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

vulnerability of low arsenic aquifers to municipal pumping in bangladesh sandy aquifers deposited 12 000 years ago some as shallow as 30 m have provided a reliable supply of low arsenic as drinking water in rural bangladesh this study concerns the potential risk of contaminating these aquifers in areas surrounding the city of dhaka where hydraulic heads in aquifers 150 m deep have dropped by 70 m in a few decades due to municipal pumping water levels measured continuously from 2012 to 2014 in 12 deep 150m 3 intermediate 90 150 m and 6 shallow 90 m community wells 1 shallow private well and 1 river piezometer show that the resulting drawdown cone extends 15 35 km east of dhaka water levels in 4 low as community wells within the 62 147 m depth range closest to dhaka were inaccessible by suction for up to a third of the year lateral hydraulic gradients in the deep aquifer system ranged from 1 7 10 4 to 3 7 10 4 indicating flow towards dhaka throughout 2012 2014 vertical recharge on the edge of the drawdown cone was estimated at 0 21 0 06 m yr the data suggest that continued municipal pumping in dhaka could eventually contaminate some relatively shallow community wells

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

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

groundwater flow dynamics and arsenic source characterization in an aquifer system of west bengal india numerical groundwater flow modeling reverse particle tracking and environmental tracers are used to locate the source of geogenic as affecting an aquifer in west bengal the aquifer is hosted by point bar sands deposited in a meandering fluvial environment wells tapping the aquifer exhibit as concentrations up to 531 g l high as groundwaters are recharged in ponds marking an abandoned river channel the source of as is traced to the underlying fine grained channel fill sediments arsenic release within these sediments is accompanied by a concomitant release of br and doc indicating that these species may be decay products of natural organobromines codeposited along with as mass transfer of as to the dissolved phase and its flushing from source sediments are described using a simplified reactive solute transport model based on this model a characteristic reaction time for mass transfer is estimated at 6 7 years average groundwater residence times in the source are estimated to have declined from 16 6 to 6 6 years with the advent of intensive irrigation pumping the ratio of residence and reaction times a damkohler number has declined correspondingly from 2 49 to 0 99 indicating a shift from transport to reaction rate limited as mobilization greater insight into the as problem in se asia may be achieved by shifting the focus of field investigations from aquifers to potential contamination sources in aquitards

reversible adsorption and flushing of arsenic in a shallow holocene aquifer of bangladesh the spatial heterogeneity of dissolved arsenic as concentrations in shallow groundwater of the bengal basin has been attributed to transport of as and reactive carbon from external sources or to the release of as from within grey sand formations we explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream within the 10 20 m depth range groundwater ages along the transect determined by the 3h 3he method increase from 10 yr in the recharge area to a maximum of 40 yr towards the stream concentrations of groundwater as within the same grey sands increase from 10 to 100 to 500 g l along this transect evidence of reversible adsorption of as between the groundwater and sediment was obtained from a series of push pull experiments traditional batch adsorption experiments and the accidental flooding of a shallow monitoring well assuming reversible adsorption and a distribution coefficient kd of 0 15 1 5 l kg inferred from these observations a simple flushing model shows that the increase in as concentrations with depth and groundwater age at this site and at other sites in the bengal and red river basins can be attributed to the evolution of the aquifer over 100 1000 years as aquifer sands are gradually flushed of their initial as content a wide range of as concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates without external inputs of reactive carbon or as from other sources

establishing chronologies from isotopic profiles in serially collected animal tissues an example using tail hairs from african elephants while the use of stable isotopes in wildlife ecological research is growing rapidly development of methods to establish time specific isotope data from continuously growing animal tissues are lacking using serially collected tail hairs from wild african elephants loxodonta africana we develop and compare four techniques to collate temporal isotope chronologies from metabolically inert tissues for which formation growth overlapped in time the influence of variation in within hair growth rates and other sources of error in the presented techniques are explored and found to be inconsequential relative to the 5 day tissue sampling interval using a floating point regression approach we find a high degree of correlation between independently derived isotope profiles from the same and different individuals in the study ecosystem remotely sensed normalized difference vegetation index ndvi data is compared with the isotope derived diet chronologies from five elephants developed independently diet shifts from browse to grass occurring at the onset of the wet season were highly synchronized while early dry season diets varied across individuals these methods are applicable across a variety of keratinous tissues and even teeth and as demonstrated by our results can be implemented using profiles from different individuals or relating profiles to environmental variation seasonality as such the presented methods allow the establishment of high resolution temporal data on diet movement and climatic conditions experienced by an organism in many research settings

