Mobilization Of Arsenic In Groundwater Research Articles

Effects of Sulfide Input on Arsenate Bioreduction and Its Reduction Product Formation in Sulfidic Groundwater.

Microbes have important impacts on the mobilization of arsenic in groundwater. To study the effects of sulfide on As(V) bioreduction in sulfidic groundwater, Citrobacter sp. JH012-1 isolated from sediments in the Jianghan Plain was used in a microcosm experiment. The results showed that sulfide significantly enhanced As(V) bioreduction as an additional electron donor. The reduction rates of As(V) were 21.8%, 34.5%, 73.6% and 85.9% under 0, 15, 75 and 150 µM sulfide inputting, respectively. The main products of As(V) bioreduction were thioarsenite and orpiment and the concentration of thioarsenite reached to 5.5 and 7.1 µM in the solution with the initial 75 and 150 µM sulfide, respectively. However, under 0 and 15 µM sulfide inputting, the dominant product was arsenite with no thioarsenite accumulation. The decrease in pH enhanced the bioreduction of As(V) and promoted the formation of thioarsenite and orpiment. In addition, the percentage of thioarsenite in total arsenic decreased with the decrease in the ratio of sulfur to arsenic, indicating that the formation of thioarsenite was limited by the concentration of initial sulfide. Therefore, the presence of sulfide had a significant effect on the transformation of arsenic in groundwater. This study provides new insights into the bioreduction of As(V) and the formation of thioarsenite in sulfidic groundwater.

Characterization and role of derived dissolved organic matter on arsenic mobilization in alluvial aquifers of Punjab, Pakistan

Biogeochemical mobilization of arsenic in groundwater depends on the presence of dissolved organic matter (DOM) that likely promotes the As release, i.e., reductive dissolution, complexation, competition, and electron shuttling. We investigated the role of DOM in As release, along with its complete characterization, in the Indus plain of Pakistan, one of the worst arsenic impacted zones in the South Asian region. In total, 60 groundwater and 15 soil samples, collected at six sites from north to south within the flood plain of the Ravi River, Lahore, Pakistan were investigated. Arsenic concentration ranged from 9.61 μg/L to 386 μg/L in the groundwater samples (high As observed in areas close to the river). Dissolved organic carbon (DOC) in 29 groundwater samples ranged between 0 and 10.1 mg-C/L. A moderately positive correlation of As with DOC and Fe in the northern part of the study area suggest the reductive dissolution of FeOOH associated with dissolved organic matter (DOM). The reductive dissolution plays an essential role for As enrichment in the area evidenced by the lower concentrations of SO42−, NO3−, and PO34−and a non-correlative pattern with As. In contrast, a positive correlation of As with PO34−, DOC, and HCO3− in the southern part indicate competitive desorption behind the As release. Fluorescence excitation-emission matrix intensity data of DOM indicate the maximum presence of humic-like substances in the northern part that gradually shifts to aromatic, fulvic and protein type towards the southern part. Specific ultraviolet absorbance and fluorescence index display aromatic and terrestrial (allochthonous) sources of DOM near the riverbank and mixed (both allochthonous and autochthonous) source away from the river. The positive correlations of As with DOC and fluorescence intensity also attest that DOM played a vital role in the As mobilization in groundwater of the study area.

Influences of the petroleum-recovering activity on the arsenic level in groundwater of Kuitun, Xinjiang, China

ABSTRACTIncreasing demands of groundwater in petroleum-recovering regions could elevate the level and mobility of arsenic in groundwater as a result of the enhanced dissolution of arsenic-bearing iron or manganese oxide due to the accelerated sulfate reduction by microorganisms in a reductive environment. To substantiate this possibility, groundwater samples were collected from 220 water wells in the nearby petroleum wells in Kuitun. Dissolved arsenic, iron, manganese, and sulfate levels and pH in groundwater samples were analyzed. The dissolved arsenic levels in groundwater varied from <2.3 to 789.4 μg·L−1, in which approximately 96.4% of the measured values exceeded the allowed limits of the World Health Organization. An inverse relation existed between dissolved arsenic and sulfate levels. Most of the high arsenic-level samples (>300 μg·L−1) were found in wells at close proximity to petroleum wells where a high iron or manganese level was also detected. The oil-exploring activity in the study region seemed to have enhanced the microbial reduction of sulfate in underground environment and hence the level of arsenic in groundwater. The microbial sulfate reduction coupled with the reduction of arsenic-bearing iron oxides in the groundwater environment may explain the spatial heterogeneity of the arsenic level in groundwater.

