SEMS-RM DOCID # 100035135
Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 1
Record of Decision
Prepared by
US EPA Region 9
November 2023
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TABLE OF CONTENTS
Page
LIST OF TABLES iv
LIST OF FIGURES v
LIST OF APPENDIXES vi
LIST OF ACRONYMS AND ABBREVIATIONS vii
1. DECLARATION 1-1
2. DECISION SUMMARY 2-1
2.1 SITE NAME, LOCATION, AND DESCRIPTION 2-1
2.2 Site History and Enforcement Activities 2-1
2.2.1 Pre-Mining History and Elem Band of Pomo Tribal Use 2-1
2.2.2 Mining Operations 2-2
2.2.3 Previous Response Actions 2-3
2.2.4 Enforcement Actions and 2012 Consent Decree 2-5
2.3 COMMUNITY PARTICIPATION 2-5
2.3.1 Tribal Consultation 2-7
2.4 SCOPE AND ROLE OF ACTION FOR OPERABLE UNIT 1 2-8
2.5 SITE CHARACTERISTICS 2-8
2.5.1 Site Features and their Origins 2-8
2.5.2 Conceptual Site Model 2-10
2.5.3 Areas of Concern 2-10
2.5.4 Overview of the Site Geology 2-16
2.5.5 Contamination and Affected Media 2-19
2.5.6 Fate and Transport of Contaminants 2-22
2.5.7 Receptor Groups 2-25
2.5.8 Remedial Investigation and Subsequent Studies 2-26
2.5.9 Groundwater Monitoring 2-27
2.5.10 Stormwater Monitoring 2-28
2.5.10.1 Historic Properties and Tribal-Cultural Resources 2-28
2.5.11 Human Health Risk Assessment Sampling 2-29
2.5.12 Pre-Mining Baseline (PMB) Conditions Surface Soil Sampling 2-30
2.6 CURRENT AND POTENTIAL FUTURE SITE AND RESOURCE USES. .. . 2-33
2.6.1 Current Land Use 2-33
2.6.2 Reasonably Anticipated Future Land Uses 2-34
2.6.3 Groundwater Resources 2-35
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2.6.4 Surface Water Resources 2-35
2.7 SUMMARY 01 SITE RISKS 2-35
2.7.1 Human Health Risks 2-36
2.7.2 Ecological Risks 2-43
2.7.2.1 ERA Conclusions 2-44
2.7.2.2 ERA Uncertainty Analysis 2-45
2.7.3 Basis for Action 2-45
2.8 REMEDIAL ACTION OBJECTIVES 2-46
2.8.1 Cleanup Levels and Remediation Goals 2-47
2.8.2 Benefits of OU-1 Remedy on OU-2 and OU-4 2-50
2.9 DESCRIPTION OF ALTERNATIVES 2-51
2.9.1 2006 OU-1 Feasibility Study 2-51
2.9.2 Changes in the Site and Conceptual Site Model After 2006 Feasibility
Study 2-52
2.9.3 2021 OU-1 Focused Feasibility Study Alternatives 2-53
2.9.4 Off-Mine Residential Soil 2-53
2.9.5 Source Areas 2-54
2.9.6 Waste Rock Dam 2-57
2.9.7 Herman Impoundment 2-58
2.10 SUMMARY OF COMPARATIVE ANALYSIS OF ALTERNATIVES 2-59
2.10.1 Off-Mine Residential Soil 2-59
2.10.2 Source Areas 2-60
2.10.3 Waste Rock Dam 2-60
2.10.4 Herman Impoundment 2-62
2.11 PRINCIPAL THREAT WASTE 2-63
2.12 SELECTED REMEDY 2-64
2.12.1 Summary of the Rationale for the Selected Remedy 2-65
2.12.2 Description of the Selected Remedy 2-66
2.12.3 Components of the Selected Remedy 2-67
2.12.4 Remedial Design Investigation and Key Design Decisions 2-74
2.12.5 Remedy Sequencing 2-75
2.12.6 Institutional Controls and Engineering Controls 2-76
2.12.7 Long-Term Operation, Maintenance, and Monitoring 2-77
2.12.8 Compliance with ARARs 2-78
2.12.9 Summary of the Estimated Remedy Costs 2-78
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2.13 STATUTORY DETERMINATIONS 2-78
2.13.1 Protection of Human and the Environment 2-78
2.13.2 Compliance with Applicable or Relevant and Appropriate Requirements
2-79
2.13.3 Cost Effectiveness 2-79
2.13.4 Utilization of Permanent Solutions and Alternative Treatment (or
Resource Recovery) Technologies to the Maximum Extent Practicable2-
79
2.13.5 Five-Year Review Requirements 2-79
2.14 DOCUMENTATION OF SIGNIFICANT CHANGES TO THE REMEDY... 2-80
3. REFERENCES 4-1
Appendix A Applicable or Relevant and Appropriate Requirements (ARARs) A-l
Appendix B RESPONSIVENESS SUMMARY B-l
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LIST OF TABLES
Table 2-1. Mining Facility Building Description
Table 2-2. Summary of Contaminants of Concern and Medium-Specific Exposure Point
Concentrations
Table 2-3. Media Specific Risk-Based Target Soil Concentration and Preliminary
Remediation Goals for OU-1
Table 2-4. Comparative Analysis Summary of Source Area Alternatives
Table 2-5. Waste Rock Dam Removal Alternatives Comparison
Table 2-6. Comparative Analysis Summary of Herman Impoundment Alternatives
Table 2-7. Area and Volume of Mine-Related Waste Piles and Remedial Alternative Extent
Table 2-8. OU-1 Site Buildings/Structures Recommended Remedial Actions
Table 2-9. Selected Remedy Cost Estimate
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LIST OF FIGURES
Figure 1-1.
Location and Site Map
Figure 1-2.
Physical Site Features and Areas of Concern
Figure 2-1.
Conceptual Site Model - All Media Combined
Figure 2-2.
Surface Geology
Figure 2-3.
Isopleth Map for Mercury in Groundwater and Hydraulic Potential
Figure 2-4.
Stormwater Catchments
Figure 2-5.
Habitat and Land Cover
Figure 2-6.
El em Indian Colony Residential Lots
Figure 2-7.
Non-Tribal Residential Lots
Figure 2-8.
Operable Unit 1 Selected Remedy
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LIST OF APPENDIXES
Appendix A: Applicable and Relevant or Appropriate Requirements - Table A-l
Appendix B: Responsiveness Summary
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LIST OF ACRONYMS AND ABBREVIATIONS
^g/L
Microgram(s) per liter
AOC
Area of concern
APE
Area of potential effect
APTIM
APTIM Federal Services
ARAR
Applicable or relevant and appropriate requirement
ARD
Acid rock drainage
BAF
Bioaccumulation factor
Basin Plan
Central Valley Regional Water Quality Control Board Basin Plan
bgs
Below ground surface
BIA
Bureau of Indian Affairs
BIA 120
Bureau of Indian Affairs Road 120
BPHE
Baseline Public Health Evaluation
CDI
Chronic daily intake
CERCLA
Comprehensive Environmental Response, Compensation, and Liability Act of
1980
CH2M
CH2M Hill
cm/s
Centimeters per second
coc
Contaminant of concern
COPC
Contaminant of potential concern
COPEC
Contaminant of potential ecological concern
CSM
Conceptual site model
cy
Cubic yard(s)
DTSC
California Department of Toxic Substances Control
E2
E2 Consulting Engineers, Inc.
EIC
Elem Indian Colony
EPA
U.S. Environmental Protection Agency
EPC
Exposure point concentration
ERA
Ecological Risk Assessment
ET
Evapotranspiration
FFS
Focused Feasibility Study
FS
Feasibility Study
ft
Foot (feet)
GI
Gastrointestinal
HDPE
High-density polyethylene
HHRA
Human Health Risk Assessment
HI
Hazard index
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HQ
Hazard quotient
IC
Institutional control
IRIS
Integrated Risk Information System
ITSI
Innovative Technology Solutions, Inc.
kg/year
Kilogram(s) per year
LLS
Lower Lake Sediments
MCL
Maximum Contaminant Level
mg/kg
Milligram(s) per kilogram
mg/L
Milligram(s) per liter
NCP
National Oil and Hazardous Substances Pollution Contingency Plan
NHPA
National Historic Preservation Act
NPL
National Priorities List
NTCRA
Non-time-critical removal action
O&M
Operations and maintenance
Oaks Arm
Oaks Arm of Clear Lake
ORNL
Oak Ridge National Laboratory
OU-1
Operable Unit 1
OU-2
Operable Unit 2
OU-3
Operable Unit 3 - No longer an active Operable Unit
OU-4
Operable Unit 4
PMB
Pre-mining baseline
PRG
Preliminary remedial goal
RAO
Remedial action objective
RCRA
Resource Conservation and Recovery Act
RD
Remedial Design
RFD
Risk reference dose
RI
Remedial Investigation
ROD
Record of Decision
RSL
Regional Screening Level
RWQCB
Central Valley Regional Water Quality Control Board
SBMM
Sulphur Bank Mercury Mine
Site
Sulphur Bank Mercury Mine Superfund Site
TAC
Technical Action Committee
TDS
Total dissolved solids
Title 27
California Code of Regulations, Title 27
TMDL
Total maximum daily load
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TRV
Toxicity reference value
TtEMI
Tetra Tech EM, Inc.
ULS
Upper Lake Sediments
USACE
U.S. Army Corps of Engineers
WRD
Waste Rock Dam
XRF
X-ray fluorescence
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1. DECLARATION
Site Name and Location—The Sulphur Bank Mercury Mine (SBMM) Superfund Site
(Comprehensive Environmental Response, Compensation, and Liability Information System
Identification number CAD980893275) is located in Lake County, California, on the eastern
shore of Oaks Arm of Clear Lake (Oaks Arm), approximately 100 miles north of San Francisco
(Figure 1-1). The physical site features are shown on Figure 1-2. SBMM can be accessed along
the northwest side of Sulphur Bank Drive, approximately 1.5 miles southwest of its intersection
with State Highway 20. SBMM is approximately 0.5 mile south of Clearlake Oaks (population is
approximately 2,400) and 5 miles north of the city of Clearlake (population is approximately
15,200).
The Site is currently organized into three operable units (OUs):
• OU-1 consists of the historical mining areas, mine waste piles, and the Herman
Impoundment; as well as the impacted residential soil areas on the Elem Indian Colony
(EIC) to the northwest of the mine and the neighborhood adjoining Sulphur Bank Mine
Road to the south.
• OU-2 consists of Clear Lake and the sediment in Clear Lake.
• OU-4 consists of two areas: the North Wetlands area and adjoining parcels to the north of
the mine Site and the OU-4 Study Area consisting of parcels to the east and south of the
mine.
The Site no longer includes an OU-3, as this OU designation was previously used to describe site
components that have been incorporated into OUs 1 and 2.
Statement of Basis and Purpose—This decision document presents the selected remedy for
OU-1 of SBMM Superfund Site, excepting a remedial decision for the groundwater media. The
selected remedy was chosen in accordance with the Comprehensive Environmental Response,
Compensation, and Liability Act of 1980 (CERCLA), as amended by the Superfund
Amendments and Reauthorization Act of 1986 and, to the extent practicable, the National Oil
and Hazardous Substances Pollution Contingency Plan (NCP). This decision is based on the
Administrative Record file for this Site, which is available for review at the Lake County Library
on Redbud Lane, Clear Lake and online at
https://cumulis.epa. gov/supercpad/cursites/csitinfo.cfm?id=0902228.
The State of California as represented by the Department of Toxic Substances Control and
Central Valley Regional Water Quality Control Board have concurred with the selected remedy
presented herein. This concurrence was received in letters both dated November 6, 2023 and
included in the administrative record. The Elem Indian Colony has raised concerns with elements
of the selected remedy and has requested close ongoing coordination moving forward but
considering the totality of factors, provided their concurrence with the selected remedy on
November 3, 2023. The EIC's concurrence letter has likewise been entered into the
Administrative Record.
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Assessment of the Site—The remedial action selected in this Record of Decision (ROD) is
necessary to protect public health or welfare or the environment from actual or threatened
releases of hazardous substances into the environment.
Description of the Selected Remedy—The selected remedy will provide long-term protection
of human health and the environment. The selected a remedy for OU-1 described in this ROD is
focused on source control and includes the following primary components:
• Off-mine Residential Soil Cleanup
• Removal of Mine Waste, Consolidation, and Capping/Covering Source Areas
• Demolition and Removal of Abandoned Structures
• Infrastructure Improvements at Herman Impoundment
• Removal of Portions of the Waste Rock Dam Area and Capping/Covering Remaining
Waste Rock Areas
• Remedy Effectiveness Monitoring
The selected remedy also includes common elements that would be required to
implement the remedy. Examples of these common elements include, but are not limited to, pre-
construction surveys; erosion and sediment control measures; dust suppression; access road
improvements (as necessary); generation, processing, and blending of uncontaminated borrow
material for construction of remedial components and access roads; and implementation of both
engineering and institutional controls (ECs and ICs). Long-term monitoring of groundwater and
surface water flow, elevations, and water quality will be conducted to evaluate the effectiveness
of the OU-1 source control remedy. Finally, ECs and ICs will be implemented that consist of
both physical controls and administrative land use restrictions.
The selected remedy for OU-1 is consistent with EPA's general site management strategy to
conduct removal actions to reduce acute risks (conducted from 1992 through 2010), followed by
large scale source control (this remedy for OU-1). Subsequent decision documents will then
address downstream contributors to excess risk. Investigations of other operable units will
continue in parallel with the source control remedy and reduction of risks in OU-1.
The Selected Remedy addresses risk posed by the OU-1 source materials by consolidating waste
and constructing a protective cap or cover, thereby removing the transport of, and subsequent
exposure to, Site contaminants, as well as eliminating direct contact with the source materials.
Statutory Determinations—This action: (1) is protective of human health and the environment
by reducing exposures of human and environmental receptors to site contaminants, (2) complies
with those federal and state requirements that are applicable or relevant and appropriate, (3) is
cost-effective, and (4) utilizes permanent solutions to the maximum extent practicable. This
action is being implemented while the remainder of the Site is investigated and actions in other
operable units are analyzed. This action is a permanent solution for the soil in residential areas
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and the mine Site source areas. The NCP states a preference for treatment alternatives to reduce
toxicity or mobility, where practicable, and use of engineering controls, such as containment, for
wastes that pose a relatively low long-term threat or where treatment is impracticable. The
majority of the solid media within OU-1 is classified as low-level threat waste for which
containment is achievable and effective and treatment would be impracticable.
Because this remedy will result in hazardous substances remaining at the mine above levels that
allow for unlimited use and unrestricted exposure, a review will be conducted to ensure that the
remedy continues to provide adequate protection of human health and the environment within
5 years after initiation of the remedial action. Because this action is on one of several operable
units, review of this remedy will be ongoing as the U.S. Environmental Protection Agency
(EPA) continues to develop remedial alternatives for the remaining operable units.
ROD Data Certification Checklist—The following information is included in the Decision
Summary section of this ROD (additional information can be found in the Site Administrative
Record).
• Contaminants of concern (COCs) and their respective concentrations (Section 2.5 and
Section 2.7).
• Baseline risk represented by the COCs (Section 2.7).
• Cleanup levels established for COCs and the basis for these levels (Section 2.8.1).
• How source materials constituting principal threats are addressed (Section 2.11).
• Current and reasonably anticipated future land use assumptions and current and potential
future beneficial uses of groundwater used in the baseline risk assessment and this ROD
(Section 2.6).
• Potential land and groundwater use that will be available at the Site as a result of the
Selected Remedy (Section 2.6).
• Estimated capital, annual operations and maintenance (O&M), and total present worth
costs, discount rate, and the number of years over which the remedy cost estimates are
projected (Section 2.12.8).
• Key factor(s) that led to selecting the remedy (Section 2.13).
Authorizing Signatures:
Digitally signed by MICHAEL
MICHAEL MONTGOMERY Montgomery
Date: 2023.11.09 18:13:00 -08'00'
Michael Montgomery, Director Date
Superfund and Emergency Management Division, Region 9
U.S. Environmental Protection Agency
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2. DECISION SUMMARY
2.1 SITE NAME, LOCATION, AND DESCRIPTION
The Sulphur Bank Mercury Mine (SBMM) Superfund Site (Site) (CAD980893275) is located in
Lake County, California, on the eastern shore of Oaks Arm of Clear Lake (Oaks Arm),
approximately 100 miles north of San Francisco (Figure 1-1). The physical site features are
shown on Figure 1-2. SBMM can be accessed along the northwest side of Sulphur Bank Drive,
approximately 1.5 miles southwest of its intersection with State Highway 20. SBMM is
approximately 0.5 mile south of Clearlake Oaks (population is approximately 2,400), 5 miles
north of the city of Clearlake (population is approximately 15,200), and immediately adjacent to
the Elem Indian Colony (EIC) reservation (population approximately 50).