contrasting dissolved organic matter quality in groundwater in holocene and pleistocene aquifers and implications for influencing arsenic mobility abstract the discontinuous nature of elevated arsenic as in drinking water wells of west bengal and other regions in the bengal basin has led to increased interest in the role that sediment derived organic matter may play in enhancing reductive dissolution and as mobilization higher as concentrations have been observed in groundwater in reduced holocene grey aquifers when compared to oxidized pleistocene orange aquifers in order to evaluate if the differences in the chemical character of dissolved organic matter dom are present in groundwater in the holocene and the pleistocene aquifers that may influence dissolved as concentrations shallow groundwater and surface water samples were collected from four study sites in murshidabad district west bengal india and analyzed for water chemistry parameters and characteristics of dom for wells known to typically contain high as concentrations in holocene sediments in beldanga 10 4622 g l at 35 45 m depth and hariharpara 5 695 g l at 6 37 depth sites as well as wells characterized by low as concentrations pleistocene sediiments in nabagram 0 16 g l at 20 45 m depth and kandi 5 50 g l at 20 55 m depth detailed dom characterization was carried out using fluorescence spectroscopy and parallel factor analysis parafac results from statistical analysis of a variety of optical absorbance and fluorescence dom properties revealed that the dom in groundwater in the holocene aquifer had high humification index hix and low freshness index values whereas groundwater in the pleistocene aquifer comprised more labile and microbial dom sources consistent with the more labile nature of dom in groundwater in the pleistocene aquifer two ratios 1 humic like to protein like components humic protein and 2 terrestrially derived to microbially derived components terr microb obtained from a four component parafac model were 1 9 and 2 9 times greater respectively in groundwater in the holocene aquifer than in that of the pleistocene aquifer which suggests that the absence of humic like dom may be an important limitation to as mobility

impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in araihazar bangladesh 1 the high degree of spatial variability of dissolved as levels in shallow aquifers of the bengal basin has been well documented but the underlying mechanisms remain poorly understood we compare here as concentrations measured in groundwater pumped from 4700 wells 22 m 75 ft deep across a 25 km2 area of bangladesh with variations in the nature of surface soils inferred from 18 500 measurements of frequency domain electromagnetic induction a set of 14 hand auger cores recovered from the same area indicate that a combination of grain size and the conductivity of soil water dominate the electromagnetic signal the relationship between pairs of individual em conductivity and dissolved as measurements within a distance of 50 m is significant but highly scattered r2 0 12 n 614 concentrations of as tend to be lower in shallow aquifers underlying sandy soils and higher below finer grained and high conductivity soils variations in em conductivity account for nearly half the variance of the rate of increase of as concentration with depth however when the data are averaged over a distance of 50 m r2 0 50 n 145 the association is interpreted as an indication that groundwater recharge through permeable sandy soils prevents as concentrations from rising in shallow reducing groundwater

