PRELIMINARY PUBLIC HEALTH ASSESSMENT
SULPHUR BANK MERCURY MINE
CLEARLAKE, LAKE COUNTY, CALIFORNIA
The Sulphur Bank Mercury Mine Site is located on the eastern shore of the Oaks Arm of Clearlake, in the City of Clearlake, Lake County, California. The site is on the Environmental Protection Agency's National Priorities List (NPL). This site was mined for sulphur beginning in 1865, and later for mercury. The most extensive surface mining occurred between 1922-1947 and 1955-1957; during these two periods mining waste containing mercury was dumped directly into Clearlake and along the shoreline. The waste (approximately 193,600 cubic yards) consists of mine tailings (residue from actual milling operations) and waste piles (overburden and low grade ore not worth milling).
The Clearlake surface waters are an important area resource for recreational purposes such as fishing and boating. The lake is generally divided into three areas: the Upper Basin, the Lower Basin, and Oaks Arm. The Oaks Arm of Clearlake has the highest level of contamination in the lake. Mercury in the sediment is taken up by biota and transformed into methyl mercury, that then accumulates in species at higher levels of the food chain. The levels of mercury in tissues of some fish and waterfowl at this lake have exceeded the maximum limits set by the U.S. Food and Drug Administration. The human exposure pathway of greatest concern is the consumption of contaminated fish. The state has issued a health advisory regarding consumption of fish from Clearlake (May 1986).
Potential on-site exposure routes include ingestion, inhalation, and dermal contact with contaminated soil and surface water. Potential off-site exposure routes include ingestion and dermal contact with surface water and groundwater; most of the mercury in the lake is contained in bottom sediment and along the shore of Oaks Arm.
Based on available information reviewed, the Agency for Toxic Substances and Disease Registry has concluded that this site is a public health hazard. However, additional information is needed to fully evaluate the threat to public health associated with exposures to potentially contaminated environmental media.
The Sulphur Bank Mercury Mine Site (SBM), located in Clearlake, California, is on the Environmental Protection Agency's (EPA) National Priorities List (NPL). The site is situated on the eastern shore of the Oaks Arm of Clearlake (see Figure 1). Sulphur Bank Road borders the site to the northeast and El-Em Drive to the northwest.
The SBM site was initially mined for sulfur from 1865 to 1868. From 1899 to 1902 and from 1915 to 1918, mercury ore (cinnabar) was mined using underground methods. The majority of the mercury ore was mined using open pit methods from 1922 to 1947 and from 1955 to 1957. Mining operations ceased in 1957 at the site, which is currently owned by Bradley Mining Company of San Francisco.
The SBM site contains functional, as well as relic buildings and processing facilities. Approximately 120 acres of the 760 acre site are covered with mining waste (tailings, disturbed overburden, and low grade ore) stored directly on the ground; the exposed piles are approximately 40 feet in height and extend into the Oaks Arm of Clearlake along 1,320 feet of shoreline.
The Herman Pit (an open unlined mining pit) covers approximately 23 acres and is filled with water to a depth of 150 feet. Sulfuric acid is produced by oxidation of geothermal hydrogen sulfide venting into the pit; the pH of surface water is approximately 3. Surface water (up to 100 gallons per minute during rainstorms) drains from the western edge of the pit into a drainage channel leading down to the Oaks Arm of Clear lake, approximately 750 feet away. A small dam was built in the drainage channel in 1978.
The Clearlake Oaks Water District well, which provides municipal drinking water for 4,731 people, may be recharged by Clearlake. Approximately 80 acres of walnut orchards are irrigated using water from the lake. Clearlake is also used for recreational purposes such as fishing and boating. There are approximately 70 houses within a 3-mile radius of the site that have undocumented sources of water. Some of these houses may be getting water directly from the surface of Clearlake.
In 1986, a coordinated investigation by the California Department of Health Services (CDHS), the Department of Fish and Game, and the California Regional Water Quality Control Board (CRWQCB) identified mercury in tailings and in biota and bottom sediments in the Oaks Arm of Clearlake. Concentrations of mercury in air at the site have been surveyed by CRWQCB.
On May 14, 1986, the state issued an advisory limiting the quantity of fish that should be consumed from Clearlake. This advisory resulted from concern about the levels of mercury in these fish. Women who are pregnant or are nursing and children under 6 years of age are advised not to eat any fish from Clearlake.
