Publication Summary

A TMDL evaluation was conducted for arsenic in the Similkameen River. The major source appears to be tailings from historical mining activity in British Columbia between Hedley and the U.S. border. The only significant sources identified in Washington were: 1) Palmer Lake, likely arising from periodic flooding by the Similkameen River and perhaps inputs from Sinlahekin Creek, and 2) resuspension of contaminated sediments.

It was determined that the Similkameen naturally exceeds EPA human health criteria for arsenic upstream of Hedley. Water quality targets of 0.4 - 0.6 ug/L total recoverable arsenic are therefore proposed and estimates provided of the load reductions needed to meet the targets. An arsenic monitoring plan is suggested.

Under Section 303(d) of the federal Clean Water Act, the Similkameen River has been listed by Washington State for non-attainment of the EPA human health criteria for arsenic. A Total Maximum Daily Load evaluation was therefore conducted for the river, as required by EPA.

The major source of arsenic appears to be tailings from historical mining activity in British Columbia between Hedley and the U.S. border. The only significant sources identified in Washington were: 1) Palmer Lake, likely arising from periodic flooding by the Similkameen River and perhaps inputs from Sinlahekin Creek, and 2) resuspension of contaminated sediments.

It was determined that the Similkameen River naturally exceeds the EPA arsenic criteria upstream of Hedley. Under these circumstances, natural conditions constitute the water quality criteria. Because the criteria are naturally exceeded, the loading capacity for the river is equal to the natural background.

Water quality targets of 0.4 - 0.6 ug/L total recoverable arsenic are proposed, and estimates are provided of the load reductions needed in British Columbia and Washington State to meet the targets. The proposed targets make no allowance for the downstream increase in arsenic concentrations that might occur naturally as the Similkameen flows through British Columbia. If new data or analysis can provide a reliable estimate of what that increase would be, the numerical targets should be revised upward accordingly. In the interim, the proposed targets appear reasonable, given the concentrations typical of other Washington rivers and streams.

An arsenic monitoring plan is suggested for the Similkameen River. The plan recommends: 1) periodical review of Canadian federal/provincial water quality monitoring data for the Similkameen, 2) renewing arsenic monitoring of the river in Washington if and when cleanups are undertaken, 3) analyzing inorganic arsenic in Similkameen River and Palmer Lake fish to assess human health risk, and 4) conducting a study of arsenic sources and cycling in Palmer Lake.

Link to EIM data for User Study ID AJOH0016