Reconnaissance of Acid Drainage Sources and Preliminary Evaluation of Remedial Alternatives at the
Copper Bluff Mine, Hoopa Valley Reservation, California
By Charles N. Alpers, Michael P. Hunerlach, Scott N. Hamlin,
and Robert A. Zierenberg
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 02-4253
Sacramento, California 2003
Prepared in cooperation with the Hoopa Valley Tribe
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Abstract
Acidic drainage from the inactive Copper Bluff mine cascades down a steep
embankment into the Trinity River, on the Hoopa Valley Reservation in northern California. The Copper Bluff mine
produced about 100,000 tons of sulfide-bearing copper-zinc-gold-silver ore during 1957–1962. This report summarizes
the results of a water-resources investigation begun by the U.S. Geological Survey in 1994 with the overall objective
of gathering sufficient geochemical, hydrologic, and geologic information so that a sound remediation strategy for
the Copper Bluff mine could be selected and implemented by the Hoopa Valley Tribe. This study had the following
specific objectives: (1) monitor the quality and quantity of the mine discharge, (2) determine seasonal variability
of metal concentrations and loads, (3) map and sample the underground mine workings to determine sources of flow
and suitability of mine plugging options, and (4) analyze the likely consequences of various remediation and
treatment options.
Analysis of weekly water samples of adit discharge over parts of two wet seasons (January to
July 1995 and October 1995 to May 1996) shows that dissolved copper (Cu) and zinc (Zn) concentrations (in samples
filtered with 0.20-micrometer membranes) varied systematically in a seasonal pattern. Metal concentrations increased
dramatically in response to the first increase in discharge, or first flush, early in the wet season. The value of Zn/Cu
in the adit discharge exhibited systematic seasonal variations; an annual Zn/Cu cycle was observed, beginning with values
between 3 and 5 during the main part of the wet season, rising to values between 6 and 10 during the period of lowest
discharge late in the dry season, and then dropping dramatically to values less than 3 during the first-flush period.
Values of pH were fairly constant in the range of 3.1 to 3.8 throughout the wet season and into the beginning of the dry
season, but rose to values between 4.5 and 5.6 during the period of lowest discharge, from October to early December 1995.
Underground reconnaissance was conducted once during dry-season conditions (September 1995) and
twice during wet-season conditions (March 1995 and March 1996). The main tunnel was accessed to a distance of about
600 feet from the portal entrance. Water samples were collected at nine locations along the floor of the main tunnel and
from several ore shoots to evaluate the contributions of water and dissolved constituents from different portions of the
mine. Values of pH ranged from 2.5 to 6.4 at different underground locations, concentrations of copper ranged from 0.020
to 44 mg/L (milligram per liter), zinc from 6.3 to 160 mg/L, and cadmium from 0.010 to 0.47 mg/L. Discharge from the ore
shoots ranged from less than 1 gallon per minute to more than 30 gallons per minute and was always a small component of
the total mine flow compared with the tunnel floor drainage. During March 1996, the main flow originated in the northernmost
portion of the underground workings (inaccessible) and mixed with an unknown quantity of water upwelling from flooded lower
workings. High-water marks observed on the tunnel walls indicate that past blockages impounded more than 100,000 gallons of
water. Sudden release of a large volume of metal-rich water could have serious effects on fish and other aquatic resources
in the Trinity River.
Because of the hydrogeologic setting, mine plugging is not likely to offer an effective long-term
solution to the problem of acid mine drainage at the Copper Bluff mine. The underground workings are close to a state highway
and underlie a 500-foot-high bluff with highly fractured rocks that seep during the wet season. Total plugging likely would
result in additional uncontrolled seepage and could potentially destabilize the highway. Partial plugging to restrict flow
during periods of highest discharge may provide benefits in terms of reduced risk of catastrophic release without the
additional risks associated with total plugging. Passive water treatment methods such as wetlands or anoxic limestone
drains are unlikely to succeed at the Copper Bluff mine because of the lack of available space. A covered conveyance for
the discharge directly from the mine portal to the Trinity River is a low-cost remedial alternative that would not reduce
metal loadings to the Trinity River, but would reduce pathways of metal exposure to humans and wildlife. Lime neutralization
or innovative, active water treatment methods such as bioreactors represent high-cost remedial alternatives that likely would
be successful if sufficient resources were available for adequate design, testing, construction, long-term maintenance, and
sludge disposal.
CONTENTS
Abstract
Introduction
Mining History
Regulatory and Environmental Concerns
Study Objectives
Acknowledgments
Regional and Local Geologic Setting
Regional and Local Hydrologic Setting
Trinity River Flows
Climate
Methods
Underground Mapping and Sampling
Surface-Water Sampling
Weekly Sampling and Flow Monitoring at Portal
Geochemical Analyses
Geochemical Modeling
Results
Geochemistry of Mine Drainage
Copper Bluff Mine Portal
Underground Reconnaissance
Surface Waters
Impact of Copper Bluff Mine Effluent on the Trinity River
Mineralogy and Geochemistry of Ores and Host Rocks
Petrographic observations—host rocks
Petrographic observations—mineralized rocks
Mineral geochemistry—silicates
Mineral geochemistry—sulfides
Geochemistry of Iron Redox Speciation
Oxidation-reduction potential
Ferric iron activity and mineral saturation
Implications for Remediation
No Action
Covered Conveyance of Adit Flow to Trinity River
Water Treatment—Passive Methods
Mine Plugging
Partial Plugging—Flow Restriction
Partial Capping
Water Treatment—Bioreactors
Water Treatment—Lime Neutralization
Summary of Remedial Alternatives and Consequences
Summary and Conclusions
References
Appendix 1. Chemical data for water samples from the Copper Bluff Mine portal, site CB1
Appendix 2. Discharge data for the Trinity River at Hoopa and the Copper Bluff mine portal
Appendix 3. Electron microprobe data for minerals from the Copper Bluff mine, Hoopa Valley Reservation
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Water Resources of California