Applied Geochemistry (v.26, #4)

Arsenic and other toxic elements in surface and groundwater systems by Abhijit Mukherjee; Prosun Bhattacharya; Alan E. Fryar (415-420).

Arsenic enrichment in unconfined sections of the southern Gulf Coast aquifer system, Texas by J.B. Gates; J.P. Nicot; B.R. Scanlon; R.C. Reedy (421-431).
► Groundwater arsenic concentrations in the Texas Gulf Coast aquifer system investigated with a hydrochemical transect. ► Volcanic sediments the dominant source based on geographic distributions and solute correlations. ► High arsenic in oxidizing recharge zones may be associated with sorption competition.Groundwater arsenic concentrations exceeding the federal drinking water standard are common in the southern Gulf Coast aquifer system in Texas, including in aerobic, unconfined groundwater which provides much of the municipal and domestic water supplies for the region. The objective of this study was to determine geochemical factors affecting the occurrence and distribution of groundwater As in unconfined portions of the southern Gulf Coast aquifer system through a comparative transect study of groundwater across three major hydrostratigraphic units (the Catahoula Formation, Jasper aquifer and Evangeline aquifer) and analysis of regional water quality data. Results show that As concentrations decrease with increasing distance from the Catahoula Formation, which is consistent with Miocene volcanic ash as the main source of As to groundwater in the region. Arsenic concentrations correlate with V, SiO2 and K, all of which were released during weathering of volcanic sediments and their degradation products. In all three units, carbonate weathering and active recharge in the unconfined zones result in circum-neutral pH and oxidizing groundwater, which are typically amenable to As immobilization by adsorption of arsenate onto mineral oxides and clays. However, As concentrations exceed 10 μg/L in approximately 30% of wells. Silica that was co-released with As may compete for sorption sites and reduce the capacity for arsenate adsorption.

► Sub-regional scale aquifers delineated in arsenic-enriched belt in the Ganga Plain. Isotopic fingerprint of the groundwater, from arsenic-enriched and arsenic-safe aquifers established for the first time in the Ganga Plain. ► Recharge processes and the water provenances of vertically separated Quaternary aquifers have been established. ► Mean residence time of groundwater in the deeper aquifers has been worked out using C-14 isotope. ► Water from the deeper aquifer has been correlated with the paleoclimatic model of the Middle Ganga Plain (Mid-Ganga Basin) for 6-2 ka.Arsenic concentrations in groundwater extracted from shallow aquifers in some areas of the Ganga Plain in the states of Bihar and Uttar Pradesh, exceed 50 μg L−1 and locally reach levels in the 400 μg L−1 range. The study covered 535 km2 of active flood plain of the River Ganga, in Bihar where a two-tier aquifer system has been delineated in a multi-cyclic sequence of Quaternary sand, clay, sandy clay and silty clay all ⩽∼250 m below ground surface. The research used isotopic signatures (δ 18O, δ 2Η, 3H, 14C) and major chemical constituents HCO 3 - , SO 4 2 - , NO 3 - , Cl - , Ca 2 + , Mg 2 + , Na + , K + , As total of groundwater to understand the recharge processes and groundwater circulation in the aquifers. Values of δ 18O and δ 2Η combined with 3H data indicate that the recharge to the As-enriched top 40 m of the deposits is modern (<50 a), predominantly meteoric, with some evaporation during infiltration, and partly from tanks and other surface water bodies. The lower part of the upper aquifer is vulnerable to mobilization of As with increasing groundwater extraction. The low As lower aquifer (max. 5 μg L−1) is hydrologically isolated from the upper aquifer and is characterized by lower 14C concentration and lower (more negative) δ 18O values. Groundwater in the lower aquifer is ∼3 ka old, occurs under semi-confined to confined conditions, with hydrostatic head at 1.10 m above the head of the upper aquifer during the pre-monsoon. The recharge areas of the lower aquifer lies in Pleistocene deposits in basin margin areas with the exposed Vindhyan System, at about 55 km south of the area.

