Applied Geochemistry (v.18, #4)

Annual yields (fluxes per unit area) of Al, Mn, Fe, Ni, Cd, Pb, Zn, Cu, Cr, Co, As and Se were estimated for two small non-tidal stream catchments on the Eastern Shore of the Chesapeake Bay, United States—a poorly drained dissected-upland watershed in the Nanticoke River Basin, and a well-drained feeder tributary in the lower reaches of the Chester River Basin. Both watersheds are dominated by agriculture. A hydrograph-separation technique was used to determine the baseflow and stormflow components of metal yields, thus providing important insights into the effects of hydrology and climate on the transport of metals. Concentrations of suspended-sediment were used as a less-costly proxy of metal concentrations which are generally associated with particles. Results were compared to other studies in Chesapeake Bay and to general trends in metal concentrations across the United States. The study documented a larger than background yield of Zn and Co from the upper Nanticoke River Basin and possibly enriched concentrations of As, Cd and Se from both the upper Nanticoke River and the Chesterville Branch (a tributary of the lower Chester River). The annual yield of total Zn from the Nanticoke River Basin in 1998 was 18,000 g/km2/a, and was two to three times higher than yields reported from comparable river basins in the region. Concentrations of Cd also were high in both basins when compared to crustal concentrations and to other national data, but were within reasonable agreement with other Chesapeake Bay studies. Thus, Cd may be enriched locally either in natural materials or from agriculture.

Degradation of carbon tetrachloride in a reducing groundwater environment: implications for natural attenuation by Andy Davis; G.G Fennemore; C Peck; C.R Walker; J McIlwraith; S Thomas (503-525).
Several laboratory experiments have demonstrated degradation of carbon tetrachloride (CT) in groundwater, but there appear to have been no corroborating long-term field studies. Investigations conducted in 1989 and 1999 at an industrial site constructed on an infilled estuarine environment in France provide data over a decade for which CT degradation could be evaluated. A Dense Non-Aqueous Phase Liquid (DNAPL) containing oil and >90% CT that was present in 1989 was absent in the extremely reducing site groundwater in both 1999 and 2000 (average Eh=−170 mV at pH 7, sulfide up to 21 mg l−1, and Fe+2 up to 3.2 mg l−1). These conditions facilitated dechlorination of CT to chloroform (CF) present at up to 46 mg l−1, and methylene chloride (up to 75 mg l−1). Carbon disulfide (CS2), a terminal degradation product in reducing environments in laboratory experiments, was present at a mass ratio averaging 2.4:1 CF:CS2, indicative of abiotic degradation. The lack of detection of the separate phase CT, the ratio of CF:CS2, the presence of low molecular weight organic acids (i.e., acetate ∼900 mg l−1; citrate 360 mg l−1; and propionate, up to 111 mg l−1) and pyrite in conjunction with excess inorganic Cl in groundwater are all indicators of ongoing degradation of the chlorinated compounds. However, while natural attenuation of chloromethanes may be a viable adjunct to strategies designed to remediate CT in reducing groundwater, its efficacy is hard to quantify in complex field environments where upgradient sources are still present.

Thermodynamic parameters for proton and metal adsorption onto a gram-negative bacterium from the genus Enterobacteriaceae have been determined and compared with parameters for other strains of bacteria. Potentiometric titrations were used to determine the different types of sites present on bacterial cell walls. Stability constants for adsorption of Pb, Cu and Zn to specific sites were determined from batch adsorption experiments at varying pH with constant metal concentration. Titrations revealed 3 distinct acidic surface sites on the bacterial surface, with pK values of 4.3±0.2, 6.9±0.5 and 8.9±0.5, corresponding to carboxyl, phosphate and hydroxyl/amine groups, with surface densities of 5.0±0.7×10−4, 2.2±0.6×10−4 and 5.5±2.2×10−4 mol/g of dry bacteria. Only carboxyl and phosphate sites are involved in metal uptake, yielding the following intrinsic stability constants: Log K carboxyl: Zn=3.3±0.1, Pb=3.9±0.8, and Cu=4.4±0.2, Log K phosphoryl: Zn=5.1±0.1 and Pb=5.0±0.9. The deprotonation constants are similar to those of other strains of bacteria, while site densities are also within an order of magnitude of other strains. The similarities in surface chemistry and metal stability constants suggest that bacteria may be represented by a simple generic thermodynamic model for the purposes of modelling metal transport in natural environments. Comparison with oxide-coated sand shows that bacteria can attenuate some metals to much lower pH values.

