Thompson Creek and S Creek

Aquatic Biological Monitoring

Client

Location

Key Elements

The Thompson Creek Mining Company (TCMC) operates an open pit mine in the Thompson Creek and S Creek watersheds in central Idaho. Biological monitoring in the study area began in 1980 with measurements of benthic macroinvertebrate and fish populations at two sites on Thompson Creek and two sites on S Creek.

The monitoring program has continued and expanded to the present to include benthic macroinvertebrate, fish, and periphyton surveys at six sites in Thompson Creek, three sites on Squaw Creek, and four sites on the Salmon River. Ongoing quantitative sampling is currently conducted by GEI Consultants, Inc. in conjunction with the operation of the Thompson Creek Molybdenum Mine and meets requirements in their NPDES discharge permit. Monitoring of these components of the aquatic environment provides information on potential effects of the Thompson Creek Molybdenum Mine operations on the stream ecosystem.

Monitoring on Thompson and S creeks and the Salmon River indicate that they contain abundant and diverse assemblages of benthic invertebrates, fish, and periphyton. The objectives of ongoing annual monitoring are to 1) describe the present status of benthic invertebrate, fish, and periphyton populations in Thompson and Squaw creeks and the present status of benthic invertebrate and periphyton populations in the Salmon River, 2) determine the factors which may influence them, and 3) monitor the potential effects of TCMC operations on the aquatic assemblages of the three streams.

The monitoring allows GEI to evaluate trends in the data over time since 1980. We have been tracking the substantial variability in aquatic populations from year-to-year. Since 1980, macroinvertebrate density and number of taxa have significantly increased in both Thompson Creek and S Creek at both reference and downstream sites, suggesting regional improvement in conditions necessary to sustain sensitive species. Thompson Creek continues to sustain one of the most diverse assemblages of benthic macroinvertebrates of all GEI biomonitoring sites across the west. The streams continue to support trout, sculpins, and low numbers of bull trout (Salvelinus confluentus), a threatened species.

GEI’s monitoring also included additional sampling for mercury and selenium as part of the requirements for the renewal of NPDES discharge permits. The primary goals are to 1) monitor bioaccumulation of total mercury and total recoverable selenium in benthic invertebrates and fish, as well as sediment metal concentrations and total organic carbon; and 2) inventory components of the aquatic habitat that potentially facilitate uptake of mercury and selenium.

Concentrations of mercury in the discharge waters have consistently been below the corresponding effluent limits. Selenium studies are ongoing and indicate trends that appear to be related to a combination of unknown upstream sources and potential input from the mine waste rock.