Ultramafic-derived arsenic in a fractured bedrock aquifer by Peter C. Ryan; Jonathan Kim; Andrew J. Wall; Jonathan C. Moen; Lilly G. Corenthal; Daniel R. Chow; Colleen M. Sullivan; Kevin S. Bright (444-457).
► Arsenic is elevated in groundwater from a fractured bedrock aquifer system in northern Vermont, USA. ► The arsenic source is serpentinized ultramafic rock. ► Antigorite, magnetite (MgCO3) and magnetite (Fe3O4) appear to be the main mineralogical hosts of arsenic in the ultramafic rock. ► Arsenic appears to be introduced to the ultramafic rock when As-bearing fluids are driven out of sediments during subduction. ► The occurrence of serpentinized ultramafic rocks in many orogenic belts suggests that similar arsenic anomalies may occur in geologically-similar terranes globally.In the fractured bedrock aquifer of northern Vermont, USA, As concentrations in groundwater range from <1 to 327 μg/L (<13–4360 nm/L) and these elevated occurrences have a general spatial association with ultramafic rock bodies. The ultramafic rocks in this region are comprised mainly of serpentinites and talc–magnesite rocks with average As concentration of 93 ppm and a range from 1 to 1105 ppm. By comparison, the other main lithologies in the study area are depleted in As relative to the ultramafics: the average As concentration in metabasaltic rocks is 4.1 ppm with a range of <1–69 ppm, and mean As concentration in meta-sedimentary phyllites and schists is 22 ppm with a range of <1–190 ppm. In the ultramafic rocks, As is correlated with Sb and light rare earth elements, indicating that As was introduced to the ultramafic rocks during metasomatism by fluids derived from the subducting slab. Evidence from sequential chemical extraction, X-ray diffraction (XRD) and stoichiometric analysis indicates that the majority of the As is located in antigorite and magnesite (MgCO3) with lesser amounts in magnetite (Fe3O4). Hydrochemistry of monitoring wells drilled into fractured ultramafic rock in a groundwater recharge area with no anthropogenic As source reveals above background As (2–9 μg/L) and an Mg–HCO3 hydrochemical signature that reflects dissolution of antigorite and magnesite, confirming that As in groundwater can be derived from ultramafic rock dissolution. Arsenic mobility in groundwater affected by ultramafic rock dissolution may be enhanced by alkaline pH values and relatively high HCO 3 - concentrations.

► Naturally occurring arsenic in Fraser River delta aquifer. ► Dissolved concentrations peak at 32 μg/L. ► Solid phase concentrations average between 2.2 and 4.1 μg/g. ► Influx of natural organic matter from peat not sufficient to release significant arsenic to solution.Naturally occurring As in the groundwater and Holocene sediments of the lower Fraser River delta was studied. At each of two sites in the delta, vertical profiles of groundwater geochemistry were characterized and sediment cores were collected for petrographic analysis, quantification of elemental abundances, and sequential extraction analysis to characterize the pools of solid-phase As. Total solid-phase As concentrations were similar to those found in other deltaic environments, ranging from 2.2 to 2.6 μg/g at a site of groundwater recharge with approximately 4 m of peat at the surface, to 2.7–4.1 μg/g at a site adjacent to the Fraser River where saline water is encountered within the aquifer at depth. Arsenic that was weakly bound or could be mobilized by reduction of Fe oxides constituted between 20% and 50% of the total As in the sediments. Groundwater As concentrations showed distinct vertical trends, and peaked at 9 μg/L at the peat-influenced site and between 29 and 32 μg/L at the site with saline water at depth. At both sites, reduction of dissolved organic matter appears to be responsible for reducing conditions, high dissolved Fe (up to 230 mg/L), high HCO 3 - , and the presence of NH 4 + and PO 4 3 - . The peat-impacted site implies that a direct source of dissolved organic C from peat is not a sufficient condition for significant As release into groundwater in the Fraser River delta.

Mineralogical profiling of alluvial sediments from arsenic-affected Ganges–Brahmaputra floodplain in central Bangladesh by A. Uddin; M. Shamsudduha; J.A. Saunders; M.-K. Lee; K.M. Ahmed; M.T. Chowdhury (470-483).
► Arsenic concentrations in groundwater of central Bangladesh are highly variable in space but generally decrease downward. ► Heavy minerals are abundant at shallow depths but less abundant at greater depths. ► Authigenic siderite that precipitates under reducing environment at greater depths decrease Fe and possibly As in groundwater. ► Absence of authigenic pyrite suggests that SO4 reduction in Manikganj groundwater is limited in contrast to the southeastern Bengal Basin where precipitation of arsenian pyrite is thought to sequester As from groundwater. ► Presence of Fe(III) minerals in aquifers shows that reduction of these minerals is incomplete.Mineral assemblages (heavy and light fractions) and sedimentological characteristics of the Quaternary alluvial aquifers were examined in the central Bengal Basin where As concentrations in groundwater are highly variable in space but generally decrease downward. Chemical compositions of sediment samples from two vertical core profiles (2–150 m below ground level, bgl) were analyzed along with groundwater in moderately As-enriched aquifers in central Bangladesh (Manikganj district), and the As mobilization process in the alluvial aquifer is described. Heavy minerals such as biotite, magnetite, amphibole, apatite and authigenic goethite are abundant at shallow (<100 m below ground level (mbgl)) depths but less abundant at greater depths. It is interpreted that principal As-bearing minerals were derived from multiple sources, primarily from ophiolitic belts in the Indus-Tsangpo suture in the northeastern Himalayan and Indo-Burman Mountain ranges. Authigenic and amorphous Fe-(oxy)hydroxide minerals that are generally formed in river channels in the aerobic environment are the major secondary As-carriers in alluvial sediments. Reductive dissolution (mediated by Fe-reducing bacteria) of Fe-(oxy)hydroxide minerals under anoxic chemical conditions is the primary mechanism responsible for releasing As into groundwater. Authigenic siderite that precipitates under reducing environment at greater depths decreases Fe and possibly As concentrations in groundwater. Presence of Fe(III) minerals in aquifers shows that reduction of these minerals is incomplete and this can release more As if further Fe-reduction takes place with increased supplies of organic matter (reactive C). Absence of authigenic pyrite suggests that SO4 reduction (mediated by SO4-reducing bacteria) in Manikganj groundwater is limited in contrast to the southeastern Bengal Basin where precipitation of arsenian pyrite is thought to sequester As from groundwater.

