Applied Geochemistry (v.16, #14)

Geochemical and mineralogical controls on trace element release from the Penn Mine base-metal slag dump, California by Michael B Parsons; Dennis K Bird; Marco T Einaudi; Charles N Alpers (1567-1593).
Base-metal slag deposits at the Penn Mine in Calaveras County, California, are a source of environmental contamination through leaching of potentially toxic elements. Historical Cu smelting at Penn Mine (1865–1919) generated approximately 200,000 m3 of slag. The slag deposits, which are flooded annually by a reservoir used for drinking water and irrigation, also may be in contact with acidic ground waters (pH<4) from the adjacent mine area. Slags vary from grey to black, are glassy to crystalline, and range in size from coarse sand to large (0.6×0.7×1.5 m), tub-shaped casts. Metals are hosted by a variety of minerals and two glass phases. On the basis of mineralogy, slags are characterized by 4 main types: fayalite-rich, glassy, willemite-rich, and sulfide-rich. The ranges in metal and metalloid concentrations of 17 slag samples are: As, 0.0004–0.92; Ba, 0.13–2.9; Cd, 0.0014–1.4; Cu, 0.18–6.4; Pb, 0.02–11; and Zn, 3.2–28 wt.%. Leachates from Toxicity Characteristic Leaching Procedure tests (acetic acid buffered at pH 4.93) on two willemite-rich slags contained Cd and Pb concentrations (up to 2.5 and 30 mg/l, respectively) in excess of US Environmental Protection Agency (USEPA) regulatory limits. Analyses of filtered (0.45 μm) water, collected within the flooded slag dump during reservoir drawdown, reveal concentrations of Cd (1.7 μg/l), Cu (35 μg/l), and Zn (250 μg/l) that exceed USEPA chronic toxicity guidelines for the protection of aquatic life. Data from field and laboratory studies were used to develop geochemical models with the program EQ3/6 that simulate irreversible mass-transfer between slag deposits and reservoir waters. These models include kinetic rate laws for abiotic sulfide oxidation and surface-controlled dissolution of silicates, oxides, and glass. Calculations demonstrate that the main processes controlling dissolved metal concentrations are (1) dissolution of fayalite, willemite, and glass; (2) sulfide oxidation; and (3) secondary phase precipitation. Close agreement between model results and measured concentrations of Al, Ba, Cu, Fe, SiO2, and SO4 in the slag dump pore waters suggests that the dissolved concentrations of these elements are controlled by solubility equilibrium with secondary phases. Differences between predicted and measured Cd and Pb concentrations imply that field weathering rates of glass and sulfides are approximately two orders of magnitude lower than laboratory rates. Overprediction of Pb release may also reflect other attenuation processes in the natural system, such as sorption or coprecipitation.

The origin of reservoir fluids in the geothermal field of Los Azufres, Mexico — isotopical and hydrological indications by Peter Birkle; Broder Merkel; Enrique Portugal; Ignacio S Torres-Alvarado (1595-1610).
The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446±206 mm/m2 for the Los Azufres geothermal area, which corresponds to a total of 82×106 m3 per a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative δ 13C values can be explained by the input of organic C from the surface, but the lack of 14C in the deep fluids reflects a pre-historic age for the infiltration event of fossil meteoric water. The dilution of the meteoric water by 14C-free CO2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocene glacial period. Strong water–rock interaction processes, such as sericitization/chloritization, caused the primary brine composition to be camouflaged. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. Strong water–rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to changes in pressure and temperature, the rising fluids get separated into liquid and vapour phases at a depth of 1500 m. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrodynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells.

Biodegradation of polycyclic aromatic hydrocarbon (PAH) was investigated in the whole matrix and in the different aggregate size fractions of a sandy soil contaminated by a mixture of 8 PAHs and incubated at water holding capacity. The distribution of PAHs and of phenanthrene-degrading bacteria were determined in the bulk soil and in 4 size aggregate fractions corresponding to sand, coarse silt, fine silt and clay. The microbial communities able to degrade phenanthrene were detected at a similar level in the different aggregate fractions of the soil before contamination. After soil contamination and incubation, a significant growth of bacteria was observed and their distribution within aggregates was modified. Bacterial communities of phenanthrene-degraders were present in a higher density in the aggregates corresponding to sand (2000–50 μm) and clay (<2 μm). Chemical analysis show that remaining PAHs (low and high molecular weight) were much more concentrated in the fine soil fractions (fine silt and clay) and were present at a very low content in the larger aggregate size fractions. The interactions of well defined aggregates with PAHs and bacteria were also studied using phenanthrene as PAH model substrate and individual aggregates corresponding to sand and clay size fractions. Incubation of sand and clay aggregate fractions enriched with phenanthrene in the presence of a bacterial isolate NAH1 led to the simultaneous solubilization and biodegradation of phenanthrene. Differences in amounts of solubilized phenanthrene between sand and clay aggregate size fractions would be related to difference in adsorption capacities of phenanthrene by clay and sand aggregates.