Personnel from the Agency for Toxic Substances and Disease Registry (ATSDR) visited the site and surrounding areas in February 1989. The site is partially surrounded by 4-foot barbed wire fence and has an entrance gate. At the time of the site visit the gate was open; paths through the site indicate that trespassers have frequently been on-site. The surface soil consists of bleached and barren soil and rock.
Mining wastes are most prominent in the area of the Herman Pit, which is not fenced. Additional information obtained during this visit will be presented throughout this Preliminary Health Assessment.
In June, 1991, the EPA Project Manager confirmed that the site has not changed substantially since the 1989 ATSDR site visit. The only physical change noted at the site relates to improvements to the dam located between the Herman Pit and the lake.
Personnel from ATSDR contacted CDHS and the Lake County Health Department in January and February, 1991, in order to determine community health concerns. The community nearest the site is a small Indian community located within a few hundred feet of the site. While there has been concern in the past, the community's current feelings are not generally known. A public meeting with state and local public health officials was sponsored by EPA on November 29, 1990. No one from the community near the site attended this meeting; EPA and CDHS are planning further efforts to involve the community. If a sufficient number of people desire to participate, the state plans to offer blood and urine tests to assess the absorption of mercury by persons living near the site.
The CDHS will review health outcome data (including the proposed symptom-and-disease prevalence study) during the preparation of a full health assessment.
The Oaks Arm of Clearlake, adjacent to the site, is a recreational area used for fishing and boating. The areas surrounding the site are characterized as a mixture of residential, cultivated land, wetlands, and hilly terrain. The Clearlake Oaks Water District supplies the community (approximately 4,731 people) with drinking water drawn from a shallow groundwater well located across the lake from the site, approximately 4,050 feet away.
The commerce associated with recreational fishing is very important to the economy of the county. An Indian community of approximately 21 homes and a school is located within several hundred feet of the western boundaries of the site. In addition, numerous other residents (including many retirees) live within two miles of the site.
A 9.5 acre freshwater wetland is located approximately 900 feet east of the site. A critical habitat for three endangered wildlife species (peregrine falcon, southern bald eagle, and yellow-billed cuckoo) is located less than one-quarter of a mile from the site.
In order to identify other possible facilities that could contribute to the contamination of Clearlake, ATSDR searched the 1987 and 1988 Toxic Chemical Release Inventory (TRI), a database developed by EPA from the chemical release information provided by certain industries. The database did not contain any information on toxic chemical releases in Lake County, California.
Preliminary sampling of the SBM site was performed by CRWQCB in 1986. Mercury was identified on-site in soil samples of mining waste (1-640 mg/kg), in sludge samples from the Herman Pit and drainage area (45-260 mg/kg), and in surface water runoff from the mine pit (0.203 mg/l). Arsenic was also detected in the surface water runoff from the mine pit (0.303 mg/l). This runoff from Herman Pit (towards the lake) may have contained mercury and arsenic in suspension, since the actual levels of mercury (0.00087 mg/l) and arsenic (non-detect) in solution in Herman Pit were much lower.
The concentrations of mercury in air at the site were surveyed by CRWQCB in 1988. The concentrations found were generally below 0.005 mg/m3). The concentrations near the gaseous hot springs of Herman Pit were not higher than surrounding areas. The highest levels (0.012 mg/m3) were found inside structures at the old mill area; the National Institute of Occupational Safety and Health recommended exposure limit (10 hour TWA) is 0.05 mg/m3.
Groundwater from the water supply wells of eight homes on the SBM property (along the Clearlake shoreline) was analyzed. The groundwater data have not been provided to ATSDR.
In 1986, a coordinated investigation by CDHS, CRWQCB, and the Department of Fish and Game, identified mercury in mining waste and in biota and bottom sediments at the Oaks Arm of Clearlake. The level of mercury contamination in lake bottom sediments varied widely between samples from Oaks Arm of Clearlake (11-250 mg/kg) and samples from other locations in Clearlake (N.D.-12 mg/kg). Since 1970, more than 380 fish have been taken from Clearlake and analyzed; 8 percent of largemouth bass and 18 percent of channel catfish analyzed exceed current U.S. Food and Drug Administration guidelines (1 ppm). Largemouth bass tend to stay in one area of a lake for life; notably, the largemouth bass taken from the Oaks Arm of Clearlake had significantly higher concentrations of mercury than those from other parts of the lake.