► Metal(loid) concentrations in mine wastes are inversely related to particle size. ► Elemental mass distributions are dictated primarily by bulk mass distributions. ► Trace metal(loid)s are often correlated, suggesting mineral phase associations. ► Fine-grained particles are enriched in metal(loids) above bulk average concentrations. ► Regulatory agencies may severely underestimate metal(loid) levels in labile size fractions.The mining and processing of metal-bearing ores has resulted in contamination issues where waste materials from abandoned mines remain in piles of untreated and unconsolidated material, posing the potential for waterborne and airborne transport of toxic elements. This study presents a systematic method of particle size separation, mass distribution, and bulk chemical analysis for mine tailings and adjacent background soil samples from the Rand historic mining district, California, in order to assess particle size distribution and related trends in metal(loid) concentration as a function of particle size. Mine tailings produced through stamp milling and leaching processes were found to have both a narrower and finer particle size distribution than background samples, with significant fractions of particles available in a size range (⩽250 μm) that could be incidentally ingested. In both tailings and background samples, the majority of trace metal(loid)s display an inverse relationship between concentration and particle size, resulting in higher proportions of As, Cr, Cu, Pb and Zn in finer-sized fractions which are more susceptible to both water- and wind-borne transport as well as ingestion and/or inhalation. Established regulatory screening levels for such elements may, therefore, significantly underestimate potential exposure risk if relying solely on bulk sample concentrations to guide remediation decisions. Correlations in elemental concentration trends (such as between As and Fe) indicate relationships between elements that may be relevant to their chemical speciation.

Surface complexation modeling for predicting solid phase arsenic concentrations in the sediments of the Mississippi River Valley alluvial aquifer, Arkansas, USA by Md. Salah U. Sharif; Ralph K. Davis; Kenneth F. Steele; Burmshik Kim; Phillip D. Hays; Tim M. Kresse; John A. Fazio (496-504).
► Surface complexation models (SCMs) can be used to predict the distribution of Arsenic (As) in natural sediments with a moderate to high level of uncertainty. ► The accuracy of the SCMs depends on the qualitative and quantitative determination of accurate solid sorbent phases, selection of proper extraction methods, inclusion of accurate surface parameters, internally consistent surface reaction constants for each sorbent phases, and detailed chemical analysis of groundwater. ► The use of default surface-site densities for individual sorbent phases reported in the literature is not applicable for sediments collected through the entire depth profile having different redox conditions at varying depths. ► It is very important to develop consensus on the appropriate extraction methods and sorption databases for interested contaminants (e.g. arsenic) that can be used in SCMs for natural sediments.The potential health impact of As in drinking water supply systems in the Mississippi River Valley alluvial aquifer in the state of Arkansas, USA is significant. In this context it is important to understand the occurrence, distribution and mobilization of As in the Mississippi River Valley alluvial aquifer. Application of surface complexation models (SCMs) to predict the sorption behavior of As and hydrous Fe oxides (HFO) in the laboratory has increased in the last decade. However, the application of SCMs to predict the sorption of As in natural sediments has not often been reported, and such applications are greatly constrained by the lack of site-specific model parameters. Attempts have been made to use SCMs considering a component additivity (CA) approach which accounts for relative abundances of pure phases in natural sediments, followed by the addition of SCM parameters individually for each phase. Although few reliable and internally consistent sorption databases related to HFO exist, the use of SCMs using laboratory-derived sorption databases to predict the mobility of As in natural sediments has increased. This study is an attempt to evaluate the ability of the SCMs using the geochemical code PHREEQC to predict solid phase As in the sediments of the Mississippi River Valley alluvial aquifer in Arkansas. The SCM option of the double-layer model (DLM) was simulated using ferrihydrite and goethite as sorbents quantified from chemical extractions, calculated surface-site densities, published surface properties, and published laboratory-derived sorption constants for the sorbents. The model results are satisfactory for shallow wells (10.6 m below ground surface), where the redox condition is relatively oxic or mildly suboxic. However, for the deep alluvial aquifer (21–36.6 m below ground surface) where the redox condition is suboxic to anoxic, the model results are unsatisfactory.

