Arsenic Concentrations In Groundwater Research Articles

Hydrogeochemical controls on arsenic mobility in an arid inland basin, Southeast of Iran: The role of alkaline conditions and salt water intrusion

Elevated inorganic arsenic concentrations in groundwater has become a major public and environmental health concern in different parts of the world. Currently, As-contaminated groundwater issue in many countries and regions is a major topic for publications at global level. However, there are many regions worldwide where the problem has still not been resolved or fully understood due to inadequate hydrogeochemical investigations. Hence, this study evaluates for the first time the hydrogeochemical behavior of the arid and previously unexplored inland basin of Sirjan Plain, south east (SE) Iran, in order to assess the controlling factors which influence arsenic (As) mobility and its distribution through groundwater resources. Total inorganic arsenic concentration was measured using inductive-coupled plasma optical emission spectrometry (ICP-OES). Arsenic content in groundwater of this region ranged between 2.4 and 545.8 μg/L (mean value: 86.6 μg/L) and 50% of the samples exceeded the World Health Organization (WHO) guideline value of 10 μg/L in drinking water. Groundwater was mainly of Na-Cl type and alkaline due to silicate weathering, ion exchange and evaporation in arid conditions. Elevated As concentrations were generally observed under weakly alkaline to alkaline conditions (pH > 7.4). Multivariate statistical analysis including cluster analysis and bi-plot grouped As with pH and HCO3 and demonstrated that the secondary minerals including oxyhydroxides of Fe are the main source of As in groundwater in this region. The desorption of As from these mineral phases occurs under alkaline conditions in oxidizing arid environments thereby leading to high levels of As in groundwater. Moreover, evaporation, ion exchange and saltwater intrusion were the secondary processes accelerating As release and its mobility in groundwater. Based on the results of this study, desorption of As from metal oxy-hydroxides surfaces under alkaline conditions, evaporation and intrusion of As-rich saline water are considered to be the major factors causing As enrichment in arid inland basins such as those in southeast Iran. This study proposes the regular monitoring and proper groundwater management practices to mitigate high levels of arsenic in groundwater and related drinking water wells of Sirjan Plain.

Spatial mapping and modeling of arsenic contamination of groundwater and risk assessment through geospatial interpolation technique

Spatial interpolation technique is useful for spatial mapping with sparse data procured from vantage in situ sampling sources. Through spatial interpolation, wall-to-wall mapping of the arsenic concentration in groundwater was accomplished for the whole of the study area by using known concentration value at nearby locations (aquifers) under homogenous terrain conditions. This present study proposes an empirical methodology through interpolation approach for spatial mapping of seasonal and annual groundwater arsenic contamination in the district North 24 Parganas, which happens to be the one of the worst arsenic-affected districts of West Bengal, India, in Bengal Basin. Two types of interpolation approach, Thiessen polygon and Kriging, have been used for spatial mapping of arsenic distribution. On the basis of spatial distribution map, classification has been done for the entire district into seven arsenic concentration zones with various levels of contaminations from arsenic in groundwater (0.01 mg/L as WHO-declared maximum limit for safe zone). In this study, a total of six seasonal (pre-/post-monsoon) data from 2006 to 2008 have been interpreted to examine temporal changes of arsenic concentration in groundwater, and finally, the future trend is projected. Future trend assessment of arsenic contamination has been performed through statistical analysis fitting a linear regression equation. Through this study, it is revealed that the unaffected blocks in the pre-monsoon season (March–April–May) of year 2006 became significantly affected by the end of year 2008. From regression model, it has been predicted that if this trend continues, then, after ten years 2/3 blocks of the said districts will be arsenic affected.

Bayesian Hierarchical Models as Tools to Evaluate the Association Between Groundwater Quality and the Occurrence of Type 2 Diabetes in Rural Saskatchewan, Canada.

