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

optical backscattering is correlated with phytoplankton carbon across the atlantic ocean 1 phytoplankton are an important component of the oceanic carbon cycle yet due to methodological constraints the carbon biomass of phytoplankton is poorly characterized to address this limitation we have explored the bio optical relationship between in situ measurements of the particle backscattering coefficient at 470 nm bbp 470 and the phytoplankton carbon concentration for cells with diameter less than 20 m cf we found a significant relationship between bbp 470 and cf for atlantic oceanic waters with chlorophyll a concentrations less than 0 4 mg m 3 or bbp 470 0 003 m 1 this relationship could be used to estimate cf from data collected by in situ autonomous platforms and from remote sensing of ocean color

reactivity and speciation of mineral associated arsenic in seasonal and permanent wetlands of the mekong delta abstract millions of people in the deltaic regions of s se asia regularly consume as contaminated groundwater arsenic persists within these groundwaters despite being flushed by several pore volumes of recharge indicative of an in situ solid phase source despite the importance of this solid phase source the identity and reactivity of the minerals hosting and releasing as in the deltaic aquifers remain poorly constrained here we seek to define the chemistry and reactivity of as and its host minerals in aquifer sediments recharged by both seasonally saturated and permanent wetlands in the mekong delta of cambodia sediment cores were retrieved to depths of up to 30 m selective extractions on near surface sediments to depths of 4 6 m indicate that more than a third 27 50 of the fe in the profile is reducible citrate bicarbonate dithionite extractable nominally consisting of fe iii oxides the fe iii oxide fraction was approximately the same under seasonal wetlands mean 39 compared with sediments from beneath permanent wetlands mean 37 correspondingly fe k edge extended x ray absorption fine structure exafs spectra analysis indicates that the fe mineral phases are predominantly goethite and hematite 30 50 analysis of solid phase as speciation by x ray absorption near edge structure xanes spectroscopy showed arsenate in the shallowest sediments upper 1 and 4 m of seasonal and permanent wetlands and arsenite and or as sulfides as the dominant phases in the deeper sediments the reducible as fraction ranged from 17 to 42 of the total as at both sites the exception was found in a peat layer at 6 m beneath the seasonal wetland in which only 1 of the total as was reducible arsenian pyrite was the predominant form of as in the peat thus a pool of as hosted by reducible mineral phases persists in sediments below both the seasonal and permanently saturated wetland sites

optical backscattering by particles in arctic seawater and relationships to particle mass concentration size distribution and bulk composition the magnitude and spectral shape of the optical backscattering coefficient of particles b bp lambda is being increasingly used to infer information about the particles present in seawater relationships between b bp and particle properties in the arctic are poorly documented and may differ from other oceanic regions which contribute the majority of data used to develop and parameterize optical models we utilize recent field measurements from the chukchi and beaufort seas to examine relationships between the spectral backscattering coefficient of particles in seawater and the mass concentration bulk composition and size distribution of the suspended particle assemblage the particle backscattering coefficient spanned six orders of magnitude from the relatively clear waters of the beaufort sea to extremely turbid waters on the mackenzie shelf this coefficient was highly correlated with the mass concentration of particles and to a lesser extent with other measures of concentration such as particulate organic carbon or chlorophyll a increased backscattering and high mass specific b bp lambda was associated with mineral rich assemblages that tended to exhibit steeper size distributions while reduced backscattering was associated with organic dominated assemblages having a greater contribution of large particles our results suggest that algorithms which employ composition specific relationships can lead to improved estimates of particle mass concentration from backscattering measurements in contrast to theoretical models however we observe no clear relationship between the spectral slope of b bp lambda and the slope of the particle size distribution in this environment

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

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

mineralogy of the ganges and brahmaputra rivers implications for river switching and late quaternary climate change late quaternary sediments of the bengal basin contain a history of river switching and climate change as revealed from sand and clay size mineralogy of boreholes and modern riverbed grabs epidote to garnet ratios e g in sand fraction sediments are diagnostic of source with high 1 e g indicating brahmaputra provenance and low 1 e g indicating ganges provenance in the clay fraction of surficial sediments smectite is diagnostic with high values 39 in the ganges and low values 3 in the brahmaputra in contrast the brahmaputra contains more kaolinite 29 vs 18 more illite 63 vs 41 and more chlorite 3 vs 1 than the ganges analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the holocene extended periods of mixed river inputs appear to be isolated to the early holocene suggesting rapidly migrating braided channels during sea level lowstand tectonically driven accommodation in the sylhet basin may have contributed to the favored easterly course of the brahmaputra during much of the holocene relative abundances of illite and chlorite ic vs smectite and kaolinite sk record varying degrees of physical and chemical weathering respectively high ic values in early post glacial deposits suggest a dominance of physical weathering at that time however a general increase in sk concentrations throughout the holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions notably a peak in sk concentrations that corresponds to an early holocene warm period 10 7 ka suggests that weathering patterns in the catchment respond quickly to climatic shifts

