late quaternary sedimentary environments and landforms in the ganges delta abstract late quaternary sediments deposited since the lowest stand of sea level during the last glacial maximum in the ganges delta and its surrounding region were classified into five units according to their sedimentary facies these are the lowest unit of sandy gravels the lower unit of sand with a few gravels and the middle upper and uppermost units consisting mainly of sand and silt with occasional peat layers the contact between the lower and middle units is fairly sharp and the upper part of the lower unit is oxidized in some places this suggests a period of subaerial exposure after the deposition of the lower unit during the low stand of sea level about the time of the last glacial maximum rivers in the ganges delta and its surrounding region dissected the surfaces of the region and deposited the lowest unit on the valley floors the lower unit was deposited over the lowest unit during an early stage of the post glacial transgression between ca 12 000 yr bp and 10 000 yr bp the surface of the delta was slightly dissected and the top of the lower unit was weathered and oxidized following this period the middle and upper units of alluvial or deltaic sand and silt with occasional peat were deposited the coastline in the early holocene retreated towards the central part of the present ganges delta in the middle and late holocene the silt and clay with occasional peat layers of the uppermost unit indicate that the lowland gradually became marshy and poorly drained as the rate of transgression became slower

the late neogene 87sr 86sr record of lowland himalayan rivers fossil shells and paleosol carbonate from ancestral himalayan river deposits provide a 87 sr 86 sr record of lowland himalayan river water during the late neogene reconstructed 87 sr 86 sr river values increased sharply in the late miocene probably marking the beginning of exhumation of high 87 sr 86 sr metalimestones more in the central than in the western himalayas these results imply that the marine 87 sr 86 sr record may not be a proxy for silicate weathering or consumption of atmospheric co 2 resulting from that weathering

seasonal evolution of titan s stratosphere during the cassini mission titan s stratosphere exhibits significant seasonal changes including break up and formation of polar vortices here we present the first analysis of mid infrared mapping observations from cassini s composite infrared spectrometer cirs to cover the entire mission ls 293 93 2004 2017 mid northern winter to northern summer solstice the north polar winter vortex persisted well after equinox starting break up around ls 60 and fully dissipating by ls 90 absence of enriched polar air spreading to lower latitudes suggests large scale circulation changes and photochemistry control chemical evolution during vortex break up south polar vortex formation commenced soon after equinox and by ls 60 was more enriched in trace gases than the northern mid winter vortex and had temperatures 20 k colder this suggests early winter and mid winter vortices are dominated by different processes radiative cooling and subsidence induced adiabatic heating respectively by the end of the mission ls 93 south polar conditions were approaching those observed in the north at ls 293 implying seasonal symmetry in titan s vortices

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

redox control of arsenic mobilization in bangladesh groundwater abstract detailed hydrochemical measurements 34sso4 and 3h analyses were performed on 37 groundwater samples collected during february 1999 january and march 2000 from 6 locations in eastern and southeastern bangladesh to examine redox processes that lead to as mobilization in groundwater the study sites were chosen based on available nation wide as surveys to span the entire spectrum of as concentrations in bangladesh groundwater and to represent 3 of 5 major geological units of the ganges brahmaputra delta uplifted pleistocene terrace fluvial flood plain and delta plain arsenic was found to be mobilized under fe reducing conditions in shallow aquifers 50 g l or 0 7 m were always accompanied by high dissolved hco3 4 mm and were close to saturation with respect to calcite groundwater enriched in as 200 800 g l or 2 7 10 7 m and phosphate 30 100 m but relatively low in dissolved fe 5 40 m probably resulted from re oxidation of reducing as and fe enriched water this history was deduced from isotopic signatures of 34sso4 and 3h2o 3h to delineate the nature of redox changes for some of the reducing groundwaters in contrast as is not mobilized in presumed pleistocene aquifers both shallow 30 60 m and deep 150 270 m because conditions were not reducing enough due to lack of sufficient o2 demand

