Å·ÃÀ¿Ú±¬ÊÓƵ

Crouch, Caitlin ¹ ; Todd, Andrew ² ; McKnight, Diane ³ ; Ling, Alison ⁴ ;Robinson, Patrick ⁵

¹ INSTAAR, Å·ÃÀ¿Ú±¬ÊÓƵ Environmental Studies Graduate Program
² INSTAAR
³ INSTAAR, Å·ÃÀ¿Ú±¬ÊÓƵ Department of Civil and Environmental Engineering
⁴ Graduate Student, Å·ÃÀ¿Ú±¬ÊÓƵ Department of Civil and Environmental Engineerging
⁵ University of Å·ÃÀ¿Ú±¬ÊÓƵ

The Snake River Watershed in Å·ÃÀ¿Ú±¬ÊÓƵ is impacted by acid rock drainage (ARD) originating from both natural sources and sources associated with historic mining in the watershed (Figure 1). Downstream ARD sources, high metal ion concentrations, low pH, and metal oxide deposition disrupt ecosystem function, impair biological diversity, and contaminate surface and groundwater drinking water supplies. While elevated concentrations of numerous trace metals are present, dissolved zinc is of particular concern because concentrations persist above trout toxicity thresholds well downstream of ARD inputs. In the Snake River Watershed, self-sustaining trout populations are quite sparse. Dissolved zinc concentrations measured during the seasonal low flows of September and October have been observed to increase significantly (four-fold increase) over the past 30 years in portions of the watershed above mining impacts (Figure 2). This trend is associated with an increase in sulfate concentrations, which indicates that these water quality changes are driven primarily by accelerated natural weathering of pyrite (FeS2) in the watershed. The observed increase in natural ARD – possibly the result of climate change – may have implications for mitigation. The purpose of this study was to analyze temporal zinc trends and delineate discrete surface water ARD sources along the Upper Snake River. Large contributions of zinc from two tributaries on the north side of the drainage were found to be an order of magnitude higher than in the main stream (Figure 3). Sulfate, hardness, and total metals varied with zinc.