When the mine site was placed on the National Priorities List (NPL) in August 1990, EPA
became the lead regulatory agency. EPA is also the primary source of funding for the cleanup.
EPA has divided the Site into three operable units (OU) to allow development of specific
cleanup plans for unique parts of the site. The OUs and the site physical features are shown in
Figure 1-2.
• OU-1 consists of the historical mining areas, mine waste piles, and Herman
Impoundment; as well as the impacted residential soil areas on the Elem Indian Colony
(EIC) to the northwest of the mine and the neighborhood adjoining Sulphur Bank Mine
Road to the south. The mine site itself is approximately 160 acres and includes
approximately 2.5 to 3 million cubic yards of mine waste distributed across 9 waste rock,
tailing, and ore piles (E2, 2021).
• OU-2 consists of Clear Lake and its sediments.
• OU-4 consists of two areas, the north area including the North Wetlands and parcels
north of the mine site, and the OU-4 Study Area consisting of parcels to the east and
south of the Site.
The site no longer includes an OU-3, as this OU designation was previously used to describe site
components that have been incorporated into OUs 1 and 2.
2.2 SITE HISTORY AND ENFORCEMENT ACTIVITIES
The following summary of the Site history and previous enforcement activities is excerpted from
the 2002 Remedial Investigation (TtEMI 2002a) and the 2021 Focused Feasibility Study (E2
2021a).
2.2.1 Pre-Mining History and the Elem Indian Colony of Pomo Indians Tribal Use
The area surrounding the Site has been inhabited by Native American people for approximately
14,000 years. At the time of European arrival, the project area belonged to the Southeastern
Pomo community of Elem. The Southeastern Pomo spoke a language belonging to the Hokan
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language family, considered the oldest language family in California and possibly in the New
World (Shipley 1978). It is likely that Hokan speaking people have inhabited California for at
least 12,000 to 14,000 years (Parker 1994).
At the time when mining activities began at Sulphur Bank Mercury Mine, the site and the greater
Clear Lake ecosystem provided robust natural resources that the Elem and other regional tribes
relied upon, including fish, waterfowl, acorns, large and small game, and a diversity of fruiting
plants, roots, bulbs and seeds.
Prior to development for mining, the geothermal activity at the Site made surface expression as a
large deposit of sulfur-rich minerals interspersed with multiple warm water springs bubbling
from the release of geothermal gases.
2.2.2 Mining Operations
As summarized in the RI ((TtEMI 2002a) Sulfur was the first mineral mined at the Site,
beginning in 1865. The California Borax Company filed a claim and began mining the surface
and near surface sulfur deposits. Sulfur mining continued until 1871, when it became non-viable
due to declining sulfur prices and increased cost to refine the sulfur due to mercury
contamination in the sulfur ore.
In 1873, the mine reopened to produce mercury. The mercury was refined using a furnace to
vaporize the mercury, followed by collection and cooling to a liquid state. The open pit was up to
60 feet (ft) deep, and two shafts were constructed to depths of 260 and 450 ft; the deeper of the
two shafts was named the Herman Shaft. Mercury production by the California Borax Company
ceased in 1883.
The Sulphur Bank Quicksilver Mining Company took over operation from 1887 to 1897 and
during that time sank two additional shafts to 155 and 100 ft.
The Empire Consolidated Mining Company assumed ownership in 1899 and operated until 1905.
During their operation, they sank two additional shafts to 240 and 150 ft. Underground
operations were abandoned in 1904, with the exception of one shaft that was dewatered until
1906.
The Sulphur Bank Association of San Francisco reopened the mine in 1917 and operated until
1919. They installed a new furnace with the intent of reprocessing the waste rock; however, the
process was not successful.
The Bradley Mining Company began open pit mining operations in 1927 (under a lease) using
power shovels and updated blasting techniques. They sank two additional mine shafts that were
soon abandoned due to hot water, gases, and lack of ore. The open pit was mined at a rate of
300 tons per day. By 1944, all underground mine workings (shafts) were caved and inaccessible.
The open pit was named the Herman Pit, and by 1943, it was mined to a depth of approximately
100 ft, which is about 60-70 ft below the water surface in Clear Lake. After World War II, the
Bradley Mining Company ceased operations, and the Herman Pit was allowed to fill with water.
In 1955, Bradley Mining had obtained ownership of the mine (rather than leasing), drained the
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Herman Pit, and resumed small scale open pit mining. They ceased operations in 1957, and the
pit filled with water again, forming what is now referred to as the Herman Impoundment.
Mining activity at SBMM distributed approximately 2.25 million tons of material were
excavated, processed, and disposed of on-mine during mining operations (TtEMI 2002a). During
mining operations, water pumped from Herman Pit was discharged to Clear Lake or the
Northwest Pit. When water reach elevated levels in the Northwest Pit, it would overflow to the
North Wetlands (EPA 1994b).
Two non-mercury mining operations also took place at the Site. An aggregate block plant
operated on the South Waste Rock Pile intermittently between 1949 and 1982. A fertilizer plant
also reportedly operated on the North Waste Rock Pile, although the period of operation is
unknown (ICF Kaiser Engineers 1994).
2.2.3 Previous Response Actions
In 1979, prior to SBMM being placed on the NPL, the Bradley Mining Company built an earth
and rock dam at the west end of the Herman Impoundment. The Central Valley Regional Water
Quality Control Board (RWQCB) had requested construction of the dam to prevent direct
discharge of the Herman Impoundment water to Clear Lake.
In 1990, the SBMM was placed on the NPL and EPA became the lead regulatory agency for the
Superfund Site. Bradley Mining Company was identified as a potentially responsible party for
SBMM.
In 1992, EPA took action under Superfund removal authority, and conducted the following
actions:
• To stabilize the waste piles adjacent to the Clear Lake shoreline, EPA graded the waste
rock to decrease the slope angle along the shoreline from 60 degrees to 20 degrees.
Approximately 130,000 cubic yards (cy) of waste rock material were excavated and
removed from the shoreline. The excavated material was used to fill in the shallow
northwest corner of the Herman Impoundment, over an area of approximately 3 acres. In
addition, the overflow outlet from the Herman Impoundment was filled with excavated
material, and drainage was rerouted around the side of the Waste Rock Dam. The newly
graded slope was revegetated to reduce erosion. To reduce the impact of wave erosion,
the beach area at the base of the piles was widened and reinforced with riprap up to the
high-water level.
• Part of the Northwest Waste Rock Pile was removed from the north wetland area. Waste
rock under the access road leading to the EIC was not removed because there was not an
exposure pathway to this waste. The remaining portion of the Northwest Waste Rock Pile
was capped with clean soil and revegetated.
• A chain-link fence was constructed along the main road leading through the mine
property to discourage visiting.
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In 1993, EPA completed a removal action and removed contaminated soil from the yards of
eleven homes at the EIC and replaced it with clean soil. Mine waste had been used by the U.S.
Department of the Interior and BIA to level home sites and build roads when community
infrastructure was constructed in the 1970s.
Between 1996 and 1997, EPA completed another removal action that included excavation of
approximately 3,300 cy of mining waste from within EIC (CH2M Hill [CH2M] 2008c). Mining
waste was excavated to a depth of approximately 18 inches below ground surface (bgs) in
thirteen residential lots. Mining waste was removed to approximately 6 inches bgs around trees,
shrubs, and flower beds. Building slabs and foundations were removed from four of the 13 lots.
In addition, mining waste was removed from the road between two lots.
Between 1999 and 2000, EPA completed a removal action that included construction of a surface
water diversion pipeline. The pipeline drains stormwater runoff from Green Pond directly to
Clear Lake ("Overflow Division System" in Figure 1-2). The pipeline intercepts stormwater
before it drains into Herman Impoundment thereby reducing the amount of water discharging
through the WRD to Clear Lake. This minimizes the interaction of the Herman Impoundment
water with waste rock between Herman Impoundment and Clear Lake which is the most
significant source of mercury loading in groundwater at SBMM.
In 2000, EPA sealed old geothermal exploratory wells located on the north edge of Herman
Impoundment. After community members expressed concern that improper seals in the
geothermal wells could fail and cause dangerous concentrations of geothermal gases, EPA
abandoned the wells.
In 2007, a non-time-critical removal action (NTCRA) was completed by EPA at the EIC (CH2M
2008c). This removal action was based on soil data collected from multiple soil sampling events
conducted in 1992, 1997, 2002, 2005, and 2006 (CH2M 2008c). These investigations showed
that there was still mining waste remaining in many of the lots that were originally remediated in
1997. The 2007 NTCRA removed more than 29,500 cy of mining waste, replaced seven homes,
refurbished six homes, demolished and removed several dilapidated structures, reconstructed
3,700 ft of roadway and sidewalks, and reconstructed the water supply system in those areas.
Mining waste and debris were hauled to an area on the northwest side of Herman Impoundment.
Post-excavation site restoration included placement of clean fill and revegetation with native
flowers and grasses, oak trees, fruit trees, and shrubs.
Also in 2008, EPA conducted a time critical removal action along Sulphur Bank Mine Road.
This TCRA was conducted in accordance with the technical memorandum Sulphur Bank Mine
Road Interim Removal Action Proposed Soil Removal Locations (CH2M 2008b) Approximately
3,549 cy of mining waste was excavated and placed in the northwest area of the Herman
Impoundment. Soil with mercury concentrations exceeding the cleanup goals for the project that
was left in place was covered with at least 1 ft of soil (fill and clean topsoil) (CH2M 2009a).
In 2010, a non-time-critical removal action was completed to address contamination along BIA
120 (also referred to as Pomo Road), the main roadway used to access the EIC along the north
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and northeast boundary of OU-1 (Innovative Technology Solutions, Inc. [ITSI] 2011). A site-
wide soil investigation conducted in 2009 concluded that elevated concentrations of arsenic and
mercury existed on the roadway and shoulders of BIA 120 and near the intersection of BIA 120
and Sulphur Bank Mine Road (CH2M 2009b). Approximately 4,000 linear feet of soil along BIA
120 was removed, replaced and/or covered. Mining waste was excavated and placed in northwest
corner of the Herman Impoundment. The excavated areas were backfilled with clean fill. Along
BIA 120, asphalt was placed over remining mining waste adjacent to the road, at least 1 ft of
compacted clean fill was placed on the shoulder of the road, and below grade utilities were
placed in a clean soil corridor (CH2M 2009b).
2.2.4 Enforcement Actions and 2012 Consent Decree
Several pre-CERCLA enforcement actions were taken by the RWQCB beginning in the 1950s.
The RWQCB completed routine inspections and collected monitoring data to evaluate
compliance with discharge limitations. The Bradley Mining Company completed several actions
in response to requests and orders from the RWQCB.
In April 2012, the United States and the EIC entered a Consent Decree with the Bradley Mining
Company. This settlement included payment of $7,200,588 by the United States to EPA on
behalf of the Bureau of Indian Affairs (BIA) to address mine waste used by BIA in construction
on and around the EIC. The Consent Decree also established the Sulphur Bank Redevelopment
Trust to hold and manage parcels transferred by Bradley Mining Company and related entities
("Bradley Mining") to the Redevelopment Trust for the benefit of the EIC and EPA. The
Bradley Mining settlement was an ability to pay settlement which required Bradley Mining to
transfer 11 parcels of land in the area to the Redevelopment Trust for the benefit of EPA and the
EIC (five parcels). The Redevelopment Trust is supposed to transfer five parcels to the EIC and
hold the other six parcels for EPA. The Consent Decree provides that EPA's parcels may be sold
to help fund the cleanup of the Site. The Consent Decree also required Bradley Mining to assign
certain insurance policies and future income to EPA. However, only nominal income has been
produced from Bradley Mining insurance and income and the proposed remedy will be fund
lead.
2.3 COMMUNITY PARTICIPATION
Since the beginning of the SBMM Superfund Site CERCLA process in August 1990, EPA has
engaged with the community, the EIC, and other regulatory agencies to facilitate the technical
review of EPA's response actions. Public participation has been encouraged in accordance with
NCP Section 300.430(f)(3). A Community Relations Plan was prepared in 1992 anda subsequent
Community Involvement Plan was prepared in 2008. Each plan set forth a strategy for
disseminating information to the public, incorporating community concerns into the RI/FS, and
involving the community in the remedy process (CH2M 2008d).
The following is a summary of community outreach activities that have been and are continuing
to be conducted for the SBMM Superfund Site:
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• Interviews are conducted with community members and public officials to share cleanup
status and gain an understanding of community concerns related to the Site.
• Community meetings are held with residents, community organizations, and tribal
members at prominent remedy milestones to discuss the cleanup activities at the Site.
• An active website is maintained for the Site including a Site history, a status of cleanup
activities, contact information, and an administrative record of technical documents and
project fact sheets for easy access by the public.
• Fact sheets are prepared to summarize Site-related information. As of 2023, EPA has
published eleven fact sheets to keep the public informed of the progress of the Site
investigation, development of removal and remedial action alternatives, and inform the
community of significant events occurring in their area. These fact sheets can be found in
the Administrative Record, which is available on EPA's SB MM Superfund Site website.
• A project distribution list is maintained that includes local community members,
regulatory agencies, public officials, and tribal members who receive notices and fact
sheets about project activities.
• Since at least 2014, EPA began convening meetings of the "Multi-Agency Environmental
Team," which is composed of EPA, California Department of Toxic Substances Control
(DTSC), RWQCB, the EIC, and has occasionally included other regulatory agencies,
such as the US Army Corps of Engineers and U.S. Geological Survey. The purpose of the
Multi-Agency Environmental Team was to provide an efficient and open dialogue
between representatives of regulatory agencies and to improve the analysis and decision-
making process. EPA conducted numerous in-person workshops and teleconference
meetings with the Team to discuss the evaluation of Site conditions and development of
the focused remedial alternatives for the focused feasibility study (FFS) (E2, 2021).
Meetings were also held to discuss the approach, process, and results of each evaluation
conducted. These periodic workshops and meetings provided a platform to discuss and
resolve the Multi-Agency Environmental Team issues.
• EPA and the Multi-Agency Environmental Team conducted a strategic planning process
during 2020, facilitated by EPA Headquarters Office of Solid Waste and Emergency
Response, which culminated in a strategic plan, risk register, and schedule for the SBMM
Site. EPA continues to work with the Multi-Agency Environmental Team to implement
the strategic plan.
• Interviews and community forums are conducted to engage the broader Clear Lake
community in discussions regarding the progress towards the OU-1 remedy and the
implementation of a fish advisory.
• EPA released the Proposed Plan for the OU-1 remedy on January 10, 2023, with a 90-day
public comment period. The community outreach associated with the Proposed Plan and
public comment included:
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— Whole community question and answer session (virtual), January 25, 2023
— Tribal focused question and answer session (virtual), February 2, 2023
— Whole community open house in Clearlake Oaks, California, February 8, 2023
— Tribal focused open house in Lakeport, California, February 9, 2023
— Tribal focused Proposed Plan meeting, February 28, 2023
— Whole community Proposed Plan meeting, March 1, 2023
— Meeting with Elem government and EPA Regional Administrator (along with EPA
staff) in Santa Rosa, California, March 16, 2023
— Meeting with Elem community members and EPA Regional Administrator (along
with EPA staff) at the SB MM Site, March 16, 2023.
2.3.1 Tribal Consultation
Consistent with EPA's 2011 Policy on Consultation and Coordination with Indian Tribes, EPA
engaged in government-to-government consultation with the EIC prior to and following the
release of the OU1 Proposed Plan. EPA's invitation to meet and engage with other area tribes
was not responded to with a request for consultation, so formal consultation was only engaged
with the EIC, though informal coordination with other tribal representatives did take place.
Informal dialog between the EIC and EPA has taken place for years. Formal government-to-
government meetings between EIC tribal leadership and EPA senior management took place
between November 2021 and August 2023. The EIC expressed a range of concerns regarding the
Proposed Plan and its likelihood of success, including in formal comments on the Proposed Plan.
In response to a request from the EIC and to ensure that the concerns raised by the EIC were
addressed in a manner consistent with the 9-criteria and acceptable for EPA, the EIC, and the
lead State agencies, a facilitated meeting series was organized to work through the EIC's most
substantive comments on the Proposed Plan. As a result of this facilitated process, including the
close staff-level coordination it entailed, substantive adjustments were made to the selected
remedy described in this ROD. These adjustments included selection maximum excavation
volume from the range of WRD actions described in the Proposed Plan, prioritization of the
cleanup of the WRD portion of the site early in remedy implementation, revisions in cleanup
levels and how comparisons to background will be performed, triggers for reevaluation of
remedy success/failure, commitments to further evaluation of background levels of COCs in
soils, and several other key provisions intended to provide assurance that issues of particular
concern for the EIC would be addressed during upcoming phases of work. Considering these
adjustments, the EIC provide concurrence on the selected remedy while also noting some
ongoing concerns. The EIC's full comments on the Proposed Plan are included in the
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administrative record and paraphrased in the Responsiveness Summary associated with this
ROD.