This paper describes a rapid procedure that allows precise analysis of Mo, Cd, U and Th in sediment samples as small as 10 mg by using a novel approach that utilizes a “pseudo” isotope dilution for Th and conventional isotope dilution for Mo, Cd and U by ICP-MS. Long-term reproducibility of the method is between 2.5 and 5% with an advantage of rapid analysis on a single digestion of sediment sample and the potential of adding other elements of interest if so desired. Application of this method to two piston cores collected near the mouth of the Patuxent River in Chesapeake Bay showed that the accumulation of authigenic Mo and Cd varied in response to the changing bottom water redox conditions, with anoxia showing consistent oscillations throughout both pre-industrial and industrial times. Accumulation of authigenic U shows consistent oscillations as well, without any apparent increase in productivity related to anoxic trends. Degrees of Mo and Cd enrichment also inversely correlate to halophilic microfaunal assemblages already established as paleoclimate proxies within the bay indicating that bottom water anoxia is driven in part by the amount of freshwater discharge that the area receives.

The radial diffusion method is a new technique that, among other applications, allows laboratory study of the chemistry of groundwater in intact aquitard materials. The computer model PHREEQC was used to simulate the changes in hydrochemistry observed in radial diffusion cells. A relatively small number of assumptions were required. In addition to using the mass measurements and the initial concentrations of aqueous species in the cells (after first addition of solution), equilibration of the aqueous solutions with the mineral calcite, and with a pool of exchangeable cations, was assumed for each step. For the majority of dilution steps, one or more of the following redox reactions were employed: oxidation of pyrite, oxidation of organic C, reduction of SO4. The simulated trends in dissolved concentrations were close to the observed data. However, the simulated levels of exchangeable cations based on tests with radial diffusion cells were generally lower than those determined by a conventional method, especially in the case of a Cretaceous “shale” core .

Sequential and selective extraction procedures for determining Hg speciation in substrates were evaluated and applied to an extraction protocol to assess Hg mobility in samples of mine waste. Pyrolytic methods were applied to determine volatile phases; leaching with Cl-solutions was utilized to determine soluble or mobile species; and acid digestions were used to extract strongly bound Hg species and for total Hg analyses. Pyrolytic extractions at greater than 80 °C were found to significantly over estimate elemental Hg. The organic content of a substrate was found to confound sequential extraction results. Inorganic amendments such as iron Fe and vermiculite also influenced extraction results but not as strongly as organic matter. This study demonstrated that sequential and selective extractions for identification of specific Hg phases should be used with caution. Comparison of HF, aqua regia, and 3 H2SO4 : 7 HNO3 digestions revealed that aqua regia was as effective as the HF digestion for determining total Hg. The 3 H2SO4:7 HNO3 digestion was significantly less effective for determining total Hg on silicified samples. Analysis of Hg in mine waste indicated that introduced Hg and naturally occurring Hg minerals are relatively immobile in terms of the percent of leachable and volatile Hg verses the total Hg concentration. Extraction methods were found to overestimate Hg released from mine waste when compared to in situ measurements.

Historical trends of metal pollution recorded in the sediments of the Culiacan River Estuary, Northwestern Mexico by A.C Ruiz-Fernández; C Hillaire-Marcel; F Páez-Osuna; B Ghaleb; M Soto-Jiménez (577-588).
The accumulation of selected trace metals (Cd, Co, Cu, Ni, Pb and Zn) was studied in a sediment core collected in a shallow sandy area at the upper estuary of the Culiacan River. The chronology was developed by using the accumulation rates determined previously from 210Pb analyses of the same core. Trace metal concentration began to increase over background levels at depths corresponding to the late 40s and reached a maximum at the present time, excepting for Cd and Pb. The observed profile trends were related to population growth, and significant relationships between organic C and Cd, Cu and Zn indicated that such metals are mainly delivered to the estuarine sediments from a common source, identified as sewage wastes. Excess metal inventories and metal concentration factors indicated a slight pollution by all the trace metals examined, although levels of enrichment for Ni and Pb (Concentration Factors ⩽1.5) were considerably less than those found for Co, Cd, Zn and Cu (CF's∼3). However, the concentrations of Cu, Pb and Ni were considered as potentially toxic and therefore, these metals must be considered pollutants of concern and further investigation (e.g. biological and chemical testing) is strongly recommended.

