Metals retention in a net alkaline mine drainage impacted stream due to the colonization of the North American Beaver (Castor canadensis).

This study investigated two aspects of Castor canadensis (North American Beaver) colonization on a net alkaline mine drainage impacted tributary to Tar Creek (Ottawa County, OK, USA). Specifically, the retention of metals due to the presence of beaver dams, and metals contamination of sediments due to long-term loading, were examined. The Unnamed Tributary (UT) has been impacted by two mine drainage sources since at least 1979, with Cd, Fe, Pb, and Zn being the primary metals of concern. This study was conducted along a 1.6 km reach of the stream. The presence of beaver was noted in the UT in 2013, and by the end of 2014, the majority of the UT had been transformed into a series of six to eight wetlands constructed by beaver. The study found the presence of beaver impoundments resulted in a decrease in aqueous Fe and Cd concentrations, with minimal influence on Pb concentrations. The most upstream beaver pond, impounding waters with the greatest initial metals concentrations, had mean Fe and Cd removal efficiencies of 57% and 63%, respectively. Utilizing geomorphic data collected during rapid habitat assessments, this beaver pond had an approximate surface area and hydraulic retention time of 3000 m and 70 h, respectively, resulting in a mean Fe removal rate of 4.12 g m day of Fe. The sediments collected from the UT contained elevated Cd, Pb, and Zn concentrations, with many of the metals concentrations greater than five times the EPA site-specific probable effect concentrations for sediment toxicity (Cd: 11.1 mg/kg, Pb: 150 mg/kg, and Zn: 2083 mg/kg). Five sediment samples contained Fe concentrations exceeding 200,000 mg/kg. Overall, this study emphasizes the ability of beaver to modify their surroundings and the potentially important role of beaver dams to help retain metals from untreated mine drainage impacted streams.