2.4 SCOPE AND ROLE OF ACTION FOR OPERABLE UNIT 1
The selected remedy for OU-1 is comprised of the final remedy for: (1) residential soil within the
EIC and residential soil south of the mine site, (2) the source areas within the mine site, (3) the
Waste Rock Dam, which lies between Herman Impoundment and Clear Lake, and (4) Herman
Impoundment (via monitoring of the impoundment and associated groundwater flow from the
Site to Clear Lake). EPA is not selecting a remedy for groundwater at this time and will make a
decision on how to address groundwater in a future remedial decision. However, the remedy
does address the flux of mercury contaminated groundwater from Herman Impoundment into
Clearlake by requiring it to be reduced to 0.5 kg/year consistent with the TMDL. Areas with
uncertainty regarding the extent of contamination will be investigated during RD, including areas
between waste piles and in off-mine residential areas and remediated if warranted.
The selected remedy is consistent with the general management strategy of addressing the
highest risks via early actions (conducted from 1992 through 2010), followed by large scale
source control (the selected remedy for OU-1). Investigations of other operable units will
continue in parallel with the OU-1 source control remedy.
Selecting a permanent remedy for OU-1 is appropriate because OU-1 is the primary source of
contaminants affecting other operable units. The OU-1 selected remedy will be designed in a
manner intended to prevent it from precluding remedial action alternatives for the other operable
units and, therefore, is consistent with the potential sitewide remedial action(s).
2.5 SITE CHARACTERISTICS
This section of the ROD describes the conceptual site model (CSM), Areas of Concern (AOCs),
Contaminants of Concern (COCs), the fate and transport of contamination, and receptor groups.
2.5.1 Site Features and their Origins
Between 1864 and 1957, SBMM was the site of underground and open pit mining operations that
spatially coincided with the hydrothermal vents and hot springs. Mining disturbed about 160
acres at SBMM and generated waste rock (rock not containing economic concentrations of
mercury and which that was removed to gain access to ore), tailings (ore that was processed to
remove the mercury), and ore (rock containing economic concentrations of mercury that was
mined and stockpiled for mercury extraction). The waste rock, tailings, and ore are distributed in
piles throughout the mine site (Figure 1-2). Remnants of the mill and other mine-related
structures are also present at the Site.
After mining stopped in 1957, Herman Pit filled with water from hydrothermal fluids,
groundwater, and surface water runoff. Herman Pit was then renamed the Herman Impoundment
after it filled with water, and it currently has a surface area of approximately 23 acres and a depth
of 90 ft. The Herman Impoundment water had a pH of about 3 from the 1980s through 2010 and
currently has a pH of 4.5-5.5. Herman Impoundment is separated from Clear Lake by the Waste
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Rock Dam (WRD) which consists of waste rock piled on to existing ground that was not mined.
The WRD is not a formal dam that was constructed to impede water flow, but instead, is an area
that was not mined between Herman Impoundment and Clear Lake where waste rock was
disposed of during mining operations.
Apart from Herman Impoundment, all the Areas of Concern (AOCs) within OU-1 are considered
"source material," which includes or contains hazardous substances, pollutants, or contaminants
that act as reservoirs for migration of contaminants to groundwater, surface water, air, or act as
sources for direct exposure. Source material is characterized as either principal threat waste or
low-level threat waste. Principal threat waste is material considered to be highly toxic or highly
mobile and, in general, cannot be reliably contained and/or would present significant risk to
human health or the environment should exposure occur. Low- level threat wastes are those
source materials that can be reliably contained and that present low risk in the event of release.
Solid media within OU-1 that contain contaminants above their respective remedial criteria
constitute source materials because they pose risks due to direct exposure and act as a reservoir
for migration of contamination to groundwater and surface water. Solid media, including mine
waste and contaminated soil, at the mining-related sources addressed by this remedy are
considered low-level threat waste, rather than principal threat waste, for the following reasons:
• Contaminants in solid media are not highly mobile.
— The contaminants present at this Site are inorganics that are generally bound as part
of mineral assemblages within the solid media and are only mobile when in contact
with acidic water over time.
• Contaminants in solid media can be reliably contained.
— The contaminants present at this Site are inorganics generally bound as part of
mineral assemblages within the solid media. Contaminants so bound are particularly
amenable to containment strategies that isolate them from water and oxygen.
The WRD Area includes a large pile of mine waste that was placed on the shore of Clear Lake
between the Herman Impoundment and the lake. The WRD is not a conventional dam and does
not prevent the Herman Impoundment water from flowing through it as groundwater into Clear
Lake. Although the surface of the WRD Area was covered with clean material placed by EPA
under removal authority, its interior contains areas with high levels of mercury and areas that
produce acidic groundwater. The movement of Herman Impoundment water, as a groundwater
flow, through the WRD is the primary pathway for site-related transport of mercury into Clear
Lake.
The WRD is currently a principal threat wastes since it is an ongoing source for contamination to
Clearlake. However, once it is removed and capped it will no longer be a principal threat wastes
because it can be reliably contained and capped along with other wastes once it is removed from
the groundwater flow path from Herman Impoundment to Clearlake.
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2.5.2 Conceptual Site Model
The CSM for SBMM has evolved over time as new data has been collected, evaluated, and
interpreted in technical discussions with the Multi-Agency Environmental Team. This section
discusses the current OU-1 CSM. Numerous investigations have been conducted at the SBMM
Superfund Site by federal, state, and local agencies, and interested groups. These investigations
have characterized contaminant concentrations in mining waste, lake sediments, surface water,
and groundwater. These studies indicate that releases of hazardous substances containing
elevated concentrations of mercury, arsenic, antimony and thallium may pose risks to human
health.
CSM's are presented in Figure 2-1 to illustrate the sources, release mechanisms, and pathways
for contaminants entering the environment and resulting in exposure to human health.
2.5.3 Areas of Concern
OU-1 includes the 160-acre mine property and off-mine contamination in residential areas. The
mine property includes the inactive mercury mine, Herman Impoundment, the Northwest Pit,
approximately 2.25 million tons of mining waste in ore piles, mine tailings, and waste rock piles,
and associated contaminated soil, groundwater, and surface water. AOCs and features of OU-1
are shown on Figure 1-2. OU-1 is divided into the following AOCs.
• Off-mine residential soil within the EIC and residential areas along Sulphur Bank Mine
Road.
• Source areas on the mine site, including waste rock piles, ore piles, mine tailings,
disturbed rock, the Northwest Pit, areas of contaminant soils between waste piles and
remains of the historical mine processing buildings.
• The WRD Area, which is also a source area, but is considered separately because it is the
primary source of mercury flux to Clear Lake via groundwater.
• Herman Impoundment, the former mine pit filled with groundwater that flows through
the WRD Area to Clear Lake.
Off-Mine Residential Soil
As described above, the nature and extent of contamination in off-mine residential areas
described in the 2002 Remedial Investigation has been substantially reduced and modified as a
result of EPA Removal Actions to address unacceptable risk to residents.
Based on review of after-action reports and subsequent soil sampling, mine-impacted soil was
left in place at some residential areas. These areas require evaluation for further action as part of
Remedial Design (CH2M 2008c, 2009a, 2009b; ITSI 2011). Preliminary remedial goals (PRGs)
have been updated (lowered) since earlier removal actions and an additional COC was added
(Thallium). In addition, chemical screening of fill material placed during EPA removal actions
on the EIC suggests that this fill should be evaluated in comparison to revised cleanup levels for
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thallium. Therefore, additional remedial actions may be necessary for protection of human health
in these areas.
The off-mine residential soil is categorized into four areas within OU-1:
1. The EIC soil within the EIC residential area;
2. EIC soil in non-residential "common areas," where specific residential lots have not been
designated and are not anticipated for future residential development.
3. BIA 120 (Pomo Road), a paved road along the northern boundary of OU-1 that cuts
through the Northwest Waste Rock Pile;
4. The Sulphur Bank Mine Road soil in two sections along Sulphur Bank Mine Road: one
directly south of the West Waste Rock Pile, and one 1,000 ft farther west near the Clear
Lake lakeshore which includes residential lots.
These soil areas are shown on Figure 1-2.
Note that these areas do not include those parts of the El em Indian Colony's lands that will be
investigated, and if necessary remediated, as part of EPA's CERCLA activities in OU-4.
Off-mine residential areas with known impacted soil are described in more detail below:
EIC Soil within the EIC Residential Area
• The area southeast of the cul-de-sac in EIC contains mining waste left in place during
previous removal actions. Mining waste was found at the edge of the EIC boundary but is
expected to extend south and east into OU-1.
• Mining waste remains beneath the pavement of some sections of roadway within EIC.
• The area west of the EIC residences and near the shore of Clear Lake contains mining
waste that was not excavated due to its culturally significant location. Instead, geotextile
and 1 ft of clean topsoil covers the mining-impacted soil.
• The area at the western boundary of the EIC residential lots contains mining waste that
was not excavated due to its culturally significant location. Instead, this waste was
covered with 6 inches of clean topsoil and revegetated.
• Small areas within EIC have deposits of mercury and/or arsenic believed to be naturally-
occurring. Based on prior sampling concurrent with EPA removal work, the observed
concentrations exceeding screening levels were found deeper than 2 ft bgs in some areas
(E2, 2021).
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• No chemical data is available about the nature and extent of contamination within the
EIC cemetery. This culturally sensitive area has been excluded from past investigations at
the request of the EIC.
BIA 120 (Pomo Road)
• Mining waste remains beneath the pavement of BIA 120. The shoulders have at least 1 ft
of clean topsoil with the exception of the Northwest Waste Rock Pile, which abuts
BIA 120.
Sulphur Bank Mine Road
• The entire area south of the West Waste Rock Pile contains mining waste that was left in
place. Mining waste was previously excavated to 1.5 ft bgs or less in this area.
• The section near the Clear Lake lakeshore contains mining waste that was left in place.
Mining waste was excavated to 1.5 ft bgs or less in this area except for one small area,
where it was excavated to 3 ft bgs. The excavated areas were backfilled with clean
topsoil.
• Several additional areas are known to have at least limited amounts of mining waste
exceeding revised remedial goals, including the dock area, portions of several residential
lots, and portions of the Bradley parcel, however additional data is needed to inform
whether the COC exceedances are widespread enough to result in exceedance of remedial
goals for the appropriate decision unit.
Source Areas on Mine Site
The source areas on the mine site include waste rock piles, ore piles, tailings piles, disturbed
rock, undisturbed native rock, the Northwest Pit, and former mine facilities and buildings
described below.
Waste Rock Piles—Waste rock consists of minerals in concentrations considered too low to be
extracted at a profit during mining activities. The waste rock contains significant concentrations
of hazardous substances, such as mercury and arsenic. These waste rock piles represent potential
exposure pathways for nearby residents, including the EIC located just north of the mine. The
locations of the waste rock piles are shown on Figure 1-2.
The waste rock poses risk due to the potential for exposure of the public directly to the
contaminated soil through direct contact or fugitive windblown dust. The waste rock contains
sulfide minerals, which may release significant quantities of mercury and other hazardous
substances. Rainwater reacts with the waste rock to produce acid rock drainage (ARD) that flows
as surface water runoff or infiltrates into the underlying groundwater toward the Herman
Impoundment, Clear Lake, or the North Wetlands.
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• The Northwest Waste Rock Pile is the smallest of the four waste rock piles (21,000 cy),
but the material contains high mercury concentrations. It is located close to the EIC and
adjacent to and within the North Wetlands.
• The North Waste Rock Pile contains moderate contaminant concentrations, but the large
volume of this waste rock pile (878,000 cy) results in a total contaminant mass
significantly greater than the other waste rock piles.
• The South Waste Rock Pile material contains low contaminant concentrations, but the
relatively large volume (730,000 cy) of the waste pile results in a high total contaminant
mass.
• The West Waste Rock Pile is smaller in total volume (260,000 cy), but the material
within this pile contains higher contaminant concentrations.
• The Satellite Waste Rock Pile consists of 33,000 cy of waste rock. This waste rock pile
was created when the WRD was re-sloped and covered in 1992 and is located within the
disturbed rock area (Figure 1-2).
Ore Piles—Unprocessed ore was placed in the East and West Ore Piles, which cover 2.9 and
1.4 acres, respectively, and contain 28,000 and 6,500 cy of material, respectively (TtEMI 2002a).
Both ore piles are on the south side of the Herman Impoundment. The locations of the ore piles
are shown on Figure 2-1. The mercury ore is highly mineralized rock that contained
concentrations of mercury high enough to be processed at a profit at the time it was mined. The
ore piles contain the highest concentrations of mercury at the site. Similar to waste rock piles,
infiltrating rainwater oxidizes and reacts with the ore to produce metal-rich ARD. ARD waters
released from the Ore Piles travel either to the Herman Impoundment or to Clear Lake. These ore
piles also represent potential exposure pathways for nearby residents, including the EIC and
residents south of the mine.
Tailings Pile—Tailings are the materials left over after the process of recovering the mercury
from the ore. SBMM has one tailings pile that covers 13.6 acres on the south side of the Herman
Impoundment and contains 45,000 cy of tailings (TtEMI 2002a). The location of the tailings
piles is shown on Figure 2-1. Meteoric surface water and infiltrating waters react with the
tailings to generate ARD. The ARD generated from the tailings is neither as acidic nor as metal
rich as the ARD from waste rock because the sulfide minerals were oxidized during ore
processing; however, they are still net acid generators. Surface water and groundwater in the
tailings pile generally travel north into the Herman Impoundment but may also migrate west to
Clear Lake (TtEMI 2006).
Disturbed Rock—The disturbed rock consists of 18.8 acres of andesite, lake sediments, and
Franciscan Complex rocks north and east of the Herman Impoundment. The rocks were
excavated or terraced during mining (TtEMI 2002a). Much of the rock has been heavily altered
by hydrothermal fluids and is mineralized. The disturbed rock has high mercury concentrations
in solid-phase material. There is the potential for groundwater to leach metals from the disturbed
rock, but the extent of this potential impact is unknown. There is also the potential for oxidation
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and dissolution of sulfide and sulfate minerals in altered rocks next to the impoundment, where
there is contact with the Herman Impoundment water.
Undisturbed Native Rock—The native rock covers the remaining 69 acres in OU-1 that were
not physically affected by mining (e.g., excavation, terracing, or covering with waste material).
This includes the undisturbed slope of Franciscan Complex material south of the tailings, ore,
and waste rock piles and the Andesite north and east of the Herman Impoundment, and Alluvium
northeast and east of the mine site. Mercury and other hazardous substances occur naturally in
the groundwater (maximum ambient groundwater mercury is 1.5 micrograms per liter [|ig/L] and
arsenic is up to 30 |ig/L), solid-phase material, stormwater runoff, and atmospheric emissions in
the areas that were not disturbed by mining. Native rock contains contaminant levels that are
generally at lower levels than in the waste rock, ore, or tailings (TtEMI 2002a).
Northwest Pit—The Northwest Pit consists of a 150,000-cy oval-shaped pit that is the result of
past mining operations. The pit contains mining wastes disposed of by the mine operator in the
course of excavating and operating in the pit, including a stockpile of large andesite boulders.
The bottom of the pit is seasonally covered by a shallow pool of acidic water. There are also
several hydrothermal gas vents in this pit. Because the pit is a depression, surface water cannot
flow out, and any water that enters the pit either evaporates or infiltrates into the saturated zone
beneath the pit. Infiltration of water from the pit into the underlying groundwater is a concern
because the pit is located near a groundwater divide (exact location of the groundwater divide is
uncertain), and groundwater in this area may flow north toward the wetlands, northwest toward
the EIC, south toward the Herman Impoundment, or southwest toward Clear Lake. Water in the
Northwest Pit is of moderate quality for the site and contains little mercury, although it is acidic.
The RI concluded the Northwest Pit was a minor source of contamination and a significant
pathway for contaminants. The 1994 RI stated the Northwest Pit also posed a significant physical
hazard due to the steep walls and treacherous slopes surrounding the pit.
Mining Facilities/Mining Buildings—Numerous buildings remain, and debris associated with
the historical mining and processing operations exist on-mine at SBMM. EPA conducted a
cultural resource survey as part of the RI/FS process that documented these building structures
(EPA 1994c). EPA conducted a site visit in September 2016 to evaluate these areas as part of the
FFS (E2, 2021). Approximately 19 features have been recorded at the Site, including equipment
pads adjacent to the Herman Impoundment, an explosives storage shack, an ore processing area,
several vehicle maintenance garage foundations, structures and debris associated with a fertilizer
plant, and a concrete block plant. Most of the structures and machinery remains have been
stripped for usable wood and metal, many have burned, and none of the buildings are intact.