tracing organic matter composition and distribution and its role on arsenic release in shallow cambodian groundwaters biogeochemical processes that utilize dissolved organic carbon are widely thought to be responsible for the liberation of arsenic from sediments to shallow groundwater in south and southeast asia the accumulation of this known carcinogen to hazardously high concentrations has occurred in the primary source of drinking water in large parts of densely populated countries in this region both surface and sedimentary sources of organic matter have been suggested to contribute dissolved organic carbon in these aquifers however identification of the source of organic carbon responsible for driving arsenic release remains enigmatic and even controversial here we provide the most extensive interrogation to date of the isotopic signature of ground and surface waters at a known arsenic hotspot in cambodia we present tritium and radiocarbon data that demonstrates that recharge through ponds and or clay windows can transport young surface derived organic matter into groundwater to depths of 44 m under natural flow conditions young organic matter dominates the dissolved organic carbon pool in groundwater that is in close proximity to these surface water sources and we suggest this is likely a regional relationship in locations distal to surface water contact dissolved organic carbon represents a mixture of both young surface and older sedimentary derived organic matter ground surface water interaction therefore strongly influences the average dissolved organic carbon age and how this is distributed spatially across the field site arsenic mobilization rates appear to be controlled by the age of dissolved organic matter present in these groundwaters arsenic concentrations in shallow groundwaters 20 m groundwaters we suggest that while the rate of arsenic release is greatest in shallow aquifer sediments arsenic release also occurs in deeper aquifer sediments and as such remains an important process in controlling the spatial distribution of arsenic in the groundwaters of se asia our findings suggest that any anthropogenic activities that alter the source of groundwater recharge or the timescales over which recharge takes place may also drive changes in the natural composition of dissolved organic carbon in these groundwaters such changes have the potential to influence both the spatial and temporal evolution of the current groundwater arsenic hazard in this region

the strontium isotopic budget of himalayan rivers in nepal and bangladesh abstract himalayan rivers have very unusual sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic we present sr o and c isotopic data from river and rain water bedload and bedrock samples for the western and central nepal himalaya and bangladesh including the monsoon season central himalayan rivers receive sr from several sources carbonate and clastic tethyan sediments high himalayan crystalline hhc gneisses and granitoids with minor marbles carbonates and metasediments of the lesser himalaya lh and miocene recent foreland basin sediment from the siwaliks group and the modern flood plain in the tethyan himalaya rivers have dissolved sr 6 mol l and 87sr 86sr 0 717 with a large contribution from moderately radiogenic carbonate rivers draining hhc gneisses are very dilute with sr 0 2 mol l and 87sr 86sr 0 74 lesser himalayan streams also have low sr 0 4 mol l and are highly radiogenic 87sr 86sr 0 78 highly radiogenic carbonates of the lh do not contribute significantly to the sr budget because they are sparse and have very low sr in large rivers exiting the himalaya sr systematics can be modeled as a mixture between tethyan rivers where slightly radiogenic carbonates mean 87sr 86sr 0 715 are the main source of sr and lesser himalaya waters where extremely radiogenic silicates 0 8 are the main source of sr hhc waters are less important because of their low sr rivers draining the siwaliks foreland basin sediments have sr 4 mol l and 87sr 86sr 0 725 weathering of silicates in the siwaliks and the flood plain results in a probably significant radiogenic 0 72 0 74 input to the ganges and brahmaputra g b but quantification of this flux is limited by uncertainties in the hydrologic budget the g b in bangladesh show strong seasonal variability with low sr and high 87sr 86sr during the monsoon sr in the brahmaputra ranges from 0 9 mol l and 0 722 in march to 0 3 mol l and 0 741 in august we estimate the seasonally weighted flux from the g b to be 6 5 108 mol yr with 87sr 86sr 0 7295

