reactions and higher pH are favored in more highly permeable portions of the spoil, where ferrihydrite and aluminum hydroxysulfate minerals (such as basaluminite) are supersaturated. In acid-producing zones at pH < 4.5, jurbanite is near equilibrium and an aluminum-sulfate phase with similar properties may limit aluminum concentrations, but become undersaturated in zones of advancing neutralization. At this particular site, ferrous iron produced by pyrite oxidation is almost completely oxidized over short transport distances, allowing hydrolysis of iron and aluminum should sufficient alkalinity be added to these acid waters.     

On elevated fluoride and boron concentrations in groundwaters associated with the Lake Saint-Martin impact structure,Manitoba

Hydrogeological investigations conducted by the Geological Survey of Canada in the Lake Saint-Martin region of Manitoba have confirmed earlier reports of naturally elevated F⁻ and B concentrations in local groundwaters. Fluoride and B concentrations are highly correlated (r² = 0.905) and reach 15.1 mg/L and 8.5 mg/L, respectively. Virtually all groundwaters with F⁻ concentrations greater than the drinking water limit of 1.5 mg/L are from wells within the Lake Saint-Martin impact structure, a 208 Ma complex crater 23 km in diameter underlying a large part of the study area. The high-F⁻ groundwaters can be classified into two groups according to their anionic and isotopic compositions. Group I samples consist of Na-mixed anion groundwaters, with Cl greater than 100 mg/L and highly depleted ¹⁸O compositions indicative of recharge under much cooler climatic conditions than at present. Samples belonging to this group exhibit a striking relationship to crater morphology, and are found in an arcuate belt within the southern rim of the impact structure. Group II high-F⁻ samples consist of Na–HCO₃–SO₄ groundwaters, with little Cl, and less depleted ¹⁸O compositions. Samples belonging to this group are associated with groundwaters recharged locally, on a low ridge within the impact structure. This paper traces the probable source of high-F⁻ groundwaters to phosphatic pellets in shales of the Winnipeg Formation, a regional basal clastic unit which sub-crops at shallow depth beneath the crater rim as a result of more than 200 m of structural uplift associated with the impact event. This extensive aquifer is known elsewhere in southern Manitoba for its naturally-softened groundwaters and locally elevated F⁻ concentrations. Group I groundwaters are interpreted as discharge from the Winnipeg Formation where it abuts against crater-fill deposits. Group II high-F⁻ groundwaters are interpreted as modern recharge from within the impact structure, displacing Group I groundwaters. Thus, elevated F⁻ and B concentrations observed in groundwaters of the Lake Saint-Martin area represent the geochemical signature of upwelling from a deep regional aquifer. The previously unsuspected discharge zone occurs within an isolated sub-crop of the aquifer formed as a result of structural uplift caused by the impact event.     

Evidence of microbially mediated arsenic mobilization from sediments of the Aquia aquifer, Maryland, USA

Sediments from the Aquia aquifer in coastal Maryland were collected as part of a larger study of As in the Aquia groundwater flow system where As concentration are reported to reach levels as high as 1072 nmol kg⁻¹, (i.e., ∼80 μg/L). To test whether As release is microbially mediated by reductive dissolution of Fe(III) oxides/oxyhydroxides within the aquifer sediments, the Aquia aquifer sediment samples were employed in a series of microcosm experiments. The microcosm experiments consisted of sterilized serum bottles prepared with aquifer sediments and sterilized (i.e., autoclaved), artificial groundwater using four experimental conditions and one control condition. The four experimental conditions included the following scenarios: (1) aerobic; (2) anaerobic; (3) anaerobic + acetate; and (4) anaerobic + acetate + AQDS (anthraquinone-2,6-disulfonic acid). AQDS acts as an electron shuttle. The control condition contained sterilized aquifer sediments kept under anaerobic conditions with an addition of AQDS. Over the course of the 27 day microcosm experiments, dissolved As in the unamended (aerobic and anaerobic) microcosms remained constant at around ∼28 nmol kg⁻¹ (2 μg/L). With the addition of acetate, the amount of As released to the solution approximately doubled reaching ∼51 nmol kg⁻¹ (3.8 μg/L). For microcosm experiments amended with acetate and AQDS, the dissolved As concentrations exceeded 75 nmol kg⁻¹ (5.6 μg/L). The As concentrations in the acetate and acetate + AQDS amended microcosms are of similar orders of magnitude to As concentrations in groundwaters from the aquifer sediment sampling site (127-170 nmol kg⁻¹). Arsenic concentrations in the sterilized control experiments were generally less than 15 nmol kg⁻¹ (1.1 μg/L), which is interpreted to be the amount of As released from Aquia aquifer sediments owing to abiotic, surface exchange processes. Iron concentrations released to solution in each of the microcosm experiments were higher and more variable than the As concentrations, but generally exhibited similar trends to the As concentrations. Specifically, the acetate and acetate + AQDS amended microcosm typically exhibited the highest Fe concentrations (up to 1725 and 6566 nmol kg⁻¹, respectively). The increase in both As and Fe in the artificial groundwater solutions in these amended microcosm experiments strongly suggests that microbes within the Aquia aquifer sediments mobilize As from the sediment substrate to the groundwaters via Fe(III) reduction.     

