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This study was designed to characterize the particle-size distribution and the concentrations of total mercury (HgT), methylmercury (MeHg), and other constituents in sediments trapped behind Daguerre Point Dam, a 28-foot-high structure on the lower Yuba River in California. The results of the study will assist other agencies in evaluating potential environmental impacts from mobilization of sediments if Daguerre Point Dam is modified or removed to improve the passage of anadromous fish. Methylmercury is of particular concern owing to its toxicity and propensity to bioaccumulate. A limited amount of recent work on hydraulic and dredge tailings in other watersheds has indicated that mercury and MeHg concentrations may be elevated in the fine-grained fractions of placer mining debris, particularly clay and silt. Mercury associated with tailings from placer gold mines is a source of continued contamination in Sierra Nevada watersheds and downstream water bodies, including the Sacramento–San Joaquin Delta and the San Francisco Bay of northern California.
Churn drilling was used to recover sediments and heavy minerals at 5-foot intervals from six locations upstream of Daguerre Point Dam. Maximum depth of penetration ranged from 17.5 to 35 feet below land surface, resulting in 31 discreet drilled intervals. Drilling in permeable, unconsolidated sediments below the streambed of the Yuba River released a significant volume of water along with the sediment, which complicated the sampling and characterization effort. Overflow of a silty fraction sampled at the drill site contained suspended sediment consisting predominantly of silt and clay, with HgT concentration ranging from 33 to 1,100 ng/g (nanogram per gram) dry weight. A sandy fraction, collected after sieving sediment through a 2-millimeter vibratory screen, contained from 14 to 82 percent sand and 1 to 29 percent silt plus clay, and had HgT concentrations ranging from 6.8 to 81 ng/g dry weight. A clay-silt fraction, sampled from material remaining in suspension after the sandy fraction settled for 15-20 minutes, contained mercury concentrations from 23 to 370 ng/g dry weight. Concentrations of MeHg were less than the detection limit (
Potential rates of mercury methylation and demethylation were evaluated in seven samples using radiotracer methods. Mercury methylation (MeHg production) potentials were generally low, ranging from less than 0.15 to about 1.6 ng/g/d (nanogram per gram of dry sediment per day). Mercury demethylation (MeHg degradation) potentials were moderately high, ranging from 1.0 to 2.2 ng/g/d. The ratio of methylation potential (MP) to demethylation potential (DP) ranged from less than 0.14 to about 1.4 (median = 0.24, mean = 0.44, number of samples = 7), suggesting that the potential for net production of MeHg in deep sediments is generally low. The MeHg production rates and MP/DP ratios were higher in the shallower interval in two of the three holes where two depth intervals were assessed, whereas the MeHg concentrations were higher in the shallower interval for all three holes. A similar spatial distribution was found for concentrations of solid-phase sulfide (measured as total reduced sulfur and likely representing iron-sulfide and iron-disulfide compounds), which were much higher in shallower samples (about 700 to about 2,100 nanomoles per gram, dry sediment) than in deeper samples (32 to 55 nanomoles per gram, dry sediment) in these three holes. If reduced sulfur compounds are oxidized to sulfate as a consequence of sediment disturbance, the activity of sulfate-reducing bacteria might be stimulated, causing a short-term increase in methylation of inorganic Hg(II) (divalent mercury). The extent of increased Hg(II)-methylation would depend on the reactivity of the inorganic Hg(II) fraction associated with these sediments, which is currently unknown.
The relation between HgT concentration and particle size was evaluated quantitatively using a suite of 15 samples, which were separated into 6 size fractions using screens with the following sieve sizes: 2,000, 500, 297, 149, 60, and 30 μm (micrometer). A trend toward higher concentrations of mercury in samples having finer particles was observed. This trend is consistent with data obtained for other bulk samples (sandy, silty, and clay-silt fractions), which indicate higher concentrations of mercury associated with larger proportions of clay-size material (less than 4 μm).
Heavy-mineral concentrates were prepared to evaluate gold concentrations and to make direct observations of elemental mercury and gold-mercury amalgam. Elemental mercury (Hg0) was observed coating gold and gold-amalgam grains and was isolated with the aid of a stereo microscope. Using a scanning electron microscope, Hg0 and mercury-rich gold amalgam were observed on the surfaces of individual gold grains. Spherules of liquid Hg0 up to 6 μm in diameter were observed in association with platy, aluminosilicate minerals and iron oxides.
In addition to mercury, concentrations of several other trace elements in the fine-grained sediment trapped behind Daguerre Point Dam are of potential environmental concern. Median concentration values of arsenic, chromium, copper, and nickel from 19 sediment samples (screened to less than 0.060 millimeter) were higher than consensus threshold effects levels for ecological toxicity, and maximum concentrations of lead, mercury, and zinc were also above the threshold effects levels.