A critical component in determining the suitability of disposing glassified, low activity waste is the identification of key mineral assemblages affecting the porosity and permeability of both the glass and near- and far-field materials. In this study, two different classes of geochemical models are used to identify mineral precipitation and dissolution potentials for an immobilized low-activity waste (ILAW) disposal facility in Hanford, Washington. The first is a static geochemical model that does not consider the effects of transport. The second model is dynamic, and combines geochemical reactions with hydrogeological processes such as advection, diffusion and dispersion. This reactive transport model also includes an innovative application of a depositional film model for determining changes in permeability due to mineral precipitation and dissolution reactions. Although both models describe solid-aqueous phase reactions kinetically, the two models identify two different sets of mineral assemblages affecting the porosity and permeability of the media. These markedly different results are due to transport considerations, the most significant of which are the spatial variability in aqueous concentrations, and advection and diffusion of dissolved glass constituents into the backfill materials. This work shows that for the prediction of geochemical behavior of engineered systems, such as the ILAW disposal facility, the traditional reaction path modeling approach is not sufficient for an accurate assessment of the precipitation of key mineral assemblages and their effect on the geochemical and hydraulic behavior of the waste glass. Reactive transport modeling improves this assessment significantly. The static model is useful in identifying potential minerals to be included in the reactive transport simulations. The dynamic model, however, ultimately determines the key mineral assemblages affecting both the geochemical behavior and the hydraulic properties of the waste glass in the presence of a flowing aqueous phase.

Weathering release of heavy metals from soil in comparison to deposition, litterfall and leaching fluxes in a remote, boreal coniferous forest by Michael Starr; Antti-Jussi Lindroos; Liisa Ukonmaanaho; Timo Tarvainen; Heikki Tanskanen (607-613).
Soil weathering release rates of Cd, Cu, Ni, Pb and Zn were determined at 4 plots in a forested catchment located in eastern Finland. The rates are compared with atmospheric deposition, litterfall and leaching fluxes at two of the plots. The soils were Haplic Podzols and developed in glaciofluvial deposits or glacial till having sandy loam to loamy sand fine-earth (<2 mm) textures. The bedrock in the area consists of granodiorites. Total concentrations of heavy metals in the <2 mm fraction of the parent material were determined from a mixed acid (HF and HClO4) digestion using ICP–MS. Weathering rates were calculated assuming that the molar ratio of heavy metal to base cation (Ca+Mg) weathering rates (previously determined by the Zr depletion method) equalled the molar ratio of heavy metal to base cation concentrations in the parent material. The mean weathering release rates were 8.3 μmol m−2 a−1 for Zn, 5.2 for Ni, 4.3 for Cu, 1.5 for Pb, and 0.011 for Cd. These rates were similar to the deposition, litterfall, and leaching fluxes for each metal. Weathering is therefore an important process in the heavy metal biogeochemistry of boreal, forest ecosystems.

A study was conducted at the Fresh Kills landfill, Staten Island, New York to investigate the use of B and Li isotopes as tracers of mixing and flow in the groundwater environment. Four end-member waters are present at the Fresh Kills: freshwater, seawater, a geochemically distinct transitional groundwater (that occurs in the zone of mixing between seawater and freshwater) and landfill leachate. The δ11B and δ6Li values of end-member waters are distinct and have isotopic compositions that reflect the solute sources: freshwater δ11B∼+30‰, δ6Li∼−22‰; transition zone groundwaters δ11B∼+20‰, δ6Li∼−27‰; seawater δ11B+40 to +75‰, δ6Li−37 to−44‰; leachate δ11B∼+10‰ (δ6Li not determined). Those wells influenced by seawater exhibited a clear chemical mixing trend, with seawater contributions ranging from 3 to 85%. Well waters with a high percentage of seawater (>30%) had δ11B values that were within 1‰ of the seawater value (+40‰), whereas a trend of increasing δ11B values (+55 to +75‰) was observed for wells with a lower percentage of seawater (<30%). δ6Li values for well waters impacted by mixing with seawater ranged from−37 to−44‰, significantly more negative than pure seawater (−31‰). This deviation from the isotopic composition of seawater, for both δ11B and δ6Li values, represents non-conservative behavior and is likely the result of isotopic fractionation during ion exchange reactions. The wide range of δ11B and δ6Li values and the distinct isotopic compositions of end-member waters makes B and Li isotopes useful for recognizing solute sources, however isotopic fractionation may limit their use as simple tracers of groundwater flow and mixing.