geomorphic controls on groundwater arsenic distribution in the mekong river delta cambodia natural arsenic contamination of groundwater is a severe public health crisis affecting nearly 60 million people in south asia and southeast asia alone contamination is spatially heterogeneous and results from the coupled microbial decomposition of organic matter and reductive dissolution of arsenic bearing iron minerals here we demonstrate that elevated arsenic concentrations arise in regions of recent organic matter deposition and thus are controlled by fluvial geomorphic processes arsenic contamination is best expressed within recent geomorphic features such as docked islands scroll bars and avulsions within these features the deposition of rapidly buried reactive organic matter facilitates microbial iron reduction and arsenic release ultimately the organic matter supply is exhausted and the conditions necessary for soluble arsenic to persist diminish

the susceptibility of uk groundwaters to acidic deposition the weathering of minerals at shallow depths provides much of the sink for acidic atmospheric deposition the geology of the uk has been divided into four classes according to the extent to which the bedrock is likely to provide buffering and possibly give rise to acidic surface or groundwaters other modifying factors including soils superficial deposits and land use are also considered the likely sources and sinks of h in shallow groundwater systems are also reviewed most available groundwater data are derived from major aquifers often in carbonate bearing lithologies which afford excellent buffering and which are unlikely ever to be adversely affected by acidic deposition relatively few data exist for shallow groundwaters widely used for domestic supplies in rural areas especially in western and northern britain where groundwaters might be most affected acidic groundwaters and or surface waters are defined on the basis of their alkalinity and calcite saturation index si c acidic groundwaters are most likely to occur where 1 acid soils are found and glacial drift is thin or absent 2 the source of acidic inputs is high 3 carbonate minerals are absent 4 the residence time of water is short low alkalinity groundwater is found in many sandstone lithologies including the sherwood sandstone a major uk aquifer many lower palaeozoic strata and granites however existing data for susceptible areas are insufficient to show whether or not there have been significant long term changes in groundwater acidity or alkalinity

groundwater chemistry of al under dutch sandy soils effects of land use and depth aluminium has received great attention in the second half of the 20th century mainly in the context of the acid rain problem mostly in forest soils in this research the effect of land use and depth of the groundwater on al ph and doc concentration in groundwater under dutch sandy soils has been studied both ph and doc concentration play a major role in the speciation of al in solution furthermore the equilibrium with mineral phases like gibbsite amorphous al oh 3 and imogolite has been considered agricultural and natural land use were expected to have different effects on the ph and doc concentration which in turn could influence the total al concentration and the speciation of al in groundwater at different depths phreatic shallow and deep an extensive dataset n 2181 from the national and some provincial monitoring networks on soil and groundwater quality was used land use type and groundwater depth did influence the ph and al and doc concentrations in groundwater samples the al concentration ranged from 7 to 1941 mol l 1 at ph 4 highest al concentrations were found for natural phreatic groundwater the doc concentration decreased and the median ph increased with depth of the groundwater natural phreatic groundwater showed lower ph than the agricultural phreatic groundwater highest doc concentrations were found for the agricultural phreatic groundwater induced by the application of organic fertilizers besides inorganic complexation the nica donnan model was used to calculate al3 concentrations for complexation with doc below ph 4 5 groundwater samples were mainly in disequilibrium with a mineral phase this disequilibrium is considered to be the result of kinetic constraints or equilibrium with organic matter log k values were derived by linear regression and were close to theoretical values for al oh 3 minerals e g gibbsite or amorphous al oh 3 except for natural phreatic groundwater for which lower log k values were found complexation of al with doc is shown to be an important factor for the al concentrations especially at high doc concentrations as was found for agricultural phreatic groundwater 2007 elsevier ltd all rights reserved

gypsum precipitation dissolution as an explanation of the decrease of sulphate concentration during seawater intrusion the precipitation of gypsum during the intrusion of seawater in a coastal aquifer is shown to be a possible cause of the decrease of the sulphate concentration with respect to the conservative freshwater seawater mixing observed in many field studies results of previously published laboratory column experiments and the application of a multicomponent reactive transport model show that gypsum precipitates during the first stages of seawater intrusion causing a decrease in sulphate concentration as the resultant front advances the water becomes undersaturated with respect to gypsum when the content of seawater 50 at that point water samples exhibit a deficit of sulphate of several mmol l in the range of the field values reported in papers on intrusion in coastal aquifers thus in addition to bacterial sulphate reduction by organic matter gypsum precipitation is a possible explanation for the non conservative behaviour of sulphate during seawater intrusion

estimation of microbial respiration rates in groundwater by geochemical modeling constrained with stable isotopes changes in geochemistry and stable isotopes along a well established groundwater flow path were used to estimate in situ microbial respiration rates in the middendorf aquifer in the southeastern united states respiration rates were determined for individual terminal electron acceptors including o2 mno2 fe3 and so42 the extent of biotic reactions were constrained by the fractionation of stable isotopes of carbon and sulfur sulfur isotopes and the presence of sulfur oxidizing microorganisms indicated that sulfate is produced through the oxidation of reduced sulfur species in the aquifer and not by the dissolution of gypsum as previously reported the respiration rates varied along the flow path as the groundwater transitioned between primarily oxic to anoxic conditions iron reducing microorganisms were the largest contributors to the oxidation of organic matter along the portion of the groundwater flow path investigated in this study the transition zone between oxic and anoxic groundwater contained a wide range of terminal electron acceptors and showed the greatest diversity and numbers of culturable microorganisms and the highest respiration rates a comparison of respiration rates measured from core samples and pumped groundwater suggests that variability in respiration rates may often reflect the measurement scales both in the sample volume and the time frame over which the respiration measurement is averaged chemical heterogeneity may create a wide range of respiration rates when the scale of the observation is below the scale of the heterogeneity

