Scotchman Creek Watershed Assessment: A Focus on Abandoned Mine Impacts

The objective of the Scotchman Creek Watershed Assessment was to understand the potential sources of contamination from past mining activities in the Scotchman Creek watershed through stakeholder outreach and the identification and quantification of mercury and turbidity within the watershed.

The Scotchman Creek watershed is a tributary to the South Yuba River. Ongoing erosion and sedimentation throughout the sub-watershed creates turbid surface water that carries particulate-bound mercury. The watershed contains two major hydraulic mine sites, Alpha Diggins and Omega Diggins, in addition to the Scotchman Debris Dam and many smaller mine sites and gravel deposits. The Omega Diggins hydraulic mine site is not remediated and privately owned while the Alpha Diggins hydraulic mine site is in the process of being remediated and is owned by the Tahoe National Forest. In addition to quantifying mercury and sediment levels across the watershed, we compared the contribution of mercury and total suspended solids during storm events directly downstream from Alpha and Omega Diggins, at an upstream control point, and the mouth of the watershed to understand whether mine remediation actions were effective at reducing contaminant sources.

Figure 2: Log transformation between total suspended sediment and particulate-bound mercury

Results indicate that particulate-bound mercury is the primary form of mercury moving through these watersheds. Controlling mercury at its source, before it methylates and incorporates into the food web is a key element to protecting the health of an entire watershed. Mercury bound to clay particles can be managed on site by decreasing erosion potential within the abandoned mine. Settling ponds, check dams, and hotspot removal are current techniques of slowing down runoff, allowing toxic sediments to filter out, or removing material. Revegetation techniques could also aid in the long-term remediation of hydraulic mines and should be considered. From a management perspective, stopping the particulate-bound mercury at the source, rather than letting it move downstream into environments where it can be methylated and become bioavailable is key.

Scotchman Falls during a storm event

A day after a storm event