Applied Geochemistry (v.15, #4)

Mechanism of arsenic release to groundwater, Bangladesh and West Bengal by R.T. Nickson; J.M. McArthur; P. Ravenscroft; W.G. Burgess; K.M. Ahmed (403-413).
In some areas of Bangladesh and West Bengal, concentrations of As in groundwater exceed guide concentrations, set internationally and nationally at 10 to 50 μg l−1 and may reach levels in the mg l−1 range. The As derives from reductive dissolution of Fe oxyhydroxide and release of its sorbed As. The Fe oxyhydroxide exists in the aquifer as dispersed phases, such as coatings on sedimentary grains. Recalculated to pure FeOOH, As concentrations in this phase reach 517 ppm. Reduction of the Fe is driven by microbial metabolism of sedimentary organic matter, which is present in concentrations as high as 6% C. Arsenic released by oxidation of pyrite, as water levels are drawn down and air enters the aquifer, contributes negligibly to the problem of As pollution. Identification of the mechanism of As release to groundwater helps to provide a framework to guide the placement of new water wells so that they will have acceptable concentrations of As.

The weathering of heavy minerals in acid soil profiles (pH 4–5) has been studied in terms of their relative mineral proportions and by using the surface etching features of apatite and hornblende grains. An increasing stability order of: apatite, titanite, hornblende, garnet, epidote, zircon is found in the 45–63 μm coarse-silt fraction of soils developed on tills in southwestern Sweden. In glaciofluvial deposits, sorting processes during deposition have largely determined the variations in heavy mineral content of soils, and only the dissolution of apatite is significant. Hornblende etching is more extensive in soils with low hornblende content, indicating that the release of cations relative to the amount of hornblende is greater in such soils.

Dissolution of silica and the development of concentration profiles in freshwater sediments by W.A. House; F.H. Denison; M.S. Warwick; B.V. Zhmud (425-438).
The dissolution of silica and diffusion of reactive dissolved Si in the porewaters of river sediments are investigated using sediments of different physical and chemical properties. Three sediments are considered: (a) from sectioned cores taken from a river-bed, (b) fine organic-rich surface sediment (<5 cm depth) installed in a fluvarium channel and, (c) coarse river sediment of low organic matter content also installed in a fluvarium channel. Dissolution rates of silica are measured at 10°C using batches of suspended material. The derived dissolution rate constants show large differences between the sediments. The river bed-sediment cores had vertical concentration profiles of dissolved Si that are consistent with the diffusion and dissolution of biogenic silica. Experiments in a fluvarium channel enabled Si fluxes to be calculated from a mass-balance of the overlying solution. The results are consistent with the attainment of a steady-state concentration profile of dissolved Si in the sediment. There are no discernible effects of water velocity over the sediment between 5 and 11 cm s−1. However, at 20 cm s−1, the flux increases as a result of either entrainment of fine particles at the surface or advective effects in the surface sediment. A fluvarium experiment with the fine sediment (<125 μm) over 61 days, produced a concentration profile with the highest concentration of 1025 μmol dm−3 at a depth of 4–5 cm in the sediment. A FORTRAN program is used to model the results of the increase in dissolved Si in the overlying water and development of a concentration profile in the porewater. This leads to a sediment diffusion coefficient of 1.21×10−9 m2 s−1 at 8.8°C at the beginning of the experiment and rate constant k=13.1×10−7 s−1 at pH=7.82 and average temperature of 7.6°C for the entire experiment. Fluxes measured at the sediment–surface interface and calculated assuming steady-state profiles had developed are typically 0.01–0.04 μmol m−2 (of river bed) s−1. The approach enables the efflux of dissolved Si from bottom-sediments to be estimated from dissolution rates measured using suspensions of bed-sediment.

A comparison of new data with historical records indicates that the chemistry of thermal springs from the Canadian Cordillera is constant through time, suggesting that water compositions develop equilibrium with the host rock. A thermodynamic model is used to evaluate the influence of water–rock interaction on the chemistry of thermal spring waters. An isotope mass-balance approach is used to evaluate biological controls on the S and C cycles in the springs.A comparison of mineral stability with water compositions suggests that the activities of major cations are controlled by equilibrium reactions with common rock forming minerals and alteration products. Sulfur has a complex redox history in thermal springs. Sulfate derived from dissolution of evaporite minerals is reduced by bacteria, causing the production of HS. The loss of HS from the system appears to be minor, instead it is reoxidized to SO4 as the spring water ascends to surface. Calculations indicate that the amount of SO4 that is reduced and reoxidized varies from 0 to 53%. There is an inverse relationship between the proportion of biological cycling of SO4 and the concentration of SO4, indicating that SO4 is not a limiting nutrient in hydrothermal systems. In low alkalinity thermal springs, HCO3 is derived from either dissolution of carbonate minerals or oxidized organic matter. However, for high alkalinity springs (>100 mg/l) HCO3 is dominantly derived from carbonate dissolution.