redox zonation and organic matter oxidation in palaeogroundwater of glacial origin from the baltic artesian basin abstract ordovician cambrian aquifer system o cm in the northern part of the baltic artesian basin bab estonia is part of a unique groundwater reservoir where groundwater originating from glacial meltwater recharge from the scandinavian ice sheet is preserved the distribution of redox zones in the anoxic o cm aquifer system is unusual strongly reducing conditions are found near the modern recharge area characterized by low concentrations of sulphate

geochemical and sr isotopic characteristics of stream and spring waters from small forested catchments at seto central japan hydrogeochemical processes controlling surface water chemistry were examined in five small 90 wt suggest slow chemical weathering the dissolved solute concentrations are generally of similar magnitude in stream waters within the catchments of similar bedrock lithology the high inverse correlation r2 0 72 between ph and so4 concentrations and the high positive correlation r2 0 90 between ba and so4 concentrations in waters draining tokai conglomerate suggest that barite baso4 is being dissolved in an acidic environment the three catchments were identified as being sensitive to acidic deposition because the bedrock conglomerate provided little capacity to buffer acidic inputs the soils from the granite area have a high cation exchange capacity cec an average of 868 mol kg and are nearly ten times greater than the soils from the conglomerate area because ion exchange besides weathering is the main source that counteracts soil acidification the sensitivity to further acidification may increase

a model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years red river floodplain vietnam aquifers on the red river flood plain with burial ages ranging from 500 to 6000 years show with increasing age the following changes in solute concentrations a decrease in arsenic increase in fe ii and decreases in both ph ca and bicarbonate these changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation the reduction kinetics of as containing fe oxides the sorption of arsenic the kinetics of siderite precipitation and dissolution as well as of the dissolution of caco3 the arsenic released from the fe oxide is preferentially partitioned into the water phase and partially sorbed while the released fe ii is precipitated as siderite the reaction network involved in arsenic mobilization was analyzed by 1 d reactive transport modeling the results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by fe oxide reduction the energy released by fe oxide reduction is strongly ph dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the ph overall it is the rate of organic carbon degradation that determines the total electron flow however the kinetics of fe oxide reduction determines the distribution of this flow of electrons between methanogenesis which is by far the main pathway and fe oxide reduction modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in as during the first 1200 years where it reaches a maximum of about 600 g l during this initial period the release of arsenic from fe oxides actually decreases but the adsorption of arsenic onto the sediment delays the build up in the groundwater arsenic concentration after 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further but diminishing release from fe oxide being reduced after 6000 years the arsenic content has decreased to 33 g l the modeling enables a quantitative description of how the aquifer properties the reactivity of organic carbon and fe oxides the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content

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

numerical modeling of subsurface release and fate of benzene and toluene in coastal aquifers subjected to tides abstract a numerical study was undertaken to investigate subsurface release and fate of benzene and toluene in a tidally influenced beach the simulations were conducted by using a numerical model biomarun which coupled a multi monod kinetic model biob to a density dependent variably saturated groundwater flow model marun the fate and transport of the contaminant plume were characterized by computing its centroid trajectory spreading area and percentage of biodegradation in beach saturated and unsaturated zone respectively key factors likely affecting this process were investigated including tide amplitude capillarity and hydraulic conductivity it was found that aerobic biodegradation was the major fate of the contaminant plume in the beach tidal action twisted the centroid of the contaminant plume during its migration in the beach which increased the residence time of the plume in the beach high tidal range significantly altered the spatial distribution of the contaminant biodegradation in the beach in contrast the capillary fringe had impacts on the percentage of benzene biodegraded in the saturated and unsaturated zone of the beach the increase in capillary fringe enhanced the percentage of the contaminant biodegraded in the unsaturated zone up to 40 which is comparable to that in the saturated zone hydraulic conductivity seemed to have large impacts on the biodegradation rate of the contaminant in the beach higher hydraulic conductivity induced faster contaminant biodegradation in the beach

iron cycling and stable fe isotope fractionation in antarctic shelf sediments king george island iron fe fluxes from reducing sediments and subglacial environments are potential sources of bioavailable fe into the southern ocean stable fe isotopes 56fe are considered a proxy for fe sources and reaction pathways but respective data are scarce and fe cycling in complex natural environments is not understood sufficiently to constrain respective 56fe endmembers for different types of sediments environmental conditions and biogeochemical processes we present 56fe data from pore waters and sequentially extracted sedimentary fe phases of two contrasting sites in potter cove king george island antarctic peninsula a bay that is affected by fast glacier retreat sediments close to the glacier front contain more easily reducible fe oxides and pyrite and show a broader ferruginous zone compared to sediments close to the icefree coast where surficial oxic meltwater streams discharge into the bay pyrite in sediments close to the glacier front predominantly derives from eroded bedrock for the high amount of easily reducible fe oxides proximal to the glacier we suggest mainly subglacial sources where fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes likely pyrite oxidation or dissimilatory iron reduction dir our strongest argument for a subglacial source of the highly reactive fe pool in sediments close to the glacier front is its predominantly negative 56fe signature that remains constant over the whole ferruginous zone this implies in situ dir does not significantly alter the stable fe isotope composition of the accumulated fe oxides the nonetheless overall light 56fe signature of easily reducible fe oxides suggests pre depositional microbial cycling as it occurs in potentially anoxic subglacial environments the strongest 56fe depletion in pore water and most reactive fe oxides was observed in sediments influenced by oxic meltwater discharge the respective site showed a condensed redox zonation and a pore water 56fe profile typical for in situ fe cycling we demonstrate that the potential of pore water 56fe as a proxy for benthic fe fluxes is not straight forward due to its large variability in marine shelf sediments at small spatial scales 2 4 at the site proximal to oxic meltwater discharge vs 0 9 at the site proximal to the marine glacier terminus both at 2 cm sediment depth the controlling factors are multifold and include the amount and reactivity of reducible fe oxides and organic matter the isotopic composition of the primary and secondary ferric substrates sedimentation rates and physical reworking bioturbation ice scraping the application of 56fe geochemistry may prove valuable in investigating biogeochemical weathering and fe cycling in subglacial environments this requires however similarly to the use of 56fe for the quantification of benthic fluxes that the spatial and temporal variability of the isotopic endmember is known and accounted for since geochemical data from subglacial environments are very limited further studies are needed in order to sufficiently assess fe cycling and fractionation at glacier beds and the composition of discharges from those areas

