USGS Publications Warehouse

Abstract

Acid drainage from abandoned mines and from naturally-acidic rocks and soil in the upper Animas River watershed of Colorado generates elevated concentrations of acidity and dissolved metals in stream waters and deposition of metal-contaminated particulates in streambed sediments, resulting in both toxicity and habitat degradation for stream biota. High concentrations of iron (Fe), aluminum (Al), zinc (Zn), copper (Cu), cadmium (Cd), and lead (Pb) occur in acid streams draining headwaters of the upper Animas River watershed, and high concentrations of some metals, especially Zn, persist in circumneutral reaches of the Animas River and Mineral Creek, downstream of mixing zones of acid tributaries. Seasonal variation of metal concentrations is reflected in variation in toxicity of stream water. Loadings of dissolved metals to the upper Animas River and tributaries are greatest during summer, during periods of high stream discharge from snowmelt and monsoonal rains, but adverse effects on stream biota may be greater during winter low-flow periods, when stream flows are dominated by inputs of groundwater and contain greatest concentrations of dissolved metals. Fine stream-bed sediments of the upper Animas River watershed also contain elevated concentrations of potentially toxic metals. Greatest sediment metal concentrations occur in the Animas River upstream from Silverton, where there are extensive deposits of mine and mill tailings, and in mixing zones in the Animas River and lower Mineral Creek, where precipitates of Fe and Al oxides also contain high concentrations of other metals. This report summarizes the findings of a series of toxicity studies in streams of the upper Animas River watershed, conducted on-site and in the laboratory between 1998 and 2000. The objectives of these studies were: (1) to determine the relative toxicity of stream water and fine stream-bed sediments to fish and invertebrates; (2) to determine the seasonal range of toxicity in stream water; (3) to develop site-specific thresholds for toxicity of Zn and Cu in stream water; and (4) to develop models of the contributions of Cu and Zn to toxicity of stream water, which may be used to characterize toxicity before and after planned remediation efforts. We evaluated the toxicity of metal-contaminated sediments by conducting sediment toxicity tests with two species of benthic invertebrates, the midge, Chironomus tentans. and the amphipod, Hyalella azteca. Laboratory toxicity tests with both taxa, exposed to fine stream-bed sediments collected in September 1997, showed some evidence of sediment toxicity, as survival of midge larvae in sediments from Cement Creek (C48) and lower Mineral Creek (M34), and growth of amphipods in sediments from these sites and three Animas River sites (A68, Animas at Silverton; A72, Animas below Silverton, and A73, Animas at Elk Park) were significantly reduced compared to a reference site, South Mineral Creek (SMC) . Amphipods were also exposed to site water and fine stream-bed sediment, separately and in combination, during the late summer low flow period (August-September) of 1998. In these studies, stream water, with no sediment present, from all five sites tested (same sites as above, except C48) caused 90% to 100% mortality of amphipods. In contrast, significant reductions in survival of amphipods occurred at two sites (A72 and SMC) in exposures with field-collected sediment plus stream water, and at only one site (A72) in exposures with sediments and clean overlying water. Concentrations of Zn, Pb, Cu, and Cd were high in both sediment and pore water (interstitial water) from most sites tested, but greatest sediment toxicity was apparently associated with greater concentrations of Fe and/or Al in sediments. These results suggest that fine stream-bed sediments of the more contaminated stream reaches of the upper Animas River watershed are toxic to benthic invertebrates, but that these impacts are less serious than toxic effects of metals in stream water. We evaluated seasonal variation in the toxicity of stream water by repeating water-only toxicity tests with amphipods in stream water during the late winter low flow period (early April) of 1999, and by conducting toxicity tests with fathead minnows, Pimephales promelas, in stream water during both sampling periods. Stream water was more toxic to fathead minnows in late winter than in late summer, with only one of five sites toxic in summer 1998 and three of four sites toxic in winter 1999. Undiluted stream water from all sites was highly toxic to amphipods in both seasons, and dilutions as low as 25% from some sites caused significant toxicity to amphipods in winter 1999. Toxic effects of stream water on amphipods and fathead minnows was associated with greater concentrations of several dissolved metals, including Zn, Cu, Fe, and Al. Complex interactions among speciation, solubility, and toxicity of Fe and Al made evaluation of the contributions of these metals to toxicity difficult. We modeled the contributions of Zn and Cu to toxicity of stream water, based on available information on dissolved Zn and Cu concentrations and on results of toxicity tests that established thresholds for toxicity of these metals at ambient water quality. We conducted toxicity tests with Zn and Cu in reconstituted waters, with ionic constituents based on stream water of the upper Animas River near Silverton (A68 and A72), to establish site-specific toxicity thresholds for toxicity of these metals. Toxicity thresholds were determined for amphipods and fathead minnows, under test conditions comparable to those used for on-site tests, and for early life stages (ELS; egg through swim-up fry) of brook trout, Salvelinus fontinalis, the most widespread and abundant fish species in the upper Animas River watershed. Reconstituted test waters were prepared to represent conditions in the Animas River at Silverton during the summer testing period (Animas soft water: hardness 110 mg/L as CaCO3) and conditions at this location during the late-fall spawning period for brook trout (Animas hard water: hardness 180 mg/L). The three taxa tested differed widely in their sensitivity to toxic effects of Cu and Zn under conditions in the upper Animas River. Amphipods were the most sensitive of the three taxa to Zn toxicity, with effects on survival at concentrations less than 100 ug/L in Animas Soft water, and brook trout were the least sensitive taxa to Zn toxicity, with effects on growth of early life stages at concentrations of 1,000 ug/L and greater and no significant effects on survival at concentrations up to 2,000 ug/L in Animas Hard water. Both brook trout and fathead minnows were highly sensitive to Cu, with significant reductions on growth in hard water at concentrations less than 10 ug/L and significant reductions in survival at concentrations between 20 and 30 ug/L. Amphipods were less sensitive to Cu, with significant reductions in survival occurring between 50 and 100 ug/L. These results indicate that the fathead minnows used for on-site toxicity tests were adequate surrogates for the responses of brook trout to Zn and Cu in stream water. The results of brook trout ELS tests suggest that Zn may not be a major contributor to the toxicity of stream water to brook trout in the Animas River and lower Mineral Creek, despite consistently high dissolved Zn concentrations (200-1,000 ug/L) at study sites in these reaches. We modeled seasonal variation in toxicity of dissolved Zn and Cu, based on site-specific toxicity thresholds and seasonal variation in concentrations of these metals at three USGS sites near Silverton. Toxicity thresholds determined in 'Animas' reconstituted waters were adjusted for seasonal variation in hardness at these sites, based on exponential regressions of relationships between hardness and metal toxicity. The resulting site-specific thresholds were compared to concentrations of dissolved Zn and Cu predicted for each site by multiple regression models based on season (Julian date) and discharge. Modeled dissolved Zn concentrations at all three locations frequently exceeded thresholds for mortality of amphipods, but did not approach thresholds for reduced growth or survival of brook trout. In contrast, modeled Cu concentrations frequently exceeded thresholds for reduced growth and survival of brook trout and fathead minnows at two of the three sites. Model predictions were consistent with results of toxicity tests with amphipods and minnows in stream water and with the distribution of brook trout in recent surveys. These findings indicate that dissolved Cu concentrations in stream water are a significant limiting factor for brook trout in the upper Animas River watershed. However, the recovery of brook trout populations at some study sites may also be limited by toxicity of other metals, such as Fe and Al, and trout populations at all three sites near Silverton are probably affected by the reduced productivity of benthic invertebrates, resulting from toxicity of dissolved Zn and from degradation of physical habitats.