Historical documentation indicates that the mine operators routinely mixed mining waste with
cement to create cinder blocks and other construction materials. Best available evidences
suggests therefore that the concrete components of these structures would be largely composed
of mining waste. The presence of potentially hazardous waste in the vicinity of these structures
has not been evaluated. Sampling under the building footprints will be required as part of the
pre-design sampling conducted during the RD. Table 2-1 describes these features in detail. The
locations of these features are shown on Figure 2-1. In October 2017, the Sulphur Fire burned a
large area on and around the SBMM site. The mining facilities could have been damaged by the
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fire; therefore, the list in Table 2-1 is approximate and will be confirmed during preparation of
the RD.
Waste Rock Dam
The WRD consists of an estimated 1,000,000 cy of waste rock that were placed on top of in-
place rock and on the sediments of Clear Lake to form a large waste pile between the Herman
Impoundment and Clear Lake. This fill was placed in uneven thicknesses over the undulating
ground surface in this area. The dam has a total surface area of approximately 24 acres. In 1992,
the dam was re-contoured, graded (to a 4H: IV slope on the Clear Lake side), and covered with
an engineered revegetated soil cover consisting of a 2-ft thick layer of soil obtained from an off-
site borrow area as part of the WRD slope stabilization effort to reduce erosion (EPA 1994b).
The soil used in this soil cover had elevated levels of nickel (TtEMI 2006).
The chemical release scenario at the WRD Area is associated with surface water and
hydrothermal water in Herman Impoundment discharging to groundwater followed by seepage
through the WRD Area and into Clear Lake. The waste rock within the WRD contains reactive
sulfides that generate acidity, sulfate, and metals when contacted by oxygenated water. Several
other reaction mechanisms operate to further mobilize contaminants, most particularly mercury,
which is highly enriched in some locations. Because Herman Impoundment's water level sits
consistently above the level of Clear Lake, there is a persistent hydraulic head moving the site-
impacted waters within the WRD towards Clear Lake. The native lithologic units underlying the
WRD Area (Upper Lake Sediments, andesite, and Franciscan in some locations) contain elevated
levels of some site contaminants either due to natural mineralization or vertical migration of
contaminants. These materials are also subject to alternating wet and dry cycles that raise and
lower groundwater levels in the WRD and potentially enhance the oxidation and transport
processes. The seepage of site-impacted groundwater through the WRD Area into Clear Lake is
the primary flow path by which contaminants are being mobilized and transported from OU-1 to
Clear Lake (E2 2021).
Herman Impoundment
The Herman Impoundment, located in the center of SBMM, is an open pit that was excavated
during mining and is now filled with a mixture of meteoric and hydrothermal water. The
1,992,550-cy impoundment has an approximate surface area of 23 acres and acts as a hydrologic
sink for most of the mine site (TtEMI 2006). The surface water catchment area of the Herman
Impoundment, which includes the Green Pond watershed, is approximately 291 acres.
Stormwater runoff from approximately 70 acres flows directly to the Herman Impoundment, and
221 acres of the Green Pond watershed is being diverted around the Herman Impoundment by
the surface water diversion piping installed in 1999 (CH2M 2013), though the placement of the
discharge pipes for this diversion result in an unknown amount of stormwater infiltration, which
is presumed to flow into Herman Impoundment.
The Herman Impoundment is hydraulically upgradient of Clear Lake due to the elevation
difference between the two waterbodies. In general, there are four water sources to the Herman
Impoundment: (1) hydrothermal fluids (gases and liquids), (2) groundwater (shallow and deep),
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(3) surface water, and (4) direct precipitation. Meteoric water includes direct precipitation to the
Herman Impoundment, surface water runoff, and shallow groundwater potentially affected by
precipitation. Both groundwater and surface water sources coming into the Herman
Impoundment are currently impacted by ARD. Any precipitation that infiltrates through the
WRD will mix with the groundwater and discharge to Clear Lake as groundwater.
In 1999, EPA conducted an NTCRA to construct surface water diversion controls (IT
Corporation 1999) that included the following diversions:
• An overflow diversion system was constructed, consisting of two high density
polyethylene (HDPE) pipelines to convey Herman Impoundment water to Clear Lake
with minimal contact with waste rock if Herman Impoundment floods above its
maximum containment level.
• An HDPE pipeline was installed along the southern perimeter of Herman Impoundment
to divert water, reducing contact with exposed rock, and then draining into the flooded
mine pit (Figure 2-2) (TtEMI 2002a).
The water from Herman Impoundment flows through the WRD as groundwater, where it
encounters the waste rock and becomes acidic. Historically, pH values were about 3 but since the
Green Pond watershed has been diverted around Herman Impoundment, the pH of the Herman
Impoundment has been rising and as of the FFS had risen to a pH of 5 (2019-2021 Stormwater
and Groundwater Monitoring Report, E2 2021). The lower pH water leaches metals, in
particular mercury, from the waste rock materials before discharging to Clear Lake; however,
mercury concentrations are relatively low in Herman Impoundment water (less than 1 |ig/L).
Mercury in Herman Impoundment is currently below the EPA Safe Drinking Water Act
maximum contaminant level (MCL) of 2 |ig/L. In addition, the rise in pH has contributed to the
lowering of the solubility of aluminum. The concentration of aluminum has dropped from
81,000 |ig/L in 2000 to 2,000 |ig/L in 2017, and an estimated value of 6,100 |ig/L in 2020. The
water within Herman Impoundment is not suitable for domestic or municipal water use due to
the high total dissolved solids (TDS) of approximately 5,000 milligrams per liter (mg/L). The
water chemistry in the Herman Impoundment is expected to change over time as the remediation
of the source areas takes place.
An estimated 50,000 to 200,000 cy of sediment are present within the Herman Impoundment.
These mining wastes include sediment that formed through chemical interaction, material placed
in the Herman Impoundment by EPA during the WRD removal action in 1992, and erosion of
mining wastes from areas adjacent to the pit (TtEMI 2006).
2.5.4 Overview of the Site Geology
As designated by EPA, OU-1 includes the 160-acre mine property and the 2 nearby residential
areas of the Elem Indian Colony and the Southern Residences along Sulphur Bank Mine Road.
The mine property includes the inactive mercury mine, the Herman Impoundment, the WRD
Area, the Northwest Pit, approximately 2.25 million tons of mining waste in ore piles, mine
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tailings, and waste rock piles; and the associated contaminated soil, groundwater, and surface
water (E2 2021).
There are five primary stratigraphic and hydrologic units within the SBMM that are currently
being monitored and are listed below from deepest (oldest) to shallowest (youngest)
(Figure 2-2):
• Franciscan Formation (Jurassic-Tertiary)
• Lower Lake Sediments (LLS) (Quaternary)
• Andesite (Quaternary)
• Upper Lake Sediments (ULS) (Quaternary)
• Waste Rock (Anthropogenic).
Each of these units is present at the WRD Area. The lithological units of most concern for the
WRD Area are the waste rock unit and the ULS unit. For reference, a brief description of all the
lithologic units is provided below.
• The Franciscan Formation is composed of muddy sandstones and black shales and
extends across the SBMM site. Very little groundwater is encountered in this unit except
near large fractures; the unit does not appear to be connected hydraulically with overlying
water bearing units. Average hydraulic conductivity values calculated for wells screened
in the Franciscan Formation range from 1.26 x 10"3 to 1.46 x 10"2 centimeters per second
(cm/s) (TtEMI 2001b).
• The LLS unit is composed of massive conglomerates, breccias, and lenses of muddy
sandstone of the pre-andesite Quaternary sediments, and unconformably overlies the
Franciscan Formation. The average saturated thickness is 30 ft and does not appear to be
hydraulically connected with adjacent water bearing units. The LLS is exposed in the
Herman Impoundment, but it is not exposed in Clear Lake near the SBMM; therefore, it
is not expected that water from the site is transmitted to Clear Lake through the LLS. The
average hydraulic conductivity values for wells completed in the LLS range from
3.97xl0"5 to 5.87xl0"4 cm/s (TtEMI 2001b).
• The andesite unit is composed of the andesitic lava flow known locally as the andesite of
Sulphur Bank. The flow overlies the LLS and is overlain by post-andesite lake sediments
in the WRD Area. The thickness ranges from 80 to 110 ft in the WRD Area, with an
average saturated thickness in the WRD Area ranging from 55 ft (TtEMI 2002a) to 81 ft
(TtEMI 2001b). Vesicles were noted in some drill cuttings; however, they do not appear
to be connected. Some vesicles were filled with chalcedonic and opaline quartz, zeolites,
and gypsum (TtEMI 2002). The andesite has relatively low matrix hydraulic
conductivity, and groundwater flow is predominantly within fractures. The average
hydraulic conductivity values calculated for wells screened in the andesite unit range
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from 4.97xl0"4 to 2.29xl0"3 cm/s (TtEMI 2001b), and these relatively high hydraulic
conductivity values are attributed to fracture flow.
• The ULS unit is composed of the massive conglomerates, breccias, and lenses of muddy
sandstone of the pre-andesite Quaternary sediments. The unit thickness is 11-48 ft.
Although pump tests were not conducted specifically in the ULS and therefore unit-
specific hydraulic conductivity is unknown, hydraulic conductivity values are have been
estimated by tests that were conducted in the LLS (3.97xl0"5 to 5.87xl0"4 cm/s) (TtEMI
2001b).
• The waste rock unit is composed of clay, sand, and gravel of the post-andesite Quaternary
sediments above the ULS and Andesite. The waste rock is mineralized rock from the
Herman pit that was piled west of the pit during open-pit mining operations (Everhart
1946). The waste rock is between 18 and 74 ft thick. The saturated thickness of the unit
ranges from 0 ft to a maximum thickness between 15 and 21 ft and does not appear to be
hydraulically connected to the underlying units. The average hydraulic conductivity
values calculated from wells screened in the waste rock unit range from 3xl0"3 to
1.5xl0"2 cm/s (TtEMI 2001b).
The hydraulic gradient in the aforementioned units making up the WRD Area between the
Herman Impoundment and Clear Lake have been estimated to range from 0.005 to 0.010 based
on water level measurements. However, the gradient flattens from east to west, and groundwater
levels in the western third of the WRD are very close to the surface elevation of Clear Lake
(Figure 2-3). The mid-range order-of-magnitude flow rate through the WRD Area estimated by
various investigators is approximately 10-100 gallons per minute. The water budget for Herman
Impoundment was analyzed by CH2M and the DTSC in 2013 (DTSC 2013), and the upper
bound cited for historical estimates was 140 gallons per minute, measured by Trumbull in 1956
(CH2M 2013).
In general, the waste piles have steep side slopes at the natural angle of repose, or roughly 1:1.
The exception to this is the WRD, which was regraded to an approximately 20% slope as part of
earlier EPA removal actions to reduce erosion of mine waste into Clear Lake. Topographic
mapping of the site before mining began and as recently as 2016 is provided in the FFS (E2
2021).
The former SB MM is in an area of hydrothermal activity at the intersection of three main faults.
The hydrothermal fluids altered the rock and deposited mercury, sulfur, and alteration minerals
(Engle et al. 2007). Past mining activities at the mine property have resulted in disturbed,
contaminated areas that present a risk to human health and the environment (E2 2017).
Initial excavations that ultimately led to the current impoundment were begun in the 1870s with
open cuts and underground mining operations. The open cuts and surface mining continued,
along with intermittent subsurface excavations, until the end of World War 1. In 1927, more
systematic and extensive surface excavations were undertaken. The bulk of the mining
excavations between 1927 and 1944 were open pit, creating much of the current impoundment
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configuration. A final episode of excavation within the impoundment occurred in 1955-1957.
The mining excavations have subsequently filled with water, forming Herman Impoundment.
2.5.5 Contamination and Affected Media
Contaminants of Concern at OU-1
COCs were initially evaluated in the RI/FS and risk assessments for SBMM (EPA 1994). The
sitewide human health risk assessment (HHRA) re-evaluated the current human health risk from
exposure to COCs (APTIM Federal Services [APTIM] and E2 2020). The following COCs have
been identified for this OU-1 remedy:
• The COCs in on-mine soil are mercury, arsenic, antimony and thallium.
• The COCs in off-mine soil are mercury, arsenic, antimony, and thallium.
• The COC for groundwater flux from the site to Clear Lake is mercury.
Affected Media
On-mine surface soil—The sources of contamination to surface soil are the following: Satellite
Waste Rock Pile, North Waste Rock Pile, Northwest Waste Rock Pile, West Waste Rock Pile,
South Waste Rock Pile, East Ore Pile, West Ore Pile, Tailings Pile, and Disturbed Rock. The
CSM identifies dust from wind erosion, dust from ground disturbance, and off-gassing as the
release mechanism for mercury. The pathways for receptor exposure to mercury include wind
and direct contact.
Off-mine residential soil—The sources of contamination to residential soil are EIC roads and
soil under adjacent residential areas along BIA 120 and Sulphur Bank Mine Road. The CSM
identifies the release mechanism for mercury and the associated pathways for receptor exposure
to mercury to be eating plants that uptake mercury from soil and/or through direct contact to the
mercury laden dust from ground disturbance.
Groundwater—The sources of contamination to groundwater are the WRD Area, waste rock
piles, ore piles, tailings, and disturbed rock. The CSM identifies infiltration, percolation, and
regional groundwater flow as the release mechanisms for these contaminants. Infiltrating
rainwater leaching contaminants through the WRD Area, flow of groundwater from Herman
Impoundment, and/or flooding of the WRD Area due to annual increases in Clear Lake water
elevation may result in mobilization of labile constituents in the waste rock material and possibly
other mining-impacted rock (i.e., the ULS) into groundwater. Figure 2-3 shows typical isoploeths
of mercury concentrations in groundwater in the WRD Area and illustrates the large seasonal
variability in these concentrations during the year. The CSM identifies groundwater flow through
the WRD Area, groundwater flow to the Herman Impoundment, ARD from waste rock piles and
ore piles, and groundwater flow through residual mineralization as groundwater pathways. The
groundwater pathways interact with each other to ultimately discharge into Clear Lake.
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Hydrothermal groundwater at SBMM has naturally occurring TDS above 3,000 mg/L and is,
therefore, potentially eligible for de-designation of the default drinking water beneficial use
assumption under the Regional Basin Plan. Groundwater flow into Clear Lake surface water is
the ultimate pathway for exposure to mercury receptors.
Seven general water types have been identified at SBMM that are distinguished by major ion
ratios, chloride and sulfate concentrations, boron-chloride ratios, and pH. The general water
types were defined in the RI (TtEMI 2002a). The description of each water type is as follows:
• Ambient groundwater—Ambient groundwater (sometimes referred to as "background
water" in previous investigations) occurs in the andesite, LLS, and Franciscan Complex
water-bearing units. Isotopic signatures are similar to that of meteoric water. It is mildly
acidic (pH 5-6.5) (TtEMI 2001a). It is low in TDS and chloride (TtEMI 2002). Ambient
groundwater contains low chloride and boron, low to moderate sulfate, moderate to high
bicarbonate, low to moderate sodium, moderate calcium, and low magnesium
concentrations (TtEMI 2001a). However, there are variations in minor and trace element
chemistry that are attributable to the natural reaction of groundwater with different
geologic units. Based on the low boron to chloride ratio, ambient groundwater does not
contain a significant hydrothermal component.
• Hydrothermal water—The active hydrothermal system at SBMM produced the sulfur
and mercury deposits where hydrothermal fluids rose (and continue to rise) along faults
and fractures in the area. Based on groundwater monitoring well data, hydrothermal
groundwater is observed in the deep andesite, LLS, and Franciscan Complex formations
near the WRD Area and along fractures north of Herman Impoundment. Hydrothermal
water at SBMM has distinctive, isotopically heavy characteristics (TtEMI 2001b). It has
high boron and high chloride concentrations. As explained in several studies (TtEMI
2001b; Engle et al. 2007; Burleson 2020), a unique feature of the hydrothermal system at
SBMM is the boron-to-chloride ratio of almost one. The SBMM hydrothermal system
contains almost equal concentrations of boron and chloride exceeding 800 mg/L. The
linear relationship between boron and chloride is a useful tool for assessing the amount of
hydrothermal water present in SBMM groundwater at a given location.
• Meteoric water—Meteoric water is from precipitation and is very dilute until it infiltrates
into the ground. For the purposes of this ROD, meteoric water is defined as water that has
an oxygen isotope, hydrogen isotope, and deuterium signatures indicating it has not been
evaporated (TtEMI 2001b). However, the isotopic signature evolves along the local
meteoric water evaporation trend when it collects at the surface and evaporates in Clear
Lake and other impoundments.