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

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

study of subsurface geology in locating arsenic free groundwater in bengal delta west bengal india over a large area of the bengal delta in west bengal india arsenic distribution patterns in groundwater were studied one hundred and ten boreholes at different target locations were made subsurface sediments were logged and analysed and arsenic values in sediments vis a vis groundwater were compared the study elucidates the subsurface geology of the western part of bengal delta and characterises the sediments that were intersected in different boreholes with contrasting values of arsenic in groundwater it reveals an existence of multiple aquifers stacked over each other depending on the color and nature of aquifer sands and their overlying clay beds six aquifer types type 1 to type 6 are classified and described sediment arsenic for all the varieties of aquifer sands are near similar but the groundwater arsenic of these six aquifers varies widely type 2 and type 5 aquifers host arsenic contaminated groundwater whereas the other four aquifers are arsenic free type 2 and type 5 aquifers are capped by a grey to dark grey soft organic matter rich clay unit which makes these aquifers semi confined to leaky confined these contribute in releasing arsenic from the sediments the results of this study are employed in a proposed georemedial measure against this hazardous toxic element

compression versus extension in the exhumation of the dora maira coesite bearing unit western alps italy abstract several recent papers suggest that uplift and exhumation of high pressure low temperature hp lt metamorphic rocks are due to crustal extension coeval with and or post dating the contraction responsible for the metamorphism a detailed survey of the coesite bearing unit of the southern dora maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra high pressure uhp rocks this uhp unit consists of pre alpine basement rocks which equilibrated at a depth of 100 km probably 120 100 ma ago it is overlain by two units with a similar lithology and eoalpine metamorphic age and high p t gradients which equilibrated at 60 and 50 km respectively this eoalpine basement complex is intercalated between blueschist facies units the whole pile was affected by a mesoalpine greenschist facies mylonitic deformation during which the early contacts were reactivated as low angle shallow dipping shear zones extension is documented only during the mesoalpine collisional period and is referred to the gravity collapse of a thickened orogenic wedge with an exhumation rate of about 1 mm yr the most important part of the exhumation of the uhp rocks occurred earlier at a higher rate close to 4 mm yr a forced return flow or extrusion tectonics of imbricate slices in an on going subduction setting seems appropriate to account for this early stage the fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed

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

arsenic in groundwater of the red river floodplain vietnam controlling geochemical processes and reactive transport modeling abstract the mobilization of arsenic as to the groundwater was studied in a shallow holocene aquifer on the red river flood plain near hanoi vietnam the groundwater chemistry was investigated in a transect of 100 piezometers results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of fe oxides and methanogenesis enhanced pco2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone the concentration of as increases over depth to a concentration of up to 550 g l most as is present as as iii but some as v is always found arsenic correlates well with nh4 relating its release to organic matter decomposition and the source of as appears to be the fe oxides being reduced part of the produced fe ii is apparently reprecipitated as siderite containing less as results from sediment extraction indicate most as to be related to the fe oxide fractions the measured amount of sorbed as is low in agreement speciation calculations for a fe oxide surface suggest as iii to constitute only 3 of the surface sites while the remainder is occupied by carbonate and silica species the evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component a redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter apart from the upper meter a constant degradation rate of 0 15 c mmol l yr could explain the redox zonation throughout the aquifer modeling also indicates that the fe oxide being reduced is of a stable type like goethite or hematite arsenic is contained in the fe oxides and is first released during their dissolution our model further suggests that part of the released as is adsorbed on the surface of the remaining fe oxides and in this way may be retarded

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

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

reply to the comment on arsenic release from biotite into a holocene groundwater aquifer in bangladesh by hossain m anawar and martin mihaljevic

