Comparative Cost Analyses of Technologies for Treating Sulfateand Metal-Contaminated Groundwater

Transcription

1 Comparative Cost Analyses of Technologies for Treating Sulfateand Metal-Contaminated Groundwater H Kempton 1,M Martin 2 and T Martin 1 ABSTRACT Mining of sulfide-based ore deposits typically creates a potential for release sulfate and acid to groundwater, and many operating mines are thus predicted to require long-term post-closure treatment of groundwater. Toward the goal of selecting an optimal treatment, this paper compares costs of four demonstrated technologies for long-term treatment of sulfate-contaminated groundwater: in situ biological reduction (ISR), in situ permeable reactive barrier (PRB) composed of compost and zero-valent iron (ZVI), and two ex situ pump-and-treat technologies ex situ biological reduction, and ex situ nanofiltration with lime precipitation. Costs for each technology are estimated for treatment of a model aquifer system containing sulfate-contaminated groundwater, with the goal of treating 1 m 3 /min of groundwater from 1500 mg/l sulfate to a target level of 500 mg/l. (Metal removal is typically achieved by these technologies, and thus excluded from this analysis.) Costs are estimated for each technology as a function of treatment duration (ten to 60 years) and, for PRB and ISR, required replacement frequency (ten to 60 years). Estimated costs include capital and operation and maintenance, using a seven per cent net discount rate. Results indicate that ex situ technologies (biological reduction and nanofiltration with lime precipitation) are the most expensive. In situ technologies may be much less expensive, with PRBs having by far the lowest potential cost, followed by ISR as the next least expensive. However, actual costs for the in situ technologies are very sensitive to poorly constrained parameters, particularly the ZVI content of the PRB, and the effective life of PRB and ISR systems. This large cost uncertainty supports delaying remedial actions where viable, and suggests that feasibility analyses are generally warranted for specific applications to focus on these sources of uncertainty. INTRODUCTION The release of acid rock drainage (ARD) by the oxidation of sulfide minerals in hardrock mine waste poses a potential need for long-term environmental management at many existing and abandoned mines. Atmospheric oxygen enters subaerial mine waste (ie tailings, waste rock, and pit benches) by diffusion or advection, reacting with sulfide minerals to form sulfate, dissolved metal cations, and, where pyrite is abundant, acidity. Simple model extrapolations suggest that sulfidic mine waste or exposed bedrock has the potential to continue generating ARD for hundreds of years or more (Kempton and Atkins, 2000). In many locations ARD leached from mine waste is neutralised near the source by reaction with naturally occurring carbonate or silicate minerals, dramatically reducing acidity and the concentrations of most dissolved solutes. However, neutralised ARD typically contains mg/l sulfate, which exceeds drinking water standards (eg the US secondary maximum contaminant level [MCL] of mg/l). As a result, discharge of neutralised ARD to groundwater is often considered degradation of drinking water. Further, regulations in two US states (Arizona and New Mexico) requiring mine pit-lake water to meet groundwater standards, meaning these too could require sulfate below 500 mg/l. As a result, a substantial number of US mines face the need for large-scale and/or long-term water treatment systems to reduce sulfate concentrations. 1. Integral Consulting, Inc, 1320 Pearl, Suite 210, Boulder CO 80302, USA. 2. Exponent, 4940 Pearl East Circle, Suite 300, Boulder CO 80301, USA. Toward the goal of identifying an optimal technology for treating sulfate-contaminated groundwater, this paper compares the costs of four technologies capable of reducing dissolved sulfate in neutralised ARD (~1500 mg/l) to below the 500 mg/l drinking water standard. Specific technologies evaluated include in situ reduction (ISR) using carbohydrate injection, in situ permeable reactive barrier (PRB) composed of organic carbon and zero-valent iron, and two ex situ (pump-and-treat) technologies ex situ nanofiltration with lime precipitation and ex situ biological reduction using carbohydrates. TECHNOLOGY OVERVIEW Ex situ nanofiltration with lime precipitation Lime precipitation is the traditional process for treating ARD, with hundreds of applications throughout the world. Groundwater is extracted and then amended with enough lime (CaO or CaOH) to increase ph to between 10 and 11. Sulfate precipitates as gypsum (CaSO 4. 