how paleosols influence groundwater flow and arsenic pollution a model from the bengal basin and its worldwide implication in the bengal basin the land surface exposed during the last lowstand of sea level around 20 ka and now buried by holocene sediment is capped by an effectively impermeable clay paleosol that we term the last glacial maximum paleosol lgmp the paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern west bengal and by implication in shallow aquifers across the bengal basin and as polluted deltaic aquifers worldwide the presence of the lgmp defines paleointerfluvial areas it is absent from paleochannel areas a paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic while that beneath paleointerfluvial areas is not paleointerfluvial aquifers are unpolluted because they are protected by the lgmp from downward migration of arsenic and from downward migration of organic matter that drives as pollution via reductive dissolution of as bearing iron oxyhydroxides horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers in which sorption of arsenic minimizes the risk of pollution

integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the mekong delta abstract arsenic is contaminating the groundwater of holocene aquifers throughout south and southeast asia to examine the biogeochemical and hydrological processes influencing dissolved concentrations and transport of as within soils sediments in the mekong river delta a 50 km2 field site was established near phnom penh cambodia where aqueous as concentrations are dangerously high and where groundwater retrieval for irrigation is minimal dissolved as concentrations vary spatially ranging up to 1300 g l in aquifer groundwater and up to 600 g l in surficial clay pore water groundwaters with high as concentrations are reducing with negligible dissolved o2 and high concentrations of fe ii nh 4 and dissolved organic c within near surface environments these conditions are most pronounced in sediments underlying permanent wetlands often found within oxbow channels near the mekong river there labile c co deposited with as bearing fe hydr oxides under reducing conditions drives the reductive mobilization inclusive of fe and as reduction of as here conditions are described under which as is mobilized from these sediments and near surface as release is linked to aquifer contamination over long time periods 100s to 1000s of years site biogeochemistry is coupled with extensive hydrologic measurements and accordingly a comprehensive interpretation of spatial as release and transport within a calibrated hydraulic flow field is provided of an as contaminated aquifer that is representative of those found throughout south and southeast asia

origin and availability of organic matter leading to arsenic mobilisation in aquifers of the red river delta vietnam abstract groundwater arsenic as concentrations in the red river delta vietnam are often patchy and related to the microbially induced reduction of fe oxy hydroxides in this study we explored the influence of the origin composition and availability of natural organic matter on the hydrochemical variability in the aquifers of van phuc carbon isotope signatures 13corg and c n ratios were assessed in combination with lithology geochemistry hydrochemistry hydrology and the distribution of specific biomarkers the elationship of c n ratios and 13corg distinguished four groups of sediment types that differ in their organic carbon sources this includes organic carbon originating predominantly from vascular c3 plants c n 15 4 21 0 13corg 28 6 to 26 7 c4 plants c n 10 6 13corg 14 8 freshwater derived particulate organic carbon c n 8 13corg 24 as well as mixtures incorporating both sources at the high as sites we found particulate organic carbon poc being 1 2 less depleted in 13corg than at low as sites more importantly however our assessment shows that the availability of organic matter has to be considered decisive with regard to groundwater as contamination fine grained clayey sediments overlaying sands generally protect organic matter from substantial degradation and its leaching into an adjacent aquifer however at the sites that are high in dissolved as in van phuc sediment layers rich in organic matter are hydraulically connected to the underlying aquifer here soluble organic matter seeping into the aquifer can induce and or enhance reducing conditions thereby mobilising as from fe oxy hydroxides our study shows that both the clay content as well as the origin of organic matter are largely controlled by the depositional environment of the sediments

thermo tectonic history of the tethyan himalayas deduced from the palaeomagnetic record of metacarbonates from shiar khola central nepal abstract palaeomagnetic measurements were carried out on low grade metamorphic carbonates of mesozoic age from the shiar area 85 1 e 28 6 n of the tethyan himalaya th in north central nepal two characteristic remanence components carried by pyrrhotite chrm 1 and magnetite chrm 2 could be identified by their unblocking temperature spectra of 270 340 and 430 580 c respectively fold tests are not significant due to the uniform bedding of all sites however according to results from other areas of the th the pyrrhotite component has been probably acquired as a secondary p trm during exhumation and cooling thus the age of remanence acquisition can be related to the last cooling event 25 17 ma in the surrounding areas the inclination of the magnetite component matches the value expected from the indian apwp this may the primary origin of the chrm 2 pyrrhotite site mean directions show a small circle distribution with a best fit parallel to the n s direction backtilting to the expected inclination i exp by intersection of the remanence small circle with the small circle of constant i exp yields a clockwise block rotation of 30 35 with respect to the indian plate characteristics of the pyrrhotite component small circle distribution of site means secondary origin p trm with unblocking temperatures below about 300 c allow the interpretation of the chronologic order of the thermo tectonic history i an earlier main folding phase at elevated temperatures ii a later event of cooling through about 300 c coinciding with the acquisition of chrm 1 iii clockwise block rotations with respect to the indian plate and iv long wave folding as the youngest tectonic event