hydrogeochemical contrasts between low and high arsenic groundwater and its implications for arsenic mobilization in shallow aquifers of the northern yinchuan basin p r china summary little is known about hydrogeochemical contrasts between low and high as groundwaters and their connection to as mobilization in the yinchuan basin investigations were carried out to evaluate as distribution and geochemical processes for as mobilization in three regions including piedmont proluvial fans pa dry farmland df and paddy farmland pf ninety two groundwater samples 4 surface water samples and 66 sediments samples were collected and analyzed for chemical and isotopic components results show that low as groundwater is generally found in pa however high as concentrations up to 105 g l 1 are mainly observed in groundwaters from df and pf which are associated with reducing conditions high as groundwater is characterized by high concentrations of nh4 dissolved mn dissolved fe and fe ii and low concentrations of no3 and so42 the intensive irrigation in pf recharges the aquifers by vertical infiltration of the diverted yellow river water and leads to the higher redox potentials and the lower dissolved as in comparison with those in df environmental isotopes 18o and d show that evaporation due to the intensive irrigation plays a minor role in as enrichment the positive correlation between as and dissolved fe suggests that groundwater as would result from the reductive dissolution of fe oxides besides dissolved p may be involved in competing with as for binding sites on fe oxide minerals sediment as ranges between 3 94 and 75 2 mg kg 1 hcl leached as accounts for 60 of total as in the sediments while h3po4 leached as accounts for 5 depth matched samples show a good correlation between dissolved as and h3po4 leached as in sediments arsenic distribution coefficient kd calculated from h3po4 leached as and dissolved as ranges between 5 08 and 17 3 cm3 g 1 which generally depends on groundwater redox potentials in reducing conditions low values are found with as being preferentially partitioned into groundwater

mineralogical profiling of alluvial sediments from arsenic affected ganges brahmaputra floodplain in central bangladesh abstract mineral assemblages heavy and light fractions and sedimentological characteristics of the quaternary alluvial aquifers were examined in the central bengal basin where as concentrations in groundwater are highly variable in space but generally decrease downward chemical compositions of sediment samples from two vertical core profiles 2 150 m below ground level bgl were analyzed along with groundwater in moderately as enriched aquifers in central bangladesh manikganj district and the as mobilization process in the alluvial aquifer is described heavy minerals such as biotite magnetite amphibole apatite and authigenic goethite are abundant at shallow 4 reduction mediated by so 4 reducing bacteria in manikganj groundwater is limited in contrast to the southeastern bengal basin where precipitation of arsenian pyrite is thought to sequester as from groundwater

sources and controls for elevated arsenic concentrations in groundwater of datong basin northern china an integrated study aiming to the distribution and occurrence of arsenic as in groundwater at datong basin northern china has been carried out the elevated as concentration was observed mainly in the low lying parts of the basin and showed high spatial variation the groundwater as concentrations ranged from 1 24 to 1058 g l which mainly originated from high as lacustrine deposits the iron bearing minerals are the major carriers of arsenic in the aquifers high arsenic groundwater is mainly nahco3 type and characterized by higher ph high na low ca2 so4 2 and no3 along with moderate alkalinity the alkaline and reducing subsurface environment facilitated leaching of as from the sediments while excessive alkalinity could restrain as enrichment in groundwater the lowering of alkalinity by calcite precipitation caused siderite dissolution due to common ion effect and facilitated as enrichment in the groundwater the evaporation condensation is not a favorable process for as enrichment