2H 2 O) and cationic metals precipitate as hydroxide and carbonate minerals. These solids, which are recovered by settling and filtration, are generally stable and non-hazardous, and thus may be disposed of in a municipal landfill. However, the effectiveness of lime for sulfate precipitation is limited by the solubility of gypsum. Equilibrium chemical modelling of sulfate precipitation by lime (using the PHREEQC code; Parkhurst and Appelo, 1999) indicates effective sulfate precipitation above ~1000 mg/l. But further sulfate removal requires dramatically higher lime addition rates (Figure 1), and the theoretical minimum attainable sulfate concentrations using lime amendment is ~650 mg/l. As a result, lime-amendment systems alone typically cannot meet regulatory standards for sulfate. Recent studies indicate that combining a membrane separation processes ( nanofiltration ) with lime treatment can reduce sulfate to drinking water standards (Kuipers 2002). In this process (licensed by HW Process Technologies), sulfate-bearing water is initially directed to a nanofiltration process, where the water is forced through an engineered membrane. This filtration process produces a concentrate stream with high sulfate concentrations (eg mg SO 4 /L) and a high purity effluent containing low concentrations of sulfate (eg 200 mg SO 4 /L). The concentrate stream is treated by lime precipitation to reduce the sulfate concentration down to ~1500 mg/l, then blended back with the high purity effluent. The resulting blended effluent meets drinking-water standards for sulfate. Ex situ biological reduction This patented ex situ treatment process removes sulfate by converting it to sulfide through the process of biologicallymediated respiration induced by the addition of electron-donor carbohydrates (Equation 1 below; Maree et al, 2000; Greben et al, 2000a). Affordable electron donors typically include methanol, ethanol, or sucrose (Greben et al, 2000b; below). The resulting sulfide is then removed, either by precipitation as an insoluble metal-sulfide minerals (eg FeS, CdS, CuS, and ZnS; Equation 2), or sparged from solution and re-oxidised to elemental sulfur or sulfate. 2 + _ ( aq ) 4 ( aq ) ( aq ) ( aq ) 3 ( aq ) CH O + 3SO 3HS + 3H + 6HCO 2 + _ + Me (aq) + HS (aq) MeS(s) + H (aq) (2) (1) 6th ICARD Cairns, QLD, July

2 H KEMPTON, M MARTIN and T MARTIN 10,000 Sulfate (mg/l) 1, ,000 10,000 15,000 20,000 25,000 Lime Added (mg/l) FIG 1 - Predicted removal of sulfate as a function of lime addition rate (modelled using PHREEQC). Costs in this analysis assume that the sulfide minerals are recovered by settling in a gravity clarifier followed by filtration. The treatment process, although novel relative to lime precipitation, is patented in various forms and is relatively well developed (Maree et al, 2000). Although the rate of solids production is low relative to lime treatment, disposal of metallic sulfides can be problematic. The sulfide-minerals are highly reactive and, if stabilisation measures are not taken, will oxidise and release sulfate, acidity, and cationic metals. At the other extreme, metal concentrations in sulfide solids recovered from metal-rich waters may allow economic metal recovery, offsetting or eliminating disposal costs. In practice, metal concentrations in neutralised ARD are often too low to ensure sulfide precipitation or solids recycling. In these cases, a cationic metal (eg Fe 2+ ) must be added to achieve a 1:1 molar ratio of cationic metal to sulfate (Equation 2) and prevent the formation of excess concentrations of hydrogen sulfide a noxious and hazardous gas. Costing here assumed passive oxidation of recovered sulfide to sulfate and disposal of the resultant materials as non-hazardous waste. In situ biological reduction (ISR) This in situ treatment uses the same reduction and precipitation processes described above in Equations 1 and 2. Soluble carbohydrates are injected into the subsurface to stimulate microbial reduction of sulfate to sulfide, which then precipitates in situ as metallic sulfide minerals. Although several patents exist for ISR applications, there are few publications on the technology in peer-reviewed literature. ISR has been tested at the field scale (Saunders et al, 2001), but examples of full-scale treatment of sulfate in groundwater have not been widely reported. ISR has the advantages of relatively low labour requirements relative to the ex situ alternatives, and of eliminating the need for solids management and disposal. Key factors affecting the feasibility of ISR for reducing groundwater sulfate are: 1. the availability of sufficient excess cationic metals to sequester the sulfide; 2. potential aquifer permeability reduction due to solids accumulation; and 3. the potential for release of secondary solutes (eg iron, manganese, arsenic, antimony, fluoride) to groundwater caused by reductive dissolution of aquifer minerals. Where groundwater metal concentrations are too low to precipitate all sulfide produced (ie the condition expected in most neutralised ARD), naturally occurring iron in the aquifer