► Distribution of arsenic in the different geochemical fractions (oxide, carbonate, sulfide + organic matter, and residual) in sediments. ► Arsenic speciation in sediments. ► Association of arsenic with the residual geochemical fraction which is less. susceptible to weathering changes. ► Biogeochemical (microbial) processes influence the distribution of arsenic in these aquifer sediments.Sediments from a core retrieved during installation of a shallow drinking water well in Ambikanagar (West Bengal, India) were analyzed for various physical and chemical parameters. The geochemical analyses included: (1) a 4-step sequential extraction scheme to determine the distribution of As between different fractions, (2) As speciation (As3+ vs. As5+), and (3) C, N and S isotopes. The sediments have a low percentage of organic C and N (0.10–0.56% and 0.01–0.05%, respectively). Arsenic concentration is between 2 and 7 mg kg−1, and it is mainly associated with the residual fraction, less susceptible to chemical weathering. The proportion of As3+ in these sediments is high and ranges from 24% to 74%. Arsenic in the second fraction (reducible) correlates well with Mn, and in the residual fraction As correlates well with several transition elements. The stable isotope results indicate microbial oxidation of organic matter involving SO4 reduction. Oxidation of primary sulfide minerals and release of As from reduction of Fe-(oxy)hydroxides do not seem important mechanisms in As mobilization. Instead, the dominance of As3+ and presence of As5+ reducing microorganisms in this shallow aquifer imply As remobilization involving microbial processes that needs further investigations.

Groundwater chemistry and redox processes: Depth dependent arsenic release mechanism by Ashis Biswas; Santanu Majumder; Harald Neidhardt; Dipti Halder; Subhamoy Bhowmick; Aishwarya Mukherjee-Goswami; Amit Kundu; Debasree Saha; Zsolt Berner; Debashis Chatterjee (516-525).
► Geochemical process responsible for mobilization of As is depth dependent. ► Concentration of As in groundwater depends on single/combined release mechanism. ► Fe and Mn hydroxide reduction, responsible for mobilization of As, is microbially mediated.Patchy occurrences of elevated As are often encountered in groundwater from the shallow aquifers (<50 m) of the Bengal Delta Plain (BDP). A clear understanding of various biogeochemical processes, responsible for As mobilization, is very important to explain this patchy occurrence and thus to mitigate the problem. The present study deals with the periodical monitoring of groundwater quality of five nested piezometeric wells between December 2008 and July 2009 to investigate the temporal changes in groundwater chemistry vis-a-vis the prevalent redox processes in the aquifer. Geochemical modeling has been carried out to identify key phases present in groundwater. A correlation study among different aqueous redox parameters has also been performed to evaluate prevailing redox processes in the aquifer. The long term monitoring of hydrochemical parameters in the multilevel wells together with hydrogeochemical equilibrium modeling has shown more subtle differences in the geochemical environment of the aquifer, which control the occurrence of high dissolved As in BDP groundwater. The groundwater is generally of Ca–HCO3 type. The dissolved As concentration in groundwater exceeded both WHO and National drinking water standard (Bureau of Indian Standards; BIS, 10 μg L−1) throughout the sampling period. The speciation of As and Fe indicate persistent reducing conditions within the aquifer [As(III): 87–97% of AsT and Fe(II): 76–96% of FeT]. The concentration of major aqueous solutes is relatively high in the shallow aquifer (wells A and B) and gradually decreases with increasing depth in most cases. The calculation of SI indicates that groundwater in the shallow aquifer is also relatively more saturated with carbonate minerals. This suggests that carbonate mineral dissolution is possibly influencing the groundwater chemistry and thereby controlling the mobilization of As in the monitored shallow aquifer. Hydrogeochemical investigation further suggests that Fe and/or Mn oxyhydroxide reduction is the principal process of As release in groundwater from deeper screened piezometric wells. The positive correlations of U and V with As, Fe and Mn indicate redox processes responsible for mobilization of As in the deeper screened piezometric wells are possibly microbially mediated. Thus, the study advocates that mobilization of As is depth dependent and concentrations of As in groundwater depends on single/combined release mechanisms.

Biogeochemical factors affecting the presence of 210Po in groundwater by Ralph L. Seiler; Lisa L. Stillings; Nichole Cutler; Laina Salonen; Iisa Outola (526-539).
210Po activities in numerous domestic wells in Fallon NV exceed 500 mBq/L. ► 210Po levels in sediment are not the primary determinant on levels in groundwater. ► δ34S measurements indicate SO4 reduction occurred in all 210Po contaminated wells. ► 210Po contaminated wells are anoxic, have high pH and low Ca. ► Po mobilization probably involves an anaerobic S cycle in which H2S dissolves MnO2.The discovery of natural 210Po enrichment at levels exceeding 500 mBq/L in numerous domestic wells in northern Nevada, USA, led to a geochemical investigation of the processes responsible for its mobilization. 210Po activities in 63 domestic and public-supply wells ranged from below 1 mBq/L to 6590 ± 590 mBq/L, among the highest reported levels in the USA. There is little spatial or depth variability in 210Pb activity in study-area sediments and mobilization of a few percent of the 210Po in the sediments would account for all of the 210Po in water. Stable-isotope measurements indicate SO4 reduction has occurred in all 210Po contaminated wells. Sulfide species are not accumulating in the groundwater in much of Lahontan Valley, probably because of S cycling involving microbial SO4 reduction, abiotic oxidation of H2S to S0 by Mn(IV), followed by microbial disproportionation of S0 to H2S and SO4. The high pH, Ca depletion, MnCO3 saturation, and presence of S0 in Lahontan Valley groundwater may be consequences of the anaerobic S cycling. Consistent with data from naturally-enriched wells in Florida, 210Po activities begin to decrease when aqueous sulfide species begin to accumulate. This may be due to formation and precipitation of PoS, however, Eh–pH diagrams suggest PoS would not be stable in study-area groundwater. An alternative explanation for the study area is that H2S accumulation begins when anaerobic S cycling stops because Mn oxides are depleted and their reduction is no longer releasing 210Po. Common features of 210Po-enriched groundwater were identified by comparing the radiological and geochemical data from Nevada with data from naturally-enriched wells in Finland, and Florida and Maryland in the USA. Values of pH ranged from <5 in Florida wells to >9 in Nevada wells, indicating that pH is not critical in determining whether 210Po is present. Where U is present in the sediments, the data suggest 210Po levels may be elevated in aquifers with (1) SO4-reducing waters with low H2S concentrations, or (2) anoxic or oxic waters with extremely high Rn activities, particularly if the water is turbid.