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

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

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

distribution and mobility of geogenic molybdenum and arsenic in a limestone aquifer matrix abstract to investigate the potential of mo and as as possible geogenic contaminants three sediment cores were examined to evaluate their mineralogical association distribution and mobility the cores were described and analyzed for total organic carbon toc ca mg si al p sr as mo fe and s content except in the uppermost segment limestone was the main lithology with the occasional presence of dolomite and clay that change in lithology was also observed in the bulk chemical composition where ca mg and sr concentrations increased with depth while si al and p concentrations decreased with depth minor minerals included pyrite fes 2 powellite camoo 4 and ferrihydrite the minimum maximum median and standard deviations for all analyzed elements including as and mo were comparable for all three cores molybdenum and as however varied significantly with depth and median as and mo values were above their respective crustal averages of approximately 1 1 mg kg and 1 5 mg kg the median values for as were 1 9 mg kg in core dep 1 3 3 mg kg in dep 2 and 1 mg kg in dep 5 the median values for mo were 2 3 mg kg in core dep 1 2 5 mg kg in dep 2 and 2 5 mg kg in dep 5 maximum concentrations for as were 101 9 mg kg 47 5 mg kg and 56 2 mg kg in cores dep 1 dep 2 and dep 5 respectively maximum concentrations for mo were 880 mg kg 123 mg kg and 225 mg kg in cores dep 1 dep 2 and dep 5 respectively electron microprobe analyses of individual minerals revealed variable concentrations of as ranging from approximately 300 to 9000 mg kg in pyrite and up to 17 600 mg kg in powellite camoo 4 the mo concentration in pyrite was consistently below the detection limit of approximately 100 mg kg in powellite the mo concentration was up to 42 wt a subset of 10 samples from different stratigraphic sections and with different as and mo concentrations was further investigated to assess as and mo mobility under changing physicochemical conditions leaching the aquifer matrix with a 1 m naoac solution at a ph of 8 1 removed more than 70 mo in 8 of the 10 samples the maximum value was 97 in contrast to mo as was mobilized to a lesser degree in 8 of the samples less than 30 were removed and the maximum was only 50 molybdenum which seemed to be loosely bound to mineral and organic matter surfaces thus could easily be removed from the aquifer matrix while as on the other hand should be much less mobile because it occurred either tightly adsorbed by hydrous ferric oxide or as an impurity in pyrite thus it is advisable to include mo in the analytical program whenever elevated as concentrations are encountered 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

geogenic as and mo groundwater contamination caused by an abundance of domestic supply wells abstract lacking a connection to a municipal water supply each household in the municipality of lithia approximately 30 km southeast of tampa florida usa is responsible for its own supply of drinking water causing a high density of private domestic supply wells dsw in this area there a multilayered aquifer system exists which can be subdivided into three distinct hydro stratigraphic units which are from the top down the surficial aquifer system sas the intermediate aquifer system ias and the upper floridan aquifer system ufa despite the relatively small area the geochemical and hydrogeological setting in lithia is complex consisting of i extensive cyclical pumping in a municipal well field to the west ii large seasonal changes in hydraulic head ii multiple aquifers with different hydraulic heads and iv a large density of domestic supply wells dsw within the zone of highest as concentrations there are approximately 100 wells in an area of 2 5 km 1 5 km most of these wells have large open screened intervals often open to all three aquifers allowing the downward flow of oxygenated and upward flow of anoxic groundwater a survey of groundwater quality found that as and mo concentrations in the dsw were up to 371 g l and 4740 g l respectively to obtain information about the individual aquifers 5 well clusters with 4 monitoring intervals approximately 50 m 65 m 80 m and 95 m below surface and 8 push core wells approximately 9 m below surface were installed and sampled in those wells as and mo were only elevated in a permeable layer within the ias at a depth of 50 m values were up to 195 g l for as and up to 5050 g l for mo using the tritium helium 3h 3he method the ages of those samples high in as and mo were determined to be 40 30 and 30 years respectively while all other samples had ages older than 50 years this indicated that mixing between young and old groundwater could be responsible for the high as and mo concentrations a good negative correlation for the whole data set was also observed between the concentration 3h and 18o values which together with hydrogeological modeling confirmed that the increased permeability created by the high density of dsw resulted in flow paths that permitted the perpetual mixing of shallow and deep groundwater the release of the as and mo appeared to be a consequence of changes to the physicochemical conditions in the aquifer either via the introduction of oxygen rich fluids into the ias or the mixing of different fluids in the ias or the introduction of oxygen depleted fluids into the ias while the mobilization of geogenic trace metals is often associated with pumping induced hydraulic gradient changes we found that a certain density of multi aquifer wells can be sufficient to alter hydrologic flow paths and induce the mobilization of geogenic trace metals even in the absence of significant pumping in lithia the dsw effectively increased the local scale permeability of the aquifer causing the mixing of oxygen rich surface and deeper anoxic groundwater across a confining unit because the alteration to the hydrologic flow paths was a consequence of changes to the physical structure of the aquifer system rather than due to pumping the alteration is not easily reversible thus significantly complicating site remediation our results provide a cautionary warning of the risks of undue private dsw development in rapidly growing communities