• ARD-influenced groundwater— Mine-impacted water/ARD forms from the interaction
of infiltrating rainwater with sulfide minerals in waste rock, tailings, ore, and native
materials. This water commonly results in the precipitation of acid sulfate minerals that
may also generate acidity by reaction with meteoric water and groundwater. Oxidation of
sulfur and sulfide minerals in the waste rock, ore, and tailings results in groundwater that
contains sulfuric acid and has low pH (TtEMI 2001a; White and Roberson 1962).
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• WRD Water—Groundwater in the WRD Area is a mixture of:
— Mine-impacted meteoric water from infiltration of precipitation through waste rock.
— Herman Impoundment water. As described above, Herman Impoundment water is a
mixture of hydrothermal water and meteoric water with an isotope ratio characteristic
of fractionation by evaporation (Burleson 2020).
— Hydrothermal water.
Meteoric water infiltrates directly into the surface of the WRD Area and intrudes from west due
to seasonal rises in Clear Lake water levels. Infiltrating meteoric water and other water types
react with minerals in the mine waste to create acid drainage that comingles with groundwater in
the WRD Area. This infiltrating mine-impacted meteoric water has an elevated redox potential
(Eh), low pH, and transports metals and sulfate to the groundwater. Cinnabar that is present in
the WRD Area is not stable at high Eh and low pH levels, especially in the presence of elevated
chloride. Therefore, mercury is mobilized in groundwater and other solid mercury minerals may
form. Additionally, hydrothermal water upwelling from below the WRD Area via fractures
combines with Herman Impoundment water and the mine-impacted meteoric water.
• Clear Lake water—Based on oxygen and hydrogen isotope studies, the water in Clear
Lake is primarily evaporated meteoric water (i.e., direct precipitation and surface water
runoff) (TtEMI 2001b). Clear Lake receives groundwater inflow from a number of areas,
including from SBMM (e.g., groundwater flowing through the WRD Area and other
surrounding catchments). There is also hydrothermal water that enters Clear Lake
through the fault that runs beneath Herman Impoundment and continues into Clear Lake.
Based on major ion ratios, the OU-1 RI (TtEMI 2002a) concluded that the influence of ARD on
groundwater quality is limited to affecting shallow groundwater in waste rock, tailings, native
materials, adjacent lake sediments south and west of Herman Impoundment (including the WRD
water-bearing unit and Clear Lake), and in the andesite north of Herman Impoundment
(northwest of the Northwest Pit and near the North Wetlands). ARD has not generally affected
deeper water-bearing units, including the LLS or Franciscan Complex, and has not affected
regional groundwater-bearing units in the vicinity of SBMM (TtEMI 2002a).
Stormwater Runoff—The sources of mercury contamination to stormwater runoff are the
WRD, waste rock piles, ore piles, tailings, and disturbed rock. The CSM identifies stormwater
runoff as one of the release mechanisms for mercury. Stormwater runoff flows into Clear Lake,
Herman Impoundment, or infiltrates into the groundwater. Stormwater is a significant source of
water to Herman Impoundment and, therefore, is also a contributor of flow from Herman
Impoundment via groundwater to Clear Lake. Reducing stormwater flow into Herman
Impoundment can reduce the amount of groundwater outflow, and thereby reduce the flux of
contamination to Clear Lake.
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2.5.6 Fate and Transport of Contaminants
This section discusses the cross-media pathways for transport of contaminants from OU-1 to the
environment and the COCs for OU-1. In addition, this section discusses the transport of
contaminants from OU-1 to OU-2 and OU-4. Identification and evaluation of the interactions
between OU-1, OU-2, and OU-4 are necessary because an effective remedy for OU-1 should
subsequently reduce the migration of contamination to OU-2 and OU-4.
The source areas within OU-1, including the waste rock, ore piles, mine tailings, and disturbed
rock, pose unacceptable risks to human health and the environment from direct contact or
ingestion of on-mine surface soils, inhalation and ingestion of gases or dust-containing airborne
contaminants, and exposure to chemicals through transport mechanisms to higher trophic level
consumers. The RI quantifies and evaluates mercury discharge rates through the groundwater
pathway, the mobility of mercury from mining wastes, atmospheric emissions of mercury from
mining wastes and native materials at the site, and associated risks to human health and the
environment (TtEMI 2002a).
Infiltrating rainwater oxidizes and reacts with the waste rock, ore, disturbed rock, and tailings to
produce metals-rich ARD that gets transported as surface water runoff or infiltrates into the
underlying groundwater. ARD can form naturally at slow rates in undisturbed systems and is not
limited to mining sites; however, in areas where mining has exposed mineralized materials to the
atmosphere (such as at the SBMM site), the formation of ARD is accelerated. In addition to
dissolved oxygen in infiltrating water, oxidation of sulfides in waste materials can occur through
atmospheric contact for unsaturated or partially wetted materials. Waste materials that are
alternately wetted and dried are more susceptible to oxidation.
ARD is produced by the oxidation of sulfide minerals, primarily iron pyrite (FeS2), which reacts
with water and oxygen to form sulfuric acid and dissolved ferrous iron (Fe2+). This reaction is
aided by certain bacteria and other microorganisms. Ferrous iron can oxidize further to form
ferric iron (Fe3+), which can accelerate the oxidation of additional iron pyrite to ferrous iron and
produce additional sulfuric acid. The acidic (low pH) water produced by these chemical
processes dissolves minerals from exposed rock, which produces an acidic discharge that has
relatively high concentrations of heavy metals.
The following describes the transport of ARD via groundwater and surface water for the source
areas, WRD, and Herman Impoundment:
Source Areas
• ARD released from the ore piles travels either as surface water runoff or infiltrates to
groundwater to Herman Impoundment or directly to Clear Lake (OU-2).
• Surface water and groundwater from the tailings generally travel north into Herman
Impoundment but may also be transported west to Clear Lake (OU-2).
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• ARD impacted stormwater runoff and groundwater from the various waste rock piles
flows toward Herman Impoundment, Clear Lake (OU-2), or the North Wetlands (OU-4),
as follows for each waste rock pile.
— ARD impacted runoff and groundwater from the Northwest Waste Rock Pile flows
toward the North Wetlands (OU-4).
— ARD impacted runoff and groundwater discharged from the South Waste Rock Pile
generally flow toward Herman Impoundment.
— ARD impacted runoff and groundwater from the West Waste Rock Pile flow toward
Herman Impoundment with some flow toward Clear Lake (OU-2).
• Surface water from the Northwest Pit cannot flow outward because the pit is a
depression. However, groundwater from beneath the pit flows north towards the
wetlands, northwest toward the EIC, or west towards Clear Lake.
• Discharges from disturbed and native rock generally flow into Herman Impoundment.
• Groundwater and surface water that interact with the source areas flow into Herman
Impoundment from the north, east, and south. Water collected within Herman
Impoundment seeps through or flows around the earthen dam at the west side or flows
directly into the WRD towards Clear Lake.
Waste Rock Dam
• Groundwater in the WRD flows from Herman Impoundment towards Clear Lake.
• ARD is generated when rainfall infiltrates through WRD material or hydrothermal
groundwater discharges into the WRD. This impacted water migrates downward through
WRD material and subsequently contaminates the underlying groundwater.
• ARD contributions in this pathway could be a primary cause for acidic water observed in
wells installed in the WRD (including those installed across waste and within underlying
formation rock).
• ARD impacted groundwater (through rainwater infiltration) in WRD and Herman
Impoundment water that enters groundwater and migrates west toward Clear Lake may
be causes of acidic water observed in some WRD wells. The cause of acidic groundwater
will be further refined during the pre-RD investigations.
• There is a groundwater divide between the Herman Impoundment and the North
Wetlands beneath the disturbed rock area (exact location of the divide is uncertain).
Groundwater south of the divide flows south toward Herman Impoundment, and
groundwater north of the divide flows north toward the North Wetlands.
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Herman Impoundment
Although there are several mechanisms for contaminant transport at the Site, water transport
from Herman Impoundment is considered to be the most prominent pathway for mercury and
other hazardous chemicals migrating from the mine site into Clear Lake. The water within
Herman Impoundment does not contain significantly elevated concentrations of mercury relative
to concentrations observed within the WRD; however, as it flows as groundwater to discharge
into Clear Lake, it passes through waste rock and becomes acidic and mobilizes mercury.
• Herman Impoundment contains a mixture of meteoric and hydrothermal waters. Meteoric
water discharges into Herman Impoundment by direct precipitation, surface runoff, and
infiltration of shallow groundwater. Waste piles contain materials that, under oxidizing
conditions, release mercury and other contaminants into groundwater and stormwater
runoff. Depending on the location of a particular waste rock pile, a portion of the shallow
groundwater or surface water runoff flows toward Herman Impoundment. In some cases,
these groundwater and surface water runoff flows are impacted by ARD.
• SBMM is at the intersection of a northwest-trending fault zone and a northeast-trending
fault zone. The most significant fault zone in the area is the South Oaks Arm Fault, the
northeast-trending fault zone extending along the axis of Herman Impoundment. The
faults, and especially the fault intersections, are the conduits through which mineralized
hydrothermal fluids rise to the surface to deposit minerals and cause hydrothermal
alteration of the bedrock. Upwelling fluids from the Sulphur Bank geothermal system
deposited various minerals, including native cinnabar, marcasite, metacinnabar, pyrite,
quartz, stibnite, and sulfur. Barite, calcite, and dolomite have also been identified, but
they are rare. Hydrothermal water discharges directly into Herman Impoundment through
submerged vents and indirectly through hydrothermal groundwater in deep water-bearing
units that flow toward Clear Lake. Also, hot gas, vapors, and sulfurous fumes escape
from the fault system and bubble up through water in Herman Impoundment.
Transport to Clear Lake
There are three pathways for discharge of site-related contaminants to Clear Lake (OU-2). These
transport pathways are discussed below.
• Transport from the WRD via groundwater flowing from Herman Impoundment to
Clear Lake—Water from the Herman Impoundment flows as groundwater to discharge
into Clear Lake. Groundwater flows from Herman Impoundment through the WRD to
Clear Lake. This is the primary pathway by which contaminants are transported from
OU-1 to Clear Lake. In the past, Herman Impoundment water was acidic (pH
approximately 3.5), but in recent years has become more neutral at a pH above 5. As this
groundwater flows through the WRD it becomes acidic and leaches metals, in particular
mercury, from the waste rock material. This water also mixes with infiltrating meteoric
waters before discharging to Clear Lake. The Herman Impoundment system (because of
its interaction with the WRD) is the most significant source and pathway for aqueous
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mercury and other dissolved contaminants migrating from OU-1 to Clear Lake. This is
due to high concentrations of dissolved mercury leached from the WRD into groundwater
flowing to Clear Lake. An evaluation to better constrain estimates of the flux of mercury
to Clear Lake via this pathway is currently underway.
• Groundwater to Clear Lake—Groundwater flows from various ore piles, the WRD,
and waste rock piles towards Herman Impoundment to Clear Lake, or directly to Clear
Lake, as described above.
• Stormwater Runoff to Clear Lake—Stormwater from various ore piles and waste rock
piles flows towards Clear Lake or the North Wetlands as described in the previous
section. Figure 2-4 shows stormwater catchments within OU-1. In the past, stormwater
runoff from the WRD flowed directly into Clear Lake; however, the 1992 WRD slope
stabilization effort re-sloped and covered the dam, which reduced contaminant migration
to Clear Lake through stormwater runoff (EPA 1994b).
EPA is currently evaluating the contamination in OU-4, with RI sampling occurring in the
summer of 2023. However, as previously mentioned, OU-1 contains most of the contaminant
mass and influences contamination in OU-4 via fugitive dust emissions, surface stormwater
runoff, and groundwater migration. These transport pathways might have additional impacts on
OU-4 while remediation takes place.
2.5.7 Receptor Groups
The future land uses guide the identification of the receptors and exposure scenarios to be
analyzed in the HHRA. The residents or land users are organized by receptor groups for the
HHRA based on common activities. The receptor groups for the HHRA are:
• Traditional Tribal Receptors— Indigenous peoples have lived on the shore of Clear
Lake for thousands of years. Today many tribes continue to depend upon the lake's
resources for sustenance and cultural practice. The Elem Indian Colony Band of Pomo
sits immediate adjacent to the Sulphur Bank Mine. The tribe's federal trust lands, also
referred to as the Elem Indian Colony, are approximately 50 acres and are bounded to the
north and west by the Oaks Arm of Clear Lake, to the south and southeast by the SBMM,
and to the northeast by the North Wetland. The EIC is tribal land with residential housing
for approximately 50 tribal members (Takata, 2006). The Traditional Tribal Receptor
group is represented by the residents of the Elem Indian Colony under an assumption that
the represented individuals live a fully subsistence lifestyle according to the lifeways and
consumption practices described in the Traditional Tribal Subsistence Exposure Scenario
and Risk Assessment Guidance Manual for the Elem Pomo (Harper, et al 2007). This
exposure scenario is conservatively assumed to entail that all traditional practices for a
member of the EIC, including eating fish, waterfowl, and local plants, are sourced from
on-site and that the sole source for drinking water is directly from Clear Lake, thus all
natural resources depended upon are assumed to be site-affected (Figure 2-1).
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• Lakeside Resident Receptors—Residents live primarily in the areas designated as
lakeside communities. Residents were evaluated for their exposure to soil in and around
their homes. SBMM roads and properties that were included in the removal action lie
within areas that were potentially disturbed during the period of active mining
(i.e., before 1957) or by the subsequent placement of mine waste used as fill material
(Figure 2-1). Several lakeside residents receive their household water from a water
supply system with intakes in Clear Lake. However, these residents have reported that
they do not use this water as their drinking water source.
• Recreational Receptors - This receptor group entails those who may swim, wade, or
fish in Clear Lake for recreation. This receptor group is only exposed to site-related
contaminants once they have reached Operable Unit 2 (Clear Lake) and therefore are
affected by OU-1 only insofar as it is the source of contamination present within OU-2.
• Trespasser Receptors - Though the Site is privately held and signage and fencing are
intended to deter entry, trespassers occasionally enter onto the Site for a range of reasons
including driving off-road vehicles on waste piles, target practice, other unknown
recreation and, according to anecdotal accounts, swimming in Herman Impoundment.
These receptors may be subject to inhalation of site-impacted dust, mercury and
hydrogen sulfide vapors; incidental ingestion of contaminated soils; skin contact with
mine waste; and exposure to physical hazards.
• Unauthorized Tribal Users Receptors— As the Site is immediately adjacent to the
El em Indian Colony and lies within the tribe's ancestral territory, tribal users are
understood to occasionally enter onto the mine site to hunt, forage, gather wood, or
recreate. As with the trespasser receptors, this scenario includes occasional swimming in
Herman Impoundment, though this is not known to routinely occur.
2.5.8 Remedial Investigation and Subsequent Studies
The RI was performed from October 1999 through August 2001, and included:
• A hydrogeologic characterization of SBMM (October to December 1999).
• Collection and analysis of water samples from monitoring wells and surface water at
SBMM (May and December 2000 through May 2001).
• Mining related rock sample collection and analysis (September through December 2000).
• Monitoring and characterization of stormwater runoff (October 2000 through April
2001).
• Sampling and modeling to quantify the atmospheric mercury emissions from geologic
and mine materials (May and June 2001).
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• Monthly measurement of groundwater elevations and the elevation of Herman
Impoundment (January 2000 through July 2001).
The RI report was published in 2002 (TtEMI 2002). Results from the RI were used in the
development of an FS for OU-1, which was published in 2006 (TtEMI 2006). These data were
further updated, combined with subsequent sampling results and used to inform the Focused FS
published in 2021.
Due to the complex hydrogeology and geochemistry present at the Site, EPA has performed
extensive additional sampling and evaluation of site conditions that inform this remedy decision.
A complete list of investigations and reports is available in the administrative record for the Site.
Several key areas of study are outlined below.
2.5.9 Groundwater Monitoring
An annual program of routine water quality sampling and water level measurements has been
conducted to monitor water chemistry changes and provide data regarding the presence and
migration of mercury and other metals in groundwater and surface water at SBMM. Annual
reports are available for the monitoring periods from 2007 through 2020. Maximum
concentrations of total and dissolved mercury at each groundwater location are presented in the
FFS (E2 2021).
The highest groundwater elevations typically occur in spring and decline through summer, with
the lowest elevations in late fall. Clear Lake and Herman Impoundment surface water elevations
had trends similar to the measured groundwater elevation (highest in mid-spring and lowest in
late fall). The general trends in the groundwater conditions are:
• Concentrations of mercury and other COCs are much higher in wells screened in waste
rock.
• Elevated COCs and low pH are frequently associated with wells screened in waste rock,
occasionally in wells screened in Andesite, and rarely in wells screened in the Franciscan
formation.
• The water level elevation between Herman Impoundment and Clear Lake ranges between
8 and 14 ft, with Herman Impoundment being higher.
• Monitoring of the groundwater each year is dependent on the groundwater elevations,
and many shallow wells have been dry in recent years due to drought conditions.
