Effects of fluvial tailings deposits on soils and surface- and ground-water quality, and implications for remediation — Upper Arkansas River, Colorado, 1992–96

Abstract

Geochemical and hydrologic studies were conducted on parts of the Arkansas River and its flood plain downstream from Leadville, Colorado, to assess the effects of fluvial tailings deposits on soil and water quality, to identify processes causing water-quality degradation, and to identify implications of these results for remediation strategies. Soil samples indicated that the tailings deposits in a 3-mile reach of the flood plain contain as much as 6,500 and 12,000 milligrams per kilogram of lead and zinc, respectively. These concentrations are as much as 380 times the reported average concentrations for soils in the Western United States. Pyrite and some of its weathering products were identified in the soils, indicating that acid-rock drainage can occur. Estimates of instantaneous mass loads for calcium, iron, lead, manganese, and zinc in unfiltered Arkansas River samples for 18 different sampling dates in 1996 showed some evidence for increases in trace-element loads in some stream reaches during a few sampling dates during snowmelt runoff. These data indicate erosion of sediments or addition of trace elements to the water column in these stream reaches. Conversely, on some dates, during low flow, downstream loads decreased, indicating deposition of sediments or formation and settling of iron and manganese oxyhydroxide minerals, or both. For most of the 5 milligrams per liter) in one of the most upgrdient wells indicate contamination of ground water from upgradient tailings sources. Durin spring, a flush occurs that increases the areal extent of degraded ground water. The spring flush is from increased infiltration of springti e snowmelt through the tailings, or from season fluctuation of the groundwater table and capil ary fringe up into tailings material, or both. An understanding of the mechanism causing groundwater contamination is impo for remedial activities. If infiltration is the contamination mechanism, then shallow arne dments of acid-neutralizing material and effort to limit infiltration might be sufficient to remedi te adverse effects of the tailings deposits. Howe er, if water-table fluctuation is the mechanism, th n deeper amendments of acid-neutralizing mate • al would be required. In this case, a remedial effi rt would require detailed information about the depth, distribution, and acid-generating capac"ty of the tailings material.