There is growing interest in the role of environmental exposures in the development of diabetes. Previous studies in rural Saskatchewan have raised concerns over drinking water contaminants, including arsenic, which has been identified as a possible risk factor for diabetes. Using administrative health and water-quality surveillance data from rural Saskatchewan, an ecological study design was used to investigate associations between concentrations of arsenic, water health standards and aesthetic objectives, and the incidence and prevalence of diabetes. Mixtures of contaminants measured as health standards or as aesthetic objectives were summarized using principal component (PC) analysis. Associations were modeled using Bayesian hierarchical models incorporating both spatial and unstructured random effects, standardized for age and sex, and adjusted for socioeconomic factors and a surrogate measure for smoking rates. Arsenic was not associated with an increased risk of diabetes. For private wells, having groundwater arsenic concentrations in the highest quintile was associated with decreased cumulative diabetes incidence for 2010-2012 (risk ratio [RR] = 0.854, 95% credible interval [CrI] 0.761-0.958) compared with the lowest quintile, a result inconsistent with other studies. For public water supplies, having a first PC score for health standards (primarily summarized selenium, nitrate, and lead) in the third quintile (RR = 1.101, 95% CrI 1.019-1.188), fourth quintile (RR = 1.088, 95% CrI 1.003-1.180), or fifth quintile (RR = 1.115, 95% CrI 1.026-1.213) was associated with an increase in 2010 diabetes prevalence compared with the first quintile. An increase in the PC scores for the third aesthetic objective in private wells (characterized primarily by iron and manganese) was associated with decreased diabetes incidence, although a meaningful dose-response relationship was not evident. No other associations between PC scores for health standards or aesthetic objectives from public or private water supplies and diabetes were identified.

Survey on Arsenic and Iron Contamination in Groundwater in Blocks of North Bank Plain Zone of Assam

Arsenic contamination in groundwater and its impact on human health has been reported as one of the world's biggest natural groundwater calamities to the mankind. In Assam, Arsenic concentration in groundwater above the permissible level has been reported from 20 districts, of which, maximum level of Arsenic concentration was found in Jorhat, Lakhimpur, Nalbari and Nagaon districts. In the present study Arsenic and Iron contamination of groundwater in Narayanpur Block and Sakhomato Block of North Bank Plain Zone of Assam was determined. Eighty seven numbers of ground water samples from Narayanpur Block and forty seven numbers from Sakhomato Block were collected from tubewells (15 feet to 75 feet deep) at different locations of the Blocks. The Arsenic content of analyzed samples from Narayanpur Block varied from 0 (zero) ppb to 102 ppb, of which, 48 (55.17%) samples were found below the WHO guideline value for drinking (less than 10 ppb); while 39 (44.83%) samples contained Arsenic above the guideline value (more than 10 ppb, WHO, 2011). In case of Sakhomato Block, all the 47 samples contain Arsenic below the critical limit of 10 ppb and hence found safe for drinking. Iron concentration of the samples having Arsenic concentration above the critical limit of 10 ppb from Narayanpur Block was analyzed with a view to ameliorating Arsenic toxicity through removal of Iron. The Iron concentration of the samples ranged from 10 ppm to 58.4 ppm. However, no significant correlation between Iron and Arsenic concentration was observed (r2=0.0403).

Evaluation of iron-rich adsorbent to remove arsenic from groundwater in decentralised water supply treatment

This paper evaluates the adsorption capacity of iron-rich adsorbent to remove arsenic from groundwater in decentralised water supply treatment; a pilot-scale with a capacity of 5 m3/day has been investigated at Cu Da Nursery School in Hanoi city, in which the concentration of arsenic in groundwater is abnormally high (175-400 μg/l). The pilot system was designed to include a series of processes, such as aeration, sedimentation, sand filtration, and adsorption, using the iron-rich adsorbent and activated carbon from coconut shells. The pilot system was tested with three operated modes. The results demonstrate that the treatment efficiency of the overall treatment processes was significantly higher than that of the traditional treatment system used by local people. After aeration, sedimentation, and sand filtration, the water quality was discovered to respond substantially in reducing iron by over 96% (from 8-12 mg/l to less than 0.3 mg/l) arsenic by 55 to 60%, but after iron-rich adsorbent, the concentration of arsenic decreased to less than 10 μg/l, which fulfills Vietnam National technical regulations concerning drinking water quality.

