Yuba River Watershed
Sierra Nevada, California
South Yuba River / Humbug Creek Sediment Characterization and Suction Dredging Impacts Study
In cooperation with the Bureau of Land Management and the California State Water Resources Control Board, during 2007-2009 the USGS characterized mercury-contaminated sediment in the area of the confluence of the South Yuba River and Humbug Creek, which drains the Malakaoff Diggins State Historic Park, one of the largest hydraulic mines in California.
See below for links to the reports from the study and a summary of key findings.
- Fleck, J.A., Alpers, C.N., Marvin-DiPasquale, M., Hothem, R.L., Wright, S.A., Ellett, K., Beaulieu, E., Agee, J.L., Kakouros, E., Kieu, L.H., Eberl, D.D., Blum, A.E., and May, J.T., 2011,
- The effects of sediment and mercury mobilization in the South Yuba River and Humbug Creek confluence area, Nevada County, California: Concentrations, speciation, and environmental fate-Part 1: Field characterization: U.S. Geological Survey Open-File Report 2010-1325A. http://pubs.usgs.gov/of/2010/1325A
- Marvin-DiPasquale, M., Agee, J.L., Kakouros, E., Kieu, L.H., Fleck, J.A., and Alpers, C.N., 2011,
- The effects of sediment and mercury mobilization in the South Yuba River and Humbug Creek confluence area, Nevada County, California: Concentrations, speciation and environmental fate-Part 2: Laboratory experiments: U.S. Geological Survey Open-File Report 2010-1325B, 54 p. http://pubs.usgs.gov/of/2010/1325B
The U.S. Geological Survey (USGS) recently completed a study to assess the effects of mercury (Hg) removal options in sediment contaminated by historical gold mining. The study was conducted at the request of the Bureau of Land Management and the State Water Resources Control Board. Field work was conducted from September 2007 through May 2009 on the South Yuba River in the vicinity of the Humbug Creek confluence in Nevada County, California. Lab studies were carried out at USGS facilities in Menlo Park and Sacramento, California. The USGS examined the potential benefits and potential effects of in-stream suction dredging with regard to Hg contamination.
The USGS made several important findings in their reports, published in January, 2011:
- Fine-grained sediment (silt and clay) contains higher concentrations of Hg than coarse-grained sediment (sand) in the main channel of the South Yuba River and in nearby stream-bank deposits. Suction dredging disturbs the sediment in the stream bed. Although dredging may remove heavy minerals (including gold, gold-mercury amalgam, and free elemental Hg) from coarse-grained sediment (sand and possibly also coarse silt), it does not remove them from fine-grained sediment (fine silt and clay), nor does it remove Hg-bearing particles of lighter density. Fine-grained, Hg-laden sediment that passes through a dredge is likely to be mobilized and transported downstream, causing increased concentrations of mercury that can potentially become methylated and then bioaccumulated in the food web.
- In lab experiments simulating the transport and deposition of Hg-bearing particles, fine-grained sediment from various locations in the South Yuba River / Humbug Creek confluence area was mixed with fine-grained material from two downstream areas, Englebright Lake and Delta Meadows. Some of these experiments resulted in increased methylmercury concentration.
- The USGS study included three excavations – two above the water line and one in the stream bed. The study found that mining above the water line (high-banking) is potentially more harmful than in-stream mining because the exposed Hg is more reactive (more likely to methylate). Suction dredging of in-stream deposits appeared to be potentially less harmful than high-banking because the Hg in the stream-bed deposits is relatively less reactive in water-saturated conditions. However, Hg-bearing particles are exposed to oxygen during downstream transport, which tends to make the transported Hg relatively more reactive and more likely to become methylated and bioaccumulated.
- Small beads of elemental Hg (less than 0.01 millimeter in diameter) were found on the surface of gold-mercury amalgam grains which had been recovered by hand panning of material from one of the excavations above the water line. There is potential for small beads of elemental Hg to be released to the water column when this kind of material is processed through a suction dredge or sluice box. Grains of gold-mercury amalgam recovered from the sluice box of a suction dredge that processed bed sediment from the South Yuba River did not have beads of elemental Hg on their surface.
- An unconsolidated deposit of hydraulic mining debris is eroding into the South Yuba River, releasing fine-grained particles with elevated concentrations of reactive Hg that is already oxidized and ready for methylation. Once methylated, the Hg is highly bioavailable and accumulates in sport fish, especially in top predator fish such as largemouth and smallmouth bass. There is a fish-consumption advisory for Englebright Lake (located about 15 miles downstream of the study area) and several other water bodies affected by historical mining in the Sierra Nevada. For more information, see: http://www.oehha.ca.gov/fish/so_cal/
- An experiment to test the efficacy of zero-discharge dredging, by recirculating water through 300-gallon tank in a closed circuit by using a venturi pump, resulted in fine-grained sediment remaining in suspension more than 40 hours after the cessation of pumping. Although the total concentration of suspended Hg declined over time as particles settled out, both the concentration of Hg on the suspended particles and the Hg reactivity increased over time as the suspended material became finer grained.
- The USGS found higher levels of methylmercury in macroinvertebrates collected during 2007 (the most recent year that suction dredging was permitted in the South Yuba River) compared with 2008 (when suction dredging was not permitted in that area). More monitoring would be needed to determine whether the observed year-to-year variation was caused primarily by suction dredging or by other factors.