50703-DIFFUSE AND DISTINCT SOURCES OF LOW-PH AND METAL-RICH WATER TO UPPER CEMENT CREEK NEAR SILVERTON, COLORADO: IMPLICATIONS FOR REMEDIATION

2000 GSA Annual Meeting -- Reno, Nevada

Abs. No. 50703

DIFFUSE AND DISTINCT SOURCES OF LOW-PH AND METAL-RICH WATER TO UPPER CEMENT CREEK NEAR SILVERTON, COLORADO: IMPLICATIONS FOR REMEDIATION

Author(s): WALTON-DAY, Katherine, U.S. Geological Survey, Box 25046, MS 415, Denver, CO 80225, kwaltond@usgs.gov; KIMBALL, Briant A., U.S. Geological Survey, 2329 West Orton Circle, West Valley, UT 84119; RUNKEL, Robert L., U.S. Geological Survey, Box 25046, MS 415, Denver, CO 80225

Keywords: acid mine drainage, remediation, tracer-injection

Cement Creek is a tributary to the Animas River in southwestern Colorado and has been affected by low-pH metal-rich inflows attributed to mining activity and to natural geologic sources. Identifying and quantifying these inflows is an essential element in making land management decisions. In the 4-kilometer headwater reach of the stream, tracer injection and synoptic sampling were used (1) to determine discharge and solute concentrations of the stream and surface-water inflows and (2) to construct profiles of pH, solute concentrations, and mass loads of metals. Stream pH ranged from 6.8 in the headwaters to 3.2 near the downstream end of the study reach. Dissolved (10,000 Dalton ultrafilter) metal concentrations (in milligrams per liter) in the stream ranged from 0.03 to 8.7 (aluminum), 0.02 to 1.8 (copper), less than 0.001 to 12.8 (iron), 0.07 to 4.8 (manganese), and 0.45 to 12.4 (zinc). Generally, metal concentrations are inversely related to pH. Results indicate that sources of acidic, metal-rich water to the stream include distinct observable inflows, as well as diffuse inputs. For example, in two segments of the stream where there were no visible inflows of acidic water, pH decreased by more than one pH unit. These segments contain hydrothermally altered, mineralized fractures and disseminated pyrite indicating that the decrease in stream pH likely results from subsurface inflow of naturally acidic water that would be difficult to remediate. In contrast, as much as 30 percent of the aluminum and manganese, and about 50 percent of the copper, iron, and zinc loads arise from two distinct surface-water inflows. If cleanup of these sources is fiscally and technologically possible, their remediation could significantly decrease metal loads and concentrations at the downstream end of the reach.

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