Spatial Variability of Groundwater Arsenic Concentration as Controlled by Hydrogeology: Conceptual Analysis Using 2-D Reactive Transport Modeling.

Combined geological, hydrogeological, and geochemical controls on the arsenic concentration of contaminated aquifers in SE Asia were explored by two‐dimensional (2‐D) reactive transport modeling of data sets from Bangladesh, Cambodia, and Vietnam. For each site, the field data are summarized and used to create a conceptual 2‐D reactive transport model that elucidates characteristic features influencing the groundwater arsenic concentration. Comparison of models for Bangladesh and Vietnam indicates that fine‐grained layers overlying young sandy aquifers generate shallow high arsenic groundwater because low vertical groundwater velocities allow sufficient time for kinetic As release from the sediment. The low vertical groundwater velocity below major river channels, predicted by the model, also creates long groundwater residence times, leading to high arsenic groundwater. Young aquifer sediments release more arsenic than older sediments, and alternating young and older sediments create complex patterns of high and low arsenic groundwater. Over time, floodplain basins will subside, and river channels migrate, causing sedimentation and erosion on the floodplain while creating local environments with evolving hydrogeology and groundwater geochemistry. We have developed a three‐step model for the evolution of the Red River floodplain with sedimentation and shifting channels over the last 6000 years. The results show comparable timescales between the dynamics of arsenic release and of river migration, causing complex groundwater As distributions, comprising geochemical palinopsia of long vanished rivers.

Concentration of arsenic in groundwater, vegetables, human hair and nails in mining site in the Northern Thai Nguyen province, Vietnam: human exposure and risks assessment

This study was aimed to examine the risk of chronic arsenic (As) exposure for the residents living in Nui Phao, Thai Nguyen in the northern Vietnam. Groundwater, vegetables, human hair, and nail samples were collected from volunteers living in Nui Phao. The results revealed that 75% of the groundwater samples had As exceeding the World Health Organization (WHO) drinking water guideline of 10 µg L−1. The result of As concentration for most of the vegetable samples was greater than the WHO/FAO safe (0.1 mg kg−1). The result of hair and nail samples in this study showed that 3.5 and 20% of the samples had As concentration exceeding the level of As toxicity in hair and nails, respectively. The result of health risks indicated that the potential health risk of As contamination is greater for groundwater than vegetables. The total hazard quotient (HQ) value through vegetables ingestion and drinking water exceeded 1.0 suggesting potential health risk for local residents. The calculation of potential carcinogenic risk through both consumption of vegetables and drinking water was low cancer risk in adults. Other food sources and the exposure pathways are needed to exactly assess health risks in this area.

The Impact of Aquifer Flushing on Groundwater Arsenic Across a 35-km Transect Perpendicular to the Upper Brahmaputra River in Assam, India.

Well testing in the floodplain of the Brahmaputra River in Golaghat and Jorhat districts of Assam, India, shows that groundwater arsenic (As) concentrations increase with distance from the river. To establish the origin of this pattern, an additional 900 wells <60 m deep were tested for As and 9 sites were drilled along a 35-km transect perpendicular to the river. The field data show no relation between groundwater As concentrations ranging from <1 to 660 μg/L along the transect and (a) As concentrations of <1-5 mg/kg in cuttings of aquifer sand recovered while drilling or (b) the degree of reduction of iron oxides in these cuttings. The drilling indicates, however, a marked increase in the thickness of a clay layer capping the aquifer starting from <1-5 m near the river to over 60 m at the most distant site towards the base of the Naga foothills. Organic radiocarbon ages of 18-46 kyr obtained from all but one of 13 clay samples indicate pre-Holocene deposition of the underlying sands across the entire transect. Radiocarbon ages of dissolved inorganic carbon of 0.2, 4.7, and 17.8 kyr were measured in groundwater from 3 monitoring wells installed to 30-60 m depth at distances of 10, 20, and 40 km from the river, respectively. A conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and, as a result, preserved higher As levels in groundwater.