Controls on elevated fluoride and arsenic concentrations in groundwater from the Yuncheng Basin, China by Matthew Currell; Ian Cartwright; Massimo Raveggi; Dongmei Han (540-552).
Chemical analysis of groundwater and sediments was carried out to investigate causes of elevated F (1.5–6.6 mg/L) and As concentrations (10–27 μg/L; one sample affected by local contamination with 4870 μg/L As), in groundwater from the Yuncheng Basin, northern China. Groundwater from 9 out of 73 wells contains both F and As concentrations above World Health Organisation safe drinking guidelines (>1.5 mg/L and >10 μg/L, respectively); F concentrations above safe levels are more widespread than As (27 vs. 12 wells). The elevated As and F concentrations represent a significant health risk, as groundwater is widely used to supply agricultural and domestic water in the region. High F and As concentrations occur in shallow groundwater affected by agriculture and deep groundwater with long residence times (>13 ka) that shows little sign of anthropogenic influence. The strong positive correlation between groundwater F/Cl and As/Cl ratios (r 2  = 0.98 and 0.77 in shallow and deep groundwater, respectively) indicates that these elements are mobilized and enriched by common processes. Positive correlations between F and As concentrations and Na/Ca ratios (r 2  = 0.67 and 0.46, respectively) indicate that groundwater major ion chemistry plays a significant role in mobilizing F and As. Mobilization likely occurs via de-sorption of As and F anions (e.g. HAsO 4 2 - and F) from hydrous metal oxides. Moderate positive correlations between pH and As and F concentrations (r 2  = 0.36 and 0.17, respectively) indicate that high pH may favour de-sorption, while HCO3 may act as a sorption competitor. High groundwater Na/Ca ratios likely result from cation exchange, while pH and HCO3 contents are predominantly controlled by carbonate weathering reactions. Sediments from the area were reacted with various water solutions, producing F concentrations between 0.49 and 2.7 mg/L and As concentrations between 0.51 and 16.7 μg/L. Up to 45% more F and 35% more As were released when sediments were reacted with a Na-rich, Ca-poor solution compared to a Ca-rich solution; this is consistent with increased mobilization of F and HAsO 4 2 - by Na-rich, Ca-poor groundwater. Increasing F and As concentrations across a wide area caused by high levels of pumping is a potential future health concern.

A field and reactive transport model study of arsenic in a basaltic rock aquifer by Bergur Sigfusson; Sigurdur R. Gislason; Andrew A. Meharg (553-564).
► As in high temperature geothermal water was predominantly bound to S. ► Thioarsenic species degraded rapidly under atmospheric conditions. ► Arsenate travelled nine times faster than arsenite in a basaltic rock aquifer.The use of geothermal energy as a source for electricity and district heating has increased over recent decades. Dissolved As can be an important constituent of the geothermal fluids brought to the Earth’s surface. Here the field application of laboratory measured adsorption coefficients of aqueous As species on basaltic glass surfaces is discussed. The mobility of As species in the basaltic aquifer in the Nesjavellir geothermal system, Iceland was modelled by the one-dimensional (1D) reactive transport model PHREEQC ver. 2, constrained by a long time series of field measurements with the chemical composition of geothermal effluent fluids, pH, Eh and, occasionally, Fe- and As-dissolved species measurements. Di-, tri- and tetrathioarsenic species ( As ( OH ) S 2 2 - , AsS3H2−, AsS 3 3 - and As ( SH ) 4 - ) were the dominant form of dissolved As in geothermal waters exiting the power plant (2.556 μM total As) but converted to some extent to arsenite (H3AsO3) and arsenate HAsO 4 2 - oxyanions coinciding with rapid oxidation of S 2 - to S 2 O 3 2 - and finally to SO 4 2 - during surface runoff before feeding into a basaltic lava field with a total As concentration of 0.882 μM following dilution with other surface waters.A continuous 25-a data set monitoring groundwater chemistry along a cross section of warm springs on the Lake Thingvallavatn shoreline allowed calibration of the 1D model. Furthermore, a series of ground water wells located in the basaltic lava field, provided access along the line of flow of the geothermal effluent waters towards the lake. The conservative ion Cl moved through the basaltic lava field (4100 m) in less than10 a but As was retarded considerably due to surface reactions and has entered a groundwater well 850 m down the flow path as arsenate in accordance to the prediction of the 1D model. The 1D model predicted a complete breakthrough of arsenate in the year 2100. In a reduced system arsenite should be retained for about 1 ka.

