Abstract

Acid drainage from the Penn Mine in Calaveras County, California, has produced a plume of contamination in ground water between Mine Run Dam and Camanche Reservoir. Historically, contaminated surface runoff from the mine flowed directly into the Mokelumne River; after the construction of Camanche Dam in 1963, the surface runoff flowed into Camanche Reservoir. Interaction of surface water with sulfide-bearing waste rock and mill tailings has produced acidic surface water with pH values between 2.3 and 2.8 and high concentrations of sulfate and metals including aluminum, cadmium, copper, iron, and zinc. Diversions and unlined impoundments were constructed in 1978 to prevent or reduce surface runoff from the mine site. Some of the impounded mine drainage infiltrates to the ground water through fractures in bedrock and flows toward Camanche Reservoir. The 1owermost impoundment was treated with lime for several months during 1993 to raise pH and to immobilize contaminants, but the impoundment has since been allowed to resume its untreated condition.

This report summarizes the findings from the flrst 2 years of study by the U.S. Geological Survey of contaminated ground water at the Penn Mine. The distribution and flow of ground water at the Penn Mine is controlled by geologic features and hydraulic properties. Geologic controls include fractures in bedrock, faults, and the contact between the principal rock types in the area. Most flow occurs through fractures in a metavolcanic unit along its contact with an underlying metasedimentary (slate) unit. The median hydraulic conductivity is about 10 times higher in the metavolcanic unit (0.1 foot per day) than in the slate unit (about 0.01 foot per day). Most flow occurs in the fractured metavolcanic rocks; hydraulic conductivity in this unit is as high as 50 feet per day. The general hydraulic gradient in ground water in the area between Mine Run Dam and Camanche Reservoir is westward toward Camanche Reservoir.

Field data show a close relation between water quality in Mine Run Reservoir and water quality in downgradient wells in an area covered by slag. Specific conductance at all wells increased between April and December 1992. During the same period, some wells showed a decrease in pH and an increase in dissolved metals concentration, reflecting a higher proportion of acid drainage in the ground water. Heavy stable isotopes of hydrogen and oxygen are enriched in the impounded surface water, as well as in the ground water downgradient from the impoundments. These stable isotope data indicate that the partially evaporated water in the impoundments is the most likely source of contamination to the fractured-rock aquifer in the slag area between Mine Run Dam and Camanche Reservoir.

Water from the underground mine workings is chemically distinct from ground water in the slag area. Exsolved-gas compositions in water from the flooded mine workings indicate somewhat reducing conditions. Ratios of dissolved concentrations of zinc to copper and of zinc to cadmium are anomalously high in the underground mine water in comparison with such ratios for ground water in the slag area. These data suggest preferential scavenging of copper and cadmium, relative to zinc, by hydrogen sulfide produced by sulfate reduction in the mine workings. Variations in stable isotopes of sulfur and oxygen in dissolved sulfate are consistent with this interpretation.

Discharge toward Camanche Reservoir within the acidic ground-water plume at the base of Mine Run Dam is estimated to be about 40 cubic feet (300 gallons) per day, using an average hydraulic gradient of 0.07 and a geometric mean value for hydraulic conductivity of 0.1 foot per day based on a total of five measurements from three wells in this immediate area. The actual rates of ground-water discharge in the contaminant plume vary with plume width, hydraulic gradient, and hydraulic properties of the fractured-rock aquifer. The hydraulic gradient varies with seasonal changes in recharge and in the water level of Camanche Reservoir. For the flow rate of 40 cubic feet per day, the corresponding loadings of dissolved metals flowing toward Camanche Reservoir were estimated to be 17 grams of copper per day, 250 grams of zinc per day, and 2.7 grams of cadmium per day.