Ecological analysis of associations between groundwater quality and hypertension and cardiovascular disease in rural Saskatchewan, Canada using Bayesian hierarchical models and administrative health data

Associations between groundwater quality and the prevalence of hypertension and ischemic heart disease were investigated in rural areas of the Canadian province of Saskatchewan. The partially ecological study was analyzed using Bayesian hierarchical models to account for spatial variability in risk. Exposure measures and health outcomes were estimated based on previously collected water quality surveillance data from public water supplies and private wells and administrative health data. Water quality exposures for each study region were estimated by applying geostatistical techniques to arsenic concentrations and principal component (PC) scores. The PC scores summarized groups of parameters measuring either health standards or aesthetic objectives described by the province. Generalized linear mixed models with a log link assessed associations between water quality and observed count of health outcomes relative to the expected value. The Bayesian models contained uncorrelated and spatially correlated random effects for each geographic region. Effect estimates were controlled for sex and age by stratification of case and expected case counts, for smoking by inclusion of sex- and age-specific prevalence of chronic obstructive pulmonary disease as a surrogate covariate, and for education and income by use of census data.There was no evidence for associations between groundwater arsenic concentrations in public or private water supplies and increased risk of hypertension or cardiovascular disease. An increase in the second aesthetic objectives PC score from public supplies was associated with a protective effect against ischemic heart disease. This PC value summarized hardness and magnesium Similarly, an increase in the second aesthetic objectives PC in private supplies was associated with decreased prevalence of hypertension.The results of this study are consistent with others demonstrating a relationship between elevated hardness and magnesium concentrations in drinking water and reduced risk of hypertension and cardiovascular disease. Further investigation is warranted with individual exposure history, particularly with respect to the potential beneficial effect of hard water on the prevalence of hypertension and cardiovascular disease.

Climatic variations and de-coupling between arsenic and iron in arsenic contaminated ground water in the lowlands of Nepal

As widely known, in several countries in South East Asia, arsenic concentrations in ground water extracted from Quaternary alluvial sediments frequently exceed the World Health Organization (WHO) drinking water guideline of 10 μg/L. The broadly accepted hypothesis states that reductive dissolution of Fe-bearing minerals releases As-oxyanions contained within these minerals. According to the results presented in this article, As and Fe concentrations in ground water in the lowlands (Terai) of Nepal are highly variable as a function of location and there is a de-coupling of As and Fe concentrations resulting in a loss of correlation between these two elements. The mean concentrations of As and of Fe in 35 wells were slightly higher in the post-monsoon than in the pre-monsoon season. Even though As is mainly associated with iron(hydr)oxides, a substantial portion of As and Fe can be retained by clay minerals (including micas). During pre-monsoon (dry season) clayey sediments are chemically weathered losing Na and K thus remaining relatively enriched in less mobile elements such as Fe and Al. As long as reducing conditions persist, As remains relatively mobile. This article includes a detailed discussion referring to the very weak correlation or decoupling between aqueous As and Fe in the ground water and addresses possible consequences for water treatment. It sheds light on the original reservoir of arsenic being incorporated and released from clay minerals as well as from iron(hydr)oxides.