Palaeo-hydrogeological control on groundwater As levels in Red River delta,Vietnam

To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta’s Quaternary geological development was reviewed. The water samples (n = 30) were analyzed for As, major ions, Fe²⁺, H₂S, NH₄, CH₄, δ¹⁸O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15–20 km wide and 50–60 m deep Holocene incised valley. The NE part of the transect consists of 60–120 m of Pleistocene yellowish alluvial deposits underneath 10–30 m of estuarine clay overlain by a 10–20 m veneer of Holocene sediments. The distribution of δ¹⁸O-values (range −12.2‰ to −6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe²⁺ (0.03–2.0 mM), Mn (0.7–320 μM), SO₄ (<2.1 μM–0.75 mM), H₂S (<0.1–7.0 μM), NH₄ (0.03–4.4 mM), and CH₄ (0.08–14.5 mM). Generally, higher concentrations of NH₄ and CH₄ and low concentrations of SO₄ were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013–11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH₄, CH₄ and SO₄. Low concentrations of As (?0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe²⁺ and H₂S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19–65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO₄ was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration.     

Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part I: Comparative hydrochemical and hydrogeological characteristics

The present study demonstrates the importance of hydrogeochemical characteristics (groundwater flow and recharge) of an aquifer in the release of As to groundwater. The study area (∼20 km²) is located in Chakdaha block, Nadia district, West Bengal, which hosts groundwaters of variable As content. The spatial distribution pattern of As is patchy with areas containing groundwater that is high in As (>200 μg L⁻¹) found in close vicinity to low As (<50 μg L⁻¹) groundwaters (within 100 m). The concentration of groundwater As is found to decrease with depth. In addition, the data shows that there is no conspicuous relationship between high groundwater As concentration and high groundwater abstraction, although the central cone of depression has enlarged over 2 a and is extending towards the SE of the study area. The river Hooghly, which forms the NW boundary of the study site, shows dual behaviour (effluent and influent during pre- and post-monsoon periods, respectively), complicating the site hydrogeology. The observed groundwater flow lines tend to be deflected away from the high As portion of the aquifer, indicating that groundwater movement is very sluggish in the As-rich area. This leads to a high residence time for this groundwater package, prolonging sediment–water interaction, and hence facilitating groundwater As release.     

Arsenic associations in sediments from the loess aquifer of La Pampa,Argentina

Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concentrations of As (up to 5300 μg L⁻¹) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concentrations in 45 loess samples collected from the aquifer have a range of 3–18 mg kg⁻¹ with a mean of 8 mg kg⁻¹. These values are comparable to world-average sediment As concentrations. Five samples of rhyolitic ash from the area have As concentrations of 7–12 mg kg⁻¹. Chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes. Results reveal a large range of porewater As concentrations, being generally higher in the horizons with highest sediment As concentrations. The displaced porewaters have As concentrations ranging up to 7500 μg L⁻¹ as well as exceptionally high concentrations of some other oxyanion species, including V up to 12 mg L⁻¹. The highest concentrations are found in a borehole located in a topographic depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective extract data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is associated with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Extract compositions also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidising, with dissolved O₂, NO₃ and SO₄ normally present and As(V) usually the dominant dissolved As species. Under such conditions, the solubility of Fe and Mn oxides is low and As mobilisation is strongly controlled by sorption–desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0–8.8) and high concentrations of potential competitors (e.g. V, P, HCO₃). PHREEQC modelling suggests that the presence of V at the concentrations observed in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by up to an order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate extract concentrations have been used to provide an upper estimate of the amount of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes investigated has been used to estimate an approximate K_d value for the sediments of 0.94 L kg⁻¹. This low value indicates that the sediments have an unusually low affinity for As.     

Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part I: Evidence from sediment profiles

This study reexamines the notion that extensive As mobilization in anoxic groundwater of Bangladesh is intimately linked to the dissolution of Fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material. Detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater As concentrations were known to span a wide range. The sediment properties that were measured include (1) the proportion of Fe(II) in the Fe fraction leached in hot 1.2 N HCl, (2) diffuse spectral reflectance, and (3) magnetic susceptibility.In parallel with local concentrations of dissolved As ranging from <5 to 600 μg/L, Fe(II)/Fe ratios in shallow (gray) Holocene sands tended to gradually increase with depth from values of 0.3 to 0.5 to up to 0.9. In deeper (orange) aquifers of presumed Pleistocene age that were separated from shallow sands by a clay layer and contained <5 μg/L dissolved As, leachable Fe(II)/Fe ratios averaged ∼0.2. There was no consistent relation between sediment Fe(II)/Fe and dissolved Fe concentrations in groundwater in nearby wells. The reflectance measurements indicate a systematic linear relation (R² of 0.66; n = 151) between the first derivative transform of the reflectance at 520 nm and Fe(II)/Fe. The magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 (x 10⁻⁹ m³/kg SI) and was linearly related (R² of 0.75; n = 29) to the concentrations of minerals that could be magnetically separated (0.03 to 0.79% dry weight). No systematic depth trends in magnetic susceptibility were observed within the shallow sands, although the susceptibility of deeper low-As aquifers was low (up to ∼200 × 10⁻⁹ m³/kg SI).This set of observations, complemented by incubation results described in a companion paper by van Geen et al. (this volume), suggests that the release of As is linked to the transformation of predominantly Fe (III) oxyhydroxide coatings on sand particles to Fe(II) or mixed Fe(II/III) solid phases with a flatter reflectance spectrum such as siderite, vivianite, or magnetite, without necessarily resulting in the release of Fe to groundwater. The very low As/Fe ratio of magnetically separated minerals compared to the As/Fe of bulk acid leachate (2 vs. 40 10⁻⁶, respectively) suggests that such a transformation could be accompanied by a significant redistribution of As to a mobilizable phase on the surface of aquifer particles.