chemical and strontium oxygen and carbon isotopic compositions of carbonates from the lesser himalaya implications to the strontium isotope composition of the source waters of the ganga ghaghara and the indus rivers samples of precambrian carbonate mostly dolomite outcrops collected across the lesser himalaya have been analysed for their mineralogy chemical composition and isotope ratios of sr o and c to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the ganga ghaghara and the indus their sr concentrations range from 20 to 363 ppm 18opdb 1 4 to 12 8 and mn 11 2036 ppm the petrography of the samples their low sr concentrations and wide range of 18o values are suggestive of their postdepositional alteration the 87sr 86sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0 7064 to 0 8935 and are generally more radiogenic than that of contemporaneous seawater comparison of the 87sr 86sr and sr ca ratios among the carbonates and silicates from the lesser himalaya and the source waters of the ganga ghaghara and the indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates an upper limit of carbonate sr in the various source waters is calculated to be between 6 and 43 assuming that all the ca in the rivers is of carbonate origin the results show that on the average weathering of the precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters however they can be a dominant supplier of radiogenic sr to some rivers on a regional scale the silicate sr component in some of the source waters of the ganga bhagirathi bhilangna alaknanda and ganga ghaghara kali and sarju and the indus sutlej was calculated from the ca na sr na ratios and strontium isotope compositions of these rivers and the silicate endmember these calculations suggest that 33 89 of sr in the bhagirathi bhilangna alaknanda ganga and sarju rivers is of silicate origin whereas in the kali and the sutlej it is much lower only 8 the remaining sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites this study underscores the importance of weathering of silicates carbonates and evaporites in contributing to the sr mass balance and 87sr 86sr of the source waters of the ganga ghaghara and the indus the present day silicate and carbonate sr contributions to the sr budget of the rivers vary considerably but among the major source waters of the ganga silicate sr exerts a more dominant control on their sr abundance and 87sr 86sr

trace elements in streambed sediments of floodplains consequences for water management measures the occurrence of trace metals was studied at the interface between groundwater and the drainage system of a large floodplain in ne germany the oderbruch region depending on the predominant hydraulic connectivity between groundwater and the drainage channels the geochemical environment creates a high variability in the accumulation of fe mn cd zn cu and as the mobility of the trace metals depends on spatial variable redox transition zones which act as geochemical barriers between the anaerobe aquifer and the oxic surface waters in drainage ditches with high exchange flow between groundwater and surface water the transition zone is small and unstable with a low retention potential for trace metals decreasing hydraulic gradients and respectively decreasing base flow cause the change for an extensive transition zone with increasing trace metal accumulation in the channel sediments the accumulation is mainly controlled by oxidation and degassing of co2 in the streambed sediments of channels which periodically run dry an effective chemical barrier can be observed this fe dominated oxic horizon controls the accumulation of mn cu as zn cd which are mainly associated with fresh amorphous fe oxyhydroxides the chemical barriers can be instable and reversible therefore water management decisions are discussed which stabilize the barriers by controlling aquifer channel exchange rates channel oxygen content and surface water levels

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

holocene and modern sediment budgets for the ganges brahmaputra river system evidence for highstand dispersal to flood plain shelf and deep sea depocenters the partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development however few quantitative sediment budgets that encompass entire dispersal systems exist for the ganges brahmaputra river system sediment discharge is estimated to be 10 9 t yr at gauging stations 300 km inland of the coast but little has been known of the downstream fate of this material geochronological geophysical and stratigraphic investigations of the lowland flood plain delta plain and shelf help to delineate the extent of holocene fill and allow calculation of a first order sediment budget results reveal that 1500 10 9 m 3 of sediment fill has been sequestered within the flood plain and delta plain since ca 7000 yr b p or about one third of the annual discharge the remaining load appears to be apportioned between the prograding subaqueous delta 1970 10 9 m 3 and transport to the deep sea bengal fan via a nearshore canyon modern 100 yr budget estimates based on short term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters the roughly equal partitioning of sediment among flood plain shelf and deep sea settings reflects the respective influence of an inland tectonic basin a wide shelf and a deeply incised canyon system the findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems furthermore results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations

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

distribution of arsenic and other trace elements in the holocene sediments of the meghna river delta bangladesh geochemical study of the holocene sediments of the meghna river delta chandpur bangladesh was conducted to investigate the distribution of arsenic and related trace and major elements the work carried out includes analyses of core sediments and provenance study by rare earth element ree analysis results showed that the cores pass downward from silty clays and clays into fine to medium sands the uppermost 3 m of the core sediments are oxidized average oxidation reduction potential orp 230 mv and the orp values gradually become negative with depths 45 to 170 mv indicating anoxic conditions prevail in the meghna sediments the ree patterns of all lithotypes in the study areas are similar and are comparable to the average upper continental crust arsenic and other trace elements pb zn cu ni and cr have greater concentrations in the silts and clays compared to those in the sands positive correlation between as and fe was found in the sediments indicating as may be adsorbed on fe oxides in aquifer sediments

