Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O₂, pH, pCO₂ and Ca²⁺ (in situ), pore-water concentration profiles of NO₃⁻, NH₄⁺, Fe²⁺, and Mn²⁺ and SO₄²⁻ (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O₂, NO₃⁻, Fe(OH)₃ and SO₄²⁻, reoxidation reactions involving Fe²⁺ and Mn²⁺, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm⁻² yr⁻¹, of which 77% were due to O₂, 17% to NO₃⁻ and 3% to Fe(OH)₃ and 3% to SO₄²⁻. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d⁻¹ and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO₃ flux to the sediment was estimated to occur at a rate of 42 g m⁻² yr⁻¹ in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m⁻² yr⁻¹, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.     

The origin of high sulfate concentrations in a coastal plain aquifer,Long Island,New York

Ion-exchange batch experiments were run on Cretaceous (Magothy aquifer) clay cores from a nearshore borehole and an inland borehole on Long Island, NY, to determine the origin of high SO₄²⁻ concentrations in ground water. Desorption batch tests indicate that the amounts of SO₄²⁻ released from the core samples are much greater (980–4700 μg/g of sediment) than the concentrations in ground-water samples. The locally high SO₄²⁻ concentrations in pore water extracted from cores are consistent with the overall increase in SO₄²⁻ concentrations in ground water along Magothy flow paths. Results of the sorption batch tests indicate that SO₄²⁻ sorption onto clay is small but significant (40–120 μg/g of sediment) in the low-pH (<5) pore water of clays, and a significant part of the SO₄²⁻ in Magothy pore water may result from the oxidation of FeS₂ by dissolved Fe(III). The acidic conditions that result from FeS₂ oxidation in acidic pore water should result in greater sorption of SO₄²⁻ and other anions onto protonated surfaces than in neutral-pH pore water. Comparison of the amounts of Cl⁻ released from a clay core sample in desorption batch tests (4 μg/g of sediment) with the amounts of Cl⁻ sorbed to the same clay in sorption tests (3.7–5 μg/g) indicates that the high concentrations of Cl⁻ in pore water did not originate from connate seawater but were desorbed from sediment that was previously in contact with seawater. Furthermore, a hypothetical seawater transgression in the past is consistent with the observed pattern of sorbed cation complexes in the Magothy cores and could be a significant source of high SO₄²⁻ concentrations in Magothy ground water.     

Redox processes in the Oderbruch polder groundwater flow system in Germany

Large scale redox processes were investigated in a river recharged aquifer in the Oderbruch polder alongside the river Oder in north-eastern Germany. Major hydraulic and hydrochemical processes were identified qualitatively. As a result of intensive drainage activities in the past 250 a, the groundwater level within the polder is situated below the river water level and a levee prevents flooding of the lowland. As a consequence, river water permanently infiltrates into the shallow confined aquifer. A sequence of redox reactions, driven by organic matter degradation, can be observed during infiltration of oxic river water into the groundwater. Up to 3 km from the river, reduction processes from O₂ respiration to SO²⁻₄ reduction dominate the groundwater chemistry. While reduction of Fe- and Mn(hydr)oxides is the source of the high amounts of dissolved Fe²⁺ and Mn²⁺, carbonate dissolution/precipitation reactions control the actual groundwater concentration of Mn²⁺. The first order rate constant for SO²⁻₄ reduction was found to be −0.0169 a⁻¹. Fe²⁺ is released into the groundwater at a rate of 0.0033 mmol l⁻¹ a⁻¹. The groundwater chemistry is strongly linked to the hydraulic conditions. Near the river, the groundwater is confined and recharged by bank-filtration only. In contrast, in the central polder the groundwater is unconfined and percolation of rainwater through the dried loam is possible because of texture changes such as shrinkage fissures. Geogenic pyrite present within the alluvial loam is oxidised and large amounts of SO²⁻₄ are released into the groundwater.     

Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O<Subscript>2</Subscript>, Mn,Fe and N-Species in Experimental Units

Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O₂, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O₂ from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn²⁺ and then Fe²⁺, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe²⁺. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO₃⁻/NO₂⁻ rich horizon at the new oxic horizon. Over the experiment period, NO₃⁻ concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.     

