Monitoring Mercury and Methylmercury in Water, Sediment and Biota in Combie Reservoir Before and After Dredging and Mercury Removal

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Dredging to maintain water storage capacity by the Nevada Irrigation District (NID) occurred at Lake Combie reservoir, one of several reservoirs along the Bear River, California over the past 40 years on an as-needed basis to maintain water storage capacity. Maintenance dredging operations were halted by the Central Valley Regional Water Quality Control Board (CVRWQCB) in 2002 because of elevated particulate mercury levels in water discharged to holding ponds adjacent to the reservoir. The mercury in the Bear River basin occurs as 1) elemental mercury, 2) gold-mercury amalgam and 3) adsorbed to the sediment (primarily silt, clay, and organics).  As a result of the historic mercury contamination, Combie Reservoir has been filling in with sediment elevated in mercury (ca. 0.5 mg/kg) and effectively lowering the water storage capacity of the reservoir. In an effort to restore Lake Combie’s water capacity and resume maintenance activities, NID must demonstrate that any sediment removal operations will not lead to increased mercury transport or bioaccumulation downstream from the maintenance activities.

On a long-term basis there is a public necessity to remove sediments from Combie Reservoir to restore and maintain water capacity while improving water quality by addressing legacy mercury contamination within the reservoir. If NID is unable to regularly maintain its reservoir capacity, in time, it would fill up with sediments, gravels and sands from upstream sources, thereby reducing water-storage capacity, hydro-electric power production opportunities and recreational uses, including fishing. NID has developed a 3-part project to fix this problem: (1) removal of sediment from Combie Reservoir via combination of excavation in the dry and in situ dredging to regain water storage capacity; (2) transport to a nearby processing plant for separation and removal of elemental and amalgamated mercury using a Knelson concentrator and a series of water treatment techniques; and (3) a net zero discharge of mercury and methylmercury to the Bear River and Lake Combie. The demonstration project removed approximately 50,000 cubic yards of sediment from the upstream part of Lake Combie reservoir (https://nidwater.com/mercury-removal-project/).

NID has requested that the USGS CAWSC characterize mercury contamination in the water, sediment, and biota of Lake Combie reservoir before and after its planned sediment and mercury removal activities. There is a need to document concentrations of mercury (Hg) and methylmercury (MeHg) in the reservoir water column, in shallow sediment, in pore water of shallow sediment, and in the food web to demonstrate potential benefits and (or) detrimental impacts of the planned sediment and mercury removal project on an ecosystem level. There is also a need to quantify changes in mercury and methylmercury loading into and out of the reservoir throughout the course of the project.

The study approach employs a two-pronged approach: 1) standard monitoring activities and supporting research efforts to characterize mercury contamination in the Lake Combie ecosystem and 2) quantification of mercury and methylmercury loads in and out of the reservoir. The within-reservoir ecosystem work is divided into ‘pre-removal’ and post-removal’ for the  water and suspended particulates, bed sediment and pore water, and zooplankton. The loading measurements and fish collections will be measured for four years throughout the duration of the project to quantify loads and bioaccumulation before, during, and after the removal operations are completed.

lake combie reservoir test sites

The aerial photos above show the test sites used for this study. On the left, are sites within the main area of Lake Combie Reservoir (CR2-CR7). The right photo shows upstream "CR1" sites where more extensive water quality measurements were collected in order to quantify mercury and methylmercury loads going into the reservoir. (Click to enlarge.)