For more than 30 years, Nevada Irrigation District (NID) has contracted with private aggregate mining companies to remove sediment that accumulates in reservoirs. At Combie Reservoir, dredging was used to remove sediment.
Dredging operations in Combie Reservoir were halted in 2003 as a result of mercury levels found in dredge effluents, affecting NID’s efforts to maintain reservoir storage capacity and operations.
Because of mercury contamination, there was a need to develop an innovative approach to sediment removal from reservoirs impacted by mercury. NID’s Combie Reservoir Sediment and Mercury Removal Project is a three step process: (1) sediment removal, (2) sediment treatment, and (3) water treatment.
1) Sediment removal can be done in the dry or in the wet. In the dry, earth moving equipment is used to remove accumulated sediments during low water conditions when the deposit is exposed. In the wet, a dredge is used to suck up material from the bottom of the reservoir and transport that material in a slurry to a treatment plant on the shore.
2) The sediment is treated using a centrifuge to remove elemental mercury.
3) The slurry water is then treated by using a combination of coagulants and polymers to settle out the fines and return clean water to the reservoir
NID began removal of accumulated sediment in 2018 under a Department of Water Resources grant, using both removal in the dry and removal in the wet methods. On-going reservoir maintenance of sediment removal is needed to maintain reservoir capacity. The project is operating under a water quality permit that requires monitoring to ensure that no water quality standards are being violated at any time in the process.
NID’s pilot project demonstrates emerging technology and improved understanding of mercury that will inform sediment removal efforts in other locations where mercury-contaminated sediment is accumulated in reservoirs. In time, a reduction of mercury contamination in reservoirs will benefit streams and rivers in the headwaters all of the way to the San Francisco Bay-Delta.
The video below and documents in the sidebar provide further information and description of the project.
Tour showcases the Combie Sediment & Mercury Removal Project
Representatives from agencies and organizations involved in the work to remove sediment and mercury at Combie Reservoir got a special 3-hour tour in August to see first-hand how the project has progressed.
This summer, a suction dredge has been used to suck up sediment from the water and then put through a centrifuge to separate any elemental mercury.About 20 participants got a tour of the different operations involved in the process.
While everyone was familiar with the project, based on their own work, seeing how the whole process has come together was rewarding.
Mercury was introduced to the Sierra Nevada during the Gold Rush period of the 1800s and used to process mining ore for gold. That mercury has remained and led to contamination of sediments throughout Sierra watersheds.
Findings from this project will become valuable to state regulators and help water managers address mercury in the aquatic food chain. When complete, this project can be replicated in other impacted reservoirs.
NID is working with these partners: the Sierra Fund, US Geological Society, the Department of Water Resources, the Cosumnes-American-Bear-Yuba Integrated Regional Water Management Group (CABY), NV5 Global, Inc., Great Lakes Environmental and Teichert Aggregates.