Iron monosulfide formation and oxidation in drain-bottom sediments of an acid sulfate soil environment

Iron monosulfide formation and oxidation processes were studied in the extensively drained acid sulfate soil environment of the Tweed River floodplain in eastern Australia. Porewater profiles of pH, Eh, SO₄²⁻, Fe²⁺, Fe³⁺, Cl⁻, HCO₃⁻, and metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) were obtained using in situ dialysis membrane samplers (`peepers'). Concentrations of acid volatile S (AVS), pyrite, total S, reactive Fe, total and organic C, simultaneously extracted metals (SEMs) and total elemental composition by X-ray fluorescence, were determined on sediment samples. The oxidation of pyrite in the surrounding landscape provides a source of acidity, Fe, Al, SO₄ and metals, which are exported into the drainage system where they accumulate in the sediments and porewaters. Negative porewater concentration gradients of SO₄²⁻ and Fe²⁺, and large AVS concentrations in the sediments, indicate Fe monosulfides form rapidly under reducing conditions and consume acidity and metals. Oxidation of the sediments during previous drought episodes has resulted in the conversion of monosulfides and pyrite to oxidised Fe minerals and the release of acidity, SO₄²⁻, Fe³⁺, and metals to the surface waters. These formation and oxidation cycles show that Fe monosulfides play an important role in controlling water quality in the drainage system.     

Thermodynamics and organic matter: constraints on neutralization processes in sediments of highly acidic waters

The controls on the internal neutralization of low productivity, highly acidified waters by sulfide accumulation in sediments are yet poorly understood. It is demonstrated that the neutralization process is constrained by organic matter quality and thermodynamic effects which control the relative rates of SO₄ and Fe reduction, and the fate of the reduced Fe and S in the sediments. The investigated sediments were rich in dissolved Fe(II) (0.005–12 mmol l⁻¹) and SO₄ (1.3–22 mmol l⁻¹). The pH ranged from 3.0 to 6.8. Contents of reduced inorganic S (0.1–9.5%), molar C/N ratios of the organic matter (12–80) and metabolic turnover rates (1–110 μeq cm⁻³ a⁻¹) varied strongly. Substantial amounts of Fe sulfides were only found at a simultaneous partial thermodynamic and solubility equilibrium of the involved biogeochemical processes. Sulfide oxidation was apparently inhibited, and SO₄ and Fe reduction coexisted. In this type of sediment increases in C availability cause enhanced neutralization rates. In the absence of a partial equilibrium, the sediments were in a sulfide oxidizing and Fe reducing state, and did not accumulate Fe sulfides. The latter type of sediment will increase neutralization rates in response to decreasing deposition of reactive Fe oxides but not necessarily in response to increases in lake productivity by e.g. fertilization measures.     

Geochemistry and mineralogy of shallow alluvial aquifers in Daudkandi upazila in the Meghna flood plain, Bangladesh

The shallow alluvial aquifers of the delta plains and flood plains of Bangladesh, comprises about 70% of total land area are mostly affected by elevated concentrations of arsenic (As) in groundwater exposing a population of more than 35 million to As toxicity. Geochemical studies of shallow alluvial aquifer in the Meghna flood plain show that the uppermost yellowish grey sediment is low in As (1.03 mg/kg) compared to the lower dark grey to black sediment (5.24 mg/kg) rich in mica and organic matter. Sequential extraction data show that solid phase As bound to poorly crystalline and amorphous metal (Fe, Mn, Al)-oxyhydroxides is dominant in the grey to dark grey sediment and reaches its maximum level (3.05 mg/kg) in the mica rich layers. Amount of As bound to sulphides and organic matter also peaks in the dark grey to black sediment. Vertical distributions of major elements determined by X-ray fluorescence (XRF) show that iron (Fe₂O₃), aluminum (Al₂O₃) and manganese (MnO) follow the general trend of distribution of As in the sediments. Concentrations of As, Mn, Fe, HCO₃ ⁻, SO₄ ²⁻ and NO₃ ⁻ in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry. Mineralogical analysis by X-ray diffraction (XRD) and scanning electron microscopy (SEM) fitted with energy dispersive X-ray spectrometer (EDS) revealed dominance of crystalline iron oxides and hydroxides like magnetite, hematite and goethite in the oxidised yellowish grey sediment. Amorphous Fe-oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of biotite is playing a critical role as the source of Fe(III)-oxyhydroxides which in turn act as sink for As. Presence of authigenic pyrite in the dark grey sediment indicates active reduction in the aquifer.     