The role of sulfide minerals in the genesis of groundwater with elevated geogenic arsenic in bedrock aquifers from western Quebec, Canada

The primary arsenic-bearing minerals and their relation to the mobilization of arsenic in groundwater are investigated in a mineralized area of the Canadian Shield (Province of Quebec, Canada). Bedrock core and groundwater samples were collected from three mining exploration boreholes completed in the fractured bedrock aquifer. Chemical analyses of groundwater reveal that the boreholes contain high arsenic concentrations ranging from 0.11 to 0.46mg/l. Mineralogical analyses of core samples indicate that arsenic is mainly present as arsenopyrite (FeAsS) and gersdorffite (NiAsS), with arsenic and sulfur contents ranging from a few mg/kg to more than 3.1 and 1.3wt%, respectively. Core leaching experiments show that the release of arsenic in oxidizing and weak alkaline water largely depends on the mineralogy of arsenic. Gersdorffite-rich samples release larger amount of arsenic than arsenopyrite-rich samples, which is mainly attributed to the high reactivity of gersdorffite compared to arsenopyrite. However, the high arsenic concentrations in groundwater are not associated with the occurrence and extent of gersdorffite-rich zones in the bedrock. These results are interpreted as evidence that the local weathering of sulfide minerals is not the main mechanism of arsenic mobilization in borehole waters. The groundwater composition strongly suggests that the mobilization of arsenic is related to the reductive dissolution of Fe and Mn oxyhydroxides in the downgradient part of the aquifer. Thus, it is proposed that the high arsenic concentrations in groundwater are mainly driven by (1) the initial release of arsenic through sulfide oxidation under oxidizing conditions (recharge areas), (2) the sequestration by sorption onto Fe-Mn oxyhydroxides and clay minerals, (3) the transport of arsenic in dissolved and particulate forms in groundwater through fractures, and (4) the (re)mobilization by dissolution of Fe-Mn oxyhydroxides under reducing conditions (downgradient areas). This study highlights the potential role of sulfide stability and the importance of redox conditions in the mobilization of arsenic in groundwater from areas of sulfide mineralization. More detailed information is needed regarding the geochemical behavior of arsenic-bearing sulfides in bedrock aquifers under natural conditions and its implication for geogenic arsenic in groundwater.

Hydrogeochemical controls on mobilization of arsenic in groundwater of a part of Brahmaputra river floodplain, India

Abstract Study region Arsenic enriched groundwater regime within low-industrialized Brahmaputra floodplains in Assam, NE India. Study focus We examined the origin, distribution and processes of As release by investigating the salient groundwater chemistry and subsurface sedimentological characteristics. Besides collection of groundwater samples from domestic and public water supply wells, sediment samples from boreholes were investigated for textural and colour linkages. New hydrological insights for the region Arsenic concentrations above the WHO guideline value of 10 μg/L were present in 33 wells and above the previous Indian national drinking standard of 50 μg/L were present in 15 wells. The green-olive colour sediments were more likely to yield As-enriched groundwater. The supersaturation of groundwater with respect to Fe(II) minerals, such as siderite and vivianite, explained the poor correlation between dissolved As and Fe. The result reinforced the phenomenon of reductive dissolution of Fe(III) oxyhydroxides releasing As to groundwater. This study throws light on the processes and mechanisms involved with As release in groundwater. The homogenous floodplain terrain makes the hydrological As imprint unambiguous and the hydrogeological signatures untarnished. Considering the absence of anthropogenic sources in the study area, the conclusions on the nature and causes for As release to groundwater looked dependable although the final contamination at specific subsurface sites would be influenced by advection–dispersion of groundwater flow accompanied by retardation, ion exchange, surface complexation and possible biodegradation.