chemical weathering in two adirondack watersheds past and present day rates rates of chemical weathering in two forested adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input output budgets based on precipitation and surface water chemistry long term rates of weathering have averaged about 500 to 600 eq ha yr for both watersheds since the glaciers retreated from the region about 14 000 yr ago present day denudation rates average 1679 eq ha yr in the panther lake watershed and only 198 eq ha yr in the woods lake watershed mineral weathering reactions in both watersheds involve primarily the dissolution of plagioclase potassium feldspar and hornblende mass balance calculations however indicate that hornblende weathers in disproportionately large amounts in the panther lake watershed relative to its abundance in soils and till cation exchange in the glacial till mantling the panther lake watershed may also play an important role in controlling surface water chemistry in the basin in the panther lake watershed the current rate of weathering which is about a factor of 3 greater than the long term average may reflect a recent adjustment for higher hydrogen ion fluxes brought about by acid deposition the capacity of the thick till deposits in the panther lake watershed to neutralize acidity is ultimately reflected by the circum neutrality of panther lake in the woods lake watershed the soils and thin till deposits cannot effectively neutralize current acid loadings and therefore woods lake is acidic with a ph below 5

the distribution and origins of extremely acidic saline groundwaters in the south of western australia groundwater and digital mapping datasets provide new insights abstract some of the largest extents of naturally occurring acidic waters are found across southern australia the origins of these systems remain poorly understood with many hypotheses for their genesis australian government agency groundwater datasets and mapping data vegetation geology regolith and soils for south western australia unavailable to previous researchers were statistically analysed to better understand the origins of acidic groundwater and guide additional fieldwork to study the origins of acidic saline groundwater the groundwater data showed a distinct bimodal distribution in ph the acid population had a median ph of 3 5 and the larger non acid population had a median ph of 6 6 acidic groundwater became progressively more common further from the coast towards the drier internally drained regions acidic groundwater was mostly confined to the lower slopes and valley floors with localised controls on distribution paradoxically subsoil alkalinity within the internally drained inland regions had the strongest correlation with acidic groundwater r2 0 85 vegetation was also a strong predictor of acidic groundwater acidic groundwater had the highest occurrence under eucalyptus woodlands and shrublands that grew on alkaline calcareous soils pre clearing soil data in areas with acidic saline groundwater showed that the upper 1 m of the unsaturated zone had a ph around 8 while the ph at depths greater than 5 m decreased to 1 5 m there the iron is reduced around roots and the alkalinity generated by microbial iron reduction is removed by biogenic calcification processes the iron moves in solution further down the profile following roots where it comes in contact with the oxygenated unsaturated zone matrix and is oxidised generating acid the resulting acidic recharging solution acidifies the unsaturated zone matrix saline groundwater moving through the matrix becomes acidified due to ion exchange or direct recharge the main chemical processes were modelled in phreeqc to test the plausibility of the hypothesis and acidic solutions with a ph of 3 8 or lower were obtained

numerical study of solute transport in shallow beach aquifers subjected to waves and tides a numerical study was conducted to investigate the fate of solute in a laboratory beach in response to waves and tides a new temporal upscaling approach labeled net inflow was introduced to address impacts of waves on solute transport within beaches numerical simulations using a computational fluid dynamic model were used as boundary conditions for the two dimensional variably saturated flow and solute transport model marun the modeling approach was validated against experimental data of solute transport due to waves and tides exchange fluxes across the beach face and subsurface solute transport e g trajectory movement speed and residence time were quantified simulation results revealed that waves increased the exchange fluxes and engendered a wider exchange flux zone along the beach surface compared to tide only forcing waves superimposed on tide caused the plume to be deeper into the beach and to migrate more seaward the infiltration into the beach was found to be directly proportional to the general hydraulic gradient in the beach and inversely proportional to the matrix retention or capillary capacity the simulations showed that a higher inland water table would attenuate wave caused seawater infiltration which might impact beach geochemical processes e g nutrient recycle and redox condition especially at low tide zone the concept of biochemical residence time maps brtm was introduced to account for the net effect of limiting concentration of chemicals on biochemical reactions it was found that waves shifted the brtms downward and seaward in the beach and subsequently they engendered different biochemical conditions within the beach