A plug flow-through reactor for studying biogeochemical reactions in undisturbed aquatic sediments

A slow flow, plug-through reactor was developed for measuring equilibrium and kinetic parameters of biogeochemical reactions on intact sections of sediment cores. The experimental approach was designed to preserve the structural, geochemical and microbiological integrity of the sediment sections and, hence, retrieve reaction parameters that apply to in-situ conditions.Inert tracer breakthrough experiments were performed on a variety of unconsolidated surface sediments from lacustrine, estuarine and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46–83%) and mean grain size (10⁻⁴−10⁻² cm). Longitudinal dispersion coefficients were determined from the breakthrough curves of Br⁻. The curves were also used to check for early breakthrough or trailing, that is, features indicative of non-ideal flow conditions. Sediment plugs that exhibited these features were eliminated from further experiments.Dimensionless equilibrium adsorption coefficients (K) of NH₄⁺, were calculated from measured retardation times between the breakthrough of NH₄⁺ and Br⁻. The values of K at 5°C vary between 0.3 and 2.3, with the highest value obtained in a fine-grained marine sediment, the lowest in a coarse-grained lake sediment. The values for the marine and estuarine sediments agree with values reported in the literature. The dependencies of K on ionic strength (range 0.2-0.7m) and temperature (range 5–25°C) in an estuarine sediment confirm that the main sorption mechanism for NH₄⁺ is ion exchange.The reactor was used in recirculation mode to measure steady-state rates of dissimilatory SO₄²⁻ reduction in a salt-marsh sediment. Recirculation homogenizes solute concentrations within the reactor, hence facilitating the derivation of reaction rate expressions that depend on solution composition. The rate of microbial S0₄⁻ reduction was found to be nearly independent of the dissolved SO₄²⁻ concentration in the range of 2.2−1 mM. Fitting of the experimental rates to a Monod relationship resulted in a maximum estimate of the half-saturation concentration, K_s, of 240 μM. This value is comparable to those reported for a pure culture of SO₄²⁻-reducing bacteria, but is significantly smaller than the only other K_s value reported in the literature for SO₄²⁻ utilization in a natural marine sediment.     

The variability of δ34S and sulfur speciation in sediments of the Sulejów dam reservoir (Central Poland)

The study was carried out on the Sulejów dam reservoir (Central Poland). Water and sediment samples were collected between February and October 2006. Sulfur compounds in the sediment were chemically extracted and subjected to isotopic analysis.Large variability of SO₄²⁻ concentration in the water column (from 10.3 to 36.2 mg/dm³) and the isotopic composition of sulfur (δ³⁴S from 2.1 to 5.4‰) was observed. The main identified sources of SO₄²⁻ were watercourses, surface runoff, and phosphorus fertilizers.Both oxidized sulfur species (SO₄²⁻) and its reduced forms were found in sediments. Particular sulfur forms were characterized by large variations in both, concentrations and the isotopic composition of sulfur. SO₄²⁻ in the sediment and in the water column had different genesis. Bacterial oxidation of organic sulfur and its binding in SO₄²⁻ were observed in the sediment. Under reducing conditions, oxidized and organic sulfur is converted to H₂S which reacted with Fe or other metallic ions leading to metal sulfide precipitation. Monosulfides were shown to have a very low concentration, ranging up to 0.07 mg/g of sediment. The transformation of elemental sulfur from sulfides through their chemical oxidation occurred in the sediment.     

Changes in the sorption capacity of Coastal Plain sediments due to redox alteration of mineral surfaces