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

source and distribution of naturally occurring arsenic in groundwater from alberta s southern oil sands regions abstract arsenic as concentrations as high as 179 g l have been observed in shallow groundwater in the alberta s southern oil sand regions the geology of this area of alberta includes a thick cover up to 200 m of unconsolidated glacial deposits with a number of regional interglacial sand and gravel aquifers underlain by marine shale arsenic concentrations observed in 216 unconsolidated sediment samples ranged from 1 and 17 ppm a survey of over 800 water wells sampled for as in the area found that 50 of the wells contained as concentrations exceeding drinking water guidelines of 10 g l higher as concentrations in groundwater were associated with reducing conditions measurements of as speciation from 175 groundwater samples indicate that as iii was the dominant species in 74 of the wells speciation model calculations showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite suggesting that reductive dissolution of fe oxyhydroxides may be the source of some as in groundwater detailed mineralogical characterization of sediment samples collected from two formations revealed the presence of fresh framboidal pyrite in the deeper unoxidized sediments electron microprobe analysis employing wavelength dispersive spectrometry indicated that the framboidal pyrite had variable as content with an average as concentration of 530 ppm reaching up to 1840 ppm in contrast the oxidized sediments did not contain framboidal pyrite but exhibited spheroidal fe oxyhydroxide grains with elevated as concentrations the habit and composition suggest that these fe oxyhydroxide grains in the oxidized sediment were an alteration product of former framboidal pyrite grains x ray absorption near edge spectroscopy xanes indicated that the oxidized sediments are dominated by as v species having spectral features similar to those of goethite or ferrihydrite with adsorbed as suggesting that fe oxyhydroxides are the dominant as carriers xanes spectra collected on unoxidized sediment samples in contrast indicated the presence of a reduced as species as i characteristic of arsenopyrite and arsenian pyrite the results of the mineralogical analyses indicate that the oxidation of framboidal pyrite during weathering may be the source of as released to shallow aquifers in this region

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

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

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

metamorphism and crustal stacking in the north indian plate north pakistan abstract the northern part of the indian plate in north pakistan is composed of a number of large scale crustal nappes each of which are stratigraphically distinct and which were stacked late in the main phase of southeasterly directed thrusting associated with the himalayan event the major nappes recognised in the swat to kaghan area of north pakistan are the besham swat hazara banna lower kaghan and upper kaghan nappes metamorphism was synchronous with early stages of deformation within each nappe the metamorphic grade increases upwards an overall inversion that represents post metamorphic imbrication within individual nappes synchronous with the main phase of nappe stacking rather than a hot iron type inversion as described under the mct in nepal and india as a result of this within nappe imbrication each thrust slice within any particular nappe contains rocks of a higher metamorphic grade than those in the slice below with sharp metamorphic breaks across the imbricating thrusts as well as across the major shears that bound the crustal scale nappes uplift along these imbricating thrusts initiated cooling of the stack k ar mica and hornblende cooling ages imply that much of this uplift was completed by 30 ma or within 20 ma of the collision

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

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

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

vulnerability of deep groundwater in the bengal aquifer system to contamination by arsenic arsenic levels in shallow groundwater in the bengal basin exceed thresholds for safe drinking water groundwater modelling indicates that deep wells that reach safe water below 150 m could remain safe for centuries if used for domestic water only whereas the intensive use of deep groundwater for irrigation could contaminate this resource within decades shallow groundwater the primary water source in the bengal basin contains up to 100 times the world health organization who drinking water guideline of 10 g l 1 arsenic as threatening the health of 70 million people groundwater from a depth greater than 150 m which almost uniformly meets the who guideline has become the preferred alternative source the vulnerability of deep wells to contamination by as is governed by the geometry of induced groundwater flow paths and the geochemical conditions encountered between the shallow and deep regions of the aquifer stratification of flow separates deep groundwater from shallow sources of as in some areas oxidized sediments also protect deep groundwater through the ability of ferric oxyhydroxides to adsorb as basin scale groundwater flow modelling suggests that over large regions deep hand pumped wells for domestic supply may be secure against as invasion for hundreds of years by contrast widespread deep irrigation pumping might effectively eliminate deep groundwater as an as free resource within decades finer scale models incorporating spatial heterogeneity are needed to investigate the security of deep municipal abstraction at specific urban locations