Geochemical interactions between agricultural soil and sea-water at a salt marsh restoration Managed Realignment site in Essex, UK were simulated through a series of laboratory experiments. Soil from the site was mixed with sea-water for 0–30 days in batch reactors. The resulting soil residue was subjected to a modified version of the Tessier sequential extraction procedure involving leaching with 5 reagents of progressively increasing strength. Differences in total metals extracted by the series of sequential extractions in comparison to a single digest hindered interpretation of data. Analysis of the sequential extracts and residual sea-water for a suite of metals revealed that Na, Mg and K were sorbed from sea-water to the soils, and from the readily exchangeable fraction of the soil, Ba, Ca, Mn, Ni and Zn were depleted. In the more strongly held fractions extractability of a number of metals, most notably Al, Fe and K by acid leaches was observed to increase as a result of contact with sea-water. The extractability of the heavy metals Cr, Pb, Ni and Zn from these tightly held fractions was also observed to increase as a result of sea-water mixing. However, results imply that tidal inundation of the soils at the site will not result in significant leaching of metals to the environment.

Cyclic voltammetry applied to evaluate reactivity in sulfide mining residues by Roel Cruz; Blanca A Méndez; Marcos Monroy; Ignacio González (1631-1640).
Oxidation of sulfide present in mining residues can generate contaminating acid effluent known as acid rock drainage. Prediction and control of acid rock drainage are critically important to the mining industry because of the environmental impact resulting from the sulfide oxidation. Due to its particular reaction kinetics, once acid drainage has begun, it is very difficult to control without a substantial economic investment. For this reason, efficient prediction and prevention programs, which monitor mining waste reactivity, are required to limit the oxidation of sulfide-bearing residues before damage to the environment occurs. In this work, the authors evaluated mining waste reactivity under oxidizing conditions as a function of its voltammetric behavior before and during alteration under simulated natural conditions produced in the laboratory. This method is supported by conventional mineralogical characterization of the mineral samples and chemical quality of the effluents produced during the simulated alteration process kinetics.

Euglena mutabilis, a benthic photosynthetic protozoan that intracellularly sequesters Fe, is variably abundant in the main effluent channel that contains acid mine drainage (AMD) discharging from the Green Valley coal mine site in western Indiana. Samples of effluent (pH 3.0–4.6) taken from the main channel and samples of contaminated stream water (pH 3.3 to 8.0) collected from an adjacent stream were analyzed to evaluate the influence of water chemistry on E. mutabilis distribution. E. mutabilis communities were restricted to areas containing unmixed effluent with the thickest (up to 3 mm) benthic communities residing in effluent containing high concentrations of total Fe (up to 12110 mg/l), SO4 (up to 2940 mg/l), Al (up to 1846 mg/l), and Cl (up to 629 mg/l). Communities were also present, but much less abundant, in areas with effluent containing lower concentrations of these same constituents. In effluent where SO4 was most highly concentrated, E. mutabilis was largely absent, suggesting that extremely high concentrations of SO4 may have an adverse effect on this potentially beneficial Fe-mediating, acidophilic protozoan.

Strontium-90 (90Sr) is one of the major radioactive contaminants found in DP Canyon at Los Alamos, New Mexico, USA. Radioactive surveys found that 90Sr is present in surface water and shallow alluvial groundwater environments in Los Alamos National laboratory (LANL). Colloids may influence the transport of this radionuclide in surface and groundwater environments in LANL. In this study, the authors investigated the sorption/desorption behavior of radioactive Sr on Ca-montmorillonite and silica colloids, and the effect of ionic strength of water on the sorption of Sr. Laboratory batch sorption experiments were conducted using 85Sr as a surrogate for 90Sr. Groundwater, collected from Well LAUZ-1 at DP Canyon and from Well J-13 at Yucca Mountain, Nevada, and deionized water, were used. The results show that 92–100% of the 85Sr was rapidly adsorbed onto Ca-montmorillonite colloids in all three waters. Adsorption of 85Sr onto silica colloids varied among the three waters. The ionic strength and Ca2+ concentration in groundwater significantly influence the adsorption of 85Sr onto silica colloids. Desorption of 85Sr from Ca-montmorillonite colloids is slower than from silica colloids. Desorption of 85Sr from silica colloids was faster in LAUZ-1 groundwater than in J-13 groundwater and deionized water. The results suggest that clay and silica colloids may facilitate the transport of Sr along potential flowpaths from DP Canyon to Los Alamos Canyon.