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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

A Toxicological Reconnaissance of the Upper Animas River Watershed near Silverton, Colorado

By Del Wayne R. Nimmo,¹ Carla J. Castle,² and John M. Besser³

Toxicity tests were conducted during 1997 with water and sediment from the mainstem and tributaries of the Animas River near Silverton, Colorado. The toxicity of stream water to rainbow trout, Oncorhynchus mykiss; daphnids, Ceriodaphnia dubia; fathead minnows, Pimephales promelas; and amphipods, Hyalella azteca, was evaluated with 48- and 96-hour lethality tests conducted onsite. Initial tests were conducted with undiluted surface water and subsurface water (interstitial or hyporheic water) from six sites. Subsurface water was collected from shallow depressions dug in gravel bars. Two sites were selected for detailed tests which used serial dilutions of test waters with reconstituted laboratory water. The toxicity of stream sediments was tested in the laboratory using 10-day tests with H. azteca and the midge, Chironomus tentans. Fine sediment (particle diameter <0.5 millimeters) was collected from all six sites by sieving stream gravels.

Initial tests indicated that both surface and subsurface water from Cement Creek and subsurface water from the Animas River below Silverton (site A72) were toxic to daphnids, minnows, and trout. Surface water from Cement and Mineral Creeks and the Animas River above Silverton (site A68), below Silverton (site A72), and at Elk Park (8 kilometers downstream from Silverton) was toxic to daphnids but not fish. Subsurface water from South Mineral Creek (reference site), Mineral Creek, site A68, and the Animas River at Elk Park was not toxic to the test species. Subsurface water from site A72 was the most toxic sample tested and daphnids were the most sensitive species, with median lethal concentrations (LC50, expressed as percent dilution) of 31 percent, followed by minnows (35 percent), trout (40 percent), and amphipods (56 percent). Calculations of Toxic Units (measured concentration of metal divided by known toxic concentration) suggest that copper and zinc could account for the toxicity of water from site A72. Multiple samples of surface water from the Animas River at Elk Park showed intermittent toxicity, which may be attributable to variation in copper concentrations.

Survival of midges and growth of amphipods were reduced significantly by exposure to sediment from several sites (relative to the reference site, South Mineral Creek). Trends in sediment toxicity were similar to those observed in water-phase tests, with greatest adverse effects caused by sediment from Cement Creek, Mineral Creek, and site A72. In contrast, neither amphipod survival nor midge growth differed significantly among sites, and the overall magnitude of effects was less in sediment tests than in water-phase tests.

¹U.S. Geological Survey, Aylesworth Hall NW, Colorado State University, Ft. Collins, CO 80523 (del_nimmo@lamar.colostate.edu)

²U.S. Geological Survey, Aylesworth Hall NW, Colorado State University, Ft. Collins, CO 80523 (carlac@cnr.colostate.edu)

³U.S. Geological Survey, 4200 New Haven Road, Columbia, MO 65201 (john_besser@usgs.gov)

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