integration of aquifer geology groundwater flow and arsenic distribution in deltaic aquifers a unifying concept groundwater arsenic as presents a public health risk of great magnitude in densely populated asian delta regions most acutely in the bengal basin west bengal india and bangladesh research has focused on the sources mobilisation and heterogeneity of groundwater as but a consistent explanation of as distribution from local to basin scale remains elusive we show for the bengal aquifer system that the numerous discontinuous silt clay layers together with surface topography impose a hierarchical pattern of groundwater flow which constrains as penetration into the aquifer and controls its redistribution towards discharge zones where it is re sequestered to solid phases this is particularly so for the discrete periods of as release to groundwater in the shallow subsurface associated with sea level high stand conditions of quaternary inter glacial periods we propose a hypothesis concerning groundwater flow silt clay layers impose hierarchical groundwater flow patterns constraining arsenic progression siha which links consensus views on the as source and history of sedimentation in the basin to the variety of spatial and depth distributions of groundwater as reported in the literature siha reconciles apparent inconsistencies between independent in some cases contrasting field observations we infer that lithological and topographic controls on groundwater flow inherent to siha apply more generally to deltaic aquifers elsewhere the analysis suggests that groundwater as may persist in the aquifers of asian deltas over thousands of years but in certain regions particularly at deeper levels as will not exceed low background concentrations unless groundwater flow systems are short circuited by excessive pumping

evidence for extension at the willimantic dome connecticut implications for the late paleozoic tectonic evolution of the new england appalachians

inverse geochemical modeling of groundwater evolution with emphasis on arsenic in the mississippi river valley alluvial aquifer arkansas usa summary inverse geochemical modeling phreeqc was used to identify the evolution of groundwater with emphasis on arsenic as release under reducing conditions in the shallow 25 30 m mississippi river valley alluvial aquifer arkansas usa the modeling was based on flow paths defined by high precision 2 cm water level contour map x ray diffraction xrd scanning electron microscopic sem and chemical analysis of boring sediments for minerals and detailed chemical analysis of groundwater along the flow paths potential phases were constrained using general trends in chemical analyses data of groundwater and sediments and saturation indices data minteqa2 of minerals in groundwater modeling results show that calcite halite fluorite fe oxyhydroxide organic matter h 2 s gas were dissolving with mole transfers of 1 40e 03 2 13e 04 4 15e 06 1 25e 01 3 11 and 9 34 respectively along the dominant flow line along the same flow line fes siderite and vivianite were precipitating with mole transfers of 9 34 3 11 and 2 64e 07 respectively cation exchange reactions of ca 2 4 93e 04 mol for na 2 51e 04 mol on exchange sites occurred along the dominant flow line gypsum dissolution reactions were dominant over calcite dissolution in some of the flow lines due to the common ion effect the concentration of as in groundwater ranged from so 4 2 with co precipitation of as into sulfide is the limiting factor controlling dissolved as in groundwater

