Charles N. Alpers and Lorraine E. Flint U.S. Geological Survey
Noah P. Snyder Boston College
Figure 2 – GLAD200 drilling platform on Englebright Lake
During May-June 2002, DOSECC and the U.S. Geological Survey collaborated on research drilling at Englebright Lake, a 70,000 acre-foot reservoir built in 1941 on the Yuba River in the northwestern Sierra Nevada of California. The drilling at Englebright Lake was part of the Upper Yuba River Studies Program, a multi-disciplinary, multi-agency effort with the overall purpose of addressing the following question:
“To determine if the introduction of wild chinook salmon and steelhead to the Upper Yuba River watershed is biologically, environmentally, and socio-economically feasible over the long term.”
Specific to Englebright Lake, the main project goal was to characterize reservoir sediments with regard to grain-size distribution and mercury concentration so that the possible effects of dam removal and sediment release downstream could be assessed. The work was funded by the Resources Agency of the State of California under the California Bay-Delta Program (CALFED) and by the U.S. Geological Survey.
Figure 1 – Location map and sampling sites
In all, 335 meters of core were collected, primarily using hydraulic piston methods. Average recovery was 86%. Thirty holes were drilled at seven locations – at six of the locations, complete sections through the reservoir bed sediment deposits were achieved. Sediment thickness ranged from 6 m near Englebright Dam to 33 m in the mid-reservoir area. Before splitting, cores were analyzed for magnetic susceptibility, bulk density (neutron log), and gamma radiation. After splitting, digital photographs were taken and archived along with descriptive information.
One set of subsamples were taken immediately for analysis of methylmercury (frozen for preservation), moisture content, and loss on ignition (organic content). After all cores were logged and stratigraphic correlations were determined, a second, more detailed round of subsampling resulting in a vertically continuous section at each location was done, for analysis of grain-size distribution, total mercury, and other trace metals. Selected subsamples were used for geochronology (210Pb and 137Cs).
Results of grain-size distribution and loss on ignition for 561 samples were used to extrapolate from one-dimensional vertical sections of sediment sampled in cores to entire three dimensional volumes of the reservoir deposit. In this manner, the mass of the reservoir sediment deposit was estimated to be 26 x 106 metric tons of material, of which 64.7 to 68.5% was sand and gravel and the remainder was silt and clay.
Total mercury concentrations in unsieved sediment increased with decreasing distance from the dam, following a trend toward finer grained sediment. Average mercury concentrations of samples rich in silt and clay was about 300 nanograms per gram (ng/g, or parts per billion, ppb), whereas average Hg in sand-gravel-rich samples was about 12 ng/g (or ppb).
The successful drilling of complete post-reservoir sediment profiles at Englebright Lake allowed the USGS to make the first quantitative assessment of the total mass of mercury contained within a foothill reservoir in a watershed greatly impacted by historical gold mining. Based on available information, about 2.3×106 kg of mercury were lost to the environment in the Yuba River watershed in association with historical gold mining. The estimated total mass of mercury in Englebright lake sediments (accumulated from 1941 to 2002) is 2,500 to 2,800 kg, which represents about 0.1% of the total amount of Hg estimated to have been lost to the environment during historical gold mining in this watershed.
Results of this project will play a significant role in determining the potential impacts of the removal of Englebright Dam on downstream environments. Also, the core samples from Englebright Lake represent an archive of the history of sediment transport and deposition processes in the Yuba River watershed over more than six decades, and are a potential resource to future researchers investigating watershed transport processes.
Alpers, C.N., Antweiler, R.A., Snyder, N.P., and Curtis, J.A., in review, Mercury transport and deposition in a watershed affected by historical gold mining: the upper Yuba River, California. Journal article for Water Resources Research.
Alpers, C.N., Hunerlach, M.P., Marvin-DiPasquale, M.C., Antweiler, R.C., Lasorsa, B.K., De Wild, J.F., and Snyder, N.P., 2006, Geochemical Data for Mercury, Methylmercury, and Other Constituents in Sediments from Englebright Lake, California, 2002: U.S. Geological Survey Data Series 151, 95 p. http://pubs.water.usgs.gov/ds151/
Snyder, N.P., and Hampton, M.A., 2003, Preliminary cross section of Englebright Lake sediments, U.S. Geological Survey Open-File Report 03-397, 1 plate. http://geopubs.wr.usgs.gov/open-file/of03-397/
Snyder, N.P., Alpers, C.N., Flint, L.E., Curtis, J.A., Hampton, M.A., Haskell, B.J., and Nielson, D.L., 2004a, Report on the May-June 2002 Englebright Lake deep coring campaign: U.S. Geological Survey Open-File Report 2004-1061, 32 p. plus 10 plates. http://pubs.usgs.gov/of/2004/1061/
Snyder, N.P., Allen, J.R., Dare, C., Hampton, M.A., Schneider, G., Wooley, R.J., Alpers, C.N., and Marvin-DiPasquale, M.C., 2004b, Sediment grain-size and loss-on-ignition analyses from 2002 Englebright Lake coring and sampling campaigns: U.S. Geological Survey Open-File Report 2004-1080, 46 p. http://pubs.usgs.gov/of/2004/1080/
Snyder, N.P., Rubin, D.M., Alpers, C.N., Childs, J.R., Curtis, J.A., Flint, L.E., and Wright, S.A., 2004c, Estimating rates and properties of sediment accumulation behind a dam: Englebright Lake, Yuba River, northern California, Water Resources Research, v. 40, W11301, doi:10.1029/2004WR003279 http://www2.bc.edu/~snyderno/snyder_etal_2004.pdf
Snyder, N.P., Wright, S.A., Alpers, C.N., Flint, L.E., Holmes, C.W., and Rubin, D.M., 2006, Reconstructing depositional processes and history from reservoir stratigraphy: Englebright Lake, Yuba River, northern California, Journal of Geophysical Research, v. 111, F04003, doi:10.1029/2005JF000451. http://www.agu.org/journals/jf/jf0604/2005JF000451/ http://www2.bc.edu/~snyderno/snyder_etal_2006.pdf