Identification of probable groundwater paths in the Amargosa Desert vicinity by Omar Al-Qudah; Arturo Woocay; John Walton (565-574).
► PHREEQC, PCFA and clustering determine groundwater chemical signatures and groups. ► Hydro-chemical signatures and groups denote surface runoff infiltration and flow-paths. ► Chemical signatures obtained at infiltration regions change slightly along flow-paths. ► Identified flow-paths are the traces of the Amargosa River and Forty mile Wash. ► A third possible flow-path is the trace of Gravity Fault, Rock Valley and Death Valley.In this study, the hydrogeochemical program PHREEQC was used to determine the chemical speciation and mineral saturation indices (SIs) of groundwater in the vicinity of the proposed high-level nuclear waste repository at Yucca Mountain, Nevada (USA). In turn, these data were used to interpret the origin and recharge mode of groundwater, to elucidate the mechanisms of flow and transport, and to determine potential sources of groundwater contamination. PHREEQC was run to determine aqueous dissolved species and minerals that would be in equilibrium with the study area’s groundwater. Selected major ions, associated SI, F and Ca/Na ion exchange were then examined using the multivariate statistical methods of principal component factor analysis and k-means cluster analysis. Analysis of dissolved ion concentrations, SIs, and Ca/Na ion exchange allows simultaneous consideration of arithmetic (raw concentrations) and logarithmic (SI, ion exchange) variables that describe the hydrochemical system and, therefore, can provide further insight into the system’s behavior. The analysis indicates that the dominant processes and reactions responsible for the hydrochemical evolution in the system are (1) evaporative concentration prior to infiltration, (2) carbonate equilibrium, (3) silicate weathering reactions, (4) limited mixing with saline water, (5) dissolution/precipitation of calcite, dolomite and fluorite, and (6) ion exchange. Principal component factor analysis and k-means cluster analysis of factor scores allow the reduction of dimensions describing the system and the identification of hydrogeochemical facies and the processes that defined and govern their evolution.Statistical analysis results indicate that the northern, west face and southern Yucca Mountain groundwater is fresh water with low concentrations of Ca2+, Mg2+, Cl, Ca2+/(Na+)2, and CaF2. The Fortymile Wash groundwater is dilute. The carbonate signature is shown in the Ash Meadows and Death Valley waters with high fluorite SI. Finally, the Crater Flat, Stripped Hills, and Skeleton Hills are dominated by Ca/Na ion exchange, Mg and Ca. The hydrochemical and statistical analyses showed three main groundwater signatures or hydrochemical processes indicating groundwater evolution, potential flowpaths, and recharge areas. The flowpaths are the trace of the Amargosa River, the trace of Fortymile Wash, and its convergence with the Amargosa River. This appears to represent not just a groundwater flow path, but traces of surface runoff infiltration as well.

Evidence of microbially mediated arsenic mobilization from sediments of the Aquia aquifer, Maryland, USA by Christine A. Pearcy; Darren A. Chevis; T. Jade Haug; Holly A. Jeffries; Ningfang Yang; Jianwu Tang; Deborah A. Grimm; Karen H. Johannesson (575-586).
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.

Mineralogy and geochemistry of shallow sediments of Sonargaon, Bangladesh and implications for arsenic dynamics: Focusing on the role of organic matter by Ashraf Ali Seddique; Harue Masuda; Muneki Mitamura; Keiji Shinoda; Toshiro Yamanaka; Shinji Nakaya; Kazi Matin Ahmed (587-599).
Mineralogy and geochemistry of modern shallow sediments (up to 5 m thick) within the zone of water table fluctuations were studied to determine the likely sources and processes responsible for releasing As into groundwater. Samples were collected from different geological settings with varying groundwater As concentrations during dry (December 2005) and wet (September 2006) seasons at Sonargaon, Bangladesh. Stratigraphic sequences of the studied sediments showed three distinct lithofacies, viz. clayey-silt, silty-clay, and silty-very fine sand, corresponding to fine-grained overbank associations. Total As concentrations of shallow sediments ranged from <1 to 16 mg/kg without a significant difference in the range of As concentrations between the seasons. Sequential chemical extraction analysis of As revealed that >80% of the As was fixed in insoluble and organic phases, while the amount of As in reducible and acid-soluble phases was very low (<20%) and varied inversely with total As content. Total As concentration varied with mica content (muscovite and biotite) and its related elements (Al, Mg and Fe), but not with total organic C, suggesting that biotite is the major host phase of As. Arsenic appears to be liberated from biotite and/or other As-bearing minerals via chemical weathering (i.e., hydration–decomposition), either from the near-surface sediments which are subject to seasonal cycling of the redox conditions, or from within the aquifer sediments. Once released, progressive diagenesis to form As-bearing organic matter may be responsible for controlling As distribution in the sediments and coexisting groundwater of the study area.