a generalized regression model of arsenic variations in the shallow groundwater of bangladesh localized studies of arsenic as in bangladesh have reached disparate conclusions regarding the impact of irrigation induced recharge on as concentrations in shallow 50 m below ground level groundwater we construct generalized regression models grms to describe observed spatial variations in as concentrations in shallow groundwater both i nationally and ii regionally within holocene deposits where as concentrations in groundwater are generally high 10 g l 1 at these scales the grms reveal statistically significant inverse associations between observed as concentrations and two covariates 1 hydraulic conductivity of the shallow aquifer and 2 net increase in mean recharge between predeveloped and developed groundwater fed irrigation periods further the grms show that the spatial variation of groundwater as concentrations is well explained by not only surface geology but also statistical interactions i e combined effects between surface geology and mean groundwater recharge thickness of surficial silt and clay and well depth net increases in recharge result from intensive groundwater abstraction for irrigation which induces additional recharge where it is enabled by a permeable surface geology collectively these statistical associations indicate that irrigation induced recharge serves to flush mobile as from shallow groundwater

delineating low arsenic groundwater environments in the bengal aquifer system bangladesh studies within the as affected bengal basin have indicated that low as groundwater can be found in a variety of geological and geomorphological settings the hydrogeological environments that host lowas groundwater may be interpreted within a geological framework determined by the quaternary evolution of the bengal aquifer system bas this provides the basis for delineating the position and extent of shallow low as groundwater low as groundwater in oxidised red bed sediments and deep low as groundwater data available on a national scale allow a preliminary delineation of these low as groundwater environments across bangladesh based on empirical associations of low as groundwater occurrences with topography water table elevation surface sediment lithology geology and the screen depth of deep wells in low as zones

sedimentogenesis and hydrobiogeochemistry of high arsenic late pleistocene holocene aquifer systems geogenic enrichment of arsenic as in groundwater has been a topic of worldwide concern over the past several decades due to the severe health threat to an estimated over one hundred million people despite the genetic diversity and the spatial variability of as concentrations the most common geogenic as enriched groundwater occurs in flat low lying river floodplains and fluvial lacustrine plains within river systems draining the himalaya tectonic movement sedimentological processes and palaeoclimatic optima after the last glacial maximum have created favorable conditions for the formation of high as aquifer systems mainly within the late pleistocene holocene deposits the co deposition of as bearing fe oxides and reactive organic matter within sediments of deltas floodplains or fluvial lacustrine plains forms the premise for as release under reducing geochemical environments however depending on the reactivity of organic matter and as bearing fe oxides as well as the availability of aqueous sulfate geochemical mechanisms controlling as enrichment in the groundwater differ between the coastal floodplains of south and southeast asia and the fluvial lacustrine plains of north and central china the indigenous geomicrobes thriving to adapt to specific aquifer environments may mediate fe s as biogeochemical cycling responsible for as speciation transformation dissolution desorption and hence mobilization hydrologic processes regulated by the local and regional groundwater flow regimes control as transport as well as its spatial distribution special attention must be paid to anthropogenic perturbation from intensive groundwater pumping and land use changes that affect the aquifer hydrological conditions and the biogeochemical processes thus altering groundwater as levels to decipher the patterns of as variation within the aquifers an integrated approach characterizing the major sedimentological hydrobiogeochemical and anthropogenic processes influencing as release and transport needs to be applied to ensure sustainable water supply for domestic and agricultural needs

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

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

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

evidence for extension in the western alpine orogen the contact between the oceanic piemonte and overlying continental sesia units abstract in the alps and other orogens major tectonic contacts must have been established during convergence and overthrusting what is less clear is how strongly these contacts were modified as the orogen evolved during alpine collision continental crust of the overriding plate the sesia unit is inferred to have been thrust northwest over oceanic material of the piemonte unit both units preserve in parts eclogite facies metamorphism however all structures along this contact in our study area indicate that final movement was southeast directed with no clear evidence of the kinematics of earlier movement these structures include se verging folds se directed shear bands and larger normal faults downthrowing to the southeast it is inferred that there was a se directed shear regime below the contact which although locally buckling and shortening earlier lithological contacts was dominantly extensional with respect to pre existing tectonic layering the lower limit of this shear regime has not been identified although se directed shear is common throughout the upper greenschist facies part of the piemonte unit we show that this shear passes beneath the sesia zone and does not re emerge it is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface it may have contributed to the unroofing of eclogite facies rocks in the lower part of the piemonte unit although additional timing data are required to clarify this nearby se directed shears buckle and imbricate earlier layering within the piemonte unit these are normally identified as backthrusts yet they do not appear to breach the base of the sesia unit and might merge with the extensional shear even though these structures shorten layering they too could have been extensional relative to the earth s surface the significance of alpine backthrusts should be reappraised in this context this study indicates that hinterland directed extension could have been an important phenomenon during alpine evolution