The mercury mass flux from groundwater discharging into Clear Lake was estimated by both a
Darcy's Law calculation method and a site 3-D numerical groundwater flow model. Based on
these evaluations, the mercury mass flux is generally understood to exceed the limit of 0.5
kilograms per year (kg/year) (1.1 pounds per year), often by an order of magnitude. The mercury
mass flux limit is based on the total maximum daily load (TMDL) established in the RWQCB
Water Quality Control Plan (Basin Plan) (RWQCB 2016).
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2.5.10 Stormwater Monitoring
Quarterly stormwater monitoring has been performed at SBMM. Stormwater samples are
scheduled to be collected one time each quarter during the rainy season, when possible, in
locations where significant runoff is observed. Annual reports are available for the monitoring
periods 2010 through 2019. Maximum concentrations of total and dissolved mercury at each
stormwater location are presented in the FFS (E2 2021).
Stormwater runoff from the mining waste-impacted areas is consistently acidic and generally has
elevated metal concentrations, especially mercury, aluminum, and zinc. Stormwater runoff in
2018-2019 also had elevated nickel, cadmium, arsenic, copper, iron, and manganese
concentrations.
2.5.10.1 Historic Properties and Tribal-Cultural Resources
The SBMM site is in a region of significant archaeological and tribal-cultural importance. For
this reason, evaluation of the Site has included survey for historic properties and tribal-cultural
resources potentially warranting protections under the National Historic Preservation Act. Such
surveys have occurred several times over the Site's history on the NPL. The first such Cultural
Resources Survey for the Site was published in April 1994 and was based upon literature reviews
and archaeological field reconnaissance. This survey also provided preliminary evaluation of the
potential effects of remedial actions being considered for the site.
In 2007, a cultural resource inspection was published that provides the results of a partial cultural
resources inspection of the EIC lands and a review of a small area within the SBMM property.
This inspection occurred after Phase 1A of the NTCRA cleanup project and before Phase IB.
This inspection was done to determine which areas within the Colony were culturally sensitive
and which areas were relatively free of historic or prehistoric cultural materials. This information
was used to update the existing archaeological site records housed with the California Historical
Resources Information System (Holson 2008).
A cultural resources study was published in 2011 describing an archaeological survey for the
SBMM Project. The goals of this study were to: (1) identify all historic resources within the
project area; (2) offer a preliminary evaluation of the significance of the identified resources;
(3) determine resource vulnerability to adverse impacts that could arise from project activities;
and (4) offer recommendations designed to protect historic resource values, as warranted (Elliot
2011).
EPA initiated a sitewide cultural and archaeological resources study in 2023, which is ongoing.
The following paragraphs provide a summary of the findings to date and the activities to be
conducted prior to implementation of the OU-1 remedy. To date, EPA has completed an Initial
Phase 1 evaluation of cultural resources, consisting of an archaeological pedestrian survey of the
non-tribal lands portion of the Area of Potential Effect (APE) for the proposed SBMM
Remediation Project. The initial fieldwork included:
• Identifying cultural resources on the surface of the APE that may qualify as historic
properties under Section 106 of the NHPA, as amended.
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• Assessing the subsurface depositional context within the APE to identify areas of low,
moderate, and moderate to high sensitivity for buried archaeological deposits.
• Identifying data gaps in the historic property identification effort.
As part of this initial field effort, a records search of the APE was conducted to identify
previously recorded pre-contact or historic period cultural resources within 1 mile of the APE.
Following the records search, an intensive archaeological pedestrian survey was conducted. A
Native American monitor representing the Elem Indians accompanied the archeological team
during the survey. The approximately 759-acre project APE was subdivided into several areas
based upon proposed inventory methods. These areas and inventory approaches are described in
the Field Closure Report (PaleoWest, LLC 2023).
Fifty-seven cultural resources were encountered and recorded during the Initial Phase I
investigation. These include 31 previously recorded and 26 newly identified cultural resources.
Of the 26 new cultural resources encountered and recorded during the field survey, 13 are
archaeological sites, 4 are linear resources, and 9 are isolated artifacts. Isolated artifacts include
flaked stone artifacts and a historic glass bottle.
Overall, most of the mine site area has low to no sensitivity for buried prehistoric deposits. These
areas are not alluvial and are underlain by near surface or exposed bedrock, may be sloping, and
have soils developed from relatively thin residuum. A total of 10.1 percent of the project area has
highly sensitive deposits for buried sites, however.
Several data gaps were identified upon completion of the Initial Phase I work that affect the
reliability of determination and are described in the Field Closure Report (PaleoWest, LLC
2023). To overcome data gaps and improve the reliability of determination related to the
horizontal and vertical limits of cultural resources within the APE, additional Phase I work is
needed during pre-design or Remedial Design phases of work at the site.
Features, midden deposits, and artifact/feature-bearing paleosols that may be impacted by the
proposed remediation work may be subject to Phase II testing during the Remedial Design
investigation.
2.5.11 Human Health Risk Assessment Sampling
Additional sampling to support the sitewide HHRA was conducted in June through December
2018. The goal of the HHRA was to evaluate current site and background conditions, further
evaluate the tribal exposure scenarios, and estimate risk from exposure to metals (APTIM and E2
2020). Samples collected and analyzed by EPA as part of this event included:
• Plants—three species (Tules, Cattails, Acorns)
• Collocated sediment samples associated with the plant species (Tules and Cattails)
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• Collocated soil samples associated with the plant species (Acorns)
• Ambient sediment samples
• Surface water samples associated with ambient sediment samples
• Fish tissue—100 samples of various species within Clear Lake.
EPA had planned to collect aquatic bird tissue samples (American coot) from Clear Lake in
October 2019. However due to a lack of birds at the time of sampling, bird tissue samples were
not collected. Sediment samples were collected from the proposed bird tissue collection areas
during the 2018 sampling event. An additional sampling event to collect the bird tissue samples
will be planned as part of the Ecological Risk Assessment (ERA) update. The Final HHRA
Sampling Program Technical Memorandum (APTIM and E2 2020) provides the details of
sample collection activities and the analytical data performed in support of the HHRA to address
data gaps.
2.5.12 Pre-Mining Baseline (PMB) Conditions Surface Soil Sampling
In conjunction with the above-described HHRA sampling effort, the 2018 sampling program and
subsequent data evaluation included sampling and evaluation aimed at refining the Agency's
understanding of background, or "Pre-Mining Baseline" conditions at the site.
The U.S. Environmental Protection Agency (EPA) Guidance for Determination of Background
(EPA, 2002) states that the hazardous substances at any Superfund site may originate from either
on-site (i.e., resulting from releases attributable to site-specific activities) or off-site (i.e.,
resulting from sources not on-site). These off-site substances may result either from natural
sources (e.g., erosion of naturally occurring mineral deposits) or anthropogenic sources that are
not related to the SBMM mining activities (e.g., widespread lead contamination from auto-
mobile exhaust in urban areas). To determine the appropriate action to take at a site, EPA must
distinguish between substances directly attributable to the site and those attributable to natural
background concentrations.
The Pre-Mining Baseline is an estimation of background but was not called background because
the term "background" is commonly considered a reference to conditions without mining or
other anthropogenic activities. A rational goal for environmental restoration of new, active, or
inactive mine sites could be the environmental conditions that existed before any mining
activities or other related anthropogenic activities. There is no place on earth without chemical
disturbance from anthropogenic activities. Hence, the terms 'baseline' and 'pre-mining' are
preferred to describe these conditions, rather than background. The term 'pre-mining baseline'
was used by the EIC Tribe technical consultants (AESE, Inc.) and adopted as the term for the
2018 sampling and the subsequent data interpretation.
The estimation of PMB conditions relies upon the selection of sampling locations which are
beyond the direct influence of the mine operations, yet still represent the site geology. The
SBMM lies within a naturally mineralized area created by the intersection of multiple faults,
with several stratigraphic units interacting via the movement of hydrothermal fluids and gases.
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The geologic conditions that led to the formation of the ore body that was the target of the
mining operations at the site are uncommon and where analogs do exist they are often subject to
significant anthropogenic influence, including mining activity unrelated to the SBMM.
Estimating the PMB conditions after the mine has operated presents challenges, not only because
of mining-related releases, but also because areas distant from the mine have been influenced by
various human-made changes such as roads, agricultural practices, air pollution, etc that may be
difficult to control for. For these reasons, in evaluating the PMB conditions at the Site, no
representative off-Site analog site could be identified.
Multiple sampling events on the lands surrounding the SBMM have included background
samples and the data set used to calculate PMB conditions incorporated results that were
reviewed for comparability in data collection, distance from mine waste, and statistical testing
for outliers. Statistical analyses were conducted using ProUCL software in collaboration with the
project Stakeholders, and followed accepted practices for identification, investigation (review of
sampling and analysis documentation), and documentation of outliers. Calculations were
performed with and without removal of outliers and documented in the PMB technical
memorandum appended to the FFS (APTIM & E2, 2021).
Summary of Background Sampling Events
• 1994 Remedial Investigation (RI): 20 samples were collected in a grid pattern on the EIC
and used as the background data set for the RI and for the Baseline Public Health
Evaluation (EPA, 1994). These samples were not considered as background in the 2020
HHRA as it has since been determined that the EIC has been contaminated by the mine
site.
• 2002 RI/Feasibility Study (FS): 28 samples were collected in the native rock system and
analyzed for antimony, arsenic, and mercury. Eight surface samples were collected from
the Franciscan Complex, southeast of the South Waste Rock Pile; 20 surface samples
were collected from native andesite, north of Herman Impoundment and west of the
North Waste Rock Pile (TTEMI, 2002). RI concluded that the native rock system
represents current background site conditions for mineralized soil, to which wastes, and
disturbed soil and rock results can be compared. This background sampling was later
found to be inadequate because most samples were collected from within the mine site
and were potentially impacted by site contamination. For this reason, these data were not
used in the PMB calculation.
• 2009 Soil Investigation: Eleven samples were collected to investigate background
concentrations in non-mineralized surface soil in near-mine site locations overlaying
post-andesite Quaternary alluvium (five sample locations), the Andesite Unit (five
sample locations), and Franciscan Complex rocks (six sample locations) in areas that
have not been disturbed by mining practices (on Parcels 01000257, 01000258, 01000232,
and 01000233) (CH2M Hill, 2009). The results from this background sampling were only
used as appropriate to determine extent of mine waste impacts. Background values were
not calculated as part of this investigation; however, these samples were considered in the
PMB dataset.
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• 2013 EIC Phase II Study: A Phase II Environmental Site Assessment was performed by
McGinnis and Associates LLC for the Bureau of Indian Affairs-Pacific Regional Office.
The study included collection of 93 soil samples and analysis for mercury, arsenic, and
antimony. The study included a number of samples that were distant from the mine site
and did not indicate the presence of mine waste (McGinnis and Associates LLC, 2013).
Seventeen of the 93 samples were included in the PMB dataset after reviewing their
locations and results.
• 2018 PMB Soil Sampling Event: To support the HHRA, 20 surface soil samples were
collected in the Andesite area and 40 were collected in the Franciscan area. The sampling
locations were documented in the HHRA Sampling and Analysis Plan and were based on
analysis of the results of prior background sampling.
Use of the Pre-Mining Baseline Results
The PMB results were used for two purposes as described below:
• To calculate the background exposure point concentration (EPC) and background
carcinogenic risk and non-carcinogenic hazard in the HHRA.
• To inform the development of Potential Remedial Goals where risk-based target
concentrations might be below the estimate of background.
Rationale for Using Pre-Mining Baseline as an Estimate of Background
The results of the PMB calculations are a reasonable estimate of background for the purposes of
the FFS for the following reasons:
• The possible mining influences were minimized by selecting samples as far from the
mine as possible while still being within the Andesite and Franciscan parent materials
footprint.
• Statistical high outliers were identified using ProUCL and researched to determine if they
should be in the background data set. If the location of the high outlier was near a
historical road or off-site construction, which may have used mine waste as construction
fill, or near a road that could have been used for hauling mine material, then it was
removed from the background data set. Comparability between the prior background
results and the 2018 results was assessed by evaluating the methods of collection,
preparation and analysis. All the results were collected within the requirements of Quality
Assurance Project Plan for the site. The methods were determined to be comparable for
samples analyzed in the laboratory. X-ray fluorescence samples were not used in the
calculation of background due to high detection limits and a high bias on the results.
• The PMB calculations included a range of data sets to allow comparison of the new data
(2018) to the prior data, and the compilation of new and prior data. The prior data sets
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were tested for outliers based on sample locations and ProUCL output (Q-Q and box
plots) when planning the new data collection.
Pre-Mining Baseline values for mercury, arsenic and antimony are provided below. These values
are based upon the 95% Upper Threshold Limit statistic for which 95% of the naturally occuring
soil concentrations in the data set would fall below this value. Other statistic methods for
establishing background concentrations exist, however UTL is a common method used by EPA
in remedial work in order to avoid remediating areas where site-related contamination may not
be present.
(ioolo^iciil l-'oi'iiiiilion
COC
I'.sliniiilod PMIi C onceii(r;i(ions
Mercury
35 mg/kg
Andesite Formation
Arsenic
22 mg/kg
Antimony
7.1 mg/kg
Mercury
98 mg/kg
Franciscan Complex
Arsenic
18 mg/kg
Antimony
5.0 mg/kg
PMB conditions were only estimated for surface soil. PMB concentrations for plant and bird
tissue representative of the SBMM Site could not be determined because the plants and birds
have been living in the post-mining environment for decades.
2.6 CURRENT AND POTENTIAL FUTURE SITE AND RESOURCE USES
2.6.1 Current Land Use
The former SBMM is inactive, and current land uses within OU-1 include visitors, residential
uses located southwest of the former mining site and in the EIC, travel on the Sulphur Bank
Mine Road, and the potential for occasional trespassing by unauthorized personnel.
There are two residential areas near the mine, and their residents are at greatest risk of contact
with site related contaminants. The EIC is immediately adjacent to (northwest of) the former
mining site and includes tribal member homes and limited open space. The SBMM directly abuts
the EIC to the south (OU-1) and west (OU-2/Clear Lake). The North Wetland (included in
OU-4) is east of the EIC and is zoned as rural and rural residential land held by the
Redevelopment Trust. Additional private residential land (along Sulphur Bank Mine Road) is
located adjacent to the former mining site to the south and west.
Currently, the EIC uses available land for subsistence hunting, fishing, and gathering as part of
their traditional lifestyle. Wildlife, including deer, turkeys, quail, waterfowl, coyotes, eagles,
osprey, owls, squirrels, and occasionally bears, have been observed in the area, including on the
mine site. Clear Lake is a productive fishery, supporting predominantly bass and catfish fishing.
Resources in the area of the former mine include open space, scenic views of Clear Lake, the
hydrothermal system beneath the former mining site, potential geologic resources, and surface
water in Clear Lake.
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2.6.2 Reasonably Anticipated Future Land Uses
The SBMM site's complex history was taken into consideration when the reasonably anticipated
future land uses were being evaluated. In particular, the requirements of the remedy and requests
from the EIC for access to the mine site needed to be balanced. The EIC expressed the
expectation that traditional tribal activities would be practiced on the mine following
remediation. The potential future land uses for the mine site and surrounding land parcels are
identified in the Sulphur Bank Mercury Mine Preliminary Reuse Evaluation (CH2M 2021).
Potential future land uses are described according to follow major Site areas:
• Mine Site — The former mine site is immediately adjacent to the Elem Indian Colony
and within the tribe's ancestral territory. Therefore, although the specific legal structure is
unclear in the long term, the future use for the mine site includes traditional tribal
practice. The extent of this use will necessarily be limited due to regulatory and practical
constraints associated with the selected remedy. The former mine site will have extensive
remediation and permanent institutional controls (ICs). Wherever engineered caps are
constructed over waste materials, use and/or access restrictions will need to be applied to
ensure the long-term protectiveness of the caps. For these reasons, the reasonably
anticipated future land use determined for the mine site is "limited traditional tribal use".
This use was incorporated into the health-based remediation goals for on-mine soil with
based upon an assumption that this entailed 2-days per week spent on-mine engaged in
traditional practices such as hunting, fishing, foraging, and pedestrian travel. Wildlife
access cannot be restricted by conventional fencing; therefore, the tribal practice of
harvesting local wildlife and game could involve wildlife and game that pass through or
live on remediated portions of the Site.
• Elem Indian Colony - The reasonably anticipated future land use on the Elem Indian
Colony is traditional tribal practice, which encompasses occupancy (residence) on the
land, collection and consumption of traditional foods from the area, and the exercise of
traditional cultural practices.
