Sediment carried into the Combie Reservoir (“Lake Combie”) contains mercury that originates, in part, from mercury use in historical gold mining performed in the upstream Bear River watershed over a century ago. The three-year Combie Reservoir Sediment and Mercury Removal Project is funded by the California Department of Water Resources (DWR) Proposition 13 Bay-Delta Multipurpose Water Management Program and the Nevada Irrigation District (NID).
This project is helping to define a standard for sediment removal at mercury-impaired reservoirs by scaling up an emergent elemental mercury extraction process and evaluating its efficiency in combination with conventional sediment removal techniques. These conventional techniques include both wet methods (suction dredging with a hydraulic cutter head) and dry methods (mechanical excavation during the reservoir drawdown period).
The project goals are (1) To remove accumulated sediment and mercury from Combie Reservoir, thus restoring reservoir capacity and reducing the mercury load in the Bear River watershed and downstream receiving waters (i.e., the Feather River and California Bay Delta); (2) to develop an efficient, compliant, and sustainable combination of processes for sediment removal at similar mercury-impacted reservoirs; (3) to monitor and document the efficiency of the system and its various components with respect to cost per unit of sediment and mercury removed; (4) to monitor mercury concentrations in biota and environmental media at locations upstream and downstream from the Project to develop a better understanding of mercury and nutrient cycling in the reservoir and to provide insight regarding the potential effects of sediment removal on these complicated systems; (5) to analyze and document the project as a reference to managers, engineers, scientists and regulators involved with maintenance dredging; and (6) to inform the public regarding the efforts of the California DWR and NID to maintain our water resources and to advance the science that protects the quality of our water.