Elevated arsenic in deeper groundwater of the western Bengal basin, India: Extent and controls from regional to local scale by Abhijit Mukherjee; Alan E. Fryar; Bridget R. Scanlon; Prosun Bhattacharya; Animesh Bhattacharya (600-613).
► First detailed study of deeper groundwater arsenic (As) in western Bengal basin. ► The study shows presence of elevated As in contrast to existing hypothesis. ► Regionally, hydrostratigraphy has a strong control on As distribution. ► As mobilization conducive sediment-groundwater chemistry is present in >200 m depth. ► Locally, prolific deep pumping can attract As-enriched shallow water to deeper depth.The deeper groundwater (depending on definition) of the Bengal basin (Ganges–Brahmaputra delta) has long been considered as an alternate, safe drinking-water source in areas with As-enrichment in near-surface groundwater. The present study provides the first collective discussion on extent and controls of elevated As in deeper groundwater of a regional study area in the western part of the Bengal basin. Deeper groundwater is defined here as non-brackish, potable (Cl  ⩽ 250 mg/L) groundwater available at the maximum accessed depth (∼80–300 m). The extent of elevated As in deeper groundwater in the study area seems to be largely controlled by the aquifer–aquitard framework. Arsenic-enriched deeper groundwater is mostly encountered north of 22.75°N latitude, where an unconfined to semi-confined aquifer consisting of Holocene- to early Neogene-age gray sand dominates the hydrostratigraphy to 300 m depth below land surface. Aquifer sediments are not abnormally enriched in As at any depth, but sediment and water chemistry are conducive to As mobilization in both shallow and deeper parts of the aquifer(s). The biogeochemical triggers are influenced by complex redox disequilibria. Results of numerical modeling and profiles of environmental tracers at a local-scale study site suggest that deeper groundwater abstraction can draw As-enriched water to 150 m depth within a few decades, synchronous with the advent of wide-scale irrigational pumping in West Bengal (India).

Delineating low-arsenic groundwater environments in the Bengal Aquifer System, Bangladesh by M.A. Hoque; W.G. Burgess; M. Shamsudduha; K.M. Ahmed (614-623).
► Arsenic concentration variation is linked to the hydrogeological framework of aquifer zones. ► Identified empirical associations to low-arsenic groundwater sources within the aquifer. ► Low-As groundwater is encountered at very shallow to very deeper depths. ► A preliminary delineation of low-As groundwater sources across Bangladesh has been made.Studies within the As-affected Bengal Basin have indicated that low-As groundwater can be found in a variety of geological and geomorphological settings. The hydrogeological environments that host low-As groundwater may be interpreted within a geological framework determined by the Quaternary evolution of the Bengal Aquifer System (BAS). This provides the basis for delineating the position and extent of shallow low-As groundwater, low-As groundwater in oxidised ‘red-bed’ sediments, and deep low-As groundwater. Data available on a national scale allow a preliminary delineation of these low-As groundwater environments across Bangladesh, based on empirical associations of low-As groundwater occurrences with topography, water table elevation, surface sediment lithology, geology and the screen depth of deep wells in low-As zones.

Dynamics of arsenic adsorption in the targeted arsenic-safe aquifers in Matlab, south-eastern Bangladesh: Insight from experimental studies by Clare Robinson; Mattias von Brömssen; Prosun Bhattacharya; Sara Häller; Annelie Bivén; Mohammed Hossain; Gunnar Jacks; Kazi Matin Ahmed; M. Aziz Hasan; Roger Thunvik (624-635).
► Adsorption behaviour of shallow oxidized sediments from Matlab Region in SE Bangladesh is investigated. ► Oxidized sediments have a high capacity to adsorb arsenic. ► Adsorption capacity will be reduced by high concentration of reactive organic C. ► Monitoring of groundwater quality over 5 year period shows relatively stable water chemistry.Targeting shallow low-As aquifers based on sediment colour may be a viable solution for supplying As-safe drinking water to rural communities in some regions of Bangladesh and West Bengal in India. The sustainability of this solution with regard to the long-term risk of As-safe oxidized aquifers becoming enriched with As needs to be assessed. This study focuses on the adsorption behaviour of shallow oxidized sediments from Matlab Region, Bangladesh, and their capacity to attenuate As if cross-contamination of the oxidized aquifers occurs. Water quality analyses of samples collected from 20 tube-wells in the region indicate that while there may be some seasonal variability, the groundwater chemistry in the reduced and oxidized aquifers was relatively stable from 2004 to 2009. Although sediment extractions indicate a relatively low amount of As in the oxidized sediments, below 2.5 mg kg−1, batch isotherm experiments show that the sediments have a high capacity to adsorb As. Simulations using a surface complexation model that considers adsorption to amorphous Fe(III) oxide minerals only, under-predict the experimental isotherms. This suggests that a large proportion of the adsorption sites in the oxidized sediments may be associated with crystalline Fe(III) oxides, Mn(IV) and Al(III) oxides, and clay minerals. Replicate breakthrough column experiments conducted with lactose added to the influent solution demonstrate that the high adsorption capacity of the oxidized sediments may be reduced if water drawn down into the oxidized aquifers contains high levels of electron donors such as reactive dissolved organic C.

