GIS ANALYSIS OF THE FUNDAMENTAL GEOLOGIC, TOPOGRAPHIC, AND MINING-RELATED FACTORS THAT INFLUENCE WATER QUALITY IN THE UPPER ANIMAS RIVER WATERSHED, SILVERTON, COLORADO

An extensive digital database for the western San Juan mountains provides a unique opportunity to quantify important geologic and topographic factors that influence water quality. Data included in the database are part of a U.S. Geological Survey, Animas River watershed abandoned mine lands study conducted between 1997-2001. The upper Animas River watershed was extensively mined from the 1870’s to the early 1990’s for base and precious metals. Natural weathering of pervasively altered and mineralized, primarily Tertiary-age, igneous rocks and veins contributes acidity and trace- and major-chemical constituents to surface water bodies that drain many subbasins. Mine waste and mill tailings also weather to produce acidity and constituent loads to the streams. A fundamental question that our GIS analysis is used to evaluate is: what is the relative importance between weathering of non-mining-affected yet highly mineralized bedrock and the surficial deposits weathered from them, and weathering of mine waste to produce acid generation and chemical constituent loading to surface and ground waters? In our subbasin analysis, we used GIS to evaluate this key question by characterizing and measuring (1) the areas of significant geologic and topographic features, (2) mines and prospects with associated estimates for the areas of disturbance, (3) potential surface water flow paths from naturally altered zones and from mine sites with use of high-resolution digital elevation models and gradient analysis, and (4) geochemical signals from surface water. For example, subbasins can be characterized by the area of quartz-sericite-pyrite alteration that has south-facing slopes of greater than 25 degrees, with greater than 30 percent sparsely vegetated and thus erosion-prone area, relative to the total subbasin area. These types of subbasins are likely to be major contributors of acid and chemical constituents as seen in a variety of surface water geochemical signals. An understanding of the physical, geologic, and geochemical characteristics of each subbasin is gained by our analysis and provides insight into how the unmined and mined subbasins contribute chemical constituent loads within the whole Animas River watershed.