Combie Reservoir Sediment & Mercury Removal Project

NOTE:  This is a project of the Nevada Irrigation District.  The Sierra Fund supports this project for its great potential to be a model for restoring water storage capacity in the Sierra’s existing reservoirs, while remediating mercury left in our watersheds from legacy mining.

The upper reaches of Combie Reservoir at low water.  The large amount of accumulated sediment is visible.

The upper reaches of Combie Reservoir at low water. The large amount of accumulated sediment is visible.

Project Overview

Written by Nevada Irrigation District

Most surface waters in the Sierra Nevada have been significantly and adversely impacted by historic gold mining activities, particularly the streams, rivers, and reservoirs in the Cosumnes, American, Bear and Yuba watersheds. As a result, water bodies in these regions contain elevated concentrations of mercury that are remnants of gold processing practices used over a century ago. Mercury is a water quality constituent of national concern; it is listed in California Toxics Rule by the Environmental Protection Agency. Consumption of fish from water bodies contaminated with mercury can lead to developmental delays in fetuses, infants and children.

The Nevada Irrigation District (NID) owns and operates two reservoirs on the Bear River which are 303d listed for mercury: Rollins Reservoir and Combie Reservoir. For more than 30 years, NID has contracted with private aggregate mining companies to remove sediments that naturally migrate toward the reservoirs. At Combie Reservoir, dredging was used to remove sediments for more than 15 years. Dredging operations in Combie Reservoir were halted in 2003 as a result of high mercury levels found in dredge effluents, affecting NID efforts to maintain reservoir storage capacity, and potentially affecting NID’s ability to supply drinking water to its customers. With the majority of California’s water supply coming from rivers and reservoirs of the Sierra Nevada Mountains, the impact of such mercury contamination, may threaten water quality for many Californians, and the prevention of dredging operations threatens water supply storage over the long-term.

The Combie Reservoir Sediment and Mercury Removal Project (hereafter referred to as the Project) is a water supply maintenance project that removes sediment from Combie Reservoir while introducing an innovative recovery process to reduce elemental mercury concentrations in the Bear River watershed. It will utilize the design, construction, and operation of an innovative mercury extraction process paired with ongoing sediment removal operations to maintain reservoir storage capacity. This project will utilize a proven patented technology, the Knelson Concentrator, in a new application in order to remove elemental mercury from dredged sediments, while monitoring and studying the effects of the operation on water quality and biota. This initial project is estimated to take between three to five years to complete. On-going maintenance dredging to maintain reservoir capacity is estimated to reoccur on 10 year intervals. Project benefits include; water supply reliability, water quality protection and improvement, ecosystem restoration and enhanced recreation. If this project demonstrates that mercury can be removed from river sediments the process can be applied at other reservoirs throughout the Sierra Nevada. In time, there could be a beneficial effect toward remediation and reduction of mercury contamination. Such remediation efforts would also be beneficial to the California Bay-Delta.

See also:

Mercury Removal Project information on NID website – This page on the Nevada Irrigation District website includes a video and public documents pertaining to the Combie Reservoir project.

SNC Video on Combie Reservoir Project – This video, produced by the Sierra Nevada Conservancy and featuring The Sierra Fund Science Director Dr. Carrie Monohan, highlights the ongoing discharge from upstream legacy mine sites and the Combie Reservoir project as a key opportunity to clean up mercury.

NID’s Video on Combie Reservoir Mercury Removal Project – Another short video about Nevada Irrigation District’s pilot project to remove mercury-contaminated sediment from their reservoir.