Identification of Hydrobiogeochemical Processes Controlling Seasonal Variations in Arsenic Concentrations Within a Riverbank Aquifer at Jianghan Plain, China

AbstractThe arsenic concentration in groundwater varies significantly with time and space in groundwater‐surface water exchange zones. Various processes have been identified that control arsenic concentration distribution and mobility in laboratory systems. However, it is still challenging to identify important processes controlling arsenic concentration distribution at the field scale due to the complex coupling of hydrobiogeochemical processes. In this study, a reactive flow and transport model was used to identify dominant processes controlling arsenic distribution and seasonal variations in groundwater‐surface water exchange zones using Jianghan Plain, China, as an example. The results revealed the importance of river water and groundwater interactions on seasonal changes in arsenic concentration; however, the affected region is limited to within 50‐m distance to the river. The modeling results, unexpectedly, revealed the predominant importance of groundwater extraction to the seasonal variation in arsenic concentration. The groundwater extraction changed the groundwater flow pattern and induced vertical leakage of oxygen‐containing surface water into the aquifer, which triggered a series of biogeochemical reactions that changed groundwater redox conditions and promoted arsenic sorption, resulting in a rapid decrease in arsenic concentrations in groundwater. After groundwater extraction ceased, aquifer recovered to anoxic condition, promoting arsenic release from the sorbed phase, leading to a rapid rebounding in groundwater arsenic concentrations. Overall, this study provided a tool to identify coupled hydrobiogeochemical processes on arsenic spatiotemporal distribution and migration in groundwater.

Overpumping leads to California groundwater arsenic threat

Water resources are being challenged to meet domestic, agricultural, and industrial needs. To complement finite surface water supplies that are being stressed by changes in precipitation and increased demand, groundwater is increasingly being used. Sustaining groundwater use requires considering both water quantity and quality. A unique challenge for groundwater use, as compared with surface water, is the presence of naturally occurring contaminants within aquifer sediments, which can enter the water supply. Here we find that recent groundwater pumping, observed through land subsidence, results in an increase in aquifer arsenic concentrations in the San Joaquin Valley of California. By comparison, historic groundwater pumping shows no link to current groundwater arsenic concentrations. Our results support the premise that arsenic can reside within pore water of clay strata within aquifers and is released due to overpumping. We provide a quantitative model for using subsidence as an indicator of arsenic concentrations correlated with groundwater pumping.

Organic carbon sources and controlling processes on aquifer arsenic cycling in the Jianghan Plain, central China

Groundwater arsenic contamination is a common environmental problem that threatens the health of over 100 million people globally. Apparent seasonal fluctuations in groundwater arsenic concentrations have been reported in various locations worldwide, including the Jianghan Plain, central China. This phenomenon has been attributed to shifts in redox conditions induced by seasonal incursions of surface water. However, it is not clear what processes during the incursion lead to changes in the redox conditions and what is the source of the organic carbon driving these processes. Therefore, we conducted a long-term investigation of stable carbon isotopic compositions in surface water and groundwater, as well as long-term monitoring of hydraulic gradients and geochemical compositions at the Jianghan Plain. Results indicated that a series of biogeochemical processes occurred during surface water incursion, including aerobic microbial respiration, nitrate and sulfate reduction. Groundwater arsenic was removed by adsorption on iron oxyhydroxides produced during oxidation of ferrous iron, resulting in dramatic decreases in arsenic concentrations during surface water recharge seasons. These processes were likely driven by organic carbon vertically transported from surface water and released from the surficial aquitard above 15 m. Groundwater pumping may accelerate the vertical infiltration of oxidizing recharge water and drive exogenous organic carbon to depth. Findings of this study advance the understandings of the mechanisms that cause temporal variations in groundwater As and the importance of exogenous organic carbon that may influence the temporal behavior of arsenic in groundwater.

