Quantification of Mercury Flux in Eroding Mining Debris, Yuba River Watershed, California

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At two priority locations on BLM-administered land in the Yuba River watershed, the USGS has documented relatively rapid erosion of river bank material that consists of mercury (Hg)-laden sediment with a significant component of mining debris. 

Map showing location of field sites

Map showing location of field sites. (Public domain.)

One location is at the confluence of the South Yuba River and Humbug Creek (SYR/HC), where a ~15 m high cliff of hydraulic mining debris is actively eroding. The eroding material has elevated Hg (1,200 ng/g) and reactive Hg(II) (Hg(II)R, 38 ng/g) in the silt-clay fraction (Fleck et al., 2011). The other location is at Stocking Flat on Deer Creek, about 3 km downstream of Nevada City, CA. At the Stocking Flat location, the USGS quantified erosion and Hg flux from December 2010 through May 2011 using Tripod Light Detection and Ranging (T-LiDAR) (Fleck et al., in preparation; Howle et al., 2012). Current levels of funding do not cover completion of a report describing results of the LiDAR work at Stocking Flat. At Humbug Creek, there is a need for new work to quantify the erosion rate of the cliff consisting of hydraulic mining debris, and to monitor baseline (pre-remediation) concentrations of Hg and methylmercury (MeHg) in water, sediment, and biota at several locations upstream and downstream of the cliff in Humbug Creek and the South Yuba River.

Objectives:

  1. Determine the erosion rate and Hg flux to the South Yuba River of hydraulic mining debris from an cliff near the Humbug Creek confluence,
  2. Determine the concentrations of Hg and MeHg in water, sediment, and biota (invertebrates and fish) in the South Yuba River upstream and downstream of the eroding cliff and publish results, and 
  3. Publish results of investigations of erosion rate and Hg flux at two sites in the Yuba River watershed.

The work has been organized into five tasks, as follows.

Task 1. Collection and analysis of biota. Invertebrates will be collected at seven sites in the South Yuba River and Humbug Creek and analyzed for THg and MeHg. Fish will be collected at three of the sites and analyzed for Hg in fillet tissue. An effort will be made to collect the same invertebrate species sampled during 2007 and 2008 (Fleck et al., 2011).

Task 2. Collection and analysis of water and bed sediment. Samples will be taken at the same sites as the biota in task 1 and will be analyzed for THg and MeHg plus several ancillary constituents.

Task 3. Estimation of erosion rate using T-LiDAR and air photos. T-LiDAR data will be collected during fall 2012 and fall 2013 at the same locations on the eroding cliff of hydraulic mining debris at the SYR/HC confluence, where USGS did an initial survey in December 2011. The annual erosion rate over two calendar years will be computed from the LiDAR data. Longer-term erosion rates will be estimated from historical air photos (1946–).The erosion rate estimates from T-LiDAR and air photos will be used to quantify the flux of THg and Hg(II)R from the eroding cliff of hydraulic mining debris to the adjacent surface-water system.

Task 4. Stage-discharge relationship and flood-frequency analysis, South Yuba River. Cross-sections of the SYR will be measured at low-flow conditions during summer 2012 using using traditional surveying methods at the same locations measured in 2002, and crest-stage gages will be installed. The resulting cross-sectional profiles, channel slope, and surface roughness measurements will be used to construct a stage-discharge relation that will make it possible to estimate the flood magnitude required to cause riverine erosion at the study site. The return interval of various flood stages will be estimated using data from the nearest active gaging station with a long-term record (South Yuba River at Jones Bar, USGS gage 11417500).

Task 5. Reporting. One report on the SYR/HC area (USGS SIR) and two reports on Deer Creek (one USGS SIR and one journal article) will be published.

Because of risk to human health and wildlife caused by consumption of Hg-contaminated fish tissue,this study will address the strategic direction "The Role of Environment and Wildlife in Human Health" in the USGS Science Strategy for the Decade, 2007-2017 (USGS Circular 1309). Specifically, the study fits into the framework of "water-quality information and research (local- to regional-scale water contamination issues)" and "earth-science information and research (locations and characteristics of …geologic terrains naturally enriched in potentially toxic substances, and contaminated water, sediment, and soil)…" (p. 44). The work will provide the BLM with information it needs to make decisions regarding remediation of Hg-contaminated sites under its CERCLA authority.