Utilisation of satellite data in identification of geomorphic landform and its role in arsenic release in groundwater

Abstract. The present study was conducted to explore the influence of geomorphic features of the area on the mobilization of arsenic in groundwater. In this study, remote sensing and GIS techniques were used to prepare the geomorphic and slope map of the area. Different geomorphic features were identified on the basis of spectral signature on the LISS III and Landsat satellite image and field survey. Groundwater samples were collected from each representative geomorphic feature to inspect the arsenic contamination in the area. The study area is drained by the Brahmaputra river and its tributaries and contain mainly fluvial geomorphic units especially older flood plain, palaeochannels, oxbow lakes, channel islands; and hilly areas at some of the places. In this study it was observed that enrichment of arsenic in groundwater varies along the geomorphic units in following trends Paleochannel&gt; Younger alluvial plain&gt; Active flood plain&gt; channel island &gt; dissected hills. The above trend shows that a higher concentration of arsenic is found in the groundwater samples collected from the fluvial landforms as compared to those collected from structural landforms. Brahmaputra River and its tributaries carry the sediment load from the Himalayan foothills, which get deposited in these features during the lateral shift of the river’s courses. Arsenic bearing minerals may get transported through river and deposit in the geomorphological features along with organic matter. The flat terrain of the area as seen from the slope map provides more residence time to water to infiltrate into the aquifer. The microbial degradation of organic matter generates the reducing environment and facilitates the dissolution of iron hydroxide thus releasing the adsorbed arsenic into the groundwater.

Role of competing ions in the mobilization of arsenic in groundwater of Bengal Basin: Insight from surface complexation modeling

This study assesses the role of competing ions in the mobilization of arsenic (As) by surface complexation modeling of the temporal variability of As in groundwater. The potential use of two different surface complexation models (SCMs), developed for ferrihydrite and goethite, has been explored to account for the temporal variation of As(III) and As(V) concentration, monitored in shallow groundwater of Bengal Basin over a period of 20 months. The SCM for ferrihydrite appears as the better predictor of the observed variation in both As(III) and As(V) concentrations in the study sites. It is estimated that among the competing ions, PO43− is the major competitor of As(III) and As(V) adsorption onto Fe oxyhydroxide, and the competition ability decreases in the order PO43− ≫ Fe(II) > H4SiO4 = HCO3−. It is further revealed that a small change in pH can also have a significant effect on the mobility of As(III) and As(V) in the aquifers. A decrease in pH increases the concentration of As(III), whereas it decreases the As(V) concentration and vice versa. The present study suggests that the reductive dissolution of Fe oxyhydroxide alone cannot explain the observed high As concentration in groundwater of the Bengal Basin. This study supports the view that the reductive dissolution of Fe oxyhydroxide followed by competitive sorption reactions with the aquifer sediment is the processes responsible for As enrichment in groundwater.

Abiotic and biotic factors influencing the mobility of arsenic in groundwater of a through-flow island in the Okavango Delta, Botswana

Summary The Okavango Delta of Botswana is a large arid-zone wetland comprising 20,000 km2 of permanent and seasonal floodplains and over 100,000 islands. It has been shown that island groundwater can have very high dissolved arsenic (As) concentration, but the abiotic and biotic controls on As mobility are not well understood in this setting. At New Island, an island located in the seasonal swamp, dissolved As concentration increased from below detection limits in the surface water to 180 μg/L in groundwater, present as As(III) species. We investigated the relative importance of hydrologic, geochemical, and geomicrobial processes, as well as influences of recent extreme flooding events, in mobilizing and sequestering As in the shallow groundwater system under this island. Our results suggest that evapotranspiration and through-flow conditions control the location of the high arsenic zone. A combination of processes is hypothesized to control elevated As in the concentration zone of New Island: high evapotranspiration rates concentrate As and other solutes, more alkaline pH leads to desorption of arsenic or dissolution of arsenic sulfides, and formation of thioarsenic complexes acts to keep arsenic in solution. Evidence from X-ray absorption near-edge structure spectroscopy (XANES) and sulfate reducing bacteria (SRB) measurements further suggests that SRBs influence arsenic sequestration as orpiment (As2S3). Although dissolved organic matter (DOM) was not significantly correlated to dissolved As in the groundwater, our results suggest that DOM may serve as an electron donor for sulfate reduction or other microbial reactions that influence redox state and As mobility. These results have important implications for water management in the region and in other large wetland environments. The processes evaluated in this study are also relevant for arsenic removal in subsurface constructed wetland systems that may exhibit rapidly changing processes over small spatial scales.