Groundwater Hazards Risk Management In West Bengal

Frequency and severity of natural hazards are increasing worldwide, in particular for hydro-meteorological extremes, posing a serious threat to the groundwater hazards in many countries and regions. Adequate fresh groundwater losses have been increasing as a result of groundwater-related hazardslike long-term depletion of groundwater level and groundwater contamination mainly due to high concentration of arsenic, fluoride, total dissolved solids etc. But it has beenexperienced that changes or variability in the climate are also responsible for the increasing losses of sufficient fresh groundwater. In particular groundwater resources are vulnerable to climate change, compounding changes in groundwater use in domestic, irrigation, industrial sector as part of its development.Groundwater conditions in some parts of West Bengal is also in vulnerable due to some hazards like groundwater crisis during frequent drought-like situation; groundwater depletion due to excessive withdrawal; and higher concentration of arsenic, fluoride and salinity in groundwater. Therefore, groundwater hazardsrisk management is the issue of groundwater security in the state of West Bengal, which must be addressed on a groundwater basin basis through a strong and properly enforced groundwater regulatory systemby modernizing the governance structure, combined with a strategic groundwater resource plan.For sustainable development, access to fresh groundwater and the protection against groundwater related hazards from quantitative and qualitative point of view are very crucial. Therefore, a productive approach to groundwater hazards risk management and adaptation strategies would reduce the risk through development of short-term action plan and long-term preventive actions e.g. a risk-based groundwater resource plan. In this paper, the review focuses on groundwater-related hazards, including impacts, current management and policies to reduce the risk, and opportunities for strengthening integrated groundwater hazards risk management in West Bengal.

Arsenic-rich shallow groundwater in sandy aquifer systems buffered by rising carbonate waters: A geochemical case study from Mannar Island, Sri Lanka

Major ion, trace elements, and stable isotope analyses were performed on groundwater samples collected from Mannar Island in the northern Indian Ocean. Arsenic concentrations up to 34μg/L have been observed in groundwater samples from the island. In addition, 23% of extensively used shallow drinking water wells showed higher arsenic contents than the recommended value by the World Health Organization (10μg/L). Groundwater in the island showed pH values between 6.9 and 8.9 and was dominated by Na+, K+, Ca2+, Mg2+, HCO3−, Cl− and SO42−. The δ18OH2O and δ2HH2O composition of most groundwater plotted very close to the local meteoric waterline, however, some wells showed enriched isotope compositions that are most likely due to evaporation. Sea water intrusion in this island was likely of minor importance as indicated by the major ion composition. An approximated mass balance calculation using chloride concentrations indicated that out of the 35 investigated wells only 6 near-shore wells were influenced by sea water intrusion up to about 15%. Even though this is a sandy aquifer, groundwaters were characterized with higher contents of dissolved inorganic carbon (DIC) (2.11–10.9mmol/L). The corresponding δ13CDIC values varied from −19.4‰ to −6.5‰. Except for a few samples with values approaching −20‰, these isotope values are more typical for carbonate dissolution and equilibration of CO2 in the aquifer. This study shows that the underlying carbonate system may buffer the aqueous geochemistry of the groundwater on the island. The high arsenic content in groundwater may have been mobilized through reductive dissolution of Fe–Mn oxides and oxy-hydroxides that are coated on sandy aquifer materials. The lower content of DOC (0.2–1.5mmol/L) provides evidence for the restricted formation of pyrite in the aquifer.

Arsenic contamination in groundwater in the Red river delta, Vietnam - a review

Arsenic contamination in groundwater and its effect on human health has been a growing concern over recent decades. Some of the most severe incidents occurred in South and Southeast Asia, including the Red river delta, Vietnam. The highest concentration of arsenic found in the Red river delta was 810 μg/L, 16 times higher than the standard guidelines given by WHO for levels of arsenic concentration in groundwater (50 μg/L). However, the contamination levels were not uniform in the whole area. The arsenic levels might be affected by natural factors such as the characteristics of the aquifer, the chemical composition of groundwater and by human activities such as the exploitation of groundwater in the urban and industrial areas or irrigation in rural areas. Due to the complex mobilisation of arsenic in sediment and groundwater, questions remain about arsenic distribution, which are yet to be answered and are in need of further study.