mobility of arsenic in the sub surface environment an integrated hydrogeochemical study and sorption model of the sandy aquifer materials groundwater and aquifer materials have been characterized geochemically at a field site located in the chakdaha municipality of west bengal india sorption experiments were also carried out on a sandy aquifer material to understand the mobility of arsenic as in the sub surface environments the result shows that the areas associated with high groundwater as mean 1 8 m is typically associated with low eh mean 129 mv and high fe mean 0 11 mm where fe2 fe oh 3 couple is controlling groundwater redox potential analysis of the aquifer material total concentrations showed the dominance of as range 8 9 22 mg kg 1 fe range 3 0 9 7 as fe2o3 and mn range 0 05 0 18 as mno in the silt clay rich sediments whereas fine medium sand rich sediment contains considerably lower amount of as 8 1 mg kg 1 fe range 1 6 3 9 as fe2o3 and mn range 0 02 0 08 as mno the acid extractable as do not correlate with ascorbate extractable fe oxyhydroxide however fe oxyhydroxide is generally high in the sediments from low groundwater as areas chemical speciation computations indicated fe ii ca ii mg ii and mn ii to be at equilibrium with respect to calcite dolomite and rhodochrosite or slightly over saturated with respect to siderite these carbonate minerals may therefore participate to the as immobilization the measured total organic carbon 1 and groundwater temperature 26 32 c coupled with sorption studies strongly favors microbially mediated fe iii oxyhydroxide reduction as the dominant mechanism for the release of as in the groundwater oscillations of as mn and fe concentrations with depth reflected pco2 oscillations consecutive to microbial respiration intensity

redox characteristics and zonation of arsenic affected multi layers aquifers in the choushui river alluvial fan taiwan summary in the light of the redox characteristics of groundwater in the arsenic affected multi layers aquifers in the choushui river alluvial fan an investigation on major redox sensitive species were carried out to explore their spatial patterns the controlling factors and the redox zonations factor analysis was applied to 16 hydrochemical parameters of 152 groundwater samples five factors including seawater salinization arsenic pollutant nitrate contamination alkaline and fe mn accounted for 86 4 of the total variance identified among them arsenic pollutant nitrate contamination and fe mn were redox sensitive factors arsenic pollutant factor has strong loading on as and toc moderate loading on nh 4 and low loading on alkalinity and fe these hydrochemical parameters closely relate to the reductive dissolution processes of arsenic release to groundwater discriminant analysis was adopted to delineate three redox zones oxidative transitional and reductive zones in 4 layers aquifers and yielded 90 4 correctness on groundwater quality data the spatial distributions of as and nh 4 were similar but showed a reverse pattern with no 3 arsenic and nh 4 were mainly distributed in the reductive zone whereas no 3 was majorly distributed in the oxidative zone suggesting the denitrification is partially responsible to the reductive dissolution of as in the groundwater

arsenic in groundwater testing pollution mechanisms for sedimentary aquifers in bangladesh in the deltaic plain of the ganges meghna brahmaputra rivers arsenic concentrations in groundwater commonly exceed regulatory limits 50 m gl 21 because feooh is microbially reduced and releases its sorbed load of arsenic to groundwater neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution the most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat concentrations of ammonium up to 23 mg l 21 come from microbial fermentation of buried peat and organic waste in latrines concentrations of phosphorus of up to 5 mg l 21 come from the release of sorbed phosphorus when feooh is reductively dissolved and from degradation of peat and organic waste from latrines calcium and barium in groundwater come from dissolution of detrital and possibly pedogenic carbonate while magnesium is supplied by both carbonate dissolution and weathering of mica the 87 sr 86 sr values of dissolved strontium define a two component mixing trend between monsoonal rainfall 0 711 6 0 001 and detrital carbonate 0 735