Adsorption and co‐precipitation reactions at the mineral‐fluid interface: natural and anthropogenic processes

The processes of adsorption and co‐precipitation at mineral surfaces during reactions with aqueous fluids play a critical role in controlling the mass transfer and bioavailability of nutrients and contaminants. Significant advances have been made in our understanding of these two processes through the application of synchrotron‐based analytical techniques including X‐ray Absorption Spectroscopy (XAS), X‐ray surface scattering methods, X‐ray fluorescence, and glancing incidence diffraction. Along with these methodologies, a wide range of other techniques involving infra‐red spectroscopy, atomic‐force microscopy, electron and particle beams have also contributed to understanding atomic‐scale processes at mineral surfaces. Here, the basics of each of these synchrotron methods are introduced along with an illustrative example from the literature. The allied techniques are also discussed. After this introduction, two case studies dealing with major contemporary environmental problems that involve adsorption and co‐precipitation are presented. The first concerns the mobility of arsenic in groundwater. Currently, dissolved arsenic species pose an environmental threat to millions of people. The second case study considers the speciation and uptake of uranium within UK radioactive waste sludge. Clean‐up of these sludges is expected to be a challenging technological problem, and the insights provided through understanding the attachment of the uranium to the sludge minerals may be critical to devising a successful remediation strategy.

Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3-48.5m) and piezometer wells (depth 12.2-79.2m) demonstrate that the groundwater is mostly the Ca-HCO3 type and anoxic in nature (mean EhSHE=34mV). Arsenic concentrations ranged from <10-538μg/L, with high concentrations only present in the shallow to medium depth (30-50m) of the aquifer along with high Fe (0.07-9.8mg/L) and relatively low Mn (0.15-3.38mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO3, SO4 and NO2 along with higher values of DOC and HCO3, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain.

Coadsorption of Arsenic(III) and Arsenic(V) onto Hydrous Ferric Oxide: Effects on Abiotic Oxidation of Arsenic(III), Extraction Efficiency, and Model Accuracy

Single solute adsorption and coadsorption of As(III) and As(V) onto hydrous ferric oxide (HFO), oxidation of As(III), and extraction efficiencies were measured in 0.2 atm O2. Oxidation was negligible for single-adsorbate experiments, but significant oxidation was observed in the presence of As(V) and HFO. Single-adsorbate As(III) or As(V) were incompletely extracted (0.5 M NaOH for 20 min), but all As was recovered in coadsorbate experiments. Single-adsorbate data were well-simulated using published surface complexation models, but those models (calibrated for single-adsorbate results) provided poor fits for coadsorbate experiments. An amended model accurately simulated single- and coadsorbate results. Model predictions of significant change in As(III) surface complex speciation in coadsorbate experiments was confirmed using zeta potential measurements. Our results demonstrate that mobility of arsenic in groundwater and removal in engineered treatment systems are more complicated when both As(III) and As(V) are present than anticipated based on single-adsorbate experimental results.

Mobilization of arsenic in groundwater of Bangladesh: evidence from an incubation study

The extensive extraction of arsenic (As)-contaminated groundwaters for drinking, household and agricultural purposes represents a serious health concern in many districts of Bangladesh. This laboratory-based incubation study investigated the sources and mechanisms of As mobilization in these groundwaters. Several incubation studies were carried out using sediments collected from the Bangladesh aquifer that were supplemented, or not, with different nutrients, followed by an analysis of the sediment suspensions for pH, ORP (oxidation-reduction potential), EC (electrical conductivity) and As and Fe(II) concentrations. In the substrate-amended sediment suspensions incubated under anaerobic environment, there was a mobilization of As (maximum: 50-67 microg/l) and Fe(II) (maximum: 182 microg/l), while the ORP value decreased immediately and drastically (as much as -468 mV to -560 mV) within 5-6 days. In the sediment suspensions incubated under control and aerobic conditions, no significant As mobilization occurred. The simultaneous mobilization of As and Fe(II) from sediments is a strong indication that their mobilization resulted from the reduction of Fe oxyhydroxide by the enhanced activity of indigenous bacteria present in the sediments; this phenomenon also provides insights on the mobilization mechanism of As in groundwater. The concentrations of As in the sediments used in the incubation studies were strongly linked to the gradients of redox potential development that was stimulated by the quantity of organic nutrient (glucose) used. The penetration of surface-derived organic matter into the shallow aquifer may stimulate the activity of microbial communities, thereby leading to a reduction of iron oxyhydroxide and As release.