formation and evolution of soils from an acidified watershed plastic lake ontario canada abstract the plastic lake watershed contains podzols developed on glacial tills deposited 12 000 years ago present day cationic fluxes from the soils are greater by a factor of 2 than long term fluxes averaged over the age of the tills the high rates of present day chemical weathering may be a result of increased input of anthropogenic acids into the plastic lake watershed time averaged proportions of cations leached from the soils are strikingly different from the proportions of cations now being leached indicating that the character of chemical weathering has changed over time weathering was and is dominated by mineral dissolution but cation exchange has become increasingly important as the soils have matured the large amount of ca now released to the soil solutions probably is derived from exchange substrates promoted by preferential uptake of al onto the substrates aluminum is derived primarily through dissolution of feldspars bulk compositional analyses of soil profiles demonstrate that feldspars of the ae horizon release base cations na k ca and al to solution in near stoichiometric proportions just as is observed experimentally for feldspar dissolution in acidic solutions furthermore surface area normalized dissolution rates of plagioclase k feldspar quartz and micas of plastic lake soils are similar to rates obtained experimentally the favourable comparison between natural and experimental dissolution rates and near stoichiometric release of base cations and al from feldspars to acidic solutions in both natural and experimental settings suggests that recent laboratory release rates can be applied to dissolution of feldspars within the ae horizon of the plastic lake podzols surface area normalized time averaged dissolution rates of primary minerals of plastic lake soils are significantly greater than present day rates measured for mature soil profiles rapid release from plastic lake soils probably results from reaction of acidic soil solutions with highly reactive biotite and ultrafine grains of feldspars produced by comminution during glaciation exhaustion of these reactive phases may well explain the low leach rates observed for mature profiles after just 12 000 years of weathering biotite has been effectively exhausted in the plastic lake soils and much of the fine grained feldspar probably has been dissolved time averaged concentrations of elements removed from the soil recast into essential mineralogy indicate that vermiculite weathers most rapidly followed closely by plagioclase k feldspar weathers less rapidly than plagioclase but more rapidly than quartz or hornblende al silicate possibly imogolite and goethite are added to the soils during weathering plagioclase dissolution consumes more acid than all other reactions accounting for 60 of the total acid consumed combined with k feldspar they account for more than 80 of the total acid neutralized the dissolution rate of quartz is one half that of plagioclase furthermore the rate is consistent with the experimentally determined rates the combination of high abundance and dissolution rate results in large amounts of quartz being dissolved from the soils of plastic lake it may also contribute significantly to total elemental fluxes from other catchments

acidification buffering and salt effects in the unsaturated zone of a sandy aquifer klosterhede denmark acidification of groundwater in a noncalcareous sandy aquifer at klosterhede denmark is the result of acid rain deposition in the 4 to 5 m thick unsaturated zone the ph ranges from 4 2 to 4 9 with al concentrations of up to 0 8 mmol l sup minus 1 the groundwater at the top of the saturated zone still has a ph below 5 deposition of sea salt affects the solute profiles and its importance varies both spatially from the forest margin to the inner part of the forest and temporally through seasonal variations in infiltration and dry deposition as a result pulses of high solute concentrations travel downward through the unsaturated zone the cation exchange capacity cec of the sediments ranges between 0 2 and 1 meq 100 g sup minus 1 and in the acidified zone base saturation is around 17 the pore waters are close to equilibrium with gibbsite supersaturated for kaolinite and strongly undersaturated for other silicate minerals mass balance calculations on increases in dissolved silica over depth suggest that the buffering effect of silicate weathering is small buffering processes and solute transport were modeled with the code phreeqm simulation of pre acid rain weathering indicates that this process operates on a timescale of thousands of years more yielding minimum ph values near 5 2 and a base saturation of greater than 70 the present leaching of al sup 3 rich acid water from the soil yields acidification rates of 7 and 10 cm yr sup minus 1 for weathering of a naturally weathered and a pristine profile respectively simulation of infiltration of sea salt pulses indicates that the cation distribution quickly becomes attenuated by the exchanger composition however due to coupling of gibbsite equilibrium with ion exchange processes downward traveling pulses with high solute concentrations will cause ph variations throughout the unsaturated zone by precipitation and dissolution of gibbsite 67 refs 16 figs 3 tabs less

the impact of variably saturated conditions on hydrogeochemical changes during artificial recharge of groundwater artificial recharge of groundwater is often used to either purify partially treated wastewater or to enhance the quality of surface water by percolation through a variably saturated zone in many cases the most substantial purification process within the infiltration water is the redox dependent biodegradation of organic substances the present study was aimed at understanding the spatial and temporal distribution of the redox reactions that develop below an artificial recharge pond near lake tegel germany at this site like at many artificial recharge sites the hydraulic regime immediately below the pond is characterised by cyclic changes between saturated and unsaturated conditions these changes which occur during each operational cycle result from the repeated formation of a clogging layer at the pond bottom regular hydrogeochemical analyses of groundwater and seepage water in combination with continuous hydraulic measurements indicate that no3 and mn reducing conditions dominate beneath the pond as long as water saturated conditions prevail manganese fe and so42 reducing conditions are confined to a narrow zone directly below the clogging layer and in zones of lower hydraulic conductivity the formation of the clogging layer leads to a steady decrease of the infiltration rate which ultimatively causes a shift to unsaturated conditions below the clogging layer atmospheric o2 then starts to penetrate from the pond fringes into this region leading to i the re oxidation of the previously formed sulphide minerals and ii the enhanced mineralisation of sedimentary particulate organic c the mineralisation of sedimentary particulate organic c leads to an increased h2co3 production and subsequent dissolution of calcite

feldspar weathering as the key to understanding soil acidification monitoring data a study of acid sandy soils in the netherlands abstract monitoring activities pose special demands on the type of survey results needed in the early 1990s a soil acidification monitoring methodology was adopted in the netherlands that leaned heavily on methods developed in more fundamental research most notably the use of proton budgets consequently various controversies still not resolved in the scientific debate reflect on the current practice of soil acidity monitoring and complicate interpretation of the monitoring results in a pilot study we address the most pressing issues capacity versus intensity parameters choice of monitoring objective and natural variation in the compartment to be monitored focus is on the major source of buffering the possible usefulness of the historic approach and the regional patterns present in the sandy soils of the netherlands in a field campaign 92 locations in sandy regions all over the country were sampled at two depths the solid phase the displaced soil solution and solid phase extractions with 0 01 m cacl2 and 0 43 m hno3 for the 184 samples were analyzed by a variety of methods aluminum release is the major source of buffering and is shown to contribute substantially to acid buffering already under natural conditions the predominant al bearing phases in dutch sandy soils are feldspars and secondary al minerals feldspars are found to be the determinative phase in acid buffering application of the historic approach using the subsoil as a proxy for the initial composition of the topsoil proved feasible for this regional dataset the average depletion of the anc s of 230 mmolc kg 1 in the topsoil matches well with estimates of the total proton load since the last ice age with the anthropogenic contribution being between 20 and 50 fuzzy c means cluster analyses of the solid phase and soil solution data show a distinct regionality that was also reflected in the parameters generally used to indicate the acidity status of soils anc s and al bc ratios a combined insight into both solid phase and soil solution based on a comprehensive set of parameters proves essential for interpreting soil acidity monitoring data