Water and gas geochemistry of the Euganean and Berician thermal district (Italy) by F Gherardi; C Panichi; S Caliro; G Magro; M Pennisi (455-474).
Between 1987 and 1995 more than 100 chemical and isotopic analyses were carried out on the thermal fluids discharged at surface from wells and springs of the Euganean and Berician thermal district. Results for δD and δ 18O in waters, δ 13C in CO2 and in C1–C4 n-alkanes, δD in CH4, 3He/4He and 40Ar/36Ar ratios in natural gases were coupled with chemical analyses in an attempt to determine the main characteristics and evolutionary trends of thermal fluids emerging in the region. The isotopic and chemical composition of thermal waters has led to the postulation of a meteoric origin of discharged thermal fluids and of a “maturation” trend as water moves from the peripheral manifestations of the Berici Hills towards those of the Battaglia, Montegrotto and Abano springs in the inner part of the geothermal field. Numerical simulation suggested that the observed evolutionary path is consistent with differentiation due to processes of water–rock interaction.The results of bulk analyses have shown that the gases are made up mainly of N2 (65–95 vol%), CO2 (0.5–20.5 vol%) and CH4 (up to 10 vol%), with relatively high Ar and He contents (up to 1.5 vol% and 0.16 vol%, respectively) and detectable amounts of C2–C6 saturated hydrocarbons. The chemical and isotopic composition of the gases suggests that both the meteoric and crustal contributions to the natural discharges are significant, while any significant magmatic contribution, possibly related to vestiges of the volcanic activity that occurred in the Abano area during the Tertiary age, can be ruled out.

Geochemical modeling approach to predicting arsenic concentrations in a mine pit lake by Regina N Tempel; Lisa A Shevenell; Paul Lechler; Jonathan Price (475-492).
Between 1968 and 1983, the North pit at the Getchell Mine, Humboldt County, NV, filled with water to form a lake. In 1983, water quality data were collected with the following results: As concentrations of 0.29 to 0.59 mg/L, pH of 7.1 to 7.9, SO4 concentrations of 1490 to 1640 mg/L, and TDS of 2394 to 2500 mg/L. Using geochemical modeling techniques presented here, pit lake waters have been theoretically allowed to react for 8.5 a, the approximate time that the North pit had been completely full by 1983. Modeling results predict pH of 7.9 to 8.2, SO4 concentrations of 1503 to 1644 mg/L, TDS of 2054 to 2366 mg/L, and As concentrations ranging from 0.57 in the hypolimnion to 96 mg/L in the epilimnion. In the epilimnion, model results do not match observed As concentrations, suggesting that mechanisms, such as precipitation of arsenate salts or adsorption to mineral surfaces, may control As levels in an actual pit lake system. Adsorption to Fe oxyhydroxide surfaces is questioned by the authors because of the low Fe content in the Getchell system, but adsorption to Al(OH)3 (gibbsite) and clay mineral surfaces may be important in controlling natural As concentrations.

A study was undertaken to explore whether the isotopic compositions of Pb and Sr are useful to distinguish mixtures of uncontaminated groundwater, seawater, and landfill leachate at the Fresh Kills landfill, Staten Island, New York. Ratios of 87Sr/86Sr ranged from 0.7088 to 0.7137 and could be used to distinguish Sr that was derived from seawater from that in uncontaminated groundwater. Lead isotopic abundances did not vary systematically among the different water sources. Plots of 87Sr/86Sr versus dissolved organic C, B, and NH4 + defined perpendicular trends, documenting where leachate or sea water mixed with uncontaminated groundwater, and demonstrating that leachate has not contaminated groundwater in aquifers beneath the landfill.

