Mercury Data for Water and Sediment from the Dutch Flat Mining District, California

PRELIMINARY DATA--SUBJECT TO REVISION

Location and description of sampling sites
Map showing sample locations in Dutch Flat area (89K)
Mercury data for sediment samples
Mercury data for water samples

Report: Mercury Contamination from Hydraulic Placer-Gold Mining in the Dutch Flat Mining District, California (PDF file)

The Dutch Flat mining district in Placer County, California was the site of considerable mining of placer gold deposits using hydraulic methods during the latter half of the 19^th century and the first part of the 20^th century (Hunerlach and others, 1999). The reader is referred to that report (and references therein) for a description of the history of hydraulic mining in the Sierra Nevada including documentation of the extensive use of liquid mercury (Hg) in the recovery of gold from placer ores using by sluicing methods.

Available data on mercury in water and sediment samples taken by the U.S. Geological Survey (USGS) from the Dutch Flat mining district are presented in the tables associated with this text. The data, including site locations and descriptions, should be considered preliminary and subject to change.

The work by USGS in the Dutch Flat area has been supported by several different entities. The samples collected during 1998 (results described by Hunerlach and others, 1999) were funded by the USGS Minerals Resources Program. During 1999, the USGS began a partnership with other public agencies with the objective of addressing contamination associated with historic mining in the Bear and South Fork Yuba River watersheds. The other agencies participating in this partnership are the the U.S. Department of Agriculture – Forest Service, the Bureau of Land Management, the California State Water Resources Control Board, and the Nevada County Resource Conservation District.

The locations and descriptions of sampling sites in the Dutch Flat mining district are given in the associated table. The samples described in this report are from two drainage tunnels that were used as sluices during hydraulic mining: the Polar Star tunnel and the Southern Cross tunnel, as shown on the associated map.

The Polar Star mine tunnel is located about ¾ mile northeast of the town of Dutch Flat, and about 20 feet from the county road that connects the town of Dutch Flat and the Dutch Flat Powerhouse (fig. 1). Three sampling locations in this tunnel were used: inlet (PS4, about 480 feet upstream of portal), midway (PS3, about 240 feet upstream of portal), and discharge (PS1), about 20 feet upstream of portal). In addition, a water sample was taken from a site (PS2) about 100 feet downstream of the portal discharge

The Southern Cross mine tunnel is located about 1,000 feet to the west of the Polar Star tunnel. Its portal is located about 400 feet north of the county road. Two sampling points were used: midway (SC2, about 400 feet upstream of portal) and discharge (SC1, at portal).

Trace metal clean techniques were used during all stages of sample collection. All containers and equipment used for sediment sampling were extensively cleaned in 30 percent nitric acid and rinsed in ultrapure Milli-Q deionized water prior to use. (The use of commercial product names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.) Sediment samples were taken with acid-leached polycarbonate core liners, stored in double zip-lock bags, and transferred to Teflon containers with an acid-cleaned stainless steel spatula. Samples were immediately stored on dry ice and remained frozen until analysis.

Wet sediment (about 0.1 grams) was weighed into 100 milliliter (mL) volumetric flasks, and reflux digested (160 degrees Celsius) in 10.0 mL of aqua regia [sulfuric acid : nitric acid, H₂SO₄:HNO₃ (2:5 by volume)] for three hours. Bromine chloride (BrCl) (1.0 mL) was added to cooled samples and trace clean water was added to the 100 mL mark. Sub-samples of this aqueous digest were assayed for total mercury (Hg) using standard methods of Gill and Fitzgerald (1987) and of Bloom and Fitzgerald (1988), which consisted of tin chloride (SnCl₂) reduction of Hg(II), gold-trapping of Hg⁰, and cold vapor atomic fluorescence detection of the analyte. Quality control consisted of two or three replicates per sample, commercial mercury chloride (HgCl₂) standards (Ultra Scientific, North Kingston, RI), marine sediment (PACS-2) certified reference material (National Research Council of Canada, Ottawa), and matrix HgCl₂ spike recovery analysis. Sediment dry weight determination was conducted on separate samples, weighed wet (about 2-4 grams), then dried to constant weight for up to 48 hours at 80 degrees Celsius (⁰C).

Water samples were collected in Teflon containers and then transferred to a Teflon-lined churn for splitting. Filtration was done in the field using either syringe or capsule filters of 0.45 micrometer pore size. Filtered and unfiltered water samples for analysis of total Hg that were taken during 1998 were shipped to Frontier Geosciences (Seattle, Washington) where techniques of Bloom and Fitzgerald (1988) and Gill and Fitzgerald (1987) were used, including the use of BrCl as an oxidant preservative. Filtered and unfiltered water samples for analysis of total mercury that were taken during 1999 were preserved with a potassium dichromate–nitric acid solution and analyzed by cold-vapor atomic fluorescence spectroscopy by the USGS research laboratory in Boulder, Colorado, using methods and quality assurance procedures described by Alpers and others (2000).

Field panning techniques utilized a suction device to obtain sediment from sluice areas (Hunerlach and others, 1999). One kilogram of wet sediment was isolated using a portable balance. The sediment was transferred to a plastic pan and standard panning techniques were used to remove the light density fraction. Residual mercury was isolated and weighed.

Alpers, C.N., Taylor, H.E., and Domagalski, J.L., editors, 2000, Metals transport in the Sacramento River, California, 1996-1997: Volume 1. Methods and data. U.S. Geological Survey Water-Resources Investigations Report 99-4286.

Bloom, N.S., and Fitzgerald, W.F., 1988, Determination of volatile mercury species at the picogram level by low temperature gas chromatography with cold-vapor atomic fluorescence detection. Analytical Chimica Acta, v. 208, p. 151-161.

Gill, G.A., and Fitzgerald, W.F., 1987, Picomolar mercury measurements in seawater and other materials using stannous chloride reduction and two-stage gold amalgamation with gas phase detection. Marine Chemistry, v. 20, p. 227-243.

Hunerlach, M.P. Rytuba, J.J., and Alpers, C.N., 1999, Mercury contamination from hydraulic placer-gold mining in the Dutch Flat mining district, California, in Morganwalp, D.W., and Buxton, H.T., editors, U.S. Geological Survey Toxic Substances Hydrology Program -- Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999, U.S. Geological Survey Water-Resources Investigations Report 99-4018B, p. 179-189.