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

delineation of arsenic contaminated zones in bengal delta india a geographic information system and fractal approach groundwater extracted from shallow aquifers in the bengal delta is contaminated with arsenic the fluviodeltaic process that creates aquifers ironically extends its role to also contaminating them with arsenic the arsenic distribution maps show a spatial association of arsenic contaminated wells with palaeo cut off abandoned channels weight on evidences analysis indicates that the zones of contamination occur around palaeo channels within a corridor of 500 700 m that contains most of the contaminated wells these corridors are interpreted to be the zone of channel shifting contaminated wells represent point fractal geometry that can be separated into isolated points and clusters clusters occur within the zone of channel shifting as obtained by weight on evidences analysis isolated points occur within floodplain or back swamp areas clusters and isolated point fractals are interpreted to reflect the process of arsenic release into groundwater the migration of biomass within the permeable sandy domain of channel deposits is proposed to be the predominant process in generating clusters the isolated points represent restricted biomass spreading in less permeable clay silt dominated floodplains

observations of pigment and particle distributions in the western north atlantic from an autonomous float and ocean color satellite profiling floats with optical sensors can provide important complementary data to satellite ocean color determinations by providing information about the vertical structure of ocean waters as well as surface waters obscured by clouds here we demonstrate this ability by pairing satellite ocean color data with records from a profiling float that obtained continuous high quality optical data for 3 yr in the north atlantic ocean good agreement was found between satellite and float data and the relationship between satellite chlorophyll and floatderived particulate backscattering was consistent with previously published data upper ocean biogeochemical dynamics were evidenced in float measurements which displayed strong seasonal patterns associated with phytoplankton blooms and depth and seasonal patterns associated with an increase in pigmentation per particle at low light surface optical variables had shorter decorrelation timescales than did physical variables unlike at low latitudes suggesting that biogeochemical rather than physical processes controlled much of the observed variability after 2 25 yr in the subpolar north atlantic between newfoundland and greenland the float crossed the north atlantic current to warmer waters where it sampled an unusual eddy for 3 months this anticyclonic feature contained elevated particulate material from surface to 1000 m depth and was the only such event in the float s record this eddy was associated with weakly elevated surface pigment and backscattering but depthintegrated backscattering was similar to that previously observed during spring blooms such seldom observed eddies if frequent are likely to make an important contribution to the delivery of particles to depth

phytoplankton mineralization in the tropical and subtropical atlantic ocean organic carbon fluxes to the deep ocean may be enhanced by association with ballast mineral material such as calcite and opal we made simultaneous measurements of the upper ocean production of calcite calcification opal silicification and organic carbon photosynthesis at 14 stations between 42 s and 49 n in the atlantic ocean these measurements confirm the light dependency of calcification and photosynthesis and the substrate dependency of silicification we estimate that mineralizing phytoplankton represent 5 20 of organic carbon fixation with similar contributions from both coccolithophores and diatoms estimates of average turnover times for calcite and phytoplankton carbon are 3 days indicative of their relatively labile nature by comparison average turnover times for opal and particulate organic carbon are 10 days rapid turnover of calcite suggests an important role for the plankton community in removing calcite from the upper ocean comparison of our surface production rates to sediment trap data confirms that 70 of calcite is dissolved in the upper 2 3 km and only a small proportion of total organic carbon 2 reaches the deep ocean

deeper groundwater chemistry and geochemical modeling of the arsenic affected western bengal basin west bengal india abstract a regional scale hydrogeochemical study of a 21 000 km2 area in the western bengal basin shows the presence of hydrochemically distinct water bodies in the main semiconfined aquifer and deeper isolated aquifers spatial trends of solutes and geochemical modeling indicate that carbonate dissolution silicate weathering and cation exchange control the major ion chemistry of groundwater and river water the main aquifer water has also evolved by mixing with seawater from the bay of bengal and connate water the isolated aquifers contain diagenetically altered water of probable marine origin the postoxic main aquifer water exhibits overlapping redox zones metal reducing sulfidic and methanogenic indicative of partial redox equilibrium with the possibility of oxidation in micro scale environments the redox processes are depth dependent and hydrostratigraphically variable elevated dissolved as in the groundwater is possibly related to fe iii reduction but is strongly influenced by coupled fe s c redox cycles arsenic does not show good correlations with most solutes suggesting involvement of multiple processes in as mobilization the main river in the area the bhagirathi hoogly is chemically distinctive from other streams in the vicinity and probably has little or no influence on deep groundwater chemistry arsenic in water of smaller streams jalangi and ichamati is probably introduced by groundwater discharge during the dry season