tensile fracture of a single crack in first year sea ice the break up of sea ice in the arctic and antarctic has been studied during three field trips in the spring of 1993 at resolute nwt and the fall of 2001 and 2004 on mcmurdo sound via in situ cyclic loading and fracture experiments in this paper the back calculated fracture information necessary to the specification of an accurate viscoelastic fictitious cohesive crack model is presented in particular the changing shape of the stress separation curve with varying conditions and loading scenarios is revealed this article is part of the theme issue modelling of sea ice phenomena

arsenic release and attenuation in a multilayer aquifer in the po plain northern italy reactive transport modeling abstract groundwater as concentrations who limit 10 g l are frequently found in the po plain n italy although several hypotheses on as mobilization exist i e reductive dissolution driven by peat degradation the mechanisms of as release and subsequent attenuation acting in the multilayer aquifer in the po plain were poorly understood the present work aims at implementing a reactive transport modeling of the aquifer system in cremona affected by as the model based on a partial equilibrium approach revealed that the observed as fe and mn chemistry could be mostly explained by the simultaneous equilibrium between fe oxide and sulfate reduction and fes precipitation and by the equilibrium of rhodochrosite precipitation dissolution model results together with litholog analysis supported the assumption of peat as the likely source of organic matter driving as release the model fitted to measured data showed that the peak in the organic carbon degradation rate at 20 40 m below surface average of 0 67 mm y corresponding to the shallow peaty aquitard and the upper portion of the underlying semiconfined aquifer is associated with the peak of net release of as average of 0 32 m y that is followed just downstream by a net precipitation in iron sulfides at 40 60 m below surface average of 0 30 m y these results support the assumptions of peaty aquifers as drivers of as release and iron sulfides as as traps the model also outlined the following aspects that could have a broad applicability in other alluvial as affected aquifers worldwide a shallow peaty aquitards may have a greater role in driving the as release since they likely have young and more reactive organic matter b the occurrence of fe oxide reduction and fes precipitation that represent the as source and sink together with sulfate reduction occurring simultaneously close to equilibrium may restrict the as mobility limiting the extent of contamination just downstream the source of organic matter that drives its release

acid groundwater in an anoxic aquifer reactive transport modelling of buffering processes abstract the acidification of groundwater due to acid rain was investigated in a quaternary sandy aquifer in the fuhrberger feld near hannover germany the groundwater recharged through an area covered by a coniferous forest had a ph in the range 4 5 down to a depth of 5 m the evolution in groundwater chemistry along the flow path was investigated in a transect of multisamplers a 2d groundwater flow model was established delineating the groundwater flow field and a groundwater flow velocity of around 80 m a along the flow path was derived speciation calculations showed the groundwater to be close to equilibrium with the mineral jurbanite alohso4 over the ph range 4 0 6 5 this suggests an accumulation of acid rain derived so 4 2 in the aquifer sediment during the decades with high atmospheric s deposition the groundwater has a ph of around 4 5 in the upstream part of the flow path increasing to near 6 further downstream 1d reactive transport modelling using phreeqc was used to analyze different combinations of buffering processes the first model contains ion exchange in combination with jurbanite dissolution at the ion exchange front al3 is adsorbed leading to the dissolution of jurbanite and an increase in ph comparison with field data showed that the simulated increases in ph and alkalinity are much lower than observed in the field the second model includes organic matter degradation in addition to ion exchange and jurbanite dissolution the model included the reduction of so 4 2 and fe oxides as well as the precipitation of fe sulfide this model matches the field data well and illustrates the importance of redox processes for ph buffering in the fuhrberg aquifer the current progress of the acidification front is about 4 m a this corresponds to an average value of 150 a of acid input which covers large historical variations remediation is expected to take the same time span because it requires desorption and neutralization of adsorbed al3 from the aquifer sediment

in situ rates of sulfate reduction in an aquifer romo denmark and implications for the reactivity of organic matter estimates of rates of organic matter degradation by bacterial sulfate reduction in aquifers are few and all are obtained by indirect means here we present the first direct radiotracer measurements of in situ rates of sulfate reduction for an aquifer romo denmark sulfate reduction occurs in distinct reaction zones within the aquifer at rates that vary greatly over short lateral distances in situ rates of sulfate reduction are significantly higher than previous indirect estimates in contrast rates of sulfate reduction in aquifers are orders of magnitude lower than in marine and limmic environments emphasizing the low reactivity of natural organic matter in aquifers