matrix acts as a scavenger, leaving a residue of iron sulfide minerals. Because available iron in aquifers is limited, preventing a sulfide plume requires that additional iron be added (eg amendment of injected groundwater with soluble iron, or emplacement of a down-stream iron-oxide PRB). Because of potential problems with in situ amendments (eg increased groundwater TDS from chloride during FeCl 2 amendment, or aquifer clogging), costing here included placement of an iron-oxide coated gravel PRB. For long-term applications, periodic replacement of the injection and sulfide scavenging system in a new location is expected as aquifer pore space in the ISR treatment zone is filled with biomass and precipitated metallic sulfides. The rate of permeability reduction is poorly known, and will depend on aquifer properties and rate of sulfide and biomass formation. (We use a ten-year replacement as a base case in our analysis, and vary replacement to determine its importance on cost.) Finally, the highly reducing condition induced by ISR favours dissolution of iron and manganese oxides in aquifers, potentially releasing these two metals, and trace constituents bound in these highly-adsorptive phases. Treatment of secondary solutes released by treatment is excluded in our cost comparison; practical applications, however, should evaluate the potential for reductive-release of solutes (eg Fe, Mn, As, Sb, F) from the aquifer, and include secondary treatment (eg air sparging downgradient of the ISR zone to re-oxidise iron and remove solutes through adsorption/coprecipitation reactions) if warranted. In Situ permeable reactive barrier (PRB) Sulfate-reducing PRBs are a patented treatment process that has been widely tested in laboratory (eg Waybrant et al, 1998) and field (eg Benner et al, 1999) for the treatment of ARD in groundwater, showing potential application to a wide variety of metals and anions (Blowes et al, 2000). Sulfate reduction occurs through bacterially-mediated reaction with organic carbon (eg municipal compost, reaction analogous to Equation 1). Zero valent iron (ZVI) may be included to increase sulfate reduction rates and PRB life. Principal advantages of PRB applications are that they are completely passive (minimising operation costs) and do not require management and disposal of a solids waste stream Cairns, QLD, July th ICARD

3 COMPARATIVE COST ANALYSES OF TECHNOLOGIES Key factors affecting PRB costs are: 1. PRB size, which relates directly to the sulfate reduction rate; 2. reactive substrate composition; and 3. effective PRB life. Sulfate reduction rates of over 160 mg/l-d have been measured in laboratory tests using organic carbon substrates (Waybrant et al, 1998); but a longer-term field application at the Nickel Rim Mine has reported sulfate reduction rates closer to 50 mg/l/day (Benner et al, 1999). Regarding composition, organic materials, such as municipal compost, are effective at reducing sulfate and typically available for very low cost. Unpublished work suggests that inclusion of ZVI with organic carbon may improve the sulfate reduction rate and operational life, but given the high cost of ZVI ($US $400 per ton), the benefits of ZVI in sulfate-reducing PRB is a critical data need. Finally, given their high installation cost, effective life of the PRB life strongly affects cost. Stoichiometric considerations (ie ratio of PRB carbon to sulfate load from groundwater) often indicate relatively long theoretical PRB lives (eg 60 years). However, it is unlikely that all of the carbon within the organic matrix will be labile, which would lead to a shorter effective life. The oldest field-scale sulfate-reduction PRBs is at the Nickel Rim Mine (installed in 1995; Benner at al, 1999). Monitoring of this PRB after six years has shown that sulfate reduction rates have remained approximately constant over this period, and that the PRB should remain effective for a total duration of ten year or more. In this paper, our base case assumes ten years for PRB life (a reasonable lower bound), and ten per cent ZVI (vol/vol); but cost sensitivity is evaluated to both of these poorly constrained parameters. METHODS Costs were developed for each of the four technologies based on a generic groundwater plume, assuming the following: Aquifer thickness = 5 m Plume width = 150 m Flow = 1 m 3 /min Treatment requirements were estimated assuming an influent groundwater sulfate concentration of 1500 mg/l and a target treatment goal of 500 mg/l. System design parameters for the ex situ reduction, in situ reduction, and in situ PRB technologies were based on the findings of laboratory and field treatability testing of the technologies at a site with groundwater conditions similar to the model plume evaluated in this paper. Unit capital and operation and maintenance (O and M) costs were estimated based on vendor quotes, standard references