Summary

Description

The Atlas Mill Tailings Pile is located adjacent to the Upper Colorado River near Moab, Utah. Milling of ore ceased in 1984 and the Atlas Corporation subsequently declared bankruptcy. The U.S. Department of Energy USDOE is the current manager of the site and is evaluating cleanup options that include remediation of groundwater at the site. This reach of the Upper Colorado River was declared as critical habitat for two endangered fish species Colorado pikeminnow, Ptychocheilus lucius and razorback sucker, Xyrauchen texanus by the U.S. Fish and Wildlife Service USFWS because it is one of the few existing areas that contains known spawning and rearing habitats for these fishes. Monitoring data indicates that the groundwater entering the Upper Colorado River is contaminated with ammonia, metals, and radiochemicals. The USFWS is concerned that contaminated groundwater from the Atlas Mill Tailings Pile may be impacting endangered fish populations within this critical habitat. The U.S. Geological Survey USGS conducted a sitespecific risk assessment to determine if groundwater entering the Upper Colorado River from beneath the tailings pile could impact populations of the endangered Colorado pikeminnow and razorback sucker. Spatial mapping of contaminant plumes in the river was conducted over several hydrologic regimes and seasons from August 1998 to August 2000. Laboratory and field toxicity tests were conducted with Colorado pikeminnow, razorback sucker, and fathead minnow Pimephales promelas to determine their sensitivity to ammonia and site waters. The effects of contaminated groundwater on macroinvertebrate communities were also assessed. Results indicated that the Atlas Mill Tailings Pile represents a localized source of groundwater input containing elevated levels of ammonia, metals, and radiochemicals that exceed Utah state water quality criteria during the lowwater hydrologic period ranging from August to March. Toxicity testing indicated that Colorado pikeminnow, razorback sucker, and fathead minnow were of similar sensitivity to ammonia with 28d LOEC values for mortality ranging from 2.19 to 4.35 mgL total ammonia at pH8.25 and temperature 25 C 0.14 to 0.31 mgL unionized ammonia. Mortality and growth endpoints were similar in sensitivity to ammonia for the three species. Onsite toxicity tests demonstrated that site waters were directly toxic to both the endangered Colorado pikeminnow as well as the standard surrogate fathead minnow. Highest observed field concentrations caused instantaneous mortality in both controlled laboratory and insitu field studies. Comparisons of laboratory and field results indicate that ammonia is the primary contaminant of concern due to high exposures and the rapid onset of toxicity. Metals and radiochemicals, although sometimes elevated above criteria, did not contribute to toxicity. There were no statistically significant differences in macroinvertebrate distributions that could be attributed to contaminated groundwater. Collectively, the data indicate that the Atlas Mill Tailings Pile represents a localized input of contaminated groundwater that threatens endangered fish species in the area. The current Utah state water quality criteria for total ammonia is 0.71 mgL assuming average conditions of pH8.25 and temperature 25 C, or approximately 0.06 mgL unionized ammonia. The Utah state water quality criteria for ammonia, if met, would be protective of Colorado pikeminnow and razorback sucker by a factor of at least 2 based on conservative toxicity endpoints. Therefore, remediation of groundwater entering the Colorado River to meet existing Utah state water quality criteria for ammonia would eliminate toxicological concerns for Colorado pikeminnow and razorback sucker.