Waterfowl indigenous to the lake (Western Grebes and American Coots) have also been tested for mercury contamination. Mercury levels in Western Grebes (fish eaters) exceeded U.S. Food and Drug Administration guidelines; mercury levels in American Coots (plant eaters) were much lower.
The Clearlake Oaks Water District well was reported not to be contaminated. Groundwater data have not been provided to ATSDR.
The conclusions presented in this Preliminary Health Assessment are based on data contained in the referenced documents. The validity of these conclusions is therefore dependent on the accuracy and reliability of the data provided.
The piles of mining waste and the Herman pit are potential hazards to persons who go on the site. The piles of mining waste are approximately 40 feet in height and the excavated pit is filled with 90 to 150 feet of water and dilute sulfuric acid (pH approximately 3).
Contaminated environmental media of concern are soil, sediment, surface water, groundwater, and the food chain. The SBM site (760 acres) has approximately 120 acres covered with mining waste and the open, unlined Herman Pit. The site is located on the eastern shore of the Oaks Arm of Clearlake. Samples of mining waste on-site contained elevated levels of mercury and arsenic. Water in the Herman Pit contains low levels of mercury and arsenic. A natural source of contamination in the pit comes through a geothermal vent located at the base of the pit which contributes (among other things) hydrogen sulfide gas. The potential exists for mercury in the waste on-site to vaporize or become entrained with dust particles from contaminated soils.
Acid mine drainage into the underlying alluvial aquifer may be a potential problem. Mining waste with high concentrations of mercury are above a permeable alluvium on the western and northern section of the site. The alluvial aquifer (composed primarily of silty sand and gravel) that serves as the primary source of drinking water for the community of Clearlake Oaks and surrounding private residences is located across the lake from the site. A hydrogeologic assessment report has recently been prepared to better characterize the groundwater at and near the site; the report has not yet been provided to ATSDR.
Contaminants are migrating off-site via surface runoff from the mining waste along the shoreline. The runoff contributes to off-site surface water, sediment, and food chain contamination. Sediments in the bottom of the lake have higher levels of mercury contamination near the SBM site. Through the food chain, methyl mercury is concentrated and passed from fish to humans. Also, other species of animals which eat fish (waterfowl) can accumulate mercury. Elevated levels of mercury were found in Western Grebes (fish eaters), but not in American Coots (plant eaters). Walnut orchards in the vicinity are irrigated with water from the lake.
Some reports have suggested that the dam between the Herman Pit and the lake may be inadequate. Improvements were made to this dam in the summer of 1990.
The potential routes for human exposure to contaminants at the SBM site are: ingestion, inhalation, and dermal contact with contaminated soil and surface water on-site. Inhalation of mercury vapors or entrained dust (by workers or site trespassers) from contaminated soil is of potential concern because mercury vapor can readily diffuse into the lungs. Although a fence partially surrounds the site, there is ample evidence (dumping and trails) of site access. Nearby residents, especially children, have access to the site and could be exposed to contaminated environmental media on-site. In the past children played on the site and adults hunted there. The current level of such activity is not known.
Potential off-site exposure routes and pathways include ingestion and dermal contact with contaminated surface water, groundwater, and food chain entities. Because fishing is one of the primary recreational uses of the lake, the human exposure pathway of greatest concern is through the consumption of contaminated fish. While it is not legal to hunt Western Grebes, human consumption of this waterfowl (if it occurs) could also lead to significant ingestion of mercury.
Exposure to the SBM site in its current state of contamination is a human health concern. Inhalation and ingestion of dust containing entrained mercury is potentially hazardous for workers or trespassers.
Fish in the lake are contaminated with mercury: a health advisory regarding the consumption of fish from Clearlake was issued in May 1986, which limits the consumption of fish to one per week. In addition, women who are pregnant or are nursing and children under 6 years of age are advised not to consume any fish taken from Clearlake. Through consumption of contaminated fish, it is likely that elevated exposures to mercury have occurred in the past and may continue to occur if guidelines in the health advisory are not followed.
One species of waterfowl indigenous to the lake (Western Grebes) is also known to contain elevated levels of mercury. While it is not legal to hunt this species, hunting and consumption may still be occurring. The level of contamination for most other species of wildlife is unknown.
Mercury exists in the environment in three principle forms: elemental, inorganic, and organic. Fish contamination is predominantly associated with the organic form (methyl mercury) and ingestion of contaminated fish has produced epidemics of mercury intoxication. Methyl mercury toxicity includes degeneration and necrosis of neurons in the central nervous system. Clinical neurological features include paresthesia (burning, itching skin sensation), ataxia, dysarthria (speech impairment), and deafness; large maternal exposures have resulted in neurologic damage to unborn children.