for costing (Means, 2002), and professional experience. Primary cost assumptions for these three technologies are summarised in Table 1. Costs for the nanofiltration with lime precipitation technology do not directly parallel the methods used for the other three technologies, but were instead estimated using the costs presented by Kuipers (2002). Specifically, all capital costs and 75 per cent of the O and M costs from Kuipers (2002) were assumed to be directly proportional to flow (the system described in Kuipers was 6 m 3 /min). To be consistent across technologies, costs for components absent from Kuipers 2002 estimate but included in this comparison (eg well installation) were then added in. Although reasonable for this screening-level evaluation, scaling the capital costs directly to flow for the nanofiltration with lime-precipitation technology may underestimate the true capital cost of this technology at the lower flow rate used in this comparison. TABLE 1 Primary assumptions used in the derivation of cost estimates for treatment of sulfate in groundwater using in situ (PRB and IRM) and ex situ (biological reduction and nanofiltration with lime precipitation) technologies. General assumptions Indirect costs were calculated as a percentage of direct costs. All net present value calculations were based upon a seven per cent discount rate. For calculation capital costs, mobilisation and demobilisation and contractor overhead and profit were assumed to be ten per cent and 15 per cent respectively, of the total of facilities and equipment for all options. Ex situ nanofilteration with lime precipation Capital costs from Kuipers (2002) are directly proportional to flow. Seventy five per cent of the annual O&M costs from Kuipers (2002) are directly proportional to flow. Ex situ biological reduction Ethonol dosage rate = 650 mg/l (assumes 50 per cent efficiency). Annual labour costs were based upon 2.5 FTE. Disposal of tons-sludge/yr at $200/ton (assumes stabilisation prior to disposal. In situ reduction Ethanol dosage rate = 650 mg/l (assumes 50 per cent efficiency). It was assumed that this alternative will create a sulfide plume; therefore, a trench containing iron-coated gravel to scavenge the sulfide would be required. Annual labour costs were based upon 0.5 FTE. Annual operation and repair costs estimated at five per cent of direct capital. A patent fee of ten per cent of direct capital costs was included in the indirect costs portion of this alternative. Permeable reactive barrier For the ten per cent ZVI option, a ten per cent ZVI, 33 per cent compost, and 57 per cent pea gravel mix was assumed. Annual labour costs were based upon 0.1 FTE. A 6.5 day residence time in the wall was assumed (both options zero per cent and ten per cent ZVI). A patent fee of ten per cent of direct capital costs was included in the costs. RESULTS Unit costs to remove sulfate from groundwater (Table 2, $US/m 3 ) under the conditions assumed in analysis indicate that PRB treatment has the potential to be the least expensive if the substrate has a low ZVI content and long substrate life (eg average NPV treatment cost = $US0.20/m 3 for a 30-year operation of a PRB that does not require replacement and contains no ZVI). The next least expensive is ISR (net present cost for a 30-year operating duration range from $US 0.32 to 0.41 per m 3, depending on injection system life). The estimated costs for the two ex situ technologies (sulfate reduction and nanofiltration with lime precipitation) are very similar ranging from $US 0.60 to 0.68 per m 3, assuming a 30-year application (Table 2). Interestingly, the PRB technology could be as costly as the ex situ technologies (eg NPV for 30-year operation = $US 0.62/m 3 ) if the substrate contains ten per cent ZVI and requires replacement at ten-year intervals, highlighting the need to refine PRB life and content for cost estimation. 6th ICARD Cairns, QLD, July

4 H KEMPTON, M MARTIN and T MARTIN Groundwater treatment costs as a function of the operating duration (Figure 2) parallel the unit cost trends (Table 2), with a zero per cent ZVI PRB predicted to be lowest cost (ten-year TABLE 2 Estimated unit cost ($US/m 3 ) for treatment of sulfate in groundwater using in situ (PRB and IRM) and ex situ (biological reduction and nanofiltration with lime precipitation) technologies. Operating duration In situ PRB (0 % ZVI) Replacement every ten years Replacement every 20 years Replacement every 60 years In situ PRB (10 % ZVI) Replacement every ten years Replacement every 20 years Replacement every 60 years In situ reduction Replacement every ten years Replacement every 20 years Replacement every 60 years Ex situ nanofiltration with lime precipitation Ex situ reduction Costs assume: 1. flow = 1 m 3 /min; 2. operating duration of 30 years; 3. treatment of 1500 mg SO 4 /L to 500 mg SO 4 L; and 4. net discount rate of