processes controlling acid attenuation in the unsaturated zone of a triassic sandstone aquifer u k in the absence of carbonate minerals abstract continuous core samples were taken through the unsaturated zone at three sites on the outcrop of permo triassic sandstone in the british west midlands sample sites were chosen for lack of recent direct anthropogenic disturbance and for differing vegetation heathland birch woodland and conifer forest interstitial water was extracted and analyzed for 32 major and trace elements solid phases were analyzed for exchangeable cations and mineralogy the rate of recharge calculated using a cl mass balance method was three times greater below heathland than below afforested sites owing to higher evapotranspiration rates in the woodlands carbonate minerals were absent from the unsaturated zone at each site soil solutions were acidic and soils at the woodland sites were more acidic ph 4 0 than those at the heathland site ph 4 5 acidic interstitial water solutions were found to up to 5 0 m depth in the unsaturated zone and are partially neutralized by two aluminosilicate mineral reactions in the unsaturated zone cation exchange and k feldspar dissolution the rate at which these acid neutralizing reactions act to neutralize acidity is revealed by the rate of depletion of base cations from the unsaturated zone in recharge solutions k dissolution ca 2 mg 2 cation exchange the total base cation depletion rate was greatest below heathland this can be attributed mainly to the greater rate of so 4 assimilation by the woodland biome

groundwater acidification in triassic sandstones prediction with magic modelling acidification of groundwater lags behind acid deposition due to the relatively long water residence time in conjunction with various buffering processes in the soil zone and deeper aquifer chemical weathering cation exchange sulfate sorption and n uptake by the biomass extensive field data from eight forested catchments in the bunter sandstone of the black forest including results from water budget studies and hydrochemical analysis of stream and spring waters were used to simulate the future evolution of ground water acidification with the magic model the present acid deposition exceeds the critical load here meaning buffering due to chemical weathering and protonation of organic acids in six of eight catchments two catchments are well buffered because they contain carbonate bearing layers in the upper bunter sandstone transient buffering i e cation exchange n uptake the sulfate sorption thus far prevents worse acidification but this effect will decline in the future for one of the poorly buffered catchments seebach a two layer simulation was carried out based on extensive data from 10 years of measurements validation of the long term simulations by hydrochemical and soil data was hampered by strong annual variations but generally supported by paleolimnological studies in the future reductions in the s deposition by 20 and the n deposition by 10 up to the year 2030 are assumed as the most probable scenario n uptake through soil and vegetation will come to an end as suggested by decreasing c n ratios of the organic matter this process is arbitrarily included in the simulations in the periglacial soil layer acidification will decrease until the year 2030 and then approach a steady state condition in the fractured aquifer acidification will also proceed at a decreasing rate however sulfate desorption up to the year 2130 the end of simulated period will prevent earlier remediation despite a significant reduction in s deposition since the mid 1980s further efforts are necessary to reduce the emission of acidifying substances liming in the recharge area is partially effective to ameliorate shallow groundwater but largely fails to ameliorate deeper groundwater in the sandstone aquifer

reactivity of organic matter in aquifer sediments geological and geochemical controls reduction rates in aquifers are commonly carbon limited but little is known about the molecular composition and degradability of sedimentary organic matter som in aquifer sediments the composition source and degradation status of som in aquifer sediments of fluvio glacial pleistocene and shallow marine pliocene origin were determined using flash pyrolysis gas chromatography mass spectrometry incubation experiments 106 d were used to assess the reactivity of som towards molecular oxygen a dominance of lignin derived components and long chain odd over even predominant alkanes indicate that terrestrial higher land plants were the main source of som even in the shallow marine sediments while bacterial lipid derived hopanoids and iso and anteiso c15 and c17 fatty acids indicate a minor contribution of microbial biomass no compositional difference was observed between som present in the fine 63 m and coarse fraction 63 2000 m a significant part of som was not present as low molecular weight compounds but was macromolecularly bound for the fluvio glacial sediments a relatively higher abundance of resistant macro molecular compounds was in agreement with stronger signs of aerobic lignin alkane and hopanoid oxidation the more degraded status of som in the fluvio glacial sediments was consistent with their significantly lower som mineralization 2 6 during incubation as compared with the shallow marine sediments 9 14 the reactivity towards oxygen of som was controlled by the extent of past aerobic oxidation not the age of som but the extent of oxygen exposure during syn and postdepositional conditions seems most important in affecting the degradation status of som in aquifer sediments and thus their ability to reduce oxidants copyright 2004 elsevier ltd

the influence of parent material on small scale spatial changes in streamwater chemistry in scottish upland catchments abstract spatial changes in streamwater chemistry in seven upland catchments in ne scotland have been studied using samples collected at 250 m intervals from the stream source to the catchment outlet the catchments contain a number of different parent materials including granites and diorite from the lochnagar complex and metasediments and metabasites from the argyll and appin groups of the dalradian supergroup samples were collected under low flow conditions between june 1989 and january 1990 and ten component ions were determined this paper concentrates on ph ca mg na and k all seven catchments show major changes in stream solute concentrations along their length samples collected immediately downstream of the source typically show a rapid increase in ph with distance reflecting degassing in the stream of co 2 from the soil solution further down the catchment small scale spatial changes in streamwater chemistry appear to be strongly influenced by the underlying geology as the stream flows over parent materials with distinctly different geochemical characteristics the ph and ca and mg concentrations in the stream show rapid changes reflecting changes in the underlying lithology in a number of the catchments k also exhibits significant spatial variability along the length of the stream the underlying geology appears to have little effect on na concentrations which typically increase from the source to the bottom of the catchment these data suggest that small scale spatial changes in streamwater chemistry may provide a useful tool for identifying changes in the underlying geology in poorly exposed areas in addition they also have implications for regional critical load mapping and prediction of streamwater sensitivity to acidification

