Date of this Version
This report presents geochemical data from two 2002 sampling campaigns conducted in Englebright Lake on the Yuba River in northern California. A deep coring campaign was done in May–June 2002 and a shallow sampling campaign was completed in October 2002. This work assessed the chemical composition of material deposited in the reservoir between 1940, the year Englebright Dam was completed, and 2002 as part of the Upper Yuba River Studies Program, an effort designed to evaluate the feasibility of introducing anadromous fish, including steelhead and spring-run Chinook salmon, upstream from Englebright Dam. Results of analyses of total mercury (HgT) in 444 subsamples, methylmercury (MeHg) in 243 subsamples, and other trace and major elements in 202 subsamples are presented. Data quality was evaluated on the basis of analyses of replicate pairs of subsamples, standard
reference materials, blanks, and spike additions.
Deep coring penetrated the full thickness of material deposited after 1940 at six locations in the reservoir; the cores reached a maximum depth of 32.8 meters below the reservoir floor. At the three deep coring sites closest to Englebright Dam, concentrations of HgT (dry basis) were consistently in the range of 100 to 500 ng/g (nanogram per gram), in sediment dominantly of silt size (median grain size of 0.004 to 0.063 mm [millimeter]). At the deep coring sites located farther upstream, the upper parts of the profile had lower concentrations of HgT, generally ranging from 2 to 100 ng/g, in sediment dominantly of sand size (median grain size from 0.063 to 2 mm). The lower part of the vertical profiles at three upstream coring sites had higher concentrations of HgT than the upper and middle parts of these profiles, and had finer median grain size.
The highest median concentration of MeHg (1.1 ng/g) was in the top 2 cm (centimeter) of the shallow box cores. This vertical interval also had the highest value of the ratio of MeHg to HgT, 0.41 percent. Median concentrations of MeHg and median values of MeHg/HgT decreased systematically with depth from 0–4 to 4–8 to 8–12 cm in the shallow cores.
However, similar systematic decreases were not observed at the meter scale in the deep cores of the MEM (MEthylMercury) series. The overall median of the ratio MeHg/HgT in the deep cores was 0.25 percent, not much less than the overall median value for the shallow cores (0.33 percent).
Mercury-203 radiotracer divalent inorganic mercury (203Hg(II)) was used to determine microbial mercury-methylation potential rates for 11 samples collected from three reservoir locations and various depths in the sediment profile. For the five shallow mercury-methylation subsamples, ancillary geochemical parameters were assayed, including microbial sulfate reduction rates, sulfur speciation (sediment acid volatile sulfide, total reduced sulfur, and pore-water sulfate), iron speciation (sediment acid extractable iron(II), amorphous iron(III), crystalline iron(III) and pore-water iron(II)), porewater chloride and dissolved organic carbon, and pH, oxidation- reduction potential (Eh) and whole-sediment organic content. The highest potential rates of microbial mercury methylation were measured in shallow (0 to 8 cm depth) sediments (5 to 30 nanograms of mercury per gram dry sediment per day), whereas potential rates for subsamples collected from depths greater than 500 cm were consistently below the detection limit of the radiotracer method (< 0.02 nanogram of mercury per gram dry sediment per day).
Chemical analyses of trace and major elements in bed sediment are presented for 202 samples from deep cores from five locations in Englebright Lake. The mean values and standard deviations for selected trace elements were as follows (in micrograms per gram): antimony, 2.4 ± 1.6; arsenic, 69 ± 48; chromium, 134 ± 23; lead, 33 ± 25; and nickel, 87 ± 24.
Concentrated samples of heavy-mineral grains, prepared using nine large-volume composite samples from deep cores, were examined using optical and scanning-electron microscopy. Estimated gold concentrations in the composite sediment samples ranged from 38 to 840 milligrams per cubic meter. Grains of gold-mercury amalgam and grains of electrum (native gold), with and without mercury staining, were observed in the heavy-mineral concentrates.
Results of this study will be used to evaluate potential effects on fish habitat and trace-element transport in relation to various scenarios regarding efforts to improve fish passage at Englebright Lake. The scenarios include dredging or release of sediments to downstream environments in association with
dam modification or removal.