Redox zonation for different groundwater flow paths during bank filtration: a case study at Liao River, Shenyang, northeastern China

The spatial and temporal distribution of redox zones in an aquifer is important when designing groundwater supply systems. Redox zonation can have direct or indirect control of the biological and chemical reactions and mobility of pollutants. In this study, redox conditions are characterized by interpreting the hydrogeological conditions and water chemistry in groundwater during bank infiltration at a site in Shenyang, northeast China. The relevant redox processes and zonal differences in a shallow flow path and deeper flow path at the field scale were revealed by monitoring the redox parameters and chemistry of groundwater near the Liao River. The results show obvious horizontal and vertical components of redox zones during bank filtration. Variations in the horizontal extent of the redox zone were controlled by the different permeabilities of the riverbed sediments and aquifer with depth. Horizontally, the redox zone was situated within 17 m of the riverbank for the shallow flow path and within 200 m for the deep flow path. The vertical extent of the redox zone was affected by precipitation and seasonal river floods and extended to 10 m below the surface. During bank filtration, iron and manganese oxides or hydroxides were reductively dissolved, and arsenic that was adsorbed onto the medium surface or coprecipitated is released into the groundwater. This leads to increased arsenic content in groundwater, which poses a serious threat to water supply security.

Vertical variability of arsenic concentrations under the control of iron-sulfur-arsenic interactions in reducing aquifer systems

High spatial variability of arsenic (As) concentration in geogenic As-contaminated groundwater has been commonly observed worldwide, but the underlying reasons remain not well understood. Selecting a sulfate-containing, As-affected aquifer at the Datong Basin, northern China as the study area and combining hydrogeochemical investigation and sediment extraction with reactive transport modeling, this work elucidated the roles of Fe-S-As interactions in regulating the vertical variation of As concentration in the groundwater. Dissolved As concentration varied between 0.05 and 18 μmol/L, but generally increased in the depth of 20–25 m and then decreased in 25–30 m. The high-As groundwater contained low Fe(II) (<0.007 mmol/L) and up to 15 μmol/L sulfide, in contrary to the S/SE Asian deltas/floodplains where high Fe(II) and As jointly occur in the groundwater devoid of sulfate reduction. The reductive dissolution of As-bearing Fe(III) oxides coupled to the degradation of organic matter with an estimated maximum rate of 0.22 mmol C/L/yr, mainly accounted for the depth-dependent increase of As concentration in the upper part of the shallow aquifer (<25 m deep). However, the decreasing reactivity of Fe(III) oxides together with the increase of pH over depth rendered the majority of electrons being transferred to sulfate reduction. The Fe(II) sulfides formed as a consequence not only helped to restrict the build-up of Fe(II) in the groundwater but also probably co-precipitated As to prompt As decrease in the depth of 25–30 m. Arsenite adsorbed on remaining Fe(III) oxides and newly-formed Fe(II) sulfides is another important pool of As in the aquifer, which varies in response to the extents of Fe(III)-oxide and sulfate reduction and consequently alters As distribution coefficient between the solid and the aqueous phases. This study highlights the importance of coupled geochemical cycling of Fe, S and As for As mobilization and reveals how it regulates As partitioning between groundwater and sediments.

How or When Samples Are Collected Affects Measured Arsenic Concentration in New Drinking Water Wells.