when mixed layers are not mixed storm driven mixing and bio optical vertical gradients in mixed layers of the southern ocean author s carranza mm gille st franks pjs johnson ks pinkel r girton jb abstract 2018 american geophysical union all rights reserved mixed layers are defined to have homogeneous density temperature and salinity however bio optical profiles may not always be fully homogenized within the mixed layer the relative timescales of mixing and biological processes determine whether bio optical gradients can form within a uniform density mixed layer vertical profiles of bio optical measurements from biogeochemical argo floats and elephant seal tags in the southern ocean are used to assess biological structure in the upper ocean within the hydrographically defined mixed layer the profiles show significant vertical variance in chlorophyll a chl a fluorescence and particle optical backscatter biological structure is assessed by fitting chl a fluorescence and particle backscatter profiles to functional forms i e gaussian sigmoid exponential and their combinations in the southern ocean which characteristically has deep mixed layers only 40 of nighttime bio optical profiles were characterized by a sigmoid indicating a well mixed surface layer of the remaining 60 that showed structure 40 had a deep fluorescence maximum below 20 m depth that correlated with particle backscatter furthermore a significant fraction of these deep fluorescence maxima were found within the mixed layer 20 80 depending on mixed layer depth definition and season results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification we hypothesize that periods of quiescence between synoptic storms which we estimate to be 3 5 days depending on season allow bio optical gradients to develop within mixed layers that remain homogeneous in density

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

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

arsenic release from biotite into a holocene groundwater aquifer in bangladesh abstract continuous core sediments to a depth of 90 1 m taken at a transitional area of holocene and pleistocene deposits in sonargaon bangladesh were characterized for their mineralogy and chemistry among the sediments of the lower part of the holocene aquifer depth 18 29 m where most domestic wells are installed as is mostly fixed in biotite and organic phases a positive correlation of as concentration with those of al and fe but not that of total organic c clearly suggests that biotite is a primary source of as although microbial reduction dissolution of as containing fe oxyhydroxides is thought to cause as enriched groundwater in the ganges brahmaputra meghna delta plain the authors conclude that chemical weathering of biotite is the primary formation mechanism and prevailing reducing conditions contribute to the expansion of as enriched groundwater in the study area

arsenic enrichment in groundwater of the alluvial aquifers in bangladesh an overview arsenic in the groundwater of bangladesh is a serious natural calamity and a public health hazard most groundwater from the shallow alluvial aquifers 150 m particularly in the holocene plai

mineralogical study of some arsenic contaminated soils of west bengal india abstract five arsenic affected soil profiles one each from ghentugachhi and gotera village of chakdah block gayeshpur kalyani block nadia district ramnagar village of baruipur block and sonarpur mouza of sonarpur block of south 24 parganas district of west bengal india covering the soils of typic haplustepts typic endoaquepts vertic haplustepts and aquic haplustepts respectively have been studied for their detail mineralogical chemical composition and also to know the level of arsenic contamination and their probable source these soils are very deep developed on alluvium and are under rice vegetable and guava cultivation sand mineralogy data of these soils are dominated by opaque and limonite whereas silt is mainly constituted of micas followed by kaolinite and chlorite the clay mineralogy of the soils indicates that smectite is in higher proportion in ghentugachhi and gotera soils followed by mica kaolinite chlorite and vermiculite and in the other soils mica is the dominant clay mineral in some of these soils the interstratified mixed layer minerals are also present the mineralogy class for the soils of ghentugachhi gayeshpur baruipur is mixed and for gotera and sonarpur it is illitic differential thermal analysis dta of clay fraction of surface soil of the profiles displayed the presence of the minerals like magnetite marcasite arsenopyrite illite montmorillonite and quartz the source of arsenic in these soils are probably due to the presence of arsenic bearing minerals marcasite and arsenopyrite and also it may occur as adsorbtion on iron hydroxide coated sand grains and clay minerals arsenic concentration is very high in the surface horizon of pedon 2 20 2 mg kg 1 and it shows a decreasing trend down the subsurface horizon the higher concentration of arsenic in the surface horizon may be due to higher abundance of opaque minerals limonite and marcasite