Chemical characteristics of grain coatings in a Coastal Plain sandy aquifer on the Eastern Shore of Virginia were investigated where sediments have been exposed to distinct groundwater redox conditions. Dissolved O₂ was 5.0 to 10.6 mg L⁻¹ in the regionally extensive aerobic groundwater, whereas in a narrow leachate plume it was only <0.001 to 0.9 mg L⁻¹. The amount of dissolved Fe in the aerobic groundwater was only 0.005 to 0.01 mg L⁻¹, but it was 12 to 47 mg L⁻¹ in the anaerobic zone. The amount of extractable Fe was an order of magnitude higher for the aerobic sediments than for the anaerobic sediments indicating that reductive dissolution removed the oxide coatings. The capacity for anion sorption on the sediment surfaces, as indicated by the sorption of ³⁵SO₄^2-, was an order of magnitude higher in the aerobic vs. anaerobic sediments. The presence of anaerobic groundwater did not significantly alter the amount of extractable Al oxides on the surface of the sediments, and those coatings helped to maintain a high surface area. The removal of the Fe oxides from the grain surfaces under anaerobic conditions was solely responsible for the significant reduction of SO₄ sorption observed. This loss of capacity for anion sorption could lead to more extensive transport of negatively charged constituents such as some contaminant chemicals or bacteria that may be of concern in groundwater.     

Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge)

Marine sediment sequences with CH₄ hydrate are characterized by an atypical depth profile in dissolved Cl⁻ squeezed from pore space: a shallow subsurface Cl⁻ maximum overlies a lengthy and pronounced Cl⁻ minimum. This pore water Cl⁻ profile represents a combination of multiple processes including glacial–interglacial variations in ocean salinity, advection and diffusion of ions that are excluded during gas hydrate formation at depth, and release of fresh water from dissociation of hydrate during core recovery. In situ quantities of gas hydrate can be determined from a measured pore water Cl⁻ profile provided the in situ pore water signature prior to core recovery can be separated. Ocean Drilling Program (ODP) Site 997 was drilled into a large CH₄ hydrate reservoir on the Blake Ridge in the western Atlantic Ocean. Previously we have constructed a high-resolution pore water Cl⁻ profile at this location; here we present a `coupled chloride-hydrate' numerical model to explain basic trends in the Cl⁻ profile and to isolate in situ Cl⁻ concentrations. The model is based on thermodynamic and ecological considerations, and uses established equations for describing chemical behavior in marine sediment–pore water systems. The model incorporates four key concepts: (1) most gas hydrate is formed immediately below the SO₄²⁻ reduction zone; (2) fluid, dissolved ions and gas advect upward through the sediment column; (3) CH₄ hydrate dissociates at the base of hydrate stability conditions; and (4) seawater salinity fluctuates during glacial–interglacial cycles of the late Pliocene and Quaternary. Rates of upward advection in the model are sufficient to account for measured Br⁻ and I⁻ concentrations as well as CH₄ oxidation at the base of the SO₄²⁻ reduction zone. In situ pore water Cl⁻ inferred from the model is similar to that determined by limited direct sampling; in situ CH₄ hydrate amounts inferred from the model (an average of about 4% of porosity) are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques. The model also predicts production of substantial quantities of free CH₄ gas bubbles (>2.5% of porosity) at a depth immediately below the lowest accumulation of CH₄ hydrate in the sediment column. Our explanation for the pore water Cl⁻ profile at Site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water Cl⁻, gas hydrate, and free gas in a marine sediment column.     

Arsenic associations in sediments from the loess aquifer of La Pampa,Argentina

Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concentrations of As (up to 5300 μg L⁻¹) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concentrations in 45 loess samples collected from the aquifer have a range of 3–18 mg kg⁻¹ with a mean of 8 mg kg⁻¹. These values are comparable to world-average sediment As concentrations. Five samples of rhyolitic ash from the area have As concentrations of 7–12 mg kg⁻¹. Chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes. Results reveal a large range of porewater As concentrations, being generally higher in the horizons with highest sediment As concentrations. The displaced porewaters have As concentrations ranging up to 7500 μg L⁻¹ as well as exceptionally high concentrations of some other oxyanion species, including V up to 12 mg L⁻¹. The highest concentrations are found in a borehole located in a topographic depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective extract data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is associated with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Extract compositions also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidising, with dissolved O₂, NO₃ and SO₄ normally present and As(V) usually the dominant dissolved As species. Under such conditions, the solubility of Fe and Mn oxides is low and As mobilisation is strongly controlled by sorption–desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0–8.8) and high concentrations of potential competitors (e.g. V, P, HCO₃). PHREEQC modelling suggests that the presence of V at the concentrations observed in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by up to an order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate extract concentrations have been used to provide an upper estimate of the amount of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes investigated has been used to estimate an approximate K_d value for the sediments of 0.94 L kg⁻¹. This low value indicates that the sediments have an unusually low affinity for As.     