Naturally occurring arsenic can adversely affect water quality in geologically diverse aquifers throughout the world. Chronic exposure to arsenic via drinking water is a human health concern due to risks for certain cancers, skin abnormalities, peripheral neuropathy, and other negative health effects. Statewide in Minnesota, USA, 11% of samples from new drinking water wells have arsenic concentrations exceeding 10 μg/L; in certain counties more than 35% of tested samples exceed 10 μg/L arsenic. Since 2008, Minnesota well code has required testing water from new wells for arsenic. Sample collection protocols are not specified in the well code, so among 180 well drillers there is variability in sampling methods, including sample collection point and sample collection timing. This study examines the effect of arsenic sample collection protocols on the variability of measured arsenic concentrations in water from new domestic water supply wells. Study wells were drilled between 2014 and 2016 in three regions of Minnesota that commonly have elevated arsenic concentrations in groundwater. Variability in measured arsenic concentration at a well was reduced when samples were (1) filtered, (2) collected from household plumbing instead of from the drill rig pump, or (3) collected several months after well construction (instead of within 4 weeks of well installation). Particulates and fine aquifer sediments entrained in groundwater samples, or other artifacts of drilling disturbance, can cause undesirable variability in measurements. Establishing regulatory protocols requiring sample filtration and/or collection from household plumbing could improve the reliability of information provided to well owners and to secondary data users.

Arsenic in Holocene aquifers of the Red River floodplain, Vietnam: Effects of sediment-water interactions, sediment burial age and groundwater residence time

Water-sediment interactions were investigated in arsenic contaminated Holocene aquifers of the Red River floodplain, Vietnam, in order to elucidate the origin of the spatial variability in the groundwater arsenic concentration. The investigated aquifers are spread over an 8 × 13 km field area with sediments that varied in burial age from <1 kyr to 11 kyr. The groundwater age ranged from less than 2 yr, up to a maximum near 90 yr. Groundwater As concentrations are between 0 and 6.5 µM and there are no simple correlations between the As concentration and groundwater age or aquifer sediment burial age. The aquifers are anoxic with up to 2 mM CH4 and up to 0.5 mM DOC. The downward advective DOC flux is too small to support both methanogenesis and the reduction of As-containing Fe-oxides and sedimentary carbon is therefore considered the main carbon source for the redox processes. The groundwater H2 concentration ranged between 0.1 and 4 nM. These values are intermediate between ranges characteristic for Fe-oxide reduction and methanogenesis and suggest that both processes take place simultaneously. The groundwater pe was calculated from the H2/H+ and CH4/CO2 redox couples, giving almost similar results that apparently reflects the pe of the bulk groundwater. The pe calculated for the As(III)/As(V) redox couple was found in disequilibrium with the other redox couples. Using the pe calculated from the CH4/CO2 redox couple we show that the groundwater has a reducing potential towards Fe-oxides ranging from ferrihydrite to poorly crystalline goethite, but not for well crystalline goethite or hematite. Hematite and poorly crystalline goethite were identified as the Fe-oxides present in the sediments. Reductive dissolution experiments identify two phases releasing Fe(II); one rapidly dissolving that also contains As and a second releasing Fe(II) more slowly but without As. The initial release of Fe and As occurs at a near constant As/Fe ratio that varied from site to site between 1.2 and 0.1 mmol As/mol Fe. Siderite (FeCO3) is the main sink for Fe(II), based on saturation calculations as well as the identification of siderite in the sediment. Most of the carbonate incorporated in siderite originates from the dissolution of sedimentary CaCO3. Over time the CaCO3 content of the sediments diminishes and FeCO3 appears instead. No specific secondary phases that incorporate arsenite could be identified. Alternatively, the amount of arsenic mobilized during the dissolution of reactive phases can be contained in the pool of adsorbed arsenite. Combining groundwater age with aquifer sediment age allows the calculation of the total number of pore volumes flushed through the aquifer. Comparison with groundwater chemistry shows the highest arsenic concentration to be present within the first 200 pore volumes flushed through the aquifer. These results agree with reactive transport modeling combining a kinetic description of reductive dissolution of As-containing Fe-oxide with adsorption and desorption of arsenite. Understanding variability in groundwater arsenic concentration requires appreciating the coupling of the chemical processes to both sedimentary and hydrogeological cycling.