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The Problem
Mercury contamination from historic gold mining operations is widespread in many rivers, lakes, and reservoirs on the western slopes of the Sierra Nevada. Miners used mercury (quicksilver) to recover gold from placer (alluvial) mines, which used hydraulic, drift, and dredging methods, and from hardrock (lode) mines. At hydraulic mining operations, placer ores were eroded with monitors (water cannons) and the resulting slurry was directed through sluices and drainage tunnels where the gold particles were combined with liquid mercury to form gold-mercury amalgam. About 10-30% of the mercury used in this process was lost each season, resulting in highly contaminated sediments downstream from the mines. The annual loss of mercury from a typical sluice was likely several hundred pounds during the operating season or roughly 3-8 million pounds statewide. Although mercury was also used in drift mining operations, in dredging operations, and at hardrock mines, the amounts lost to the environment by these sources have not been estimated. The northwestern Sierra Nevada region has been mined extensively for both hardrock and placer deposits of gold. In this region, the highest average concentrations of mercury measured in fish tissues occur in the Bear River and South Yuba River watersheds. Other investigators reported that mercury concentrations were highest in upper-trophic-level predators (largemouth bass Micropterus salmoides, smallmouth bass M. dolomieui, and spotted bass M. punctatus) from Camp Far West Reservoir and Lake Combie on the Bear River and Lake Englebright on the South Yuba River, with lower concentrations occurring in benthic omnivores (channel catfish, Ictalurus punctatus) and intermediate-trophic-level predators (bluegill, Lepomis macrochirus; green sunfish, L. cyanellus; and black crappie, Pomoxis nigromaculatus). Moreover, 14% (8 of 57) of the basses had mercury concentrations that exceeded 1.0 mg/kg (wet weight basis), the
Objectives Specific objectives are as follows: (i) to determine if mercury body burdens vary temporally and spatially in selected fishes and fish-forage macroinvertebrates; (ii) to determine how mercury body burdens vary according to chronological ages of fishes; (iii) to determine how mercury body burdens vary according to the trophic status of fishes; and (iv) to determine if methylmercury:total mercury ratios differ among fishes and fish-forage macroinvertebrates. Methodology
Fish and macroinvertebrates will be sampled from Camp Far West Reservoir at three general locations as follows: the Bear River arm, the Rock Creek arm, and the lower reservoir adjacent to the dam. Fish will be sampled at bimonthly intervals from March 2002 to August 2003 whereas macroinvertebrates will be sampled only during July 2002 and July 2003. Fish will be measured for total length and weight, and scale samples will be taken from a standard "scale sample area." The fish will then be preserved for analysis of gut contents. Excess fish and all non-target species will be counted and released alive.
During August 2002 and August 2003, the first 10 fish of selected species and size categories from each of three sampling sites will be identified with unique codes, measured for total length and weight, sampled for scales, then individually wrapped and bagged in plastic and placed on wet ice. Within 12 hr after returning to the field station, the gastrointestinal tracts (anterior end of esophagus to pyloric sphincter) of all sampled fish will be opened by dissection to remove gut contents. During August 2002, fish will be processed further by dissecting skinless and boneless fillets from both sides of each individual. After weighing, the fillets will be wrapped and bagged in plastic, then frozen. The remaining carcass (including dissected organs) of each fish will be weighed, then wrapped and bagged in plastic and frozen.
Bottom sediments and associated vegetation collected during July 2002 and July 2003 will be sorted for macroinvertebrates (crayfish, mayfly nymphs, and midge larvae). After sorting, the macroinvertebrates will be composited according to taxa and sampling site, then stored frozen. During each sampling date (July 2002 or July 2003), an attempt will be made to collect three composite samples of each macroinvertebrate taxon from each of the three sampling sites. Each composite sample should weigh at least 2.0 g (wet weight).
A maximum of 509 samples or subsamples of fish and macroinvertebrates will be analyzed for moisture content and total mercury concentration, whereas a maximum of 79 subsamples will be analyzed for methylmercury concentration. Frozen samples packaged at the Field Station will be shipped overnight to the Columbia Environmental Research Center (CERC), Columbia, Missouri, or USGS-approved laboratories for all mercury measurements. Standard techniques will be used to determine fish ages from scale samples and food habits from gut samples. Highlights and Key Findings Results are not yet available for this recently initiated study. Where Are We Headed In 2003 We will sample fish at bi-monthly intervals throughout FY-03 for determinations of age and gut contents. In addition, fish and macroinvertebrates will be sampled in July-August 2003 for mercury analysis. Project Contact Michael Saiki Email: michael_saiki@usgs.gov Publications |