groundwater dynamics and arsenic contamination in bangladesh although arsenic contaminated groundwater in bangladesh is a serious health issue little is known about the complex transient patterns of groundwater flow that flush solutes from aquifers and carry solutes into the subsurface hydrologic modeling results for our field site in the munshiganj district indicate that groundwater flow is vigorous flushing the aquifer over time scales of decades to a century and also transporting solute loads into the aquifer with recharge from ponds rivers and rice fields the combined hydrologic and biogeochemical results from our field site imply that the biogeochemistry of the aquifer system may not be in steady state and that the net effect of competing processes could either increase or decrease arsenic concentrations over the next decades modeling results suggest that irrigation has greatly changed the location timing and chemical content of recharge to the aquifer flushing water through the system more quickly and also cycling large fluxes of water through rice fields during the dry season that could mobilize arsenic from oxides in near surface sediments furthermore the hydrologic model reveals that ponds many of which have been excavated over the last 50 years now provide much of the groundwater recharge these ponds receive most of the waste from the villages and thus provide another potential source of organic carbon to the groundwater system

hydrologic control of temporal variability in groundwater arsenic on the ganges floodplain of nepal abstract elevated arsenic in groundwater affects some 100 million people in south asia yet mitigation efforts are hindered by persistent uncertainty about the proximal source of arsenic and mechanisms for its mobilization at the core of this uncertainty are the relative roles of surficial organic clays vs deeper aquifer matrix iron oxyhydroxides temporal variations in groundwater chemistry can serve to distinguish the contributions of these two sources and such variation is especially pronounced in headwater areas of the ganges floodplain immediately adjacent to the himalayan foothills e g the terai of nepal tubewells down to 50 m in the terai commonly exhibit cyclical temporally correlated variation in dissolved arsenic iron and other species in nawalparasi the most arsenic affected district these wells tap thin 2 m gray sand aquifers embedded in a thick 50 m sequence of organic clays monsoon recharge refreshes these aquifers temporarily minimizing arsenic concentrations post monsoon average groundwater compositions exhibit increasing trends in water rock interaction higher tds with cation exchange to form increasingly na hco 3 waters arsenic and iron this cycle can be repeated during dry season precipitation events as well revealing direct correlation between trends in degree of clay interaction sodium fraction of major cations and arsenic concentrations during the year reversals in vertical head gradient yield reversals in arsenic temporal trend and downward gradients in the dry season correlate with increases in arsenic collectively these observations strongly support a model of reductive mobilization of arsenic from adjacent clays into aquifers tempered by repeated flushing during periods of appreciable rainfall

spatial vertical and temporal variation of arsenic in shallow aquifers of the bengal basin controlling geochemical processes a detailed understanding of the geochemical processes that regulate the spatial temporal and vertical variation of dissolved arsenic as in shallow aquifers 50m is a prerequisite for sustainable drinking water management in the bengal basin the present study conducted at chakdaha block of the nadia district west bengal india combined a high resolution hydrogeochemical monitoring study over 20months from two sets of piezometers 2 5 to the sediment geochemistry at areas with high average 146 g l n 5 and relatively low average 53 3 g l n 10 dissolved as concentrations in groundwater the determination of the isotopic composition of 2h and 18o in groundwater of the two sites indicated the recharge of evaporative surface water to the aquifer the concentrations of major aqueous solutes ca2 mg2 na k hco3 and cl and electrical conductivity were considerably higher in wells at the high as site compared to the low as site additionally at the high as site the major ions fe so4 2 electrical conductivity 2h and 18o showed markedly greater enrichment in the shallowest part 24m of the aquifer compared to the deeper part reflecting vertical layering of groundwater composition within the aquifer the oxidation of pyrites has been attributed to the high rate of mineral dissolution resulting in such greater enrichments in this part of the aquifer in addition the anthropogenic input with recharge water possibly increased the concentrations of cl in this part of the aquifer the vertical layering of groundwater was absent in the aquifer at the low as site the absence of such layering and relatively low major ion concentrations and electrical conductivity could be linked to the enhanced aquifer flushing and decreased water sediment interactions influenced by local scale groundwater abstraction the seasonal variations of as concentrations in groundwater were observed only in the shallowest part of the aquifers 30m furthermore the as concentrations in groundwater at the uppermost part of the shallow aquifers 21m increased continuously over the monitoring period at both sites this study supports the view that the reductive dissolution of fe oxyhydroxides coupled with competitive po4 3 sorption reactions in the aquifer sediment enriches as in groundwater of the bengal basin however the additional fe released by the weathering of silicate minerals especially biotite or the precipitation of fe as secondary mineral phases such as siderite vivianite and acid volatile sulfides may result in the decoupling of as and fe enrichment in groundwater the redox zonation within the aquifer possibly regulates the vertical distribution of as in the groundwater