The aqueous solubility of trichloroethene (TCE) and tetrachloroethene (PCE) as a function of temperature by Kevin G Knauss; Michael J Dibley; Roald N Leif; Daniel A Mew; Roger D Aines (501-512).
Using a flexible Au bag autoclave and a precision high-pressure liquid chromatography pump to control pressure, the liquid–liquid aqueous solubilities of TCE and PCE were measured as a function of temperature from 294 to 434 K (at constant pressure). The results were used to calculate the partial molal thermodynamic quantities of the organic liquid aqueous dissolution reactions: Δ soln, Δ soln, Δ soln and Δ p soln. Calculated values for these quantities at 298 K for TCE are: Δ soln=11.282 (±0.003) kJ/mol, Δ soln=−3.35 (±0.07) kJ/mol, Δ soln=−49.07 (±0.24) J/mol K, and Δ p soln=385.2 (±3.4) J/mol K. Calculated values for these quantities at 298 K for PCE are: Δ soln=15.80 (±0.04) kJ/mol, Δ soln=−1.79 (±0.58) kJ/mol, Δ soln=−59.00 (±1.96) J/mol K and Δ p soln=354.6 (±8.6) J/mol K. These thermodynamic quantities may be used to calculate the solubility of TCE and PCE at any temperature of interest. In the absence of direct measurements over this temperature range, the Henry's Law constants for TCE and PCE have been estimated using the measured aqueous solubilities and calculated vapor pressures.

Metal contamination of soils at Scott Base, Antarctica by D.S Sheppard; G.G.C Claridge; I.B Campbell (513-530).
Soil samples taken from excavated pits on traverses across New Zealand’s Scott Base, Antarctica, were leached with water and 0.01 M HNO3 and the leachates analysed for Ag, Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn. The soils had high conductivity and pH values generally increasing with depth and in the range 8.3–10.1. The water leachate generally contained most of the extractable metals except Mn and Cd, and As. Linear relationships were observed between some metals leached into alkaline solution and the Fe in those solutions. The ratios to Fe were comparable to those of the host basanite, and this observation is interpreted as showing that these metals are incorporated in fine mineral particulates derived directly from the rock mass. Outliers in leachable metal concentrations in the soils indicated appreciable contamination of the soil from anthropogenic sources with Ag, Cd, Cu, Pb and Zn as well as As. In some locations high concentrations of Ag and Cd correspond to specific sources and drainage channels. High concentrations of Pb were widely spread and in the top soil layers whereas the elevated concentrations of Zn were distributed throughout the soil profiles indicating atmospheric sources and different mobilities within the soils. Transport within the soils is evident for some metals, as is lateral movement over and through the soils.

Regional geochemical reconnaissance of the Cordillera Occidental of Ecuador: economic and environmental applications by T.Martin Williams; Peter N Dunkley; Edgar Cruz; Victor Acitimbay; Alina Gaibor; Edgar Lopez; Napoleon Baez; John A Aspden (531-550).
A regional geochemical reconnaissance survey of the Cordillera Occidental of Ecuador was initiated in 1995 as a sub-component of a wider Mining Development and Environmental Control Technical Assistance Project (PRODEMINCA) in Ecuador. The 36,000 km2 survey area encompasses oceanic and continental-margin volcano-sedimentary terranes with known occurrences of porphyry-style Cu-Mo, exhalative massive sulphide, epithermal Au and mesothermal polymetallic mineralisation. A survey sample medium of <177 μm stream sediments was selected following an orientation study in the vicinity of known porphyry Cu mineralisation. In the 2–4°S sector of the Cordillera Occidental for which data are presented, 4850 drainage samples were collected at an average density of 1 per 2.57 km2. All were analysed for 36 major and trace elements (Au, Ag, Cu, Pb, Zn, Mo, Ni, Co, Cd, Bi, As, Sb, Fe, Hg, Mn, Te, Ba, Cr, V, Sn, W, La, Al, Mg, Ca, Na, K, Sr, Y, Ga, Li, Nb, Sc, Ta, Ti, Zr). A stringent quality-control procedure included the systematic analysis of certified reference samples, field duplicates and replicates, data for which were used to calculate analytical precision, temporal drift and practical detection limits. Results for this part of the cordillera highlight the contrasting lithogeochemical signatures of the ocean-floor basalt terrane (Pallatanga Unit), the island-arc terrane of the Macuchi Unit, the continental volcanics of the Saraguro Group and the acid and intermediate lavas extruded from the Late Miocene to the Quaternary. Regional geochemical images for Au and associated pathfinder elements are dominated by anomalies relating to known mines and prospects. New exploration targets, often inconspicuous at the regional scale, have however been identified through the normalisation of data for individual lithological units against their respective geochemical backgrounds. In addition to mineral exploration, the drainage geochemical dataset for the Cordillera Occidental provides an unparalleled environmental baseline against which the impacts of future anthropogenic activities (including mining) may be assessed. A basis for the formulation of pragmatic sediment quality criteria and for the identification of natural geochemical hazards is also provided.