Environmental geochemistry of high arsenic groundwater at western Hetao plain, Inner Mongolia

Environmental geochemistry of high arsenic groundwater at Hetao plain was studied on the basis of geochemical survey of the groundwater and a core sediment. Arsenic concentration in groundwater samples varies from 76 to 1093 μg/L. The high arsenic groundwater mostly appears to be weakly alkaline. The concentrations of NO₃⁻ and SO₄²⁻ are relatively low, while the concentrations of DOC, NH₄⁺, dissolved Fe and sulfide are relatively great. Analysis of arsenic speciation in 21 samples shows that arsenic is present in the solution predominantly as As(III), while particulate arsenic constitutes about 10% of the total arsenic. Methane is detected in five samples with the greatest content being 5107 μg/L. The shallow aquifer in Hangjinhouqi of western Hetao plain is of strongly reducing condition. The arsenic content in 23 core sediment samples varies from 7.7 to 34.6 mg/kg, with great value in clay and mild clay layer. The obvious positive relationship in content between Fe₂O₃, Mn, Sb, B, V and As indicates that the distribution of arsenic in the sediments may be related to Fe and Mn oxides, and the mobilization of Sb, B and V may be affected by similar geochemical processes as that of As.     

Interpreting the borehole water chemistry of the Permo-Triassic sandstone aquifer of the Liverpool area,UK

Using hydrogeological data, historical chemical data and the results of studies in adjacent aquifers, an interpretation of the water chemistry from a sparse network of boreholes is presented for the Liverpool area. The chemistry of the fresh groundwater samples is influenced by geology, pollution and pumping history. The oldest waters, present where the sandstone is covered by Quaternary deposits, are calcite-saturated, contain little NO⁻₃ and have low SO²⁻₄ and Cl⁻ concentrations. However, water from the Collyhurst Sandstone are depleted in HCO⁻₃ whatever the concentrations of the other anions. Samples from boreholes in areas where the sandstones are not covered by Quaternary deposits are characterized by very low alkalinity and pH, and by high NO⁻₃, SO²⁻₄, and Cl⁻. In the regions of the aquifer close to sandstone outcrop, or where the Quaternary deposits are thin, the water samples have higher alkalinity and pH, and lower anion concentrations. Scattered throughout the region are boreholes yielding waters with very high SO²⁻₄ concentrations: where associated with industrial sites, these waters also have high NO⁻₃ concentrations and industrial pollution is suspected. In rural areas the high SO²⁻₄ concentrations are derived from leakage through the sulphur-bearing tills in response to pumping-induced lowering of the piezometric surface. The distribution of borehole water types can be described with the help of a set of rules relating water type to hydrogeological features; these rules allow a map of hydrochemical distributions to be constructed. Saline groundwaters occur in the aquifer adjacent to the Mersey Estuary and have chemistry compositions equivalent to slightly modified, diluted Estuary water. With the exception of a single deep borehole sample, there is no indication of the widespread presence of ancient saline groundwaters in the base of the sandstone sequences as is found in the sandstones to the east of the study area. However, slightly saline, reduced waters occur below the Mercia Mudstone Group in the north of the area. Historical records give some indication of the changes in water chemistry distributions through time.     

Calcite dissolution driven by benthic mineralization in the deep-sea: in situ measurements of Ca2+, pH,pCO2 and O2

In situ measured microprofiles of Ca²⁺, pCO₂, pH and O₂ were performed to quantify the CaCO₃ dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m⁻² d⁻¹ and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m⁻² a⁻¹. The metabolic CO₂, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca²⁺ microprofiles were 0.6 mmol m⁻² d⁻¹ for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO₂ produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O₂, pH and Ca²⁺ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw⁻¹ a⁻¹ (mol per kg water and year), which equals 95% d⁻¹. This rate constant is at the upper end of reported in situ values.     

