LANDSCAPE GEOCHEMISTRY OF CENTRAL COLORADO—RESULTS FROM STREAM-SEDIMENT DATA

Federal land management agencies are required to manage their lands for the benefit of the public as a whole. One component of that activity requires effective protection and improvement of the aquatic and riparian habitat for wildlife and sports fisheries. The USGS is conducting an environmental assessment of the central portion of Colorado, including the core of the Rocky Mountains, much of which is on Federal lands. Historical mining activity has been extensive in the Colorado Mineral Belt portion of the study area and has affected water and sediment quality, riparian function, and fishery productivity. In the past three years, we have collected more than 300 multimedia samples from about 220 first- or second-order streams. In each watershed, stream water, sediment, and biofilm were collected from the stream bottom for chemical analysis using IC, ICP-AES and ICP-MS methods. In addition, detailed benthic macroinvertebrate data were collected in half of the watersheds to determine the ecological effects. To determine the effects of underlying lithology on the sediment geochemistry, the geochemical data were collected as much as possible from drainage basins underlain by a single rock type. However, not all of the 15 lithologic classes in the study area are sufficiently well represented to determine a geochemical signature for each lithology. Hydrothermal alteration and mineralization is clearly associated with some but not all of the late Cretaceous and Tertiary intermediate to felsic intrusive rocks. Elevated concentrations of base metals are present in sediment from drainage basins underlain by these mineralized intrusive rocks with geometric mean values of the base metals (Cd, Cu, Pb, and Zn) significantly greater than that for the unaltered Precambrian orthogneisses, plutonic rocks, and migmatites/metapelite rock groups. Metal concentrations among unaltered rock types showed small geochemical variations. Base-metal concentrations in sediment from watersheds underlain by hydrothermally altered rock were elevated above those from watersheds underlain by the same unaltered rock types. In watersheds where historical mining had occurred, the concentrations of metals in sediment often exceeded the Probable Effects Concentrations recommended for toxicity effects for sensitive species of aquatic life.