a tectonic model for the late paleozoic of southeastern new england hornblende and biotite sup 40 ar sup 39 ar age spectra from rocks in south central connecticut help define a permian triassic cooling curve for the area together with petrologic and structural information a time temperature pressure strain path is established similar data for the narragansett basin in rhode island and massachusetts allow correlation of the late paleozoic histories of the two areas together these data suggest that in the late paleozoic south central new england was part of a fold thrust belt and the narragansett basin was a retroarc foreland basin nw se compression during the final assembly of pangaea resulted in se directed thrusting causing the development of clastic wedges in adjacent rhode island and massachusetts in the late carboniferous early permian a clockwise rotation of this deformation from nw to nne led to northward underthrusting and concomitant uplift of both eastern connecticut and rhode island in the permian and triassic

recharge of low arsenic aquifers tapped by community wells in araihazar bangladesh inferred from environmental isotopes more than 100 000 community wells have been installed in the 150 300 m depth range throughout bangladesh over the past decade to provide low arsenic drinking water 90 m systematic relationships between 18o 16o 2h 1h 13c 12c and 14c 12c and variations in noble gas temperatures suggest that changes in monsoon intensity and vegetation cover occurred at the onset of the holocene when the sampled water was recharged thus the deeper low as aquifers remain relatively isolated from the shallow high as aquifer

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

monsoonal influence on variation of hydrochemistry and isotopic signatures implications for associated arsenic release in groundwater summary the present study examines the groundwater and surface water geochemistry of two different geomorphic domains within the chakdaha block west bengal in an attempt to decipher potential influences of groundwater abstraction on the hydrochemical evolution of the aquifer the effect of different water inputs monsoon rain irrigation and downward percolation from surface water impoundments to the groundwater system and concomitant as release a low land flood plain and a natural levee have been selected for this purpose although the stable isotopic signatures of oxygen 18o and hydrogen 2h are largely controlled by local precipitation the isotopic composition falls sub parallel to the global meteoric water line gmwl the cl br molar ratio indicates vertical recharge into the wells within the flood plain area especially during the post monsoon season while influences of both evaporation and vertical mixing are visible within the natural levee wells increase in mean doc concentrations from 1 33 to 6 29 mg l from pre to post monsoon season indicates possible inflow of organic carbon to the aquifer during the monsoonal recharge concomitant increase in ast fe ii and hco3 highlights a possible initial episode of reductive dissolution of as rich fe oxyhydroxides the subsequent sharp increase in the mean as iii proportions by 223 particularly in the flood plain samples during the post monsoon season which is accompanied by a slight increase in mean ast 7 may refer to anaerobic microbial degradation of doc coupled with the reduction of as v to as iii without triggering additional as release from the aquifer sediments

thrust sequences and the significance of low angle out of sequence faults in the northernmost moine nappe and moine thrust zone nw scotland in common with most thrust belts the moine thrust zone scotland preserves a foreland propagating sequence of thrusting however significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial remapping of the northernmost moine thrust zone has defined a thick belt of mylonites in the centre of which lies the ductile wnw directed moine thrust the base of the mylonite belt however is marked by a regionally significant low angle west directed brittle fault here termed the lochan riabhach thrust that truncates thrust related structures in its footwall comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at durness indicates that the lochan riabhach thrust cuts down section in its footwall and therefore appears to be extensional locally it is unequivocally breached by underlying imbricate thrusts we prefer to interpret the lochan riabhach thrust as an extensional fault that formed during a hiatus in thrust stacking we relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common although possibly unrecognized features in many other thrust belts

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