Evaluation of the origin and fate of nitrate in the Abbotsford Aquifer using the isotopes of15N and18O in NO3−

Extensive NO₃⁻ contamination of groundwater in the Abbotsford aquifer to levels above drinking water limits is a major problem in the Fraser Lowlands of southwestern British Columbia, Canada. Nitrate concentrations in the aquifer ranged from 0 to 151 mg/l NO₃⁻, with a median concentration of 46 mg/l NO₃⁻. Of 117 wells sampled, 54% had NO₃⁻ concentrations exceeding the drinking water limit of 45 mg/1. Approximately 80% of the study area had groundwater NO₃⁻ concentrations exceeding 40 mg/1 NO₃⁻. Potential NO₃⁻ source materials were poultry manure N and synthetic NH₄ based fertilizers. Theδ¹⁵N of solid poultry manure samples ranged between + 7.9 and + 8.6‰ (AIR). Four brands of synthetic fertilizers commonly used hadδ¹⁵N values between −1.5 and −0.6‰. Ammonia volatilization caused theδ¹⁵N of groundwater NO₃⁻ produced from poultry manure N to range between +8 and +16‰. Theδ¹⁸O values of groundwater NO₃⁻, by contrast, mostly ranged between +2 and +5‰ (SMOW). This narrow range ofδ¹⁸O values fell within the expected range of NO₃⁻ produced by nitrification of reduced N forms such as poultry manure N and NH₄ fertilizers, and had a similar range ofδ¹⁸O values as NO₃⁻ in the upper part of the unsaturated zone below raspberry fields and beneath former manure piles. Theδ¹⁵N-NO₃⁻ andδ¹⁸O-NO₃⁻ data confirmed that NO₃⁻ in the aquifer was predominantly derived from poultry manure and to a lesser extent from synthetic fertilizers. Theδ¹⁸O-NO₃⁻ data further suggested the nitrification process occurred mainly in the summer months, with the soil NO₃⁻ produced subsequently flushed into the aquifer during fall recharge. Theδ¹⁵N-NO₃⁻andδ¹⁸O-NO₃⁻ data conclusively indicated that no significant bacterial denitrification is taking place in the Abbotsford aquifer.     

Fe and Mn levels regulated by agricultural activities in alluvial groundwaters underneath a flooded paddy field

Iron and Mn concentrations in fresh groundwaters of alluvial aquifers are generally high in reducing conditions reflecting low SO₄ concentrations. The mass balance and isotopic approaches of this study demonstrate that reduction of SO₄, supplied from agricultural activities such as fertilization and irrigation, is important in lowering Fe and Mn levels in alluvial groundwaters underneath a paddy field. This study was performed to investigate the processes regulating Fe and Mn levels in groundwaters of a point bar area, which has been intensively used for flood cultivation. Four multilevel-groundwater samplers were installed to examine the relationship between geology and the vertical changes in water chemistry. The results show that Fe and Mn levels are regulated by the presence of NO₃ at shallow depths and by SO₄ reduction at the greater depths. Isotopic and mass balance analyses revealed that NO₃ and SO₄ in groundwater are mostly supplied from the paddy field, suggesting that the Fe-and Mn-rich zone of the study area is confined by the agricultural activities. For this reason, the geologic conditions controlling the infiltration of agrochemicals are also important for the occurrence of Fe/Mn-rich groundwaters in the paddy field area.     

Hydrochemical characterization,mechanism of mobilization,and natural background level evaluation of arsenic in the aquifers of upper Gangetic plain,India

Although arsenic (As) contamination has been extensively investigated in the aquifers of the lower and middle Gangetic plains, less attention has been given to the distribution and fate of As in the groundwater of the upper Gangetic plain, India. In the current study, groundwater samples (n = 40) were collected from Moradabad district in the upper Gangetic plain and analyzed for several physicochemical parameters to characterize the groundwater chemistry and evaluate various geogenic and anthropogenic factors controlling the occurrence, mobilization, and fate of As in the plain. Arsenic concentrations in groundwater ranged from 0.17 μg/L to 139 μg/L, with the majority of high-As groundwater associated with high Fe, Mn, and HCO₃⁻ and low NO₃⁻, SO₄²⁻, and negative Eh values, implying that As was released via reductive dissolution of Fe and Mn oxyhydroxides in reducing conditions under the influence of organic matter degradation. Interrelationships between various geochemical variables and the natural background level (NBL) quantification of As suggested the influence of anthropogenic processes on the mobility of As in groundwater. Piper and Gibbs diagrams and various bivariate plots revealed that the majority of groundwater was of the Ca²⁺ − Mg²⁺ − HCO₃⁻ type and that the major ions in groundwater were derived from carbonate and silicate weathering, cation exchange and reverse ion exchange processes, and anthropogenic activities. Moreover, the results of principal component analysis (PCA), and hierarchical cluster analysis (HCA) also suggested geogenic and anthropogenic sources for the ion concentration in groundwater. The health risk assessment showed a higher non-carcinogenic risk for children and a higher carcinogenic risk for adults, respectively, due to the daily intake of As contaminated groundwater. Overall, this study represents the first systematic investigation of the distribution, geochemical behavior, and release process of As in groundwater in the study area and provides a strong base for future research in the alluvial aquifers of the upper Gangetic plain.     