mineralogical profiling of alluvial sediments from arsenic affected ganges brahmaputra floodplain in central bangladesh abstract 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 4 reduction mediated by so 4 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

river bank geomorphology controls groundwater arsenic concentrations in aquifers adjacent to the red river hanoi vietnam many aquifers that are highly contaminated by arsenic in south and southeast asia are in the floodplains of large river networks under natural conditions these aquifers would discharge into nearby rivers however large scale groundwater pumping has reversed the flow in some areas so that rivers now recharge aquifers at a field site near hanoi vietnam we find river water recharging the aquifer becomes high in arsenic reaching concentrations above 1000 g l within the upper meter of recently 50 g l aqueous arsenic concentrations are found in aquifer regions adjacent to zones where the river has recently deposited sediment and low arsenic concentrations are found in aquifer regions adjacent to erosional zones high arsenic concentrations are even found adjacent to a depositional river reach in a pleistocene aquifer a type of aquifer sediment which generally hosts low arsenic water using geochemical and isotopic data we estimate the in situ rate of arsenic release from riverbed sediments to be up to 1000 times the rates calculated on inland aquifer sediments in vietnam geochemical data for riverbed porewater conditions indicate that the reduction of reactive poorly crystalline iron oxides controls arsenic release we suggest that aquifers in these regions may be susceptible to further arsenic contamination where riverine recharge drawn into aquifers by extensive groundwater pumping flows through recently deposited river sediments before entering the aquifer

hydrological and sedimentary controls leading to arsenic contamination of groundwater in the hanoi area vietnam the impact of iron arsenic ratios peat river bank deposits and excessive groundwater abstraction groundwater contamination by arsenic in vietnam poses a serious health threat to millions of people in the larger hanoi area elevated arsenic levels are present in both the holocene and pleistocene aquifers family based tubewells predominantly tap the holocene aquifer while the hanoi water works extract more than 600 000 m3 day of groundwater from the pleistocene aquifer detailed groundwater and sediment investigations were conducted at three locations exhibiting distinct geochemical conditions i e i high levels of dissolved arsenic av 121 g l at the river bank ii low levels of dissolved arsenic av 21 g l at the river bank and iii medium levels of dissolved arsenic 60 g l in an area of buried peat and excessive groundwater abstraction seasonal fluctuations in water chemistry were studied over a time span of 14 months sediment bound arsenic 1 3 22 g g is in a natural range arsenic correlates with iron r2 0 8 with variation related to grain size sediment leaching experiments showed that arsenic can readily be mobilized at each of the three locations low levels of arsenic in groundwater 10 g l generally exhibit manganese reducing conditions whereas elevated levels are caused by reductive dissolution under iron and sulphate reducing conditions average arsenic concentrations in groundwater are twofold higher at the river bank than in the peat area the lower levels of arsenic contamination in the peat area are likely controlled by the high abundance of iron present in both the aqueous and sediment phases with median molar fe as ratios of 350 in water and 8700 in the sediments of the peat area reduced iron possibly forms new mineral phases that resorb or sequester previously released arsenic to the sediment despite similar redox conditions resorption is much less significant at the river bank fe as aq 68 fe as s 4700 and hence arsenic concentrations in groundwater reach considerably higher levels drawdown of holocene water to the pleistocene aquifer in the peat area caused by the pumping for the hanoi water works clearly promotes reducing conditions in pleistocene groundwater this demonstrates that excessive abstraction of water from deep wells i e wells tapping water below the arsenic burdened depth can cause a downward shift of iron reducing conditions and concurrently mobilize arsenic along the way vertical migration of reduced groundwater may also impact aquifers under natural hydrological conditions seepage of doc enriched groundwater derived from degradation of organic matter in the clayey sediments at the river bank was observed to enhance and maintain iron reducing conditions in the aquifer where organic matter is scarce once the aquifer becomes reduced arsenic is released from the aquifer solid hosts but additionally derives from the arsenic enriched groundwater seeping from the clay into the aquifer this behaviour is an important mechanism for arsenic contamination in aquifers that might not necessarily contain enough organic matter in their sediments to induce reducing conditions independently

