By Scott N. Hamlin and Charles N. Alpers
Water-Resources Investigations Report 96-4257
INTRODUCTION
Background
The Penn Mine is an abandoned copper-zinc mine in the Foothill copper-zinc belt in northwestern Calaveras County, California (Peterson, 1985) (fig. 1 below). The mine property encompasses about 140 acres (Finlayson and Rectenwald, 1978) in the drainage basin of Mine Run, an intermittent tributary to Camanche Reservoir. Mining activity at Penn Mine was active from the early 1860's to the late 1950's (Clark and Lydon, 1962). The area has 20 or more shafts, several adits, and numerous open pits and cuts; two smelters and several mills operated at the site (Clark and Lydon, 1962). About 10.5 mi of underground workings were excavated to a depth of 3,300 ft (Heyl and others, 1948). Several acres of mill tailings and unmilled waste rock are exposed on the surface (Bond, 1988). Slag from the smelters was dumped in a 1,500-ft-long area immediately adjacent to the former channel of the Mokelumne River (Finlayson and Rectenwald, 1978). The history of the Penn Mine was presented in more detail in a report by Hamlin and Alpers (1995).
In response to incidents of fish mortality, several diversion channels were constructed in 1978 at the Penn Mine site to divert relatively unpolluted surface runoff around the Hinckley Run and Mine Run drainages (California Regional Water Quality Control Board Central Valley Region, written commun., 1978, 1979). Hinckley Run drains from the northeast into Mine Run Reservoir and contains impoundments HR1, HR2, and HR3 (fig. 2 below). Mine Run drains from the southeast into Mine Run Reservoir and contains impoundments MR1, MR2, and MR3 (fig. 2 below). These seven impoundments (Mine Run Reservoir, HR13, and MR13) were constructed in 1978 to capture contaminated runoff from the Penn Mine site, replacing two or more previously existing impoundments. The Mine Run Dam was constructed from nonreactive earth materials with a clay core. The remaining dams or dikes were constructed from available sulfide bearing waste rock, mill tailings, and soil; the impoundments were not lined, with the exception of an ineffective plastic liner in pond MR1.
Purpose and Scope
The groundwater investigation by the USGS in the Penn Mine area began in October 1991 and has the overall goal of characterizing the distribution of metals, sulfate, and acidity in ground water flowing through a fractured-rock aquifer. This report summarizes the findings from the first 2 years of study. In a previous report, Hamlin and Alpers (1995) presented initial findings from the firstyear of study. The specific objectives of the investigation are to evaluate (1) the quantity and quality of ground water flowing from the Penn Mine area toward Camanche Reservoir; (2)ground-water transport of metals, sulfate, and acidity between Mine Run and Camanche Reservoirs; and (3) the hydrologic interactions between the flooded mine workings and other ground water and surface water in the Penn Mine area. Accomplishment of these objectives will facilitate the development of effective pollution-abatement measures (such as those described by Golder Associates, Inc., 1996) for the site.
Hamlin and Alpers (1995) described ground water in the Mine Run drainage basin between Mine Run Dam and Camanche Reservoir on the basis of data from eight boreholes drilled in 1991. This report includes data from 12 additional boreholes drilled in 1992 (eight boreholes in the vicinity of Mine Run Dam and four boreholes in the vicinity of shafts 3 and 4 in the area adjacent to pond HR2; (fig. 2). Geologic, hydrologic, and water-quality data were collected during 1991-93 from the 20 USGS boreholes and 2 boreholes (fig. 3 below) constructed by the Utility District and its contractors (Brown and Caldwell Consultants, 1991) [available from East Bay Municipal Utility District]. These data were used to describe geologic structure, ground-water movement, and the distribution of acid mine drainage in the ground-water system.
The authors acknowledge several USGS colleagues for assistance with this study. Paul A. Hsieh (Menlo Park, Calif.) assisted with the design, procurement, and installation of downhole inflatable packers. Frederick L. Paillet (Denver, Colo.) supervised field measurements and data analysis of borehole geophysics, including conventional logs (caliper, gamma, etc.), acoustic-televiewer logs, and heatpulse flowmeter tests; assistance from Roger H. Morin and Richard E. Hodges on these tasks also is appreciated. Gas analyses were done by William C. Evans (Menlo Park, Calif.) Stable isotopes of oxygen and sulfur in dissolved sulfate were analyzed under the supervision of Robert O. Rye (Denver, Colo.). Interpretation of the hydrogeology benefitted from discussions with Clark Londquist (Sacramento, Calif.) and Anthony Buono (San Diego, Calif.). Interpretation of geochemical correlations was improved by discussions with Terry Rees (San Diego, Calif.).