By Joseph L. Domagalski, Charles N. Alpers, Darell G. Slotton, Thomas H. Suchanek, and Shaun M. Ayers
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5037
Sacramento, California 2004
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Abstract
Concentrations and mass loads of total mercury and methylmercury in streams draining abandoned mercury mines and near geothermal discharge in Cache Creek Basin, California, were measured during a 17-month period from January 2000 through May 2001. Rainfall and runoff averages during the study period were lower than long-term averages. Mass loads of mercury and methylmercury from upstream sources to downstream receiving waters, such as San Francisco Bay, were generally the highest during or after winter rainfall events. During the study period, mass loads of mercury and methylmercury from geothermal sources tended to be greater than those from abandoned mining areas because of a lack of large precipitation events capable of mobilizing significant amounts of either mercury-laden sediment or dissolved mercury and methylmercury from mine waste. Streambed sediments of Cache Creek are a source of mercury and methylmercury to downstream receiving bodies of water such as the Delta of the San Joaquin and Sacramento Rivers. Much of the mercury in these sediments was deposited over the last 150 years by erosion and stream discharge from abandoned mines or by continuous discharges from geothermal areas. Several geochemical constituents were useful as natural tracers for mining and geothermal areas. These constituents included aqueous concentrations of boron, chloride, lithium, and sulfate, and the stable isotopes of hydrogen and oxygen in water. Stable isotopes of water in areas draining geothermal discharges were enriched with more oxygen-18 relative to oxygen-16 than meteoric waters, whereas the enrichment by stable isotopes of water from much of the runoff from abandoned mines was similar to that of meteoric water. Geochemical signatures from stable isotopes and trace-element concentrations may be useful as tracers of total mercury or methylmercury from specific locations; however, mercury and methylmercury are not conservatively transported. A distinct mixing trend of trace elements and stable isotopes of hydrogen and oxygen from geothermal waters was apparent in Sulphur Creek and lower Bear Creek (tributaries to Cache Creek), but the signals are lost upon mixing with Cache Creek because of dilution.
Abstract
Introduction
Purpose and Scope
Awknowledgments
Description of Study Area and Selection of Sampling Sites
Methods
Sampling and Sample Processing
Analytical Methods
Quality Assurance
Results and Discussion
Mercury Concentrations and Loads
Chemical Signatures of Water Sources
Chloride-to-Sulfate Ratios
Boron-to-Alumium Ratios
Boron-to-Lithium Ratios
Boron-to-Chloride Ratios
Boron, Chloride, and Sulfate
Stable Isotopes of Hydrogen and Oxygen
Dissolved Organic Carbon
Summary and Conclusions
References Cited
Appendix 1
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