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"Effects of Small-Scale Gold Dredging on Arsenic, Copper, Lead, and Zinc Concentrations in the Similkameen River"

Washington State Department of Ecology

March, 2005

A just released study confirms what previous studies by the EPA and others have shown -- discharges from small-scale dredges are not a threat to aquatic life.

The Washington State Department of Ecology conducted a field study "to determine if small-scale gold dredges operating in the Similkameen River exacerbate current exceedances of the human health criteria for arsenic or result in violations of aquatic life criteria for arsenic, copper, lead, or zinc. Dredge effluents were analyzed from 14 sites on the river, and discharge plumes were sampled below three dredges. Data were also obtained on ambient metals concentrations, total suspended solids, and turbidity."  

The following conclusions are made on page 24 (page 34 of the pdf):

"Results showed that the metals concentrations discharged from small-scale gold dredges are not a significant toxicity concern for aquatic life in the Similkameen River. Although this activity will exacerbate exceedances of arsenic human health criteria, it would take very large numbers of dredges to effect a 10% change in the river’s arsenic levels, even at low-flow conditions." (emphasis added)

Small-scale miners and prospectors in Washington State have been struggling to comply with current regulations for years. The "Gold & Fish" book, which spells out those regulations, is administered by the Washington Department of Fish & Wildlife (WDFW). The miners have repeatedly asked the WDFW for the "science" to justify such severe restrictions. (See "Progress Report on Washington State Small-Scale Regulations" in our upcoming April 2005 issue.)

Finally, the Washington State Department of Ecology has provided the scientific evidence that miners have been asking for. From the study:

"Based on analyzing 14 effluents and 27 plume samples, it appears that small-scale gold dredges have little or no potential to cause exceedances of aquatic life criteria in the Similkameen River. (page 21 of the study; page 31 of the pdf; emphasis added)

"During average September flows, it is estimated that somewhere between 17 and 57 dredges operating continuously would be required to increase dissolved zinc, lead, and copper concentrations in the Similkameen River by 10%.

It would take between approximately 200 and 520 dredges to have the same effect on total recoverable and dissolved arsenic, respectively. In order for zinc, lead, or copper concentrations to be doubled in the river, anywhere from 170 to 570 dredges would need to be operating.

Arsenic concentrations in the dredge effluents are too low to cause an increase of that magnitude, regardless of river flow.

At the 7-day, 10-year low flow in the Similkameen, relatively few dredges could effect a 10% change in copper, lead, and zinc concentrations. It would take 50 or more continuously operating dredges to double concentrations of these metals.

As demonstrated elsewhere in this report, a 100% increase in the ambient arsenic, copper, lead, or zinc concentrations in the Similkameen River would not result in exceedances of aquatic life criteria." (page 22 of the study; page 32 of the pdf; emphasis added)

The complete study is available on our website. Adobe Acrobat Reader is needed to read the .pdf file. You can download a free version here if needed. Read the complete study (.pdf).

A big "Thank you!" goes to the Washington State Department of Ecology for making the time and effort to complete this scientific study.