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• and (2) organic cazbon not consumed in the reductive column was also oxidized or incorporate ,,,{ s~'" ~°~ into microbial biomass. 5°, ~~ ~~4~ ~ ~~ The laboratory tests indicate that in a system that utilizes an initial in situ reduction step, followed by an ex situ oxidation step, concentrations can be achieved for sulfate of less than 50 mg/L, manganese less than 0.05 mg/L, and TDS less than 500 mg/L. At the present time, the reductive reactions aze continuing in the columns that simulate in situ treatment, with continued removal of manganese and TDS. It is not yet possible to make a final prediction of the water quality that is achievable by the in situ treatment, other than to indicate that sulfate concentrations less than 50 mg/L aze realistic. 1.2 Results to be obtained from Pilot Scale Testing The column results have provided "proof-of-principle" evidence that that it is possible to achieve • highly effective reductive removal of sulfate, followed by oxidative removal of manganese and TDS. Afield pilot study is now needed to obtain the further critical information concerning in- situ treatment. To obtain these additional data, the following field test is proposed: ~~G Test 1: In-Situ Treatment: Injection of organic cazbon through BF-7 in the Pink Gneiss Pit. This test will be monitored by testing the water quality in the zone affected by the nutrient addition, and by microbial tests of pit materials. A conceptual model of the in Fitu test is n~ (, shown in Figure 1. ~ ~ ~ ~ S - 1^ C'" ~(~~p,~'fhis test will last as long as necessary to ascertain reali~ti~final concentrations for sulfate, \~~{ manganese, and TDS in systems treated by organic cazbon. It is expected that sufficient data `~ ~~ could be obtained by Summer or Fall 2000 to allow for a complete design of a full-scale system ,` that would go on-line by the Spring 2001. • Basle Moun(ain Rvources S6ephnd Milier, !nc Ilgranitelpdrive170076Trepor[sldmglal/achmenib.doc 4 June B, 1999