• Residential Areas on Sulphur Bank Mine Road - Residential use is the anticipated future
land use for the areas currently zoned for and used as residential properties in this area,
with the exception of those held by the Sulphur Bank Redevelopment Trust. For those
properties, the Consent Decree provided EPA authority to restrict their use to
accommodate the needs of the remedy. For instance, the Bradley Parcel (APN 01000232)
is currently used as a residential property, however due to its close proximity to multiple
waste piles and a potential soil borrow area, this parcel may not be suitable for residential
use after remedial action implementation is completed, in which case it would revert to
the same use expectation for the mine property - as open space expected to experience
limited traditional tribal practice.
• North of BIA 120— The portion of OU-1 that is north of BIA 120 is the northern lobe of
the Northwest Waste Rock Pile. The remainder of the lands north of this boundary are
part of Operable Unit 4 and will be subject to investigation and potential remediation
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according to the CERCLA process for that OU. To the extent affected by OU-1, the area
north of BIA 120 is primarily wetland and pastureland. Future land use expectations
include traditional tribal practices as well as agriculture, or potentially residential
development on portions outside the wetlands, subject to the wishes of the EIC. Wildlife
will not be restricted by conventional fencing and, therefore, the wildlife that pass
through or live on the remediated portions of OU-1 may be encountered in this area.
2.6.3 Groundwater Resources
As described above, groundwater at the SBMM site is heavily influenced by naturally occuring
hydrothermal constituents. Groundwater at the site is either of insufficient quality (due to the
natural hydrothermal constituents boron, chloride, ammonia, and TDS) or of insufficient quantity
to be a practical source of drinking water or irrigation supply without treatment.
California State Water Resources Board Resolution 88-63 states that all surface and
groundwaters of the state are considered to be suitable, or potentially suitable, for municipal or
domestic water supply and should be so designated by the RWQCBs with the exception of
surface and groundwaters where TDS exceeds 3,000 mg/L and it is not reasonably expected to
supply a public water system. The naturally occurring TDS in the hydrothermal groundwater
may make site groundwater eligible for beneficial use de-designation in a Basin Plan
Amendment as provided under Resolution 88-63 and the Basin Plan.
There are not currently any known users of the potentially site-affected groundwater at the site,
as residents near SBMM receive water from the municipal water supply via Clear Lake Oaks
Water District or obtain water from directly from Clear Lake.
While groundwater is not a component of the OU-1 remedy, addressing source areas under the
OU-1 ROD is expected to improve the groundwater quality. Capping of exposed waste piles will
minimize future transport of site-related constituents to groundwater.
2.6.4 Surface Water Resources
Clear Lake is a large and important surface water resource used for municipal water supply,
habitat for waterfowl, and for fishing. OU-2 will address contamination in the sediments of Clear
Lake, whereas OU-1 addresses contributions of contamination to Clear Lake from source areas
within the mine site. There are no surface water resources within OU-1. The Herman
Impoundment, the former open pit, is now flooded with groundwater and stormwater. The
naturally occurring concentrations of some constituents in the groundwater that enters Herman
Impoundment exceed several secondary MCLs, meaning that the water would require extensive
treatment to be useable. Furthermore, Herman Impoundment has naturally occurring TDS above
3,000 mg/L and is, therefore, not considered suitable or potentially suitable for domestic or
municipal use.
2.7 SUMMARY OF SITE RISKS
Potential risks posed to human health and the environment at the Site include:
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• Exposure to arsenic, antimony, and thallium in soil.
• Exposure to arsenic in Clear Lake (if presumed to be a source of drinking water without
treatment), sediment, and surface water.
• Exposure to arsenic and thallium in plants.
• Exposure to mercury in fish.
Potential risks posed to the environment at the Site include:
• Exposure of plants to antimony and mercury in soil.
• Exposure of birds (including raptors) to aluminum, lead, and mercury in soil.
• Exposure of mammals to aluminum, antimony, arsenic, lead, and mercury in soil.
• Exposure of mammals to aluminum in surface water.
2.7.1 Human Health Risks
A baseline human health risk assessment (HHRA) estimates the risks a site would pose if no
further action were taken. It provides the basis for taking action and identifies the contaminants
and exposure pathways that need to be addressed by a remedial action. Four elements are
required in a baseline risk assessment: identification of contaminants of potential concern
(COPCs), exposure assessment, toxicity assessment and risk characterization.
For the Sulphur Bank Mercury Mine site, EPA evaluated the risks to human health on several
occasions, most recently in an updated Sitewide HHRA published in 2020 (E2, 2020). Results
from the 2020 HHRA are used to support risk management decisions to address potential cancer
risks from exposure to carcinogens and non-cancer health effects associated with exposure to
mercury and other metals in soil at OU-1. For the evaluation of non-cancer health hazards, a
hazard index (HI) is calculated and a value of 1 or less is considered protective of non-cancer
health hazards. Cancer risk is generally expressed as a probability. For example, a cancer risk
probability of 5 in 100,000 (5 x 10"5) indicates that out of 100,000 people, five cancer cases may
occur because of exposure to contaminants. EPA uses the federally established risk management
range of 1 x 10"6 to 1 x 10"4 to evaluate site cancer risks. The risk characterization process allows
the identification of contaminants which contribute significantly to site-related risks and hazards.
These key contaminants are called Contaminants of Concern, or COCs, and are the focus of this
risk summary.
2.7.1.1 Identification of Contaminants of Concern
The Sitewide Human Health Risk Assessment identified arsenic, antimony, thallium and
mercury as the contaminants that result in a risks to human health and the environment outside
the risk management range, and thus warranting consideration for remedial action. Affected
media evaluated for the presence of these metals within OU-1 includes soil (which is not
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differentiated from mine waste for purposes of this section), surface water, drinking water,
sediment and plants.
2.7.1.2 Exposure Assessment
The objective of the exposure assessment is to estimate the type and magnitude of exposure to
the COPCs (EPA 1989) present at, or migrating from, the site. The exposure assessment also
identifies the receptors that might be exposed; the routes by which these individuals might
become exposed; and the magnitude, frequency, and duration of potential exposures. When
combined with chemical-specific toxicity information summarized in the toxicity assessment,
these exposures produce estimations of potential risks and hazards.
The CSM (Figure 2-1) presents the current and future exposure scenarios, including present
receptors, exposure routes, exposure pathways, and sources. The detailed rationale for the
selection (or non-selection) of current and/or future receptor populations is presented in the
HHRA.
This decision document is limited to OU1, however the HHRA included risk from all operable
units. Risks described below that result from exposure to site contamination in other operable
units (e.g. ingestion of Clear Lake and fish) will be addressed in separate decision documents.
Three human receptor groups for OU1 were identified in the HHRA:
• Traditional Tribal receptors (adult and child)- including traditional tribal exposure to soil,
sediment, surface water, and potential future drinking water from Clear Lake; exposure
during fishing; and consumption of tules, cattails, acorns, fish, waterfowl, and upland
birds.
• Lakeside resident receptors (adult and child)- including residential exposure to soil and
drinking water. This drinking water is conservatively assumed to come a water system
drawn directly from Clear Lake, however residents have reported that they do not use this
water for drinking.
• Unauthorized user receptors: Users with unauthorized access to the mine-site consisting
of two groups, recreational visitors and unauthorized tribal users.
Exposure pathways that were evaluated included:
• Ingestion, inhalation, and dermal contact with mine-site surface soil and off-mine
residential soils;
• ingestion and dermal contact due to groundwater and storm water flow to Herman
Impoundment; and
• ingestion and dermal contact via storm water runoff.
2.7.1.3 Toxicity Assessment
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The toxicity assessment provides information regarding the potential of a chemical/metal to
cause cancer and other adverse health effects. Toxic chemical effects are separated into
carcinogenic effects and non-carcinogenic effects, based on the understanding that the
mechanisms of action for these types of chemicals differ. The following sources provided the
basis for evaluating the toxicity of the four COCs at OU1:
The OU1 toxicity assessment provides information regarding the potential of a chemical to
cause cancer and other adverse health effects. Toxic chemical effects are separated into
carcinogenic effects and non-carcinogenic effects based on the understanding that the
mechanisms of action for cancer-causing and non-cancer-causing chemicals differ.
Toxicity information was provided by Superfund Technical Support Center. Health effects were
classified using methodology recommended by EPA procedures for health effects from exposure
to chemicals (EPA 2019b, 2019c). The health effects analysis considers chronic (long-term)
exposures. The Final Sitewide HHRA, as corrected by the 2023 HHRA Errata (EPA, 2023), was
informed by EPA's standard chronic toxicity criteria hierarchy (EPA, 2003).
For risk calculations in the 2020 HHRA and the later development of risk-based target
concentrations for remedial alternatives, a site-specific decision was made to use the most
protective toxicity criteria available between the USEPA and CalEPA (CA Office of
Environmental Health and Hazard Assessment) values.
Dermal toxicity factors were estimated from oral toxicity data using chemical-specific oral to
dermal adjustment factors recommended by EPA (2019b).
Bioavailability Factors
Not all environmental contaminants are equally available for biological uptake. For this reason,
the toxicity assessment includes the development of bioavailability factors where appropriate.
The bioavailability factors incorporated into the 2020 HHRA and their rationale for each site
COC are as follows:
• Arsenic: Arsenic bioavailability studies conducted in animal models show that the
bioavailability of arsenic in soil is typically less than that of highly water-soluble forms
of arsenic (e.g., sodium arsenate dissolved in water). These results suggest that
bioavailability of arsenic in soil will typically be less than that of arsenic dissolved in
drinking water, resulting in relative bioavailability (RBA) of soil/water less than 100%.
OU1 oral aresenic RBA of 60% was assumed in the derivation of the RSL for the
ingestion of soil and sediment. Likewise, the intake equations for arsenic were adjusted
by a factor of 0.6 or 60%. The RBA was applied to intake equations for evaluating the
incidental ingestion of arsenic in soil and sediment in both site and PMB/ambient
calculations. It is noted that the RBA does not apply to dermal exposures to arsenic in
soil and sediment for which the absorbed dose is calculated using a dermal absorption
fraction (ABSd) of 0.03 (EPA 2019b; EPA 2004).
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• Mercury: The mercury at the Site is the result of a natural ore deposit and is therefore
present in mineral complexes containing several different forms of mercury compounds.
Many of these mercury compounds are not readily available for biological uptake (e.g.
mercury sulfide). Mercuric chloride is a common form of mercury which is highly
bioavailable and for which robust toxicity data exists. For this reason, mercuric chloride
was used as a surrogate for the bioavailable fraction of mercury. Based upon limited site-
specific mercury speciation data and experience at other mining sites, a conservative
assumption was made that 10 percent of measured total mercury levels were in the form
of mercuric chloride.
• Antimony and Thallium. Bioavailability for all other constituents, including the COCs of
antimony and thallium was conservatively assumed to be 100%.
2.7.1.4 Risk Characterization
For carcinogens, risks are generally expressed as the incremental probability of a population of
individuals developing cancer over a lifetime as a result of exposure to the carcinogen. Excess
lifetime cancer risk due to exposures to carcinogens through ingestion and skin contact is
calculated from the following equation:
Risk = CDI x SF
where:
Risk = a unitless probability (e.g., 2 x 10-^) of a population of individuals developing
cancer
CDI = chronic daily intake averaged over 70 years (milligrams per kilograms per day
[mg/kg-day])
SF = slope factor, expressed as (mg/kg-day)
Quantitative estimates of risk due to inhalation are evaluated using the URF. Using the same
equation shown immediately above, dose is replaced by the exposure level based on the
contaminant concentration in air and the length of exposure. The exposure level is multiplied by
the URF instead of the SF. An excess lifetime cancer risk of 1 x 10"6 indicates that a population
of individuals experiencing the reasonable maximum exposure estimate has a 1 in 1,000,000
chance of developing cancer as a result of exposure to site - related contaminants. This is
referred to as an "excess lifetime cancer risk" because it would be in addition to the risks of
cancer individuals face from other causes such as smoking or exposure to too much sun. The
chance of a population of individuals developing cancer from all other causes has been estimated
to be as high as one in three. U.S. EPA's risk management range for site - related exposures is 1
xlO'^tolxlO"^. Determination of what constitutes acceptable levels of residual risk within
this range is made on a site-specific basis.
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Health effects due to exposures to non-carcinogens are estimated using the hazard quotient (HQ)
approach. The HQ is the ratio of exposure to toxicity. An HQ less than 1 indicates that toxic non-
carcinogenic effects from that chemical are unlikely. The HI is generated by adding the HQs for
all COCs to which a given population of individuals may reasonably be exposed. An HI less than
1 indicates that toxic non-carcinogenic effects from all contaminants are unlikely. An HI greater
than 1 indicates that site-related exposures may present a risk to human health.
The HQ is calculated as follows:
Non-cancer HQ = CDI/RfD
where:
CDI = chronic daily intake
RfD = reference dose
As in risk estimates due to inhalation of carcinogens, the hazard quotient due to inhalation of a
non-carcinogen is calculated by replacing the dose with the exposure level based on the COPC
concentration in air and the length of exposure. The RfD is replaced by the RfC.
In general, calculated cumulative cancer risks greater than 1 x 10"4 and hazard indices greater
than 1 require consideration of cleanup alternatives. Cancer risks between 1 x 10"4 and 1 x 10"6
(between 1 in ten- thousand and 1 in one million) fall within EPA's risk management range.
Cumulative incremental lifetime cancer risk related to site contamination below 1 x 10"6 is
considered a de minimis level and typically does not warrant active risk/exposure mitigation.
2.7.1.5 Summary of Human Health Risks
The HHRA was performed to evaluate the potential human health effects associated with
chemicals at the SBMM. Receptors evaluated included lakeside residents, recreational users,
unauthorized users, and traditional tribal receptors. A summary of carcinogenic and non-
carcinogenic risk is below.
The following are estimated risks (if-applicable) and hazards (total and by exposure media) for
the RME scenarios outlined in the HHRA (2020) for adult and child receptors (if-applicable):
• Lakeside Residents (Off-Mine)
— Total cancer risk for adults is 7E-04, and hazard index is 39. For cancer risk and
noncancer hazard the primary exposure pathway is soil.
- Drinking water (Clear Lake) has a cancer risk of 5E-04 and an HI of 34; for both
cancer risk and noncancer hazard the primary COC is arsenic.
- Soil cancer risk is 2E-04, and an HI of 5.3; for both cancer risk and noncancer
hazard the primary COC is arsenic.
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— Total cancer risk for children is 7E-04 and an HI is 107; for cancer risk and noncancer
hazard the primary exposure pathway is soil.
- For children, drinking water (Clear Lake) poses a cancer risk of 5E-04 and HI of
56; arsenic is the primary COC for both cancer risk and noncancer hazard.
- For children, soil has a cancer risk of 2E-04 and HI of 51; for cancer risk the
primary COC is arsenic, while for noncancer hazard the primary COCs are
arsenic, antimony, and thallium.
• Unauthorized Tribal User (On-Mine)
— The total cancer risk is 3E-05 to 7E-05 with an hazard index of 7 to 13, the primary
exposure pathway is soil for both cancer risk and noncancer hazard.
- Soil presents a cancer risk of 3E-05 and HI of 6, and the primary COC is arsenic.
- Unauthorized tribal user's cancer risk from soil is 7E-05 and HI is 13, with
arsenic as the driving COC.
- Surface Water (swimming) in Herman Impoundment has a cancer risk of 8E-05
and HI of 0.6. No COC results in an HQ>1.
• Traditional Tribal Receptors (Off-Mine)
— The total traditional tribal adult's cancer risk is 2E-03 with an HI of 76, with drinking
water as the primary exposure pathway is drinking water.
- EIC soil has a cancer risk of 2E-04 and HI of 5, with arsenic as the primary COC.
- Drinking water (Clear Lake) has cancer risk of 1E-03 and HI of 46, with arsenic
as the primary COC.
- Recreational sediment (Clear Lake) has a cancer risk of 3E-05, arsenic is the
primary COC.
- Recreational surface water (Clear Lake) has a cancer risk of 8E-06 and HI of 1.7
with arsenic as the primary COC.
- Plant ingestion (multi-OU) has a cancer risk of 2E-04 and HI of 9, for both cancer
risk and noncancer hazard the primary COC is arsenic, while thallium is also a
primary noncancer COC.
- Waterfowl ingestion (multi-OU) has an HI of 4, with methyl mercury as the
primary noncancer COC.
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— The total cancer risk for tribal children is 2E-03, with an HI of 133; primary exposure
pathway is drinking water.
- EIC soil has a cancer risk of 2E-04 and HI of 28, with arsenic as the primary
carcinogenic COC and arsenic and thallium both as noncarcinogenic hazardous
coc.
- Drinking water (Clear Lake) has cancer risk of 1E-03 and HI of 74, with arsenic
as the primary COC.
- Recreational sediment (Clear Lake) has a cancer risk of 3E-05 and HI of 3.9;
arsenic is the primary COC.
- Recreational surface water (Clear Lake) has a cancer risk of 8E-06 and HI of 2,
with arsenic as the primary COC.