adsorption and desorption of arsenic to aquifer sediment on the red river floodplain at nam du vietnam abstract the adsorption of arsenic onto aquifer sediment from the red river floodplain vietnam was determined in a series of batch experiments due to water supply pumping river water infiltrates into the aquifer at the field site and has leached the uppermost aquifer sediments the leached sediments remain anoxic but contain little reactive arsenic and iron and are used in our experiments the adsorption and desorption experiments were carried out by addition or removal of arsenic from the aqueous phase in sediment suspensions under strictly anoxic conditions also the effects of hco3 fe ii po4 and si on arsenic adsorption were explored the results show much stronger adsorption of as v as compared to as iii full reversibility for as iii adsorption and less so for as v the presence or absence of hco3 did not influence arsenic adsorption fe ii enhanced as v sorption but did not influence the adsorption of as iii in any way during simultaneous adsorption of as iii and fe ii as iii was found to be fully desorbable while fe ii was completely irreversibly adsorbed and clearly the two sorption processes are uncoupled phosphate was the only solute that significantly could displace as iii from the sediment surface compiling literature data on arsenic adsorption to aquifer sediment in vietnam and bangladesh revealed as iii isotherms to be almost identical regardless of the nature of the sediment or the site of sampling in contrast there was a large variation in as v adsorption isotherms between studies a tentative conclusion is that as iii and as v are not adsorbing onto the same sediment surface sites the adsorption behavior of arsenic onto aquifer sediments and synthetic fe oxides is compared particularly the much stronger adsorption of as v than of as iii onto red river as well as on most bangladesh aquifer sediments indicates that the perception that arsenic phosphate and other species compete for the same surface sites of iron oxides in sediments with properties similar to those of for example a synthetic goethite probably is not correct a simple two component langmuir adsorption model was constructed to quantitatively describe the reactive transport of as iii and po4 in the aquifer

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

decoupling of as and fe release to bangladesh groundwater under reducing conditions part i evidence from sediment profiles this study reexamines the notion that extensive as mobilization in anoxic groundwater of bangladesh is intimately linked to the dissolution of fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater as concentrations were known to span a wide range the sediment properties that were measured include 1 the proportion of fe ii in the fe fraction leached in hot 1 2 n hcl 2 diffuse spectral reflectance and 3 magnetic susceptibility in parallel with local concentrations of dissolved as ranging from 5t o 600g l fe ii fe ratios in shallow gray holocene sands tended to gradually increase with depth from values of 0 3 to 0 5 to up to 0 9 in deeper orange aquifers of presumed pleistocene age that were separated from shallow sands by a clay layer and contained 5 g l dissolved as leachable fe ii fe ratios averaged 0 2 there was no consistent relation between sediment fe ii fe and dissolved fe concentrations in groundwater in nearby wells the reflectance measurements indicate a systematic linear relation r 2 of 0 66 n 151 between the first derivative transform of the reflectance at 520 nm and fe ii fe the magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 x 10 9 m 3 kg si and was linearly related r 2 of 0 75 n 29 to the concentrations of minerals that could be magnetically separated 0 03 to 0 79 dry weight no systematic depth trends in magnetic susceptibility were observed within the shallow sands although the susceptibility of deeper low as aquifers was low up to 200 10 9 m 3 kg si

hydrochemistry on the yilgarn block western australia ferrolysis and mineralisation in acidic brines abstract in the southeastern part of western australia internal drainage and a semi arid climate have resulted in extensive development of playas the brines beneath the playas wedge outwards and force local infiltration to crop out at playa margins as discharge zones the discharging water contains dissolved fe2 oxidation and hydrolysis ferrolysis of which generates very acidic water ph 2 8 at least some of the alkalinity generated by the original reduction of fe3 has been sequestered as subsurface calcrete within local soils the acidity causes the dissolution of kaolinite which is abundant in playa sediments and as a thick capping on weathered bedrock giving water with a low ph and high concentrations of dissolved al and si evaporation and mixing with playa brines containing high concentrations of k and so4 causes alunite to precipitate as ph rises from a minimum of 2 8 at chlorinities of 1 to 2 m to 6 at chlorinities of 4 8 m the alunite deposits formed by this process are many orders of magnitude larger than occurrences elsewhere in the world the large scale mobilisation of si during this process may be one mechanism for the formation of silica cemented rocks silcrete that are widespread in the australian regolith