redox controls on arsenic enrichment and release from aquifer sediments in central yangtze river basin abstract more than 100 million people in asia are presently exposed to groundwater with arsenic as concentrations exceeding the world health organization standard of 10 g l 1 arsenic contaminated groundwater within basins of the central portion of the yangtze river has recently been reported but the processes controlling arsenic concentrations have yet to be resolved we examined the hydrologic and geochemical factors controlling arsenic within the jianghan plain an inland sedimentary basin of the yangtze river where arsenic concentrations exhibit strong seasonal variability driven by surface and groundwater mixing schaefer et al 2016 hydrologic fluctuations alter redox conditions in the aquifer leading to oscillations between arsenic iron reduction and oxidation here we investigate the depth distribution of solid and aqueous phase iron and arsenic species and through a series of laboratory manipulations constrain the biogeochemical processes controlling seasonal changes in groundwater arsenic concentrations in sediment incubations from 20 m below the surface where solid phase arsenic concentrations exceed 100 mg kg 1 both unamended and glucose amended sediment samples result in arsenic release to the aqueous phase in situ carbon was capable of promoting as release in the sediment in contrast sediment batch incubations from other depths resulted in limited as release solid phase arsenic in the enriched zone was relatively oxidized but may become reduced over short time periods in sediments below the as enriched zone glucose amendment resulted in arsenic reduction but arsenic release to the aqueous phase was restricted by the subsequent formation of arsenic sulfide minerals buried sedimentary arsenic coupled with anaerobic microbial respiration of subsurface organic carbon within the jianghan plain aquifer leads to rapid release of as to groundwater arsenic release from sediments at 20 m depth is sufficient to explain arsenic concentrations throughout the aquifer and provides a mechanism to explain how shifts in hydrology result in seasonally variable arsenic concentrations in groundwater

controlling geological and hydrogeological processes in an arsenic contaminated aquifer on the red river flood plain vietnam abstract geological and hydrogeological processes controlling recharge and the mobilization of as were investigated in a shallow holocene aquifer on the red river flood plain near hanoi vietnam the geology was investigated using surface resistivity methods geophysical borehole logging drilling of boreholes and installation of more than 200 piezometers recharge processes and surface groundwater interaction were studied using i time series of hydraulic head distribution in surface water and aquifers ii the stable isotope composition of waters and iii numerical groundwater modeling the red river and two of its distributaries run through the field site and control the groundwater flow pattern for most of the year there is a regional groundwater flow towards the red river during the monsoon the red river water stage rises up to 6 m and stalls the regional groundwater flow the two distributaries recharge the aquifer from perched water tables in the dry season whilst in the flooding period surface water enters the aquifer through highly permeable bank sediments the result is a dynamic groundwater flow pattern with rapid fluctuations in the groundwater table a transient numerical model of the groundwater flow yields an average recharge rate of 60 100 mm a through the confining clay and a total recharge of approximately 200 mm a was estimated from 3h 3he dating of the shallow groundwater thus in the model area recharge of surface water from the river distributaries and recharge through a confining clay is of the same magnitude being on average around 100 mm a the thickness of the confining clay varies between 2 and 10 m and affects the recharge rate and the transport of electron acceptors o2 no 3 and so 4 2 into the aquifer where the clay layer is thin an up to 2 m thick oxic zone develops in the shallow aquifer in the oxic zone the as concentration is less than 1 g l but increases in the reduced zone below to 550 g l in the holocene aquifer as is mobilized at a rate of around 14 g l a an as mass balance for the field site shows that around 1100 kg of as is annually leached from the holocene sand and discharged into the red river corresponding to 0 01 of the total pool of as now present in the holocene sand

crab burrows as conduits for groundwater surface water exchange in bangladesh groundwater recharge affects water budgets and groundwater quality on the deltas and floodplains of south and southeast asia rain and flooding rivers recharge groundwater during the monsoon irrigated rice fields and surface water bodies recharge aquifers during the dry season groundwater throughout the region is severely contaminated by arsenic and recent research suggests that quantifying and characterizing recharge is important to understand whether recharge flushes or mobilizes arsenic from aquifers at a field site in bangladesh we found that burrows of terrestrial crabs short circuit low permeability surface sediments providing the primary conduit for recharge we combine field observations along with a model that couples isotope and water balances to quantify the effect of crab burrows on aquifer recharge given the wide distribution of burrowing crabs and the surficial geology we suggest that crab burrows provide widespread conduits for groundwater recharge

influence of irrigation practices on arsenic mobilization evidence from isotope composition and cl br ratios in groundwater from datong basin northern china environment isotopes 18o and 2h and cl br ratios in groundwater have been used to trace groundwater recharge and geochemical processes for arsenic contamination in datong basin the arsenic concentrations of groundwater samples ranged from 0 4 to 434 9 g l with the average of 51 2 g l which exceeded china s drinking water standard 10 g l all the groundwater samples are plotted on or close to the meteoric water line of the 18o vs 2h plot indicating their meteoric origin the relationship between 18o values and cl br ratios and cl concentrations demonstrate that leaching and mixing are the dominant processes affecting the distribution of high arsenic groundwater in this area the observed non linearity in the trend between 18o and arsenic concentration is due to combined effects of mixing and leaching the similarity of the trend in cl br ratios and 18o values for high arsenic groundwater demonstrate that extensive leaching of irrigation return and salt flushing water flow could be the dominant process driving arsenic mobilization in the groundwater system moreover the long term irrigation practice can cause the drastic change of the biogeochemical and redox condition of in the aquifer system which in turn promotes the mobilization of arsenic therefore groundwater pumping for irrigation in this area of waterborne endemic arsenic poisoning should be under strict control to protect groundwater quality in this area

perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in west bengal india excess of 4600 m gk g 1 stable isotope ratios of waters from constructed perennial ponds indicate the ponds are chiefly recharged during the summer monsoon and subsequently undergo extensive evaporation during the dry winter season in contrast groundwaters with high as concentrations plot along the local meteoric water line lmwl near where the annual volume weighted mean precipitation values for d 2 ha ndd 18 o would plot the stable isotope data demonstrate that groundwaters are directly recharged by local precipitation without significant evaporation and thus are not recharged by nor mixed with the pond waters furthermore reactive transport modeling indicates that dissolved organic matter dom derived from pond waters does not fuel microbial respiration and as mobilization at depth in the underlying aquifer because travel times for pond derived dom exceed groundwater ages by thousands of years instead organic matter within the aquifer sediments must drive dissimilatory iron reduction and as release to groundwaters citation datta s a w neal t j mohajerin t ocheltree b e rosenheim c d white and k h johannesson 2011 perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in west bengal india geophys res lett 38 l20404 doi 10 1029 2011gl049301

spatial relationship of groundwater arsenic distribution with regional topography and water table fluctuations in the shallow aquifers in bangladesh the present study has examined the relationship of groundwater arsenic as levels in alluvial aquifers with topographic elevation slope and groundwater level on a large basinal scale using high resolution 90 m 90 m shuttle radar topography mission srtm digital elevation model and water table data in bangladesh results show that high as 50 g l tubewells are located in low lying areas where mean surface elevation is approximately 10 m similarly high as concentrations are found within extremely low slopes 0 7 in the country groundwater elevation weekly measured by bangladesh water development board was mapped using water table data from 950 shallow depth 100 m piezometers distributed over the entire country the minimum maximum and mean groundwater elevation maps for 2003 were generated using universal kriging interpolation method high as tubewells are located mainly in the ganges brahmaputra meghna delta sylhet trough and recent floodplains where groundwater elevation in shallow aquifers is low with a mean value of 4 5 m above the public works datum pwd level extremely low groundwater gradients 0 01 0 001 m km within the gbm delta complex hinder groundwater flow and cause slow flushing of aquifers low elevation and gentle slope favor accumulation of finer sediments as carrying iron oxyhydroxide minerals and abundant organic matter within floodplains and alluvial deposits at low horizontal hydraulic gradients and under reducing conditions as is released in groundwater by microbial activity causing widespread contamination in the low lying deltaic and floodplain areas where as is being recycled with time due to complex biogeochemical processes

geochemical study of arsenic release mechanisms in the bengal basin groundwater 1 to investigate arsenic mobility in the bengal basin groundwater we sampled water wells and sediments throughout the region there are strong correlations among high levels of dissolved arsenic and iron ammonia and methane especially in samples from a single site laxmipur no linkage is seen between as and agricultural tracers such as phosphate the association of as and fe occurs because arsenic strongly adsorbs onto feooh particles in river water they flocculate with other fine grained particles at the freshwater saltwater transition zone subsequent bacterially mediated reduction of feooh in the clay releases the adsorbed arsenic weathering of as bearing mica plays a significant role in the as budget the correlated presence of as ch4 and nh4 in water supply wells is the result of diffusion out of organic rich clay into the more permeable zones arsenic is mainly released from recent sediments at 50 m depth deposited in the gbr floodplain as sea level rose throughout the holocene

aqueous and solid phase arsenic speciation in the sediments of a contaminated wetland and riverbed abstract mobility of as in the environment is controlled by its association with solid phases through adsorption and co precipitation to elucidate the mobilization potential of as deposited in wetland and riverbed sediments of the wells g h wetland in woburn ma as the result of decades of industrial activity as retention mechanisms were inferred from aqueous and solid phase geochemical measurements of sediment cores testing included a sequential extraction method designed for and standard tested with as phases and pe ph equilibrium modeling the uppermost sediments in the wells g h wetland contain elevated concentrations of both dissolved and solid phase as up to 2 000 g l and 15 000 g g respectively and a maximum concentration between 30 and 40 cm depth measurements obtained in this study suggested that as in the wetland sediments was predominantly adsorbed likely onto amorphous fe hydr oxide phases and mixed valence fe phases in the riverbed sediments however a relatively greater proportion of the solid as was associated with more reduced and crystalline phases and adsorbed as was more likely associated with al oxide or secondary reduced fe phases ph pe modeling of the fe as s system was consistent with observations the association of as with more oxidized phases in the wetland compared with the riverbed sediments may result from a combination of plant activities including evapotranspiration driven water table depression and or root oxygenation

natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water the example of west bengal and its worldwide implications in order to investigate the mechanism of as release to anoxic ground water in alluvial aquifers the authors sampled ground waters from 3 piezometer nests 79 shallow 80 m wells in an area 750 m by 450 m just north of barasat near kolkata calcutta in southern west bengal high concentrations of as 200 1180 mug l 1 are accompanied by high concentrations of fe 3 13 7 mgl 1 and po4 1 6 5 mg l 1 ground water that is rich in mn 1 5 3 mg l 1 contains 50 mug l 1 of as the composition of shallow ground water varies at the 100 m scale laterally and the metre scale vertically with vertical gradients in as concentration reaching 200 mug l 1 m 1 the as is supplied by reductive dissolution of feooh and release of the sorbed as to solution the process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit in well waters thermotolerant coliforms a proxy for faecal contamination are not present in high numbers 10 cfu 100 ml in 85 of wells showing that faecally derived organic matter does not enter the aquifer does not drive reduction of feooh and so does not release as to ground water arsenic concentrations are high much greater than50 mug l 1 where reduction of feooh is complete and its entire load of sorbed as is released to solution at which point the aquifer sediments become grey in colour as feooh vanishes where reduction is incomplete the sediments are brown in colour and resorption of as to residual feooh keeps as concentrations below 10 mug l 1 in the presence of dissolved fe sorbed as released by reduction of mn oxides does not increase as in ground water because the as resorbs to feooh high concentrations of as are common in alluvial aquifers of the bengal basin arise because himalayan erosion supplies immature sediments with low surface loadings of feooh on mineral grains to a depositional environment that is rich in organic mater so that complete reduction of feooh is common c 2004 published by elsevier ltd

arsenic movement and traces in the groundwater from the hetao area inner mongolia in the hetao area of inner mongolia quaternary alluvial aquifers used for the water supply are contaminated by naturally occurring arsenic which heavily affects the health of 200 000 local residents this study on the isotopes of strontium and relevant elements contained in the groundwater as well as the arsenic in the groundwater and residents hair indicates that the arsenic originally derives from the upper reaches of this area where arsenic levels are high in groundwater rock and soil over respectively 44 km work line aa and 36 km work line bb away from there the levels of arsenic in the water corresponding to the trend of the residents arseniasis decrease along the direction of the flow from 0 251 to 0 05 mg l and 0 232 to 0 036 mg l the result of this research suggests that long term strategies to deal with this arseniasis should involve finding hydrous terrains uncontaminated by water from the upper reaches and developing routes to prevent water from taking arsenic

solid phases and desorption processes of arsenic within bangladesh sediments abstract arsenic is a contaminant in the groundwater of bangladesh and west bengal india where an estimated 57 million people may be drinking water with unsafe arsenic levels the source of arsenic appears to be natural solid phase arsenic in the sediments and various theories have been put forth regarding the modes of arsenic release ranging from the oxidative or reductive degradation of arsenic bearing solids to competitive ligand displacement by phosphate currently however reductive dissolution of fe iii hydr oxides and concomitant arsenic release is the most widely accepted explanation of high arsenic groundwater concentrations using micro x ray fluorescence elemental mapping and micro x ray absorption near edge structure spectroscopy we detect arsenic bearing sulfides in the aquifer sediments from our field site in munshiganj bangladesh reduction of fe and as in surface soil layers is apparent but fe iii hydr oxides are not detected in the holocene aquifer materials rapid abiotic desorption of arsenic from sediments is observed in batch experiments and positive controls with ferrihydrite dismiss the role of ferric hydr oxides in arsenic retention within the aquifer sediments based on our laboratory results we do not see evidence for ferric hydr oxide reductive dissolution at well depths in the holocene aquifer in contrast our data suggest that arsenic is only released via redox cycling in surface soils sediments and thus must then be transported to well depth through the sandy aquifer