The geochemical study of fluvio-lacustrine aquifers in the Kathmandu Basin (Nepal) and the implications for the mobilization of arsenic

The geochemical analyses of fluvial-lacustrine aquifer sediments of the Kathmandu Valley have been made as a step in assessing the environment for the mobilization of arsenic in groundwater. Elements measured by X-ray fluorescence (XRF) include 4 major oxides (Fe2O3, TiO2, CaO, P2O5) and 14 trace elements (As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc and TS). Elution tests of 15 selected core samples were also carried out to determine the potential leaching of arsenic from the sediments. The XRF results show that average bulk concentrations of the major oxides and trace elements are similar to modern unconsolidated sediments and average upper continental crust. However, the abundance of elements varies with grain size, with higher concentrations in finer-size grades. Variations in elemental abundances within the basin are strongly controlled by sediment facies. The elution tests show that greater amounts of arsenic are generally eluted from the fine sediments, although the rates are variable. The results overall suggest that As concentrations in the bulk sediments are not a controlling factor for elevated As in the Kathmandu Basin groundwater, and the roles of other factors such as redox conditions and organic matter contents are likely to be more significant.

Occurrence of Arsenic-contaminatedGroundwater in Alluvial Aquifers from Delta Plains, Eastern India: Options for Safe Drinking Water Supply

Arsenic contamination in groundwater used for drinking purposes has been envisaged as a problem of global concern. Exploitation of groundwater contaminated with arsenic within the delta plains in West Bengal has caused adverse health effects among the population within a span of 8-10 years. The sources of arsenic in natural water are a function of the local geology, hydrology and geochemical characteristics of the aquifers. The retention and mobility of different arsenic species are sensitive to varying redox conditions. The delta plains in West Bengal are characterized by a series of meander belts formed by the fluvial processes comprising different cycles of complete or truncated fining upward sequences (sand-silt-clay). The arseniferous groundwater belts are mainly located in the upper delta plain and in abandoned meander channels. Mineralogical investigations have established that arsenic in the silty clay as well as in the sandy layers occurs as coatings on mineral grains. Clayey sediments intercalated with sandy aquifers at depths between 20 and 80 m are reported as a major source of arsenic in groundwater.Integrated knowledge on geological, hydrologicaland geochemical characteristics of the multi-level aquifer system of the upper delta plain is therefore necessary in predicting the origin, occurrence and mobility of arsenic in groundwater in West Bengal. This would also provide a basis for developing suitable low-cost techniques for safe drinking water supply in the region.

Mobility of arsenic in groundwater in the Obuasi gold-mining area of Ghana: some implications for human health

Arsenic in drinking water from streams, shallow wells and boreholes in the Obuasi gold-mining area of Ghana range between 0.5), probably as a result of methylation and reduction reactions mediated by bacteria and algae. Concentrations of As in groundwaters reach up to 64 μgl−1, being highest in deeper (40–70 m) and more reducing (220–250 mV) waters. The As is thought to build up as a result of the longer residence times undergone by groundwaters in the deeper parts of the aquifer. The proportion of As present as As(III) is also higher in the deeper groundwaters. Deep mine exploration boreholes (70–100 m) have relatively low As contents of 5–17 μgl−1, possibly due to As sorption onto precipitating ferric oxyhydroxides or to localized low As concentrations of sulphide minerals. Median concentrations of inorganic urinary As from sample populations in two villages, one a rural streamwater-drinking community and the other a suburb of Obuasi using groundwater for potable supply, were 42 μgl−1 and 18 μgl−1 respectively. The value for the community drinking groundwater is typical of background concentrations of urinary As. The slightly higher value for the streamwater-drinking community probably reflects different provenance of foodstuffs and higher As concentrations of water sources local to the village. The low value obtained for the inhabitants of the Obuasi suburb, living close to and abstracting groundwater from the area of major mining activity, suggests that groundwater can form a useful potable supply of good inorganic quality provided that deep, long residence time sources are avoided.