Abstract. The Empire Mine State Historic Park near Grass Valley, California offers the ever-curious public a unique step back in time by preserving the underground mining heritage of the western slope of the Sierra Nevada. Perennial flow of a neutral pH, mining influenced water containing iron, arsenic, manganese, and other trace metals through the Magenta Drain adit is an unfortunate legacy of that heritage. Peak Magenta Drain flows requiring treatment are expected to be 4,740 L/min (1,200 gallons per minute). Several active treatment options were evaluated, including traditional lime dosing and green sand; alternative passive treatment technologies, including biochemical reactors, were also considered. However, bench scale test results suggested that simple settling of suspended iron oxy-hydroxide (with co-precipitated arsenic) and passive aerobic precipitation of manganese oxide could satisfy discharge targets.Land area in the vicinity of the Magenta Drain portal was inadequate to site the passive treatment system in a manner that would allow gravity flow. Land was available, however, near the crest of a nearby ridge. Consequently, the design included an influent pumping system, overland pipelines, settling pond, and two styles of aerobic wetlands. Once the decision to embrace a passive treatment system concept based on life-cycle cost and appropriateness for a public historic park was made, design plans and specifications were developed on a fast-track basis. The paper summarizes the rationale for the selection of passive treatment and provides design details that include trace metal removal in an aerobic environment.

Abstract. Sulfate and metals are commonly found in mining influenced water (MIW). A biochemical reactor (BCR) is an established technology that can remove sulfate and metals. Three organic mixtures were bench-tested for approximately six months to decrease sulfate concentration in a circum-neutral pH MIW containing low metal concentrations. Organic materials included wood pellets, oat straw, biochar, and manure as an inoculum. These were blended with limestonedolomite sand. Additionally, sulfide polishing units (SPUs), which were charged with native soil from the site, zero valent iron (ZVI), or magnetite, were evaluated for removal of dissolved sulfide discharged from each of the BCRs. Median MIW influent contained about 3000 mg/L of sulfate and very low concentrations of metals. The flow rates varied from 144 to 1,231 mL/day. Among all the BCRs tested, the hydraulic retention times varied from 5 to 75 days. All BCRs demonstrated similar removal rates of about 1.3 (BCR 1), 1.4 (BCR 2), and 1.6 (BCR 3) mol SO4 -2 /m 3 -day during the last week of testing. While the SPUs removed dissolved sulfide from the BCR effluents as expected, they removed sulfate as well. Dissolved organic carbon in the BCR effluents promoted sulfatereducing microbial activity in the SPUs where the inorganic materials functioned as a solid support for the microbial community. The magnetite was not an effective medium for post-BCR sulfate removal. Sulfate removal efficiencies in the BCRs were 55% (BCR 1), 57% (BCR 2), and 67% (BCR 3) during the final week of the bench-scale testing. Sulfate removal in the SPUs (from the BCRs effluents) was 35% and 37%, for SPU 1 and SPU 2, respectively. Novel reactor charging configurations in single units may therefore be much more effective and efficient than approaches exclusively using lignocellulosic or inert supports. Sulfate reducing microbial populations were still increasing when the test was concluded.

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