Effects of cations and anions on iron and manganese sorption and desorption capacity in calcareous soils from Iran

This study investigated the effect of cations and anions on the sorption and desorption of iron (Fe) and manganese (Mn) in six surface calcareous soil samples from Western Iran. Six 10 mM electrolyte background solutions were used in the study, i.e., KCl, KNO₃, KH₂PO₄, Ca(NO₃)₂, NaNO₃, and NH₄NO₃. NH₄NO₃ and NaNO₃ increased the soil retention of Fe and Mn, whereas Ca(NO₃)₂ decreased the soil retention of Fe and Mn. Iron and Mn sorption was decreased by NO₃ ^? compared with H₂PO₄ ^? or Cl^?. The Freundlich equation adequately described Fe and Mn adsorption, with all background electrolytes. The Freundlich distribution coefficient (K _F) decreased in the order H₂PO₄ ^? > Cl^? > NO₃ ^? for Mn and H₂PO₄ ^? > NO₃ ^? > Cl^? for Fe. The highest sorption reversibility was for Fe and Mn in competition with a Ca²⁺ background, indicating the high mobility of these two cations. A MINTEQ speciation solubility model showed that Fe and Mn speciation was considerably affected by the electrolyte background used. Saturation indices indicated that all ion background solutions were saturated with respect to siderite and vivianite at low and high Fe concentrations. All ion background solutions were saturated with respect to MnCO_3(am), MnHPO₄, and rhodochrosite at low and high Mn concentrations. The hysteresis indices (HI) obtained for the different ion backgrounds were regressed on soil properties indicating that silt, clay, sand, and electrical conductivity (EC) were the most important soil properties influencing Fe adsorption, while cation exchange capacity (CEC), organic matter (OM), and Mn-DTPA affected Mn adsorption in these soils.     

Redox control of arsenic mobilization in Bangladesh groundwater

Detailed hydrochemical measurements, δ³⁴S_SO4 and ³H analyses were performed on 37 groundwater samples collected during February 1999, January and March 2000 from 6 locations in eastern and southeastern Bangladesh to examine redox processes that lead to As mobilization in groundwater. The study sites were chosen based on available nation-wide As surveys to span the entire spectrum of As concentrations in Bangladesh groundwater, and to represent 3 of 5 major geological units of the Ganges-Brahmaputra Delta: uplifted Pleistocene terrace, fluvial flood plain and delta plain. Arsenic was found to be mobilized under Fe-reducing conditions in shallow aquifers (<35 m depth), presumably of Holocene age. It remained mobile under SO₄-reducing conditions, suggesting that authigenic sulfide precipitation does not constitute a significant sink for As in these groundwaters. The redox state of the water was characterized by a variety of parameters including dissolved O₂, NO₃⁻, Mn²⁺, Fe²⁺ concentrations, and SO₄²⁻/Cl⁻ ratios. High dissolved [As] (> 50 μg/l; or > 0.7 μM ) were always accompanied by high dissolved [HCO₃⁻] (> 4 mM), and were close to saturation with respect to calcite. Groundwater enriched in As (200–800 μg/l; or 2.7–10.7 μM) and phosphate (30–100 μM) but relatively low in dissolved Fe (5–40 μM) probably resulted from re-oxidation of reducing, As and Fe enriched water. This history was deduced from isotopic signatures of δ³⁴S_SO4 and ³H₂O (³H) to delineate the nature of redox changes for some of the reducing groundwaters. In contrast, As is not mobilized in presumed Pleistocene aquifers, both shallow (30–60 m) and deep (150–270 m), because conditions were not reducing enough due to lack of sufficient O₂ demand.