USGS

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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