Mine Inventory and Compilation of Mine-Adit Chemistry Data

An inventory of inactive mines and mine-related sites in the Animas River watershed study area was compiled from published and unpublished State and Federal sources. Site locations were spatially verified using digital orthophoto quadrangles to ensure an accurate, geographically referenced inventory. The inventory provides, where available, descriptive information for mine-related sites, the estimated volume of mine-waste piles and mill-tailings deposits, and the discharge and pH of mine adits with flowing water. Water-quality and discharge data for 232 samples collected at 110 mine sites during low-flow conditions in 1995-99 have been compiled. In addition, time-series data are presented for water quality at seven adits sampled over the annual hydrologic cycle. These data provide site-specific information about potential sources of deposit-related acidity and trace elements that can be used to support an environmental evaluation of the effect of historical mining in the study area. Analysis of the spatial variation in water-quality data from mine adits at low flow shows considerable variability. Mine-adit water chemistry is controlled by the degree of hydrothermal alteration in the surrounding bedrock and the type and extent of base-metal sulfides in the mine workings. Time-series data collected from seven draining adits show that increases in discharge generally are not associated with spring runoff and that concentrations of many metals remain very nearly constant or show a slight increase during this seasonal event. Increases in discharge from a few mines during spring runoff may be controlled by surface water that infiltrates into the mine, forcing mine-pool water out at the adit. Mine-adit flow may represent a long-term source of metals that affect surface water quality.