Bioremediation of mine area groundwater inorganic contamination (BioMAGIC): final report.

Abstract

The occurrence of metal-contaminated acid mine drainage (AMD) is one of the most problematic situations facing the mining industry today (Hutchinson and Ellison 1992; Ledin and Pedersen 1996). Sources of AMD include the walls of underground and open pit mining sites, waste rock and ore stock piles, tailings waste, and spent heap leach piles from leaching operations. If left uncontrolled and untreated, AMD has the potential to contaminate surface and groundwater broadly enough to adversely affect regional water quality, as well as indigenous fish and wildlife populations (Hutchinson and Ellison 1992; Orabkowski 1993). The chemical composition of AMD is determined principally by mine waste mineralogy, and the amount of water (meteoric and/or groundwater) infiltrating and percolating through the waste (Filion and Ferguson 1990). Iron sulfide minerals, such as pyrite, begin to undergo chemical oxidation almost immediately upon exposure to atmospheric oxygen and water (Evangelou and Zhang 1995). In the absence of neutralizing agents (e.g., carbonates or clays), these chemical reactions produce acid and liberate various metals whose concentrations will vary as a function of the composition of the ore and waste rock (Blowes and Ptacek 1994). As pH decreases, bacteria and ferric iron catalyze the overall process to a rate 10 to 20 times the abiotic chemical rate (Stumm and Morgan 1996; Okereke and Stevens 1991). Rainfall, snow melt, and other hydrological processes then leach the acid solutions from the waste sites into downstream and groundwater environments. Typically, AMD exhibits pH values well below 5.6 (the pH of meteoric water in equilibrium with atmospheric carbon dioxide) that are accompanied by high concentrations of dissolved sulfate, ferrous iron, and metals such as Al, Cr, Ni, Cu, Zn, Ag, Au, and Pb (Hutchinson and Ellison 1992; Orabkowski 1993). The primary goal of AMD treatment is to prevent acid generation at new sites, and to abate acid drainage where it occurs at existing facilities. Two basic approaches are used. These are (I) control of acid generation and leachate migration, and (ii) collection and treatment of AMO (Ledin and Pedersen 1996). Technologies currently employed to control acid generation and leachate migration include liming or carbonate treatment, application of bactericides, inundation (i.e., flooding) with water to maintain anaerobic conditions that prohibit sulfide mineral oxidation, and installation of dry covers to prevent water infiltration. Collection and treatment of AMO contamination in surface waters is accomplished using alkaline recharge trenches or wetland systems (natural or engineered, for example, the Boojum ARUM [acid reduction using microbiology] treatment). Treatment protocols for in situ Bioremediation of Mine Area Groundwater Inorganic Contamination (BioMAGIC) are not presently available, but are urgently required as field studies have demonstrated that infiltration of AMD into groundwater is common at mine sites (Ledin and Pedersen 1996). This study is focused on assessing the feasibility of an in situ BioMAGIC treatment process to mitigate the􀀃 potential adverse impact of AMD contamination along groundwater flow paths. The concept is based on using urea-degrading bacteria to increase the pH and alkalinity of acidic groundwater containing high concentrations of dissolved metals. Specifically, the enzymatic hydrolysis of urea by bacteria is apt to be of considerable benefit as the reaction consumes protons and promotes the development of alkaline conditions according to the following reactions: CO(NH2)2 + 2H20 = 2NH/ + C03- (1) H + C03- = HC03- (2) The intention is not only to reduce the extent of acidification that normally accompanies the hydrolysis of dissolved metals in mine drainage, particularly ferric iron, but also to initiate the removal of dissolved metals through mineral precipitation and sorption reactions (Stumm and Morgan 1996; Chapman et al. 1983; Herbert 1994; Herbert 1996). Anticipated benefits include restoration of normal pH values and substantially reduced metal-loading in water that is apt to move off mine-site property. Moreover, urea is produced as a bulk commodity agricultural fertilizer that is readily available (Tisdale et al. 1985). The target site for this investigation is the South Bay mine site in northwestern Ontario. Boojum Research has identified a plume of AMD contaminated groundwater that ultimately discharges into a small lake located approximately 800 m from the n􀀁arest surface source of AMD. A detailed site description is found in Appendix A. The basic premise for BioMAGIC at South Bay is that urea would be introduced into the contaminated groundwater system (most likely through installation of infiltration galleries across the groundwater flow path) to stimulate the activity of indigenous urea-degrading bacteria to a point where a significant increase in groundwater pH can be recognized. The approach used to evaluate the potential use of urea-degrading bacteria for BIOMAGIC involved both geochemical modeling and microbiological investigations. Flow-through column work has also been undertaken to learn about the potential impact of bacteria on groundwater flow in porous geological media. Construction of the geochemical model using the MINEQL + program was based on South Bay groundwater chemistry provided exclusively by Boojum Research. The objective of the modeling exercise was to determine (i) the extent to which groundwater pH would change in response to urea degradation, (ii) the minimum amount of urea that must be degraded to restore near-neutral pH, and (iii) the impact of anticipated pH changes on the aqueous chemistry and solubility of dissolved metals. Microbiological studies encompassed a survey of South Bay groundwaters to determine whether any urea-degrading bacteria existed in the contaminated aquifer, and to identify specific locations with conditions favorable to their growth. Subsequent work involved single colony isolation of South Bay urea-degrading bacteria to compare their activity with that of a reference bacterial culture (Bacillus pasturil) from the American Type Culture Collection (ATCC). This microbiological information is needed to identify areas where BioMAGIC, is most likely to be implemented successfully, and whether natural populations of bacteria might be used or, alternatively, whether a need exists to add urea­degrading bacteria to the groundwater system.