Additional characterization of the impact of suction dredging, South Yuba River – Humbug Creek

Science Center Objects

The potential impacts of suction dredging on water quality remain largely undetermined, especially with regard to trace metals including Hg. Several State of California regulatory agencies have expressed concerns that suction dredging may cause deleterious impacts with regard to turbidity and Hg contamination in downstream areas.

Areas of historical gold mining are the primary target for suction dredgers and these same areas are generally contaminated with mercury (Hg) because of the extensive historical use of Hg for gold recovery by amalgamation (Alpers et al., 2005). Suction dredging regulation is being evaluated by the California Department of Fish and Game (CDFG) in a Subsequent Environmental Impact Statement that will rely in part on results of this study. 

The planned California State Water Resources Control Board (SWRCB) -funded work expands the experimental approach of the U.S. Bureau of Land Management (BLM) study by investigating a wider variety of sediment samples collected at the site as "spiking" sediment, and by improving the evaluation of Hg methylation by using other "receiving" sediment types, collected from a foothill reservoir (Englebright Lake) and from the central Sacramento–San Joaquin Delta (Delta Meadows), both of which have higher LOI (8 to 12%). These additional receiving sediments are from areas where fine sediment fractions mobilized by dredging activities (i.e., silt- and clay-sized particles) may be deposited. They are also more likely to promote Hg methylation than the South Yuba River sediment used in the BLM-funded experiments because of their higher organic content and other environmental factors such as lower oxidation-reduction potential. 

The overall objective of the proposed work is to gather information regarding potential environmental consequences of suction dredging in areas affected by historical placer gold mining and in areas downstream, including Sierra Nevada foothill reservoirs and the central Sacramento-San Joaquin Delta. Specific objectives include: (1) Explore whether interaction with oxygenated water causes an increase in reactive Hg(II) in fine-grained, Hg-contaminated sediment; (2) Explore whether mixing of fine-grained, Hg-contaminated sediment with organic-rich sediment will cause an increase in net MeHg production; and (3) Characterizing sediment with regard to grain-size distribution, Hg speciation, and presence/absence of micrometer-sized beads of elemental Hg, to provide information useful in the interpretation of experimental results.

The additional SWRCB-funded work is divided into four tasks: (1) Project Administration, (2) Sediment Biogeochemistry (laboratory experiments), (3) Sediment Characterization, and (4) Reporting. The laboratory experiments will use methods similar to those used in the previous, BLM-funded part of the study. The difference in approach is that a larger variety of sediments will be tested, including more diverse spiking sediment from the SYR-HC field site and receiving sediment from environments farther downstream (Englebright Lake and Delta Meadows). The sediment characterization will include determination of grain-size distribution using laser-scattering methods, sequential extractions to determine Hg speciation, and examination of heavy-mineral concentrates using scanning electron microscopy.