Stable-isotope data indicate that there are three sources of water that effect the composition and Hg concentration of waters in Harley Gulch: (1) meteoric water that dominates water chemistry during the wet season; (2) thermal water effluent from the Turkey Run mine that effects the chemistry at sample site HG1; and (3) cold connate groundwater that dominates water chemistry during the dry season as it upwells and reaches the surface. The results from sampling executed for this study suggest four distinct areas in Harley Gulch: (1) the contaminated West Fork of Harley Gulch, consisting of the stream immediately downstream from the mine area and the wetlands upstream from Harley Gulch canyon (sample sites HG1-HG2, (2) the East Fork of Harley Gulch, where no mining has occurred (sample site HG3), (3) sample sites HG4-HG7, where a seasonal influx of saline groundwater alters stream chemistry, and (4) sample sites HG7-HG10, downstream in Harley Gulch towards the confluence with Cache Creek.
West Fork: Mine Area and Wetlands
The concentration of Hg in both storm sediment and active channel sediment was highest at sample site HG1, immediately downstream from the mine. The highest concentrations of total Hg (HgT) in water also occurred at site HG1, and they decreased systematically downstream from the mine. The high concentration of HgT at site HG1 reflects input of thermal-water effluent from the Turkey Run mine which comprises most of the flow at this site during the dry season. During the May 2011 low-flow sampling, HgT concentration was very high at site HG1, but the maximum in HgT concentration occurred at sample site HG1.5 in the middle of the wetland area. The high concentration of HgT and isotopic chemistry at this site indicates that a significant input of connate groundwater into the creek at this location contributes to the high Hg concentration in water. At site HG1, just downstream from the thermal water input from the Turkey Run mine, water sampled in June 2010 was almost entirely composed of thermal-water effluent. During the storm sampling in March 2011, which resulted in the highest flows of the winter, thermal effluent was virtually undetectable at site HG1, and the water was all meteoric. During the May 2011 sampling event, the input of connate groundwater in the middle of the wetland area at site HG1.5 was dominant. Discharge from the adit and runoff from the mine contributes to the high Hg concentration at site HG1 under both high and low-flow conditions.
East Fork: Background
Hg levels in waters collected from the East Fork of Harley Gulch, where no mining has occurred, were as high as 32.8 parts per trillion (pptr). These levels of Hg in water are significantly higher than regional background Hg concentrations, which range from 4-7 pptr. These anomalous Hg concentrations are partially explained by the abundance of Hg-enriched groundwater in Harley Gulch.
Downstream from the wetland, the aqueous concentration of HgT decreased, but remained above background levels as another input of connate groundwater occurs in the creek segment between sample sites HG4 and HG7. The input of connate groundwater in this segment of the creek is reflected in the increase in dissolved constituents characteristic of the connate groundwater, such as sulfate (SO4), chloride (Cl) and magnesium (Mg). Stable-isotope data for heavy isotopes d18O and d2D also confirm two areas of input of connate groundwater into Harley Gulch: the creek segment in the West Fork near sample site HG1.5 and the segment between sample sites HG4 and HG7. Downstream from the second area of input of connate groundwater, both HgF and HgT concentrations decrease similarly, but the percentage of Hg in the filtered fraction increases. The decreases in HgT and HgF between sample sites HG5 and HG7 suggests that this second source of connate groundwater to Harley Gulch is distinct from the Hg-enriched source that enters the middle of the wetlands at sample site HG1.5. During low-flow conditions in June 2010, input of connate groundwater increased from sample site HG4 and reached a maximum near sample site HG7, where it dominated creek water chemistry. Waters collected from sample site HG7 during the June 2010 sampling event were the heaviest isotopically and contained high concentrations of Cl and SO4, constituents that are characteristically high in the connate groundwater. Both above and below sample site HG7, the amount of connate groundwater in the creek water decreased.
Sediment with high Hg concentration is present throughout the West Fork of Harley Gulch below the mine and in the upper part of the Harley Gulch main stem to just above sample site HG10. At the sample site furthest downstream, HG10, Hg concentration is at background levels, as are cobalt (Co), nickel (Ni), and tungsten (W), indicating that the sediment is not significantly contaminated with Hg from the mine.
Additional publication details
USGS Numbered Series
Impact of mine and natural sources of mercury on water, sediment, and biota in Harley Gulch adjacent to the Abbott-Turkey Run mine, Lake County, California
U.S. Geological Survey
Western Mineral and Environmental Resources Science Center-Menlo Park Office