Evaluation of the extent of contamination caused by historical mining in catchments of central Colorado

The U.S. Geological Survey conducted an assessment of stream water and sediment quality in central Colorado, an area of about 54,000 km2. The study area is focused on small tributary catchments in the Rocky Mountains. The Colorado Mineral belt, a northeast-trending mineralized zone that experienced base- and precious-metal mining at the beginning of the late 1800s and early 1900s, cuts diagonally across the geologic trend in the study area. The goal of this study was to compare water and sediment quality in background catchments with those which have been mined. Water and sediment data from 200 catchments, and data from macroinvertebrates from more than 100 catchments, provided ample data for evaluation of the effects of mining on water and sediment quality. Focused sampling was conducted during low-flow conditions in the summers of 2004-2007. Samples were collected from catchments that (1) were underlain largely by a single lithologic unit, (2) contained hydrothermally altered rock and had been prospected, and (3) contained historical mines. Geochemical data determined from catchments that did not contain hydrothermal alteration or historical mines met water-quality criteria and recommended sediment-quality guidelines and showed small variations in base-metal concentrations. Hydrothermal alteration and mineralization typically are associated with igneous rocks that have intruded older bedrock. Base-metal concentrations were elevated in sediment from catchments underlain by hydrothermally altered rock. Catchments affected by historical mining contained highly elevated base-metal concentrations. Classification of catchments on the basis of mineral deposit types proved to be an efficient and accurate method for discriminating catchments that had degraded water and sediment quality. Only about 4.5 percent of the study area has been affected by historical mining, whereas a larger portion of the study area is underlain by hydrothermally altered rock. Weathering of QSP-altered catchments release metals and result in naturally elevated geochemical background concentrations in both sediment and water. The presence of hydrothermal alteration is shown to be a major consideration in the selection of sites for the determination of geochemical background.