► Sorption and mobility of As in the sediment-water interface of the Holocene deposit. ► Leaching by more reactive As leads to groundwater pollution in the shallower aquifer. ► Slower adsorption/desorption reaction rate enables deeper aquifer to be safe. ► 99% of groundwater As can be reduced by oxidizing sand, Fe and Al oxide minerals. ► For 100 years deeper aquifer could be an adequate source of sustainable, safe water.The importance of accessing safe aquifers in areas with high As is being increasingly recognized. The present study aims to investigate the sorption and mobility of As at the sediment-groundwater interface to identify a likely safe aquifer in the Holocene deposit in southwestern Bangladesh. The upper, shallow aquifer at around 18 m depth, which is composed mainly of very fine, grey, reduced sand and contains 24.3 μg/g As, was found to produce highly enriched groundwater (190 μg/L As). In contrast, deeper sediments are composed of partly oxidized, brownish, medium sand with natural adsorbents like Fe- and Al-oxides; they contain 0.76 μg/g As and impart low As concentrations to the water (4 μg/L). These observations were supported by spectroscopic studies with SEM, TEM, XRD and XRF, and by adsorption, leaching, column tests and sequential extraction. A relatively high in-situ dissolution rate (Rr ) of 1.42 × 10−16  mol/m2/s was derived for the shallower aquifer from the inverse mass-balance model. The high Rr may enhance As release processes in the upper sediment. The field-based reaction rate (Kr ) was extrapolated to be roughly 1.23 × 10−13  s−1 and 6.24 × 10−14  s−1 for the shallower and deeper aquifer, respectively, from the laboratory-obtained adsorption/desorption data. This implies that As is more reactive in the shallower aquifer. The partition coefficient for the distribution of As at the sediment–water interface (Kd -As) was found to range from 5 to 235 L/kg based on in-situ, batch adsorption, and flow-through column techniques. Additionally, a parametric equation for Kd -As (R 2  = 0.67) was obtained from the groundwater pH and the logarithm of the leachable Fe and Al concentrations in sediment. A one-dimensional finite-difference numerical model incorporating Kd and Kr showed that the shallow, leached As can be immobilized and prevented from reaching the deeper aquifer (∼150 m) after 100 year by a natural filter of oxidizing sand and adsorbent minerals like Fe and Al oxides; in this scenario, 99% of the As in groundwater is reduced. The deeper aquifer appears to be an adequate source of sustainable, safe water.

Natural Red Earth as a low cost material for arsenic removal: Kinetics and the effect of competing ions by Anushka Upamali Rajapaksha; Meththika Vithanage; Lakmal Jayarathna; Chanaka Kapila Kumara (648-654).
► The effect of reaction time and competing ions on arsenic sorption to Natural Red Earth was examined. ► Arsenate on Fe–O sites with PO43- showed inner-sphere bond formation. ► Weak complexation was observed for NO 3 - . ► As(III) adsorbed better onto NRE than As(V) with all the competing ions. ► Time taken to the equilibrium was 90 min for both As species.This study investigates the effect of reaction time and competing ions on As retention on Natural Red Earth (NRE). The initial As [As(III) or As(V)] concentrations were varied between ∼10−5 and ∼10−4 M for competitive adsorption studies while samples were spiked with ∼2.67 μM As for kinetic studies. Batch experiments were performed for solutions with different concentrations of PO 4 3 - , NO 3 - and SO 4 2 - (5.26 × 10−5, 8.06 × 10−4, and 2.60 × 10−3  M, respectively) as competing ions for the two systems. One system had controlled conditions (pH 5.5, 0.01 M NaNO3) while the second is uncontrolled (no pH control and no NaNO3). Kinetic data were best described by a pseudo-second order model demonstrating strong interaction between As species and >FeOH and AlOH sites on the NRE surface. The equilibrium solid phase concentrations for As(III) and As(V) were observed as ∼20 and ∼12.5 μg/g, respectively. The time taken to equilibrium was the same (90 min) for both As species. Competitive adsorption isotherm experiments showed a greater effect of PO 4 3 - on the reduction of adsorption of both As species than with SO 4 2 - and NO 3 - . Arsenic(III) agreed with the Langmuir equation signifying monolayer formation while As(V) adsorption was in accord with a Fruendlich isotherm indicating multilayer adsorption. FTIR spectra indicated an inner sphere bonding of arsenate and Fe–O sites with PO 4 3 - while an outer-sphere weak complexation was observed with NO 3 - . The substrate appears to show a potential for a similar rate of adsorption under both controlled and uncontrolled conditions indicating its possible use in domestic water filters to remove As from water.