seven per cent. replacement interval for PRB and ISR), followed by ISR, ex situ reduction, and ex situ nanofiltration with lime precipitation (Figure 2). (Note that unit costs [Table 2] decrease with increasing operating duration because a higher fraction of the treatment is paid for with discounted future money; but that total net-present values are cumulative [Figure 2], and thus increase with longer operating periods). Capital costs for the ISR treatment are low relative to the other treatment alternatives, as substantial equipment and materials are not required. Operations and maintenance costs, however, would be considerable, largely due to reagent (carbohydrate) and operational requirements. An itemised component cost summary of each technology (Figure 3) identifies those parameters most critical for feasibility analysis. While both ex situ technologies require substantial capital expenditure, operations and maintenance costs generally represent the largest component of all alternatives (Figure 3). This reflects primarily the costs associated with labour, chemical reagents, and for in situ technologies, periodic system replacement (emphasising again the importance of system life in in situ technology costs). Solids disposal represents a significant component of the cost for treatment by both the nanofiltration with lime precipitation and the sulfate reduction technologies (estimated to be ~20 per cent of the total costs). In the case of the nanofiltration with lime precipitation technology, it may be possible to reduce these costs if an alternative to landfill disposal is available, such as land application of the solids or use of the solids as backfill to an open mine pit. The solids generated by sulfate reduction are unlikely to be amenable to these alternative disposal options. However, in some cases, the sulfide-based solids may be sufficiently enriched with a precious metal (eg Cu, Zn) to permit economic metal recovery. In these cases, solids management costs would be substantially lower than those estimated based on disposal of the solids and, in some cases, may even represent a profitable component. In general, attempts to reduce treatment costs should focus on increasing the operating life of in situ systems, increased automation of active systems (ie reduced labour requirements), and, for PRB technology, reducing the cost of the reactive substrate. FIG 2 - Estimated total water treatment costs (net present value, millions $US) for in situ (PRB and ISR) and ex situ (biological and nanofiltration with lime precipitation), ten to 60 years of operation Cairns, QLD, July th ICARD

5 COMPARATIVE COST ANALYSES OF TECHNOLOGIES FIG 3 - Estimated capital and operations and maintenance costs (net present value, millions $US) for four water-treatment technologies. FIG 4 - Estimated costs (net present value, millions of $US) for ISR treatment of sulfate in water as a function of operating duration and system replacement frequency. Cost estimates for the two in situ technologies considered here (ISR and PRB) are quite different in their sensitivity to the operating life (and associated replacement frequency) of the systems (Figures 4 and 5, respectively). For an ISR treatment system, decreasing the effective operating life from 60 years to ten years only increases the net present value cost of a 60-year operation by ~40 per cent (Figure 4). This relatively low sensitivity to replacement frequency reflects the low cost of capital construction relative to the long-term costs for labour and materials. The capital and O and M costs for the ISR treatment (including re-installation) could be lowered further if the aquifer contains sufficient iron to scavenge the sulfide, delaying or eliminating the need for a sulfide-scavenging PRB. In contrast, estimated PRB treatment costs (again using NPV in a 60-year operating duration) increase by 100 per cent as the effective operating life of a PRB system decreases from 60 to ten years. The high sensitivity of PRB cost to operating life reflects the dominance of PRB construction costs capital costs represent approximately half of the total costs for PRB treatment (Figure 3). (Replacement costs for PRB and ISR were considered an operation and maintenance cost in Figure 3). Also clear in this analysis is the high sensitivity of PRB treatment costs to the ZVI content of the reactive media (Figure 5) net present value costs for PRB treatments increase by over 80 per cent as ZVI content increase from zero to ten per cent (vol/vol). Thus PRB treatment ranges between the least and most expensive of the technologies evaluated here as ZVI content ranges between zero and ten per cent (Figure 2). 6th ICARD Cairns, QLD, July