high resolution regional hydrogeochemical baseline mapping of stream water of wales the welsh borders and west midlands region abstract the stream water hydrogeochemical database produced by the geochemical baseline survey gbase of the british geological survey has recently been enhanced in the light of experimental pilot studies in north wales to meet a wider range of economic and environmental objectives which require modern integrated and strategic geochemical surveys for their implementation hydrogeochemical data are therefore now collected in conjunction with geochemical data for stream sediments soil samples and mineral concentrates the density of sampling based on the collection of stream water at near baseflow conditions during the late summer period each year has been increased to one site per 1 75 km 2 and a broader spectrum of geochemical determinants introduced provisional regional datasets are being prepared for an extensive region covering wales the welsh borders and part of the west midlands representing over 17 000 sample sites bedrock geology and base metal sulphide mineralisation are particularly well reflected in the stream water chemistry at the regional scale the influence of secondary factors such as geomorphology atmospheric controls and anthropogenic contamination due to agriculture urban industrial and military developments can also be readily distinguished the data obtained by systematic high resolution sampling of first and second order streams vary in concentration over three or four orders of magnitude for many of the analytes studied here this compares with a range of only one or two orders of magnitude for many of the analytes in stream sediment samples the extended range in values for stream water is an important factor in the gridding plotting and production of relatively stable maps they are relatively unaffected either by short temporal changes in stream water flow which are attributable either to storm events noted during the sampling campaign or by annual differences between wet and dry summers in different years this has enabled a series of robust surface hydrogeochemical maps to be prepared for analytical data collected during the summer sampling campaigns conducted annually from 1989 to 1994 these maps provide a unique source of synoptic baseline information for a wide range of economic and environmental applications especially when combined with other geoscience datasets in a gis environment

groundwater acidification near the water table of the superficial aquifer gnangara mound swan coastal plain western australia abstract this study improves the spatial understanding of the magnitude of the groundwater acidification issue in a poorly buffered sandy aquifer in western australia the study provides more information to assess the risk the acidification poses to groundwater dependent ecosystems of the gnangara mound groundwater acidification ph so 4 2 cl ratios 0 5 mg l units low ca to al molar ratios and elevated concentrations of al and fe acidic groundwater in the capture zones of wetlands may pose a long term risk to ecological values acidic groundwater in poorly buffered systems also poses a risk for users of untreated groundwater land and water use management should consider poorly buffered aquifers to be highly sensitive particularly where rainfall and recharge trends have declined over a period of time a comprehensive and consistent groundwater sampling program should be developed to fully delineate the groundwater acidification issue near the water table of the gnangara mound to assist the formulation of groundwater allocation plans

towards more detailed methods for quantifying the acid susceptibility of rocks and soils the sensitivity of rocks and soils to acidification is best described by their acid base titration curves these curves tend to be relatively featureless but can be usefully summarized in terms of their initial ph the buffer capacity at this ph and the total acidity to ph 8 3 unfortunately the shapes of soil titration curves depend on the type and concentration of background electrolyte and the rate of titration therefore while a set of standard titration conditions can be defined the resulting titration curves will not simulate those applicable to in situ field conditions as required for modelling purposes to progress it is necessary to gain a better understanding of the sources of and sinks for acidity in rocks and soils for soils partitioning of the acidity into soil solution neutral salt extractable and solid phase but not neutral salt extractable components provides a promising approach this is demonstrated using a series of soil samples derived from an extremely acid humus iron podzol overlying eocene sand near wokingham berks

near surface geochemical processes in quaternary sediments pore water analyses and mineralogical investigations from the unsaturated zone of quaternary glacial outwash sediments were carried out to investigate the major geochemical reactions that control the chemistry of seepage and groundwater the kinetics of rock water interaction especially that of dissolution precipitation are strongly influenced by secondary minerals and amorphous phases on grain surfaces the role of buffer systems such as calcite dissolution ion exchange and silicate weathering is discussed with help of a lumped model the kinetics of these systems and the methods for their investigation are presented

hydrogeochemistry as an aid to geological interpretation the glen roy area scotland the hydrochemistry of first and second order streams sampled at a density of 1 km 2 has been used to investigate ways of enhancing information obtained during regional geological mapping the glen roy area of scotland is a pristine area with large variations in bedrock geology ranging from dalradian metasediments including carbonates to granite granodiorite and appinite rocks coarse grained lamprophyre but with relatively minor drift alkalinity hco 3 is used to determine the extent of water rock interaction with both silicate and carbonate rocks the ratio na cl is also used to measure the extent of silicate reaction dolomitic facies are picked out in areas with carbonate by mg mn fe and y are useful indicators of the weathering of biotite and garnet a strong and well defined hydrogeochemical anomaly of ba sr and k outline the outcrops of the appinites together with hco 3 these three elements define an extended nw se lineament strath ossian lineament which includes possible concealed areas of appinite the bicarbonate probably reflecting the presence of a carbonate aureole derived from mantle co 2 in particular the bedrock characteristics in poorly exposed ground can be inferred and information obtained on the volume of bedrock through which the water has passed

hydrochemical evolution of the east midlands triassic sandstone aquifer england a detailed hydrogeochemical interpretation has been made of the red bed sherwood sandstone triassic aquifer of the east midlands of england on which a radiocarbon model has been used to establish a chronology of groundwaters along the flow line groundwater evolution is dominated by carbonate and sulphate mineral solution and precipitation and by redox reactions cation exchange reactions are negligible and the hydrogeochemistry has evolved in the absence of residual saline water three distinct zones are recognised 1 i recent groundwater mainly 100 y oxidising dominated by congruent dissolution of dolomite and by inputs from atmospheric and anthropogenic sources 2 ii groundwater age 103 104 y predominantly reducing dominated by incongruent dissolution of dolomite 3 iii groundwater age 104 y reducing conditions approach to isotopic and chemical equilibrium in the carbonate system dominance of gypsum dissolution the minor and trace element variation can be interpreted in relation to the principal geochemical reactions and recharge history na cl f and b are controlled by atmospheric inputs sr2 and mn2 trends can be explained by carbonate and sulphate mineral dissolution ba2 is derived from carbonates but controlled by barite equilibrium fe so42 no3 and u are strongly influenced by redox reactions the concentrations of transition metals and other elements remain very low only li and k are considered to represent additions by reaction of silicate minerals the extent of direct anthropogenic influence is probably limited to enhanced so4 no32 and possibly cu zn and pb in zone i