redox zonation and oscillation in the hyporheic zone of the ganges brahmaputra meghna delta implications for the fate of groundwater arsenic during discharge abstract riverbank sediment cores and pore waters shallow well waters seepage waters and river waters were collected along the meghna riverbank in gazaria upazila bangladesh in jan 2006 and oct nov 2007 to investigate hydrogeochemical processes controlling the fate of groundwater as during discharge redox transition zones from suboxic 0 2 m depth to reducing 2 5 m depth then suboxic conditions 5 7 m depth exist at sites with sandy surficial deposits as evidenced by depth profiles of pore water n 7 and sediment n 11 diffuse reflectance fe iii fe ratios and fe iii concentrations the sediment as enrichment zone up to 700 mg kg 1 is associated with the suboxic zones mostly between 0 and 2 m depth and less frequently between 5 and 7 m depth the as enriched zones consist of several 5 10 cm thick dispersed layers and span a length of 5 15 m horizontally from the river shore depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved fe 11 6 11 7 mg l 1 and as 118 91 g l 1 mostly as arsenite between 2 and 5 m depth but lower concentrations between 0 and 2 m depth 0 13 0 19 mg l 1 fe 1 1 g l 1 as and between 5 and 6 m depth 1 14 0 45 mg l 1 fe 28 17 g l 1 as because it would take more than a few hundred years of steady groundwater discharge 10 m yr 1 to accumulate hundreds of mg kg 1 of as in the riverbank sediment it is concluded that groundwater as must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s not only does this lend unequivocal support to the argument that as occurrence in the ganges brahmaputra meghna delta groundwater is of geogenic origin it also calls attention to the fate of this as enriched sediment as it may recycle as into the aquifer

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groundwater flow in an arsenic contaminated aquifer mekong delta cambodia abstract to advance understanding of hydrological influences on as concentrations within groundwaters of southeast asia the flow system of an as rich aquifer on the mekong delta in cambodia where flow patterns have not been disturbed by irrigation well pumping was examined monitoring of water levels in a network of installed wells extending over a 50 km2 area indicates that groundwater flow is dominated by seasonally variable gradients developed between the river and the inland wetland basins while the gradient inverts annually net groundwater flow is from the wetlands to the river hydraulic parameters of the aquifer k 10 4 ms 1 and overlying clay aquitard k 10 8 ms 1 were determined using grain size permeameter and slug test analyses when coupled with observed gradients they indicate a net groundwater flow velocity of 0 04 0 4 ma 1 downward through the clay and 1 13 ma 1 horizontally within the sand aquifer producing aquifer residence times on the order 100 1000 a the results of numerical modeling support this conceptual model of the flow system and when integrated with observed spatial trends in dissolved as concentrations reveal that the shallow sediments upper 2 10 m of fine grained material are an important source of as to the underlying aquifer

geochemical evolution of groundwater flowing through arsenic source sediments in an aquifer system of west bengal india the source of geogenic arsenic as contaminating a shallow aquifer in west bengal was traced to fine grained sediments deposited in an abandoned river channel along with as bearing phases these sediments contain 0 46 co deposited organic carbon the release of as and the geochemistry of groundwater within the channel fill deposits is investigated using a detailed mass balance model supported by aqueous solid phase and mineralogical data the model describes the evolution of groundwater chemistry along a flow path extending from its recharge in an abandoned channel pond through the channel fill to the underlying aquifer variations in groundwater composition are explained in terms of mineral weathering of host sediments driven by organic carbon decay arsenic is released through the reductive dissolution of goethite and the weathering of chlorite concomitantly some as is sequestered in precipitating vivianite these competing processes reach equilibrium deeper in the channel fill sequence as groundwater as concentrations stabilize the model yields estimates of mineral reaction or precipitation rates including rates of organic carbon oxidation 1 15 mmol c l 1 a 1 and net as release 4 57 10 4 mmol l 1 a 1 fine grained slightly permeable deposits such as channel fill containing reactive organic carbon and as bearing goethite and phyllosilicates are centers of intense chemical weathering conducive to as mobilization

mobility of arsenic in a bangladesh aquifer inferences from geochemical profiles leaching data and mineralogical characterization aquifer geochemistry was characterized at a field site in the munshiganj district of bangladesh where the groundwater is severely contaminated by as vertical profiles of aqueous and solid phase parameters were measured in a sandy deep aquifer depth 150 m below a thick confining clay 119 to 150 m a sandy upper aquifer 3 5 to 119 m above this confining layer and a surficial clay layer 3 5 m in the deep aquifer and near the top of the upper aquifer aqueous as levels are low 10 g l but aqueous as approaches a maximum of 640 g l at a depth of 30 to 40 m and falls to 58 g l near the base 107 m of the upper aquifer in contrast solid phase as concentrations are uniformly low rarely exceeding 2 g g in the two sandy aquifers and never exceeding 10 g g in the clay layers solid phase as is also similarly distributed among a variety of reservoirs in the deep and upper aquifer including adsorbed as as coprecipitated in solids leachable by mild acids and reductants and as incorporated in silicates and other more recalcitrant phases one notable difference among depths is that sorbed as loads considered with respect to solid phase fe extractable with 1 n hcl 0 2 m oxalic acid and a 0 5 m ti iii citrate edta solution appear to be at capacity at depths where aqueous as is highest this suggests that sorption limitations may in part explain the aqueous as depth profile at this site competition for sorption sites by silicate phosphate and carbonate oxyanions appear to sustain elevated aqueous as levels in the upper aquifer furthermore geochemical profiles are consistent with the hypothesis that past or ongoing reductive dissolution of fe iii oxyhydroxides acts synergistically with competitive sorption to maintain elevated dissolved as levels in the upper aquifer microprobe data indicate substantial spatial comapping between as and fe in both the upper and deep aquifer sediments and microscopic observations reveal ubiquitous fe coatings on most solid phases including quartz feldspars and aluminosilicates extraction results and xrd analysis of density magnetic separates suggest that these coatings may comprise predominantly fe ii and mixed valence fe solids although the presence of fe iii oxyhydroxides can not be ruled out these data suggest as release may continue to be linked to dissolution processes targeting fe or fe rich phases in these aquifers copyright 2004 elsevier ltd