6 H KEMPTON, M MARTIN and T MARTIN FIG 5 - Estimated costs (net present value, in millions $US) for PRB treatment of sulfate in groundwater as a function of operating duration, system replacement frequency, and ZVI content. CONCLUSIONS AND RECOMMENDATIONS This paper presents a comparative cost analysis of four demonstrated technologies for remediation of a model groundwater system containing elevated levels of sulfate. Traditional ex situ lime precipitation technology has been most widely applied to the treatment of sulfate in groundwater. However, lime treatment is not capable of reducing sulfate to below the 500 mg/l drinking water standard without inclusion of nanofiltration or other concentrating process. Ex situ biological sulfate reduction represents a reasonable alternative to nanofiltration with lime precipitation technology, and is capable of meeting lower sulfate targets than traditional lime treatment for comparable cost. Where concentrations of valuable metal-cations (eg Cu, Zn) are also elevated, the sulfide solids recovered from biological treatment may have recycle value; but more typically, disposal will be a challenge due to the reactive nature of sulfide waste, and should be considered carefully in feasibility costing. The two in situ treatment technologies evaluated here, ISR and PRB, have the potential to provide comparable sulfate treatment at lower cost, particularly where sulfate concentrations are relatively low (eg below ~2000 mg/l). However, reliable feasibility assessment of these in situ technologies is limited by uncertainty in a few critical parameters. Economically, the potential for dramatically lower long-term treatment costs using in situ technologies argues for delaying capital expenditures if possible, waiting until key parameters are refined enough to support confident selection of an optimal technology. Cost-sensitivity analysis indicates that site-specific evaluations of alternatives to remediate sulfate (and metal) contaminated groundwater would benefit most by focusing on obtaining accurate estimates for the parameters listed below. Critical parameters for evaluating in situ reduction technology 1. potential for release of solutes from the aquifer under strongly reducing conditions; 2. the balance of required sulfate reduction to available cationic metals in the aquifer system necessary to provide for metal-sulfide precipitation; and 3. expected operating life of the ISR treatment zone (ie before aquifer permeability reductions necessitate replacement of the treatment system). Critical parameters for evaluating permeable reactive barriers 1. sulfate reduction rate within the PRB; 2. effect of ZVI in the reactive mixture on PRB life and sulfate reduction rate; and 3. effective PRB life. Beyond specific site applications, these six parameters represent general research needs, and are recommended as points of focus for broad-based research in the field of in situ remediation of metal and sulfate-contaminated groundwater. REFERENCES Blowes, D W, Ptacek, C J, Benner, S G, McRae, C W T, Bennett, T A and Puls, R W, Treatment of inorganic contaminants using permeable reactive barriers, Journal of Contaminant Hydrology, 45: Benner, S G, Blowes, D W, Gould, W D, Herbert, R B, Jr and Ptacek, C J, Geochemistry of a permeable reactive barrier for metals and acid mine drainage, Environ Sci Technol, 33: Greben, H A, Maree, J P, Singmin, Y and Mnqanqeni, S, 2000a. Biological sulphate removal from acid mine effluent using ethanol as carbon and energy source, Water Science and Technol, 42(3-4): Greben, H A, Maree, J P and Mnqanqeni, S, 2000b. Comparison between sucrose, ethanol and methanol as carbon and energy sources for biological sulphate reduction, Water Science and Technol, 41(12): Kempton, H and Atkins, D, Delayed environmental impacts from mining in semi-arid climates, in Proceedings Fifth International Conference on Acid Rock Drainage, Vol 2, pp (Society for Mining, Metallurgy and Exploration, Inc: Littleton) Cairns, QLD, July th ICARD

7 COMPARATIVE COST ANALYSES OF TECHNOLOGIES Kuipers, J R, Water treatment as a mitigation method for pit lakes, Southwest Hydrology, 1(3): Maree, J P, Strobos, G, Greben, H, Günther, P and Christie, A D M, Biological treatment of mine water using ethanol as energy source, Conference on Environmentally Responsible Mining in South Africa, Muldersdrift, South Africa, September. Parkhurst, D L and Appelo, C A J, User s guide to PHREEQC (version 2) - A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, US Geological Survey Water-Resources Investigations Report Denver, Colorado. Saunders, J A, Lee, M, Whitmer, J M and Thomas, R C, In situ bioremediation of metals-contaminated groundwater using sulfate-reducing bacteria: a case history, in Proceedings Sixth International In Situ and On-Site Bioremediation Symposium, San Diego, CA, 4-7 June 2001, pp Waybrant, K R, Blowes, D W and Ptacek, C J, Selection of reactive mixtures for use in permeable reactive walls for treatment of mine drainage, Environ Sci Technol, 32: th ICARD Cairns, QLD, July