Executive Summary

The South Yuba River (SYR), located on the western portion of the Sierra Nevada in California, is highly contaminated with mercury (Hg) as a result of historical gold (Au) mining that took place throughout this region starting in the mid 1800s and continuing into the early 1900s. During this period, the hydraulic mining of alluvial Au deposits formed during the Tertiary period (65.5 to 2.6 million years before present) was responsible for mobilizing hundreds of millions of tons of hydraulic mining debris (HMD), which was and continues to be redeposited in the SYR, its tributaries, and the San Francisco Bay Delta. Hydraulic mining was used in combination with the mercurygold (Hg-Au) amalgamation process. Elemental mercury (Hg(0)) was introduced into Au recovery sluices to trap Au flakes, which were mixed with the sediment-water slurry produced from the hydraulic mobilization of sediment. As a result of inefficient trapping, some amount of both Hg(0) and Hg-Au amalgam was lost in this process. And along with the HMD, both can still be found throughout the SYR watershed, downstream of the major historic mining areas.

Today, there is concern about the level of Hg bioaccumulation in the SYR watershed and similarly affected former Au-mining locations throughout the Sierra Nevada. Further, Hg-contaminated HMD and sub-aqueous Hgcontaminated sediment can be remobilized both by natural processes and human activities, including recreational suction dredging. Thus, resource managers, anglers, and other watershed stakeholders are concerned that remobilizing Hg-contaminated sediment and HMD may exacerbate Hg bioaccumulation in downstream environments.

To address the above concerns, a study was conducted to assess the potential effects of recreational suction dredging in terms of 1) its viability as an approach to clean up riverbed sediment contaminated with legacy Hg from historical Au-mining activity, and 2) its effects on Hg remobilization, speciation (chemical form), and the potential for the stimulation of toxic methylmercury (MeHg) production and bioaccumulation in downstream environments. As part of this larger study, a series of laboratory experiments were conducted with various types of sediment material collected in and around the South Yuba River−Humbug Creek (SYR-HC) confluence area. This report describes the results of these laboratory experiments.

First posted January 25, 2011