depositional and paleohydrogeological controls on the distribution of organic matter and other reactive reductants in aquifer sediments abstract the reactivity of sedimentary reductants is the main control for the natural attenuation of common groundwater contaminants such as nitrate or chlorinated hydrocarbons middle miocene to late pleistocene 0 01 20 my old aquifer sands of marine fluvial fluvio glacial and aeolian origin were characterized to determine the control of sediment depositional environment sediment age and paleohydrology on the distribution and reactivity of sedimentary reductants in addition to the variability in the molecular composition of sedimentary organic matter som the reduction reactivity of these sediments was determined oxygen consumption and carbon dioxide production during sediment incubations indicated that som pyrite and ferrous iron bearing carbonates were the main reductants bulk 13 c org values 25 indicated that terrestrial higher land plants were the main precursor of som regardless of sediment origin however pyrolysis gc ms analysis showed that som in the marine tertiary sands contained lignin with preserved side chains while in the pleistocene fluvial and fluvio aeolian sediments highly degraded lignin and recalcitrant macromolecular aliphatic structures dominated som indicative of aerobic degradation the higher dynamics of these terrestrial depositional environments as compared with their marine counterpart likely allows for increased oxygen exposure times and thus more intense aerobic degradation of som the aquifer sediment that fills a large erosional valley created during the saale ice age likely consists of fluvio glacially reworked marine tertiary sediments as supported by carbonate isotopic evidence the more degraded status and decreased reactivity of som in this sediment than in its precursor is likely due to the increased oxygen exposure during dynamic fluvio glacial reworking despite the highly degraded nature of som and the absence of pyrite in the pleistocene fluvial and fluvio aeolian sediments oxygen consumption rates were high during the incubation of sediments from the shallowest part of the aquifer a reactive ferroan carbonate phase was inferred as the main source of oxidant demand in shallow fluvial and aeolian sands depleted oxygen and carbon isotopes indicated that this phase was groundwater derived aerobic degradation during sediment deposition appears to control the molecular composition and reactivity of som in aquifer sediments and to affect the potential of subsequent diagenetic pyrite formation in addition paleo hydrological conditions may result in the accumulation of a ferrous iron bearing carbonate phase precipitated during the exfiltration of fe ii containing anoxic groundwater

major minor and trace element mobility in the acidic upland forested catchment of the upper river severn mid wales variations in the major minor and trace element concentrations in two streams within a coniferous forest are described and are related to hydrological and hydrogeochemical factors the rainfall chemistry reflects a maritime influence although there are occasional pollutant inputs ph 4 5 streamwater chemistry variations are strongly dependent upon flow baseflow chemistry determined by reactions in the inorganic soils and bedrock is of a mg ca hco 3 type ph 6 7 5 mineralization yields up to a four fold increase in ca hco 3 zn and so 4 concentrations ph 0 5 units higher stormflow chemistry reflects hydrogeochemical processes in the organic soil horizons the stormflow waters are acidic ph 4 5 and are enriched by an order of magnitude in h ai y co and mn mineral equilibrium calculations have been applied to aluminium hydroxides crystalline and amorphous aluminosilicates and hydroxy aluminium sulphates to determine the degrees of mineral saturation in the streamwaters ion exchange reactions have been evaluated to describe the consequences of natural variations in atmospheric salt loadings and evapotranspiration on the stream water chemistry and to explain low na cl ratios in storm waters

semi analytical solution of land derived solute transport under tidal fluctuation in a confined aquifer abstract a one dimensional semi analytical solution of land derived solute transport subject to tidal fluctuation in a coastal confined aquifer was derived using the generalized integral transform technique gitt to investigate the plume migration of land derived contaminants within a tidally influenced aquifer both spatially and temporally varying expressions of the darcy velocity and dispersion coefficients obtained from the analytical solution of the groundwater head response which were subject to sinusoidal boundary conditions due to tidal fluctuation were considered this new semi analytical solution was verified against a numerical solution as well as the peak location trajectory obtained using the predictor corrector method sensitivity analyses of tidal amplitude hydraulic conductivity and storage coefficient using the proposed solution were performed to understand plume behavior with regard to plume shape plume spatial moments and macrodispersion coefficients to gain a better understanding of the transport mechanisms as the tidal amplitude hydraulic conductivity and storage coefficient were increased the peaks were travelled faster and peak concentrations were decreased in addition an increase in tidal amplitude hydraulic conductivity and storage coefficient caused an increase in variance as well as the macrodispersion coefficient it was observed that negative macrodispersion appeared when the storage coefficient was largest as well as when the difference between landward directed advective velocity at the leading and trailing edges of the plume was greatest this newly developed semi analytical solution provides a useful mathematical tool for validating numerical models and understanding the physical mechanism of the migration of plume discharge to the sea or estuaries within a tidally influenced aquifer

driving mechanisms for groundwater flow and salt transport in a subterranean estuary this paper presents field measurements and numerical simulations of groundwater dynamics in the intertidal zone of a sandy meso tidal beach the study focusing on vertical hydraulic gradients and pore water salinities reveals that tides and waves provide important forcing mechanisms for flow and salt transport in the nearshore aquifer such forcing interacting with the beach morphology enhances the exchange between the aquifer and ocean the spatial and temporal variations of vertical hydraulic gradients demonstrate the complexity and dynamic nature of the processes and the extent of mixing between fresh groundwater and seawater in a subterranean estuary these results provide evidence of a potentially important reaction zone in the nearshore aquifer driven by oceanic oscillations land derived contaminants may undergo important biogeochemical transformations in this zone prior to discharge