mineralogy and geochemistry of shallow sediments of sonargaon bangladesh and implications for arsenic dynamics focusing on the role of organic matter abstract 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 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

simulation of flowpaths and travel time of groundwater through arsenic contaminated zone in the multi layered aquifer system of bengal basin use of deep groundwater is becoming an important issue in bengal basin due to water quality problems in upper aquifers in many areas still 50 of cultivable land in the model area has the potential for future irrigation prior to development of deep aquifers it is important to know the trend of flowpaths from recharge zone and travel time of stored water in field geologic conditions and development stresses which is the principal objective of this study a three dimensional model was used for this study to simulate groundwater flow model simulation results depict that flowpaths and travel time of groundwater are primarily controlled by hydrogeologic characteristics however the recharge zone for different aquifer units is about the same in different conditions with variation of travel time continued irrigation abstraction from the shallow aquifer may decrease vertical percolation of recharge water to deeper aquifers the water of the shallow i e the 1st aquifer generally has a higher head than water in the main i e the 2nd aquifer water from the shallow aquifer may move downward into the deeper units through aquitard windows the water level of the deep i e the 3rd aquifer is higher than that of the main aquifer the arsenic or chloride rich groundwater in the upper aquifers is not likely to be drawn into the deep aquifer under conditions of moderate use of the deep aquifer

groundwater chemistry and redox processes depth dependent arsenic release mechanism 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 processe

hydrological control of as concentrations in bangladesh groundwater 1 the elevated arsenic as content of groundwater from wells across bangladesh and several other south asian countries is estimated to slowly poison at least 100 million people the heterogeneous distribution of dissolved arsenic in the subsurface complicates understanding of its release from the sediment matrix into the groundwater as well as the design of mitigation strategies using the tritium helium 3h 3he groundwater dating technique we show that there is a linear correlation between groundwater age at depths 20 m and dissolved as concentration with an average slope of 19 g l 1 yr 1 monitoring wells only we propose that either the kinetics of as mobilization or the removal of as by groundwater flushing is the mechanism underlying this relationship in either case the spatial variability of as concentrations in the top 20 m of the shallow aquifers can to a large extent be attributed to groundwater age controlled by the hydrogeological heterogeneity in the local groundwater flow system

14c dating of deep groundwater in the bengal aquifer system bangladesh implications for aquifer anisotropy recharge sources and sustainability summary environmental isotopes and 14 c dating were applied to estimate ages and recharge sources of deep groundwater 150 m bgl in south east bangladesh with one exception deep groundwater is shown to have been recharged more recently than 10 ka range 3 9 ka mean 7 6 ka under climatic conditions indicated by 18 o and 2 h as similar to the present day groundwater age distributions have been used to infer the scale of aquifer hydraulic anisotropy 2d groundwater flow modelling is able to reproduce the observed vertical profiles of groundwater age as determined at two locations when the aquifer is assigned a permeability anisotropy k x k z of at least 10 3 under these conditions deep groundwater originates as recharge in the hill regions at the eastern boundary of the basin recharge rates estimated from the groundwater ages are close to an estimate of the current rate of deep groundwater abstraction cautious development and careful monitoring are therefore necessary as excessive deep groundwater pumping could draw dissolved arsenic from the shallow levels of the bengal aquifer system bas and contaminate the deep groundwater resource

effect of groundwater flow on forming arsenic contaminated groundwater in sonargaon bangladesh summary three dimensional groundwater flow in sonargaon bangladesh is numerically simulated in order to evaluate the flow paths of as contaminated drinking groundwater in the holocene aquifer of the ganges blamaptra meghna delta plain over a recent 30 year period the model indicates that vertical infiltration of surface groundwater into the shallow holocene aquifer occurs frequently in the ganges blamaptra meghna delta plain it predicts that the water recharged from ground surface moves approximately 10 20 m vertically downward beneath the flood plain with a gradually increasing horizontal flow toward the underlying pleistocene middle mud layer aquitard the model also predicts that groundwaters containing highest as concentrations 700 g l are formed on the vertical groundwater flow paths where surface water recharges the holocene aquifer and not on the horizontal flow paths combining with the groundwater chemistry reducing groundwater condition is not essential for the as contaminated groundwater of the studied area in the ganges delta plain