INTEGRATED RESULTS FROM THE ANIMAS RIVER WATERSHED PROJECT, SOUTHWESTERN COLORADO

Studies of the effect of historical mining on the water chemistry of the Animas River watershed in southwestern Colorado indicate that premining metal concentrations in streambed sediment exceeded crustal abundance, were indicative of exposed mineralization in the watershed, and limited aquatic habitat in some reaches prior to mining. Present-day streambed-sediment geochemistry is dominated by tailings released during milling between 1917 and 1931. The upper Animas River upstream from the confluence with Cement Creek (A-68) to Minnie Gulch supports relatively diverse and abundant stream biological communities, including brook trout (Salvelinus fontinalis). Near neutral pH values reflect the predominance of propylitically altered rock in the upper Animas River basin. Stream segments and measured inflows in the upper Animas River account for 24 percent of the dissolved zinc load and 14 percent of the dissolved copper load measured at the gage downstream from Mineral Creek (A-72). Toxicity studies indicate that benthic invertebrates may be adversely affected by the elevated zinc concentrations in the vicinity of A-68, but that early life-stages of brook trout can tolerate exposure to stream water from this site year-round. In contrast, water quality in Mineral and Cement Creeks do not support aquatic life. The rocks are altered phyllic and acid-sulfate assemblages that generate acidic waters. Weathering of altered rock in these tributary basins has resulted in formation of ferricrete, which indicates that natural weathering processes have affected water chemistry throughout the Holocene. Biological communities in Cement Creek are limited to acid-tolerant algae. Stream segments and measured inflows from Cement Creek account for 39 percent of the dissolved zinc load and 30 percent of the dissolved copper load at A-72. Toxicity tests indicate that the high concentrations of dissolved copper and zinc in water at the mouth of Cement Creek are toxic to fish and aquatic invertebrates. Stream segments and measured inflows in Mineral Creek account for 29 percent of the dissolved zinc and copper loads at A-72. Stream water from the gage on Mineral Creek was found to be toxic to fish and aquatic invertebrates, which is likely due to elevated concentrations of dissolved copper and dissolved and colloidal aluminum.