linking organic matter deposition and iron mineral transformations to groundwater arsenic levels in the mekong delta cambodia enriched as in drinking water wells in south and southeast asia has increased the risk of cancer for nearly 100 million people this enrichment is generally attributed to the reductive dissolution of fe oxides however the complex expression of as enrichment in these areas is not yet well understood here the coupled sedimentological and geochemical factors that contribute to the extent and spatial distribution of groundwater as concentrations in the mekong river delta cambodia in an avulsed scroll bar sequence are examined x ray absorption spectroscopy xas was used to determine fe and as speciation in redox preserved sediment collected from drilled cores dissolved as fe and s solution concentrations in existing and newly drilled wells cores differed considerably depending on their source sedimentology the rapid burial of organic matter in the scroll bar sequence facilitated the development of extensive fe reducing conditions and as release into the aquifer in older features organic c levels are high enough to sustain extensive fe reduction and provide ample so4 which is reduced to sulfide this s reduction impacts as levels as is sequestered in sulfide minerals outside of the scrollbar sequence decreasing pore water concentrations in contrast as is depleted in sediments from the scroll sequence and associated with elevated pore water aqueous concentrations the concentration and form of organic c in the scrollbar sequence is related to depositional environment and can facilitate fe and s mineral transformations distinct sedimentary environments explain a portion of the inherent heterogeneity of aquifer as concentrations

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

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

arsenic incorporation into authigenic pyrite bengal basin sediment bangladesh abstract sediment from two deep boreholes 400 m approximately 90 km apart in southern bangladesh was analyzed by x ray absorption spectroscopy xas total chemical analyses chemical extractions and electron probe microanalysis to establish the importance of authigenic pyrite as a sink for arsenic in the bengal basin authigenic framboidal and massive pyrite median values 1500 and 3200 ppm as respectively is the principal arsenic residence in sediment from both boreholes although pyrite is dominant ferric oxyhydroxides and secondary iron phases contain a large fraction of the sediment bound arsenic between approximately 20 and 100 m which is the depth range of wells containing the greatest amount of dissolved arsenic the lack of pyrite in this interval is attributed to rapid sediment deposition and a low sulfur flux from riverine and atmospheric sources the ability of deeper aquifers 150 m to produce ground water with low dissolved arsenic in southern bangladesh reflects adequate sulfur supplies and sufficient time to redistribute the arsenic into pyrite during diagenesis

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

groundwater arsenic concentrations in vietnam controlled by sediment age arsenic contamination of groundwater threatens the health of millions of people in southeast asia measurements in an arsenic contaminated aquifer in vietnam point to sediment age as a key determinant of groundwater arsenic concentrations

distribution of c n and p in the sediments of the ganges brahmaputra meghna river system in the bengal basin the bengal basin is one of the geologically youngest and tectonically active drainage regimes of the world which includes the total lower ganges brahmaputra meghna gbm river system one of the highest sediment dispersal systems in the world the sediment load in this system comprises exclusively fine sand silt and clay river bed sediments and suspended matter from stations situated uniformly all over the basin have been analyzed in order to understand the spatial distribution and biogeochemical processing of the c n and p in the system among the ganges brahmaputra and meghna subbasins variations in total phosphorus tp and nitrogen tn contents in the bulk and 63 m fraction of the bed sediments are statistically insignificant but total carbon tc content varies significantly among the three subbasins there is a gradual increase in the concentration of tc from the meglna towards the ganges subbasin attributable to the nature of the floodplain and bank materials which supply major sediment load to the river channels the tp concentration is 2 to 5 times more in the suspended matter than that in any particle size fraction of the sediments although excellent correlation between corg and tn suggests their coorigin the influence of inorganic nitrogen is evident in very low c n ratios the c n ratio varies from 2 to 11 suggesting complete degradation of the organic matter and or enrichment of inorganic nitrogen in the sediments the plant available phosphorus pap varies from 1 78 to 6 98 of tp and indicates a significant anthropogenic contribution the corg tn and c n ratio in the bed sediments of the lower gbm system are in the range comparable with other river basins of the subcontinent

