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of arsenic varied from about 45-90~, while the removal of sulfate reached about 80% in column 2. The results indicated that the more expensive mushroom compost or peat mixtures did not provide a higher level of treatment. in addition, a retention time of between 15-30 days appeared to be optimal. The pH remained steady and neaz neutral throughout the study with increasing alkalinity's being noted in the effluents from the columns. A large negative Eh was noted in all of the columtu, except the first one which contained straw only. The laboratory study is more clearly defining the optimal retention time needed and also examines the performance of the Biopass System under intermittent flow conditions, as would be seen in the field. In addition, the organic material will be enhanced chemically in order to promote further removal of metals, particularly arsenic. 5.0 PROCESS DESIGN AND CONFIG URATION Since the Biopass System is contained, the retention period for solution is on the order of many weeks, providing maximum contact time for bacterial treatmen[ and chemical precipitation depending on site climate and infiltra[ion. Therefore, there is no need to incorporate the volume of organic material into the Biopass System to provide greater than a 30-day hydraulic retention period, but there remains a need to incorporate sufficient organic material to complete the bacterial treatment without periodically replenishing the carbon source. The organic material must be carefully selected on the basis of both efficiency and economics. From an efficiency standpoint, the organic material mus[ not only provide the appropriate growth substrates for the individual bac[eria needed, but also a suitable matrix for initial sorption of cyanide and metals present in the incoming drainage. Consideration of economics mus[ be included in an assessment, due to the relatively large volume of organic material needed and the fact tha[ specialty composted organic material can -13- cost several tern of dollars per ton. In general, the Biopass System costs only a fraction of that needed for chemical de[oxification. Asufficient quantity and volume of material must be incorporated into the Biopass System to provide a long term supply of nutrients and substrate for treatment. The organic material for vazious in- situ passive treatment systems can be obtained from a variety of sources including peat, specialty compost, wood chips, straw or hay, food processing wastes, and digested municipal sludges or animal waste. The organic material can be further bioaugmented with specially adapted microbial seeds obtained in bulk form from specific suppliers. In the Curren[ situation, the most suitable organic material would include a combination of composted or animal waste mixed with 5- 10% by weight straw or hay. The composted manure provides the organic substrate, while the hay or straw provides interstitial surfaces onto which bacterial films can develop and grow. The volume of organic matter required is dependent upon the total mass of sulfate and nitrate that could be degraded over time. The anticipated [oral mass of these two constituents would be derived by multiplying their concen[rations in the drainage by the estima[ed volume of drainage exiting the pads. By assuming a required mass of organic matter per unit macs of sulfate or nitrate reduced, a total volume and weight of organic [natter can be calcula[ed for incorporation into the Biopass System design. Calculation of the total mass of organic matter needed on the basis of the total mass of sulfate alone in the drainage is conservative, since the sulfate concentra[ion is typically an order of magnitude or greater than chat of nitrate and otily a portion of the sulfate will be bacterially reduced over time. The mass of organic matter required for sulfate reduction would be compared with the volume needed to provide an appropriate treaunent or retention period in days within the Biopass System. The limiting factor of the two (i.e.,