We have previously defined in situ biogeochemical transformation as the biogenic formation of reactive minerals that are capable of abiotically degrading chlorinated solvents such as trichloroethene without accumulation of degradation products such as vinyl chloride (AFCEE et al. ). This process has been implemented in biowalls used to intercept contaminated groundwater. Abiotic patterns of contaminant degradation were observed at Altus Air Force Base (AFB) and in an associated column study, but not at other sites including Dover AFB. These abiotic patterns were associated with biogenic formation of reactive iron sulfide minerals. Iron sulfides in the form of small individual grains, coatings on magnetite, and sulfur‐deficient pyrite framboids were observed in samples collected from the Altus AFB biowalls and one of the EPA columns. Larger iron sulfide grains coated with oxide layers were observed in samples collected from Dover AFB. Altus AFB and the EPA column differed from Dover AFB in that groundwater flow at Dover AFB was relatively slow and potentially reversing. High volumetric sulfate consumption rates, an abiotic pattern of trichloroethene (TCE) degradation, and the formation of small, high surface area iron sulfide particles were associated with relatively high rates of TCE removal via an abiotic pattern. Geochemical modeling demonstrated that iron monosulfides such as mackinawite were near saturation, and iron disulfides such as pyrite were supersaturated at all sites. This environmental condition can be supportive of nucleation of small particles rather than crystal growth leading to larger particles. When nucleation is dominant, small, high surface area, and reactive particles result. When crystal growth dominates the crystals are larger and have lower specific surface area and reactivity. These results taken together suggest that creation of a dynamic environment can promote biogeochemical transformation based on generation of reactive iron sulfides.

In situ biogeochemical transformation involves biological formation of reactive minerals in an aquifer that can destroy chlorinated solvents such as trichloroethene (TCE) without accumulation of intermediates such as vinyl chloride. There is uncertainty regarding the materials and geochemical conditions that are required to promote biogeochemical transformation. The objective of this study was to identify amendments and biogeochemical conditions that promote in situ biogeochemical transformation. Laboratory columns constructed with plant mulch were supplemented with different amendments and were operated under varying conditions of water chemistry and hydraulic residence time. Four patterns of TCE removal were observed: (1) no removal, (2) biotic transformation of TCE to cis‐1,2‐dichloroethene (cis‐1,2‐DCE), (3) biogeochemical transformation of TCE without accumulation of reductive dechlorination products, and (4) mixed behavior where a combination of patterns was observed either simultaneously or over time. Principal coordinates analysis and analysis of variance (ANOVA) identified factors that promoted biogeochemical transformation: (1) high influent sulfate concentration, (2) relatively high hydraulic retention time, (3) supplementation of mulch with vegetable oil, and (4) addition of hematite or magnetite. The combination of the first three factors promoted complete sulfate reduction and a high volumetric sulfate consumption rate. The fourth factor provided a source of ferrous iron and/or a surface to which sulfide could react to form reactive iron sulfides. Many columns demonstrated either no TCE removal or a biotic TCE transformation pattern. Modification of column operation to include all four factors identified above promoted biogeochemical transformation in these columns. These results support the importance of the factors in biogeochemical transformation.

Acid rock drainage from a closed gold-mining operation in northern California was studied first in the laboratory and then on the pilot scale to determine the technical feasibility of passive u-eatment. The drainage has a pH of 3.8, and concentrations of Cu, Fe, Mn, Ni, and Zn of 140, 190, 28, 0.93, and 40 mg/L respectively. The laboratory studies concentrated on the question of whether local organic and soil materials could be used to support sulfate reduction in a passive treatment system. Samples were incubated at laboratory temperatures for a period of 4 weeks. Soil and wood processing wastes from the immediate vicinity proved to be too acidic to maintain a large population of sulfate reducers. The most reasonable material for sulfate reduction was a mixture of equal amounts by weight of cow manure, planter mix soil, and limestone chips. The final solutions had pH's of 6.5 to 6.9, and average Cu, Fe, Mn, Ni, and Zn concentrations of 0.02, I, 5, 0.05, and 0.1 mg/L, respectively. Based on the laboratory results, a pilot system was constructed that consisted of a lined steel container filled with a substrate volume that measured 2 by 3 by 12 m. The substrate mixture was the same as used in the laboratory tests. Raw manure from a dairy farm was mixed into the substrate for the sulfate-reducing bacterial (SRB) inoculum. Loading of the system was based on the estimate that 0.3 mo! sulfide per cubic meter of substrate per day would be generated, and the inflow of heavy metals should not exceed the sulfide generated. Using these principles, the flow was set at approximately 800 mL/min. Over the course of 9 months, the pilot system achieved removal of Cu and Ni below the effluent standards of 1.0 and 0. 7 mg/L. Dissolved Zn concentrations in the effluent averaged approximately 0.1 mg/L, compared with an effluent standard of 0.02 mg/L. Dissolved Fe concentrations in the effluent varied with the seasons, reaching a minimum of 1 mg/Lin the summer and rising to a maximum of 120 mg/Lin the winter. There is a significant increase in concentrations of Fe in unfiltered waters. This implies that, in a full-scale system, a settlingpolishing pond will be needed.

No abstract

The Avoca Mine in County Wicklow, Ireland is a volcanogenic massive sulfide deposit which has been mined for copper intermittently over the past 300 years using both underground and open pit methods. As a result of the mining activities, large areas of the site are covered with mine spoils, and the Avoca River has been impacted by ARD containing metals concentrations which have made the stream habitat unsuitable for native trout and salmon. An evaluation of the bioavailability of lead and arsenic within the spoils material was performed using a bioassay technique developed by the University of Colorado, ICP, and electron microprobe (EMP) analyses, while the leach ability of zinc and copper within the spoils was evaluated using ICP, and EMP techniques. While the spoils contained on average 900 mg/kg arsenic and 15,000 mg/kg lead, only 0-28% of the lead (average of 6%) and 0-8% of the arsenic (average of 0.5%) was bioavailable. EMP results showed the lead and arsenic to be present mainly within plumbojarosite and other hydroxysulfate phases, which are stable under the low pH conditions within the spoils pore water. The copper and zinc concentrations within the spoils averaged 1750 mg/kg and 420 mg/kg, respectively. EMP results showed that copper and zinc was present in the spoils mainly as primary sulfide minerals. Copper and zinc concentrations were low within the secondary minerals such as plumbojarosite and other hydroxysulfate, reflecting their higher mobility in the system and explaining why copper and zinc are present in much higher concentrations in the ARD and the Avoca River than arsenic and lead.