initial responses of carbonate rich shelf sediments to rising atmospheric pco2 and ocean acidification role of high mg calcites abstract carbonate rich sediments at shoal to shelf depths 3 reservoir that can rapidly react to the decreasing saturation state of seawater with respect to carbonate minerals produced by the increasing partial pressure of atmospheric carbon dioxide p co 2 and acidification of ocean waters aragonite is usually the most abundant carbonate mineral in these sediments however the second most abundant typically 24 wt carbonate mineral is high mg calcite mg calcite whose solubility can exceed that of aragonite making it the first responder to the decreasing saturation state of seawater for the naturally occurring biogenic mg calcites dissolution experiments have been used to predict their stoichiometric solubilities as a function of mol mgco 3 the only valid relationship that one can provisionally use for the metastable stabilities for mg calcite based on composition is that for the synthetically produced phases where metastable equilibrium has been achieved from both under and over saturation biogenic mg calcites exhibit a large offset in solubility from that of abiotic mg calcite and can also exhibit a wide range of solubilities for biogenic mg calcites of similar mg content this indicates that factors other than the mg content can influence the solubility of these mineral phases thus it is necessary to turn to observations of natural sediments where changes in the saturation state of surrounding waters occur in order to determine their likely responses to the changing saturation state in upper oceanic waters brought on by increasing p co 2 in the present study we investigate the responses of mg calcites to rising p co 2 and ocean acidification by means of a simple numerical model based on the experimental range of biogenic mg calcite solubilities as a function of mg content in order to bracket the behavior of the most abundant mg calcite phases in the natural environment in addition observational data from bermuda and the great bahama bank are also presented in order to project future responses of these minerals the numerical simulations suggest that mg calcite minerals will respond to rising p co 2 by sequential dissolution according to mineral stability progressively leading to removal of the more soluble phases until the least soluble phases remain these results are confirmed by laboratory experiments and observations from bermuda as a consequence of continuous increases in atmospheric co 2 from burning of fossil fuels the average composition of contemporary carbonate sediments could change i e the average mg content in the sediments may slowly decrease furthermore evidence from the great bahama bank indicates that the amount of abiotic carbonate production is likely to decline as p co 2 continues to rise

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

effect of wintertime low pressure systems on the atlantic inflow to the nordic seas 1 the response of the atlantic inflow to the nordic seas to a variable wind field is investigated using storm track data and observed current meter data from the three inflowing iceland faroe and shetland branches and from the svinoy section off the west coast of norway the atlantic inflow is found to be anomalously large in the shetland and svinoy sections for 4 5 days after a storm passage with maximum response after 2 3 days no similar inflow response was found in the iceland and faroe sections on the contrary the faroe current becomes weaker 2 days after the inflow maxima in the shetland and svinoy sections we speculate that this is a lagged response to increased sea level height north of the greenland scotland ridge for longer timescales most of the winter anomalies in the volume transports through the shetland and svinoy sections are related to cyclone variability with the number of storms the residence time and the intensity of the storms being critical factors

arsenic pollution in groundwater of west bengal india an insight into the problem by subsurface sediment analysis abstract arsenic values in groundwater above the maximum permissible limit of 0 05 mg l have been reported from several areas of the lower delta region of the ganga padma river system it is confined to areas east of hooghly river and to the shallow aquifer arsenic values in sub surface sediments from arsenious water zones are significantly higher than values from safe water zones the major part of the sediments consisting of non magnetic and some feebly magnetic minerals is arsenic free some iron rich clastic minerals like illite biotite chlorite fe coated grains and also the authigenic siderite concretions which together constitute only a small part of the sediments are carriers of arsenic and contribute the element to contaminate groundwater illite biotite and chlorite degenerate to produce iron oxide hydroxide coating on their surface and also on the surface of some other sand particles arsenic is adsorbed on the iron hydroxide coating creating one kind of sink for the element bacteria induced growth of siderite concretions grow initially on these clastic minerals and after attaining bigger size fall off as individual globules arsenic gets adsorbed on these concretions forming a second kind of sink for the element arsenic carrying river water inundates the lower delta at the time of seasonal flood arsenic percolates with the water downwards to the shallow aquifer it gets fixed in the aquifer sediments in the two ways described and is retained in the traps thus formed locally developed reducing condition causes dissolution of iron hydroxide coating on the surface of clastic arsenic traps and also of the siderite concretions this leads to increase in arsenic level in groundwater at these sites a currently active process of creation and periodic enrichment of the arsenic traps followed by their subsequent depletion through desorption and dissolution is suggested to be the reason for arsenic contamination of groundwater in this part of the delta