geochemistry and arsenic behaviour in groundwater resources of the pannonian basin hungary and romania abstract groundwater resources in the pannonian basin hungary romania croatia and serbia are known to contain elevated naturally occurring as published estimates suggest nearly 500 000 people are exposed to levels greater than the eu maximum admissible concentration of 10 g l in their drinking water making it the largest area so affected in europe in this study a variety of groundwaters were collected from romania and hungary to elucidate the general geochemistry and identify processes controlling as behaviour concentrations ranged from so 4 2 reduction containing low as levels so 4 2 ongoing so 4 2 reduction will release s2 removing as from solution either by the formation of as sulfides or from sorption onto fe sulfide phases in methanogenic waters as released by reductive dissolution is not removed from solution and can rise to the high levels observed levels of organic c are thought to be the ultimate control on the redox conditions in these 2 groups high levels of organic c as found in group 1 would quickly exhaust any so 4 2 present in the waters driving the system to methanogenesis and subsequent high levels of as group 2 has much lower concentrations of organic c and so so 4 2 is not exhausted therefore as levels in waters of the pannonian basin are controlled not by release but by retention mechanisms ultimately controlled by levels of toc and so 4 2 in the waters d and 18o analysis showed that groundwaters containing elevated as dated mostly from the last ice age and are sourced from late pliocene to quaternary aquifers the importance of toc and retention capabilities of so 4 2 reduction have only previously been suggested for recent holocene sediments and groundwater most notably those in se asia as these are the most likely to contain the right combination of factors to drive the system to a redox situation leading to high aqueous as concentrations in contrast it is shown here that a much older system containing as bearing fe oxides also has the potential to produce elevated levels of as if the toc is suitable for the microbial population to drive the system to the correct redox situation and so 4 2 is either absent or wholly consumed

high resolution la icp ms trace element mapping of igneous minerals in search of magma histories we report experiments on optimisation of la icp ms mapping as a tool for visualising and quantifying internal structure of trace element concentration in igneous minerals the experimental design was refined with maps on clinopyroxene and amphibole macrocrysts mainly antecrysts from a porphyritic lamprophyre in ne spain as well as on a high precision metal wire grid in terms of spatial resolution we demonstrate with scanning electron microscope and white light interferometry that a full ablation removes between 0 4 and 0 7 mu m of material depending on ablation parameters maps were produced with square laser beam spots of 12 and 24 mu m it was found that complexities can be resolved in the sample even though they are smaller than the beam diameter e g 7 10 mu m discontinuities using 12 mu m laser beam resolution in x and y was found to be identical probably reflecting the fast washout of the two volume ablation cell and the short total dwell time of the analyte menu selected due to the excellent stage reproducibility and the limited ablation depth it is feasible to re ablate the identical map area many times employing different instrument parameters or analyte menus on the magmatic crystals la icp ms maps define very sharp compositional zoning in trace elements highlighting complex crystallisation histories where normal magmatic fractionation is not the only process events of mafic recharge are easily recognised as zones enriched in compatible metals such as cr ni or sc further trace element maps reveal complexities in mineral zoning previously undetectable with petrography or major element data these include resorbed primitive cores and oscillatory zoning within apparently homogeneous mineral zones therefore la icp ms mapping opens a new window of opportunity for analysis of magmatic histories the wide combination of instrumental parameters such as laser beam size scan speed and repetition rate make it possible to carry out experiments at different levels of detail we recommend a two step approach to mapping the initial step involves rapid maps to gain an overview of potential complexities in the sample this enhances representativeness of the analysed materials as a large number of crystals and trace elements can be tested in little time subsequently detailed maps can be carried out on areas of interest an additional functionality is to create 1d profiles from 2d maps the potential of the technique to unveil compositional complexities efficiently and at greater detail than traditional microanalysis will help to improve our understanding of processes in the magmatic environment and beyond c 2015 elsevier b v all rights reserved