Surface water on-site, which drains from the excavated pit, is contaminated with both mercury and arsenic at concentrations that exceed EPA's maximum contaminant levels for drinking water. Although ingestion of surface water may be of health concern, it is unlikely that significant ingestion of drain water will occur. Dermal contact with contaminated soils and drainage water on-site is probably not of health concern because metals tend to be poorly absorbed through intact skin. However, dermal exposure to inorganic arsenic compounds can cause dermatitis.
Ingestion of off-site groundwater via the municipal well supply or private wells is a potential health concern, particularly if the water column in Clearlake becomes significantly contaminated. Fortunately, most of the mercury in the lake is contained in the bottom sediment of the Oaks Arm and the municipal water supply has reportedly not become contaminated to date. However, some of the houses located within a 3-mile radius of the site may be using private wells or taking water directly from Clearlake.
Generally, the water column in Clearlake is not heavily contaminated with mercury, even in the Oaks Arm. However, storm conditions or recreational activities in the Oaks Arm could lead to temporary suspension of significant quantities of mercury.
The potential health effects resulting from exposure to potentially contaminated environmental media cannot be fully evaluated at this time. Additional information is needed to characterize the groundwater at and near the site, the recreational uses of the Oaks Arm of Clearlake, the extent of contamination of wildlife, and the potential impact on human health.
Based on available information reviewed, ATSDR has concluded that this site is a public health hazard because of the risk to human health resulting from potential exposure to hazardous substances at concentrations that may result in adverse human health effects. It is likely that human exposure to mercury occurred in the past and continues to occur, primarily through consumption of contaminated fish. No adverse health effects have been documented.
Potential routes for human exposure to contaminated environmental media on-site include ingestion, inhalation, and dermal contact with soil and surface water. Workers or trespassers may come in contact with exposed waste at the site. While the ambient concentrations of mercury found at the site are generally not of public health concern, workers may have been exposed to higher concentrations of mercury in air during mining operations.
Potential off-site exposure routes and pathways are ingestion and dermal contact with surface water, groundwater, and any food chain entities contaminated with the metals found at the SBM site. Additional information is needed to fully evaluate the potential health threats associated with exposure to potentially contaminated environmental media.
The recommendations for this site are as follows:
- Further restrict access to the site; consider appropriate safeguards for physical hazards such as the Herman Pit.
- Encourage adherence to the 1986 state Health Advisory regarding consumption of fish from Clearlake; investigate any reports of commercial fishing.
- Consider posting roads accessing the lake with signs regarding the mercury contamination and the state Health Advisory.
- Prevent off-site migration of contaminants to the extent possible.
- Monitor private and municipal wells on and near the site.
- Further characterize contamination at and near the site.
- Further characterize the contamination of wildlife in the area.
In accordance with the Comprehensive Environmental Response, Compensation and Liability Act of 1980, as amended, the Sulphur Bank Mercury Mine, Clearlake, Lake County, California site has been evaluated for appropriate follow-up health activities by the ATSDR Health Activities Recommendations Panel. There are indications that human exposure to on-site/off-site contaminants are likely to have occurred in the past and may continue to occur. There are currently plans (by CDHS) to offer the Indian community an exposure study (including blood and urine tests for mercury) and possibly a symptom-and-disease prevalence study; ATSDR concurs with these plans. The ATSDR Division of Health Studies will offer technical assistance.
There is a cooperative agreement between ATSDR and the state of California to perform health assessments on NPL sites; we will continue to monitor this site with assistance from CDHS. Efforts should continue to ascertain the concerns of communities living near this site. When indicated by public health needs, and as resources permit, the evaluation of additional relevant health outcome data and community health concerns, if available, is recommended.
Environmental and Health
Sharon O. Williams-Fleetwood, Ph.D.
Health Science Section
Division of Health Assessment and Consultation
Dannie C. Middleton, M.D.
Health Science Section
Division of Health Assessment and Consultation
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- California Department of Health Services. Methyl mercury in Northern Coastal Mountain Lakes: DHS Guidelines for Sport Fish Consumption for Clearlake (Lake County), Lake Berryessa (Napa County), and Lake Herman (Solano County). Berkeley, CA: State of California Department of Health Services, April 1987.
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