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drainage (Smith and Mudder, 1991). The orgatilc layer and drain pipe is then covered with fill and a reclaimed soil and vegetative cover to enclose the entire system to minim;~e infiltration, while promoting evapotranspiration and the necessary anaerobic conditions. Figures 2 and 3illustrate across-section of the Biopass System. In the course of evaluating the treatment performance of natural and manmade (i.e., engineered or constructed) wetlands with respect to pH adjustment and metals removal, researchers found that the vegetation communities removed only a few percent of the metals, generally well less than ten percent (Sencindiver and Bhumbla, 1989; Envirortment Canada, 1990). The neutralization of acidity and the precipitation of metals were occurring primazily in the anaerobic sediments beneath the wetland, in which the bacterial reduction of sulfate yielded alkalinity and sulfide as biochemical by-products (IGeittrnan, et al, 1986; Kleinman and Girds, 1986). The sulfide produced precipitated the metals as their very insoluble complexes, yielding very low residual dissolved concentrations and very high removal efficiencies up to 99 percent. Although wetlands have proved successful in treating ARD, several disadvantages have been noted, which have limited their application. First, their effectiveness was limited in cold climates in which the ground was frozen for extended periods of the year. Second, the wetlands did not respond well to rapidly changing and elevated flows, which often occur in the spring in mountainous regions, just as the plant and bacterial communities within the wetland are re-establishing themselves. Third, the source of organic matter often became exhausted due to ongoing bacterial activity, and required replacement from time to time. Fourth, the organic matter or submerged anaerobic zone lost its metals removal capacity with time as its available sorptive sites were occupied. Fifth, the metal removal efficiencies decreased with time due to short-circuiting, which resulted in pan from the continued -6- precipitation of iron and aluminum hydroxides within the submerged anaerobic zone. With the Biopass System, and in this particular situation, many of the disadvantages of wetlands can be circumvented, while taking advantage of the virtues of the bacterial and chemical processes that occur in an anoxic environment. The performance of the Biopass System is not readily affected by temperature as it is designed as an in-situ system in an area of moderate climate. Second, the flows that potentially can enter the system are small and relatively constant. Third, sufficien[ organic matter or compost can be placed in the system to accommodate the anticipated lone term flux of constituents in the drainage, if it occurs. The fact that the organic layer will be placed well below a compacted fill and vegetated surface promotes the necessary anoxic conditions. Fourth, since the heap leach drainage is not acidic and does not contain elevated iron or aluminum levels, the problems associated with precipitation of a large mass of metal hydroxides within the organic layer is avoided. The surface vegetation could be selected on the basis of specific metal uptake, as has been done successfully in the rehabilitation of mine wastes (Money, 1993 and 1994). However, in the current situation, the approach is to bacterially degrade the residual cyanide, while partially reducing the residual rti[rate and sulfate levels, and permanently fixing the precipitated the metals within the subsurface organic layer. The fill and vegetative cover above the organic layer promotes runoff and uptake of solution through capillary action. From a design perspective, the primary concerns are associated with providing sufficient nutrients, moisture, and reten[ion time for stimulation of bacterial growth, acclimation, and degradation. During construction of the system, the organic matter is wetted [o promote anaerobic conditions, to supply the initial moisrure needed, and to encourage bacterial growth. The next section provides a discussion of the basic biochemical processes that occur within the Biopass System.