- Plant ingestion (multi-OU) has a cancer risk of 2E-04 and HI of 12, for both
cancer risk and noncancer hazard the primary COC is arsenic, while thallium is
also a primary noncancer COC.
- Fish ingestion (Clear Lake) has an HI of 9, with methyl mercury as the primary
COC. Waterfowl ingestion has an HI of 4, with methyl mercury as the primary
COC.
2.7.1.6 HHRA Uncertainty Analysis
As with any risk assessment, there are numerous assumptions and uncertainties that may impact
the conclusions of the HHRA. Where assumptions need to be made in the HHRA process, those
that result in more conservative results, and therefore might overestimate site-specific
parameters, are preferred. This conservatism is important to ensure that no contaminants are
dismissed when they may potentially pose a risk to human health. The most significant
assumptions or uncertainties in the 2020 HHRA are outlined below. For a full discussion of
HHRA uncertainties, refer to the complete 2020 Sitewide HHRA in the administrative record.
• Mercury Speciation/Bioavailability: As described above, mercury is present in several
different forms at the Site. A conservative bioavailability factor of 10% was used in
assessing risk from soil ingestion based upon Smucker, 2012. This assumption is
conservative based on limited site-specific bioavailability data. This may result in an
overestimation of some risks associated with exposure to mercury contaminated soil at
the Site.
• Arsenic Bioavailability. As discussed in the 2020 HHRA a conservative arsenic relative
bioavailability value of 60% was used for assessing risk from soil ingestion. Arsenic is a
primary contributor to risk; therefore this assumption may result in overestimation of risk
for some exposure scenarios.
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• Tribal Cultural Use Risks and Hazards: As discussed in the 2020 HHRA, unique
exposure factors have been used in the HHRA for this sensitive receptor population (EPA
2007; Harper 2018). Many of these unique exposure factors lead to elevated risks and
hazards (Tables 6-1, 6-2, and 6-3 HHRA 2020). The associated enhanced exposures, such
as an assumption of consuming 4 liters of Clear Lake surface water per day, an incidental
soil ingestion rate of 400 milligrams per day, and a diet based on harvesting 100 percent
of food items from SBMM COC-impacted areas (for 365 days per year for 70 years, for
an adult), assumes resource availability and utilization that may exceed current or
probable future conditions. For this reason, the traditional tribal use scenario is
conservative.
2.7.2 Ecological Risks
A terrestrial ERA (TtEMI 2002b) was conducted as part of the 2002 RI to evaluate ecological
risk in a Tier 1 risk assessment of current site conditions. The terrestrial ERA provided
information on the biological resources associated with the SBMM and surrounding areas in
support of the FS (E2 2013). The assessment provided information on the terrestrial and wetland
habitats (Figure 2-5) and a list of wildlife and common plants observed during site
reconnaissance surveys. It also identified potential special-status species that may occur in the
project area.
SBMM and surrounding Clear Lake offer suitable habitat for a variety of terrestrial mammals
and avian wildlife. The landscape is highly disturbed due to mining activities and subsequent
emergency response actions. Three main plant communities in the SBMM area include desert
saltbush, scrub, and cismontane alkali marsh; however, for the ERA, only the terrestrial habitats
were addressed (TtEMI 2002).
Bird species observed at SBMM include scrub jay, California towhee, rufus sided towhee,
sparrows, goldfinch, Nuttall's woodpecker, osprey, bald eagle, golden eagle, northern harrier,
northern goshawk, dark-eyed junco, purple martin, and gallinaceous birds such as the California
quail. Other bird species that are known to use the lake surrounding the mine site include double
crested cormorant, western and Clark's grebes, buffiehead, herons, ruddy duck, and other duck
species.
Mammals that have been observed at SBMM include raccoon, skunk, black-tailed jackrabbit,
domestic cats and dogs, and black-tail deer. Other mammals which may inhabit the site include
the mink and coyote (University of California Davis 1993).
Special status species that have been positively observed at the site include the raptors noted
above. A special status plant, little mousetail, was observed in a small seasonal wetland area in
the northeast part of the study area. Little mousetail is a California Rare Plant Rank 3.1 species,
which means it is potentially seriously threatened but the necessary information to assign this
species to one of the other ranks is lacking. No federal or state listed threatened, endangered or
candidate species were observed during the February and May 2013 reconnaissance surveys nor
an April 2023 follow up attended by a USFWS Wildlife Biologist.
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The ERA indicated the current level of metals in soil at the site pose probable risk to plant, avian,
and mammalian receptors. The metals identified as contaminants of ecological concern (COECs)
in soil include:
• Plants may be adversely affected by exposure to antimony in the East Ore Pile, West Ore
Pile, Tailings, South Waste Rock Pile, North Waste, Rock Pile, and West Waste Rock
Pile and mercury throughout the SBMM site.
• Birds may be adversely affected by exposure to aluminum, lead, and mercury at SBMM.
• Mammals may be affected by exposure to aluminum, antimony, arsenic, lead, and
mercury at SBMM.
Potentially complete exposure pathways to terrestrial receptors were evaluated based on the fate
and transport processes associated with each COEC. Because distributions of COECs and plants
at the site overlap, the following activities were conducted to evaluate whether metal exposure
had occurred:
• Complete metal exposure pathways for ecological receptors were developed.
• Site soil concentrations were compared to benchmark values protective of ecological
receptors.
• Spatial variation in the EPC was evaluated by calculating HQs (Hazard Quotients) for
individual sample locations and mean HQs for site areas.
Bioaccumulation Factors (BAFs) were used to evaluate whether site-specific oak or grass tissues
contained metal concentrations greater than those in soil, suggesting that significant uptake or
accumulation (exposure) had occurred.
In surface water, aluminum was determined to pose potential risk to small mammals. However,
the low pH of site waters renders the aluminum biologically unavailable. Mercury in air was
found unlikely to pose a significant risk to populations of burrowing animals.
2.7.2.1 ERA Conclusions
Throughout OU 1, elevated concentrations of aluminum, antimony, arsenic, and lead in soil are
all collocated with mercury contamination. An action level of 805 mg/kg to protect ecological
receptors from mercury in soil was developed in the ERA. The RI concluded that a response
action that would result in capping of wastes identified as posing hazard to human
trespassers/users would likely result in reducing exposure of ecological receptors leading to an
associated reduction in ecological risk.
A spatial analysis of risks based on the action level of 805 mg/kg showed the highest overall
risks are in the East Ore Pile and West Ore Pile, which have the overall highest levels of
mercury. Individual exceedances of action levels occurred in mostly unvegetated or poorly
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vegetated areas. Average mercury concentrations in both ore piles are greater than 1,000 mg/kg.
Average mercury concentrations in the waste piles are less than 600 mg/kg. However, the highest
concentration of mercury detected (4,296 mg/kg) was at the West Waste Rock Pile.
In developing an action level for the site, a balanced approach between the use of conservative
(i.e., protective) assumptions and consideration of protection at the population level was used.
The action level was chosen as 805 mg/kg based on the following:
• A conservative assumption of 1% methylmercury.
• The use of an HQ of 10 to represent protection of receptors.
• The use of a low TRV to be adequately protective given the use of an HQ of 10.
2.7.2.2 ERA Uncertainty Analysis
Uncertainty is inherent in ecological risk assessment. EPA made many assumptions in the ERA
process, most of which were conservative. These may result in overestimation of site-specific
ecological risk. A full discussion of ERA uncertainties is present in the ERA (TtEMI 2002b) and
includes factors such as:
• The assumption that all receptors live and feed solely on site likely increases the doses
used in risk assessment and so overestimates risk.
• It was conservatively assumed that 100 percent of the most contaminated diet items
composed 100 percent of the receptor diet.
• Risk calculations used the smallest body weight and highest ingestion rate reported for
each receptor, which may represent an overestimation of exposure and risk.
• The use of an HQ of 10 to represent protection of receptors likely underestimates risk to a
limited degree.
2.7.3 Basis for Action
The response action selected in this ROD is necessary to protect the public health or welfare or
the environment from actual or threatened releases of hazardous substances into the
environment. The following primary environmental issues should be addressed by the OU-1
remedy:
• Surface soil concentrations in residential areas result in chemical risk exceeding EPA's
risk management range for residential use and traditional tribal practice. Cumulative site-
wide risk for traditional tribal users is particularly high. Because EPA's ability to
effectively mitigate the risks associated with traditional tribal use of Clear Lake (OU-2) is
uncertain and unlikely to occur within a reasonable timeframe, remedial action in OU-1 is
essential to address those sources of risk that can be more readily managed.
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• Surface soil concentrations within the mine site result in risk greatly exceeding EPA's
risk management range for current and reasonably anticipated future land uses,
particularly unauthorized tribal use. Because EPA cannot reliably exclude users of the
mine site, remedial action is needed to protect impacted receptors.
• As stated by the El em Indian Colony in their comments on the SBMM OU1 Proposed
Plan, "Elem has been forced to bear a vastly disproportionate share of the environmental
consequences resulting from the industrial, governmental, and commercial operations and
policies at SBMM, and therefore is a textbook example of what EPA would call an
"overburdened community." (Elem Indian Colony, 2023) The EIC have traditional tribal
practices that include foraging and hunting on their tribal trust lands and adjacent lands
within their ancestral territory, including nearly all areas within OU-1 of the SBMM
Site. Even if access to OU-1 could be reliably restricted, wildlife still accesses the site,
and, if harvested, could expose traditional tribal receptors to additional real or perceived
burdens associated with site contamination. This could lead to impacts to the EIC health
or restrict their ability to practice their traditional lifeways.
• Ongoing releases of site contaminants, particularly mercury, results in unacceptable risk
within other Operable Units. Mercury released into Clear Lake (OU-2) via groundwater
transport through the WRD is subject to methylation and bioaccumulation, resulting in
excess risk for recreational and traditional tribal users of Clear Lake fisheries. Source
control is needed before mitigation of the downstream receiving water is practicable.
Current mercury loading exceeds the TMDL of 0.5 kg/year promulgated in the applicable
Regional Basin Plan.
• The surface soil concentrations in the source areas exceed the ecological risk assessment
(ERA) criteria for impacts to wildlife (based on mercury). This could cause negative
impacts to wildlife that passes through, feeds, lives, or nests on the land within OU-1.
• The exposed waste piles at the Site are a continuing source COCs in windblown dust. In
addition, some unpaved roads are built on and cross over waste piles, specifically the
South Waste Rock Pile. The ongoing dust is a source of contaminant migration and
exposure.
• Exposed mining waste piles at the site are not in compliance with several applicable
regulations such as the "Closure and Post-Closure Maintenance of Mining Units"
requirements under California Title 27. Remedial action is needed to bring these site
features into compliance with applicable law.
2.8 REMEDIAL ACTION OBJECTIVES
RAOs are overarching goals to be met by the cleanup plan to address the COCs, exposure routes,
and affected groups (receptors), and that identify the acceptable chemical level for each exposure
route. RAOs are based on considerations such as:
• Protecting human health and the environment
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• Where site contamination occurs and how it moves
• How the community and environment come into contact with the contamination
• What the future use of the site is expected to be, i.e., the reasonably anticipated future
land use.
In evaluating future use of the SBMM property, the EIC expressed its desire to use the Site for
traditional tribal purposes following Site remediation. Although future property ownership is
uncertain, future Site use is reasonably understood and, therefore, informs the RAOs described
below. During design of the Selected Remedy consolidation of waste away from the Elem Indian
Colony will be incorporated to the extent practicable.
Based on the current and anticipated future land use, EPA developed the following RAOs for this
remedy with input from the EIC, the California RWQCB, and the California DTSC. RAOs for
the OU-1 remedy are:
1. Reduce risk posed by contact with mining wastes/on-mine soil to acceptable levels for
reasonably anticipated future land uses.
2. Control off-site transport of COCs in surface water and dust, at concentrations that would
result in unacceptable degradation of surface and groundwater resources or at levels not
in compliance with applicable or relevant and appropriate requirements (ARARs).
3. Reduce risk posed by contact with contaminated soil to acceptable levels in the
residential areas and for users of associated roads.
4. Minimize leachate generation by surface water or geothermal water infiltration through
mining waste.
5. Limit transport of mercury entering Clear Lake from the Site via groundwater to 0.5
kg/year in accordance with the Clear Lake TMDL.
2.8.1 Cleanup Levels and Remediation Goals
Cleanup levels provide a quantitative means of identifying areas for remedial action and
assessing compliance with the RAOs. Cleanup levels are also the performance standards for the
remedy and the main basis for measuring the success of the response actions.
Within the CERCLA process, the FS uses the phrase "preliminary remediation goals" for the
development of the remedial alternatives. Following the evaluation of alternatives and the
selection of the Preferred Alternative, EPA selects cleanup levels in the ROD. Cleanup levels
and performance standards for this remedy are summarized in Table 2-3 and are discussed
below.
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To develop cleanup levels for soils, EPA compared risk-based target soil concentrations to the
estimated background level for each COC. Risk-based target concentrations were calculated in
the FFS based on an individual non-cancer hazard of 1 for each identified COC. In recognition of
the potential role of cumulative risk from the combined effects of multiple COCs on the at-risk
populations, risk-based target soil concentrations were also calculated based on a total
cumulative hazard equal to 1 assuming the hazards for each COC can be summed. This second
target concentration calculation was presented in the 2023 Focused Feasibility Study Errata (E2,
2023) included in the administrative record. This approach is intended to provide greater
protectiveness where uncertainty may exist regarding the additive effects of exposure to multiple
COCs. Cumulative hazard risk-based target concentrations were developed for off-mine
traditional tribal (child), Lakeside resident (child) and on-mine unauthorized tribal users because
these are the risk-driving exposure scenarios that inform the selection of cleanup levels for the
remedy.
Where risk-based targets were below background concentrations, the background value was
selected as the cleanup level. As discussed above, the calculated Pre-mining Baseline (PMB)
concentrations provide the best available estimate of background. Where PMB values were
selected, the value used was based upon the parent geologic formation underlying the remedial
area indicated (Andesite, Franciscan or Upper Lake Sediments), with the exception of arsenic on
the EIC, where the Franciscan PMB is used to be conservative.
Should future data collection result in a revision to the PMB values, the revised PMB values will
be compared to risk-based target soil concentrations to determine if any changes to associated
cleanup levels are warranted.
On-mine soils: Of the on-mine exposure scenarios evaluated, the unauthorized tribal user leads
to the more protective, lower remediation goals for all COCs and is therefore the exposure
scenario that informs the selection of cleanup levels for on-mine soils for all the COCs.
For arsenic, the risk-based target concentration for unauthorized tribal use is calculated to be 2
mg/kg for non-carcinogenic hazard and 0.36 mg/kg for carcinogenic risk. The PMB value for the
on-mine soils, where the parent geologic material is Andesite, is 22 mg/kg. Since the PMB value
is higher than the risk-based target, EPA is selecting 22 mg/kg as the cleanup level for arsenic for
on-mine soils.
For mercury, the risk-based target for the unauthorized tribal user (on the mine site) is 204
mg/kg, which is higher than the PMB value of 35 mg/kg for Andesite and 98 mg/kg for
Franciscan, and less than the ecological risk target; therefore, EPA is selecting the risk-based
value of 204 mg/kg for mercury as the cleanup level for on-mine soils.
For antimony, the risk-based target is 51 mg/kg, which is higher than the PMB value for
Andesite; therefore, EPA is selecting the risk-based target of 51 mg/kg as the cleanup level for
on-mine soils.
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For thallium, the risk-based target of 1.3 mg/kg is higher than the HHRA EPC background value
(upper confidence limit [UCL]); therefore, EPA is selecting the risk-based target of 1.3 mg/kg as
the cleanup level for on-mine soils.
Off-mine residential soils: Two potential human exposure scenarios for off-mine residential
soils were considered for calculation of risk-based target concentrations: a) exposure to soil at
the off-mine residential area (EIC area) for the traditional tribal use by tribal residents (based on
child risk), and b) exposure to soil at the off-mine residential area (Sulphur Bank Mine Road
area) for the residential use by the Lakeside Residents (based on child risk). Because the
reasonably anticipated future land use for these two areas aligns with their respective exposure
scenarios, separate cleanup levels were established for each area.
EIC Residential Soils
For arsenic, the risk-based target concentrations for traditional tribal child use are calculated to
be 0.2 mg/kg for non-carcinogenic hazard and 0.03 mg/kg for carcinogenic risk. The PMB value
for the off-mine soils, where the parent geologic material is Franciscan, is 18 mg/kg. Since the
PMB value is higher than the risk-based target concentration, EPA is selecting the PMB of 18
mg/kg as the cleanup level for arsenic for off-mine residential soils in this area.
For mercury, the risk-based soils target considering cumulative risk for the off-mine residential
soils for the traditional tribal child exposure scenario is 30 mg/kg for total mercury (assuming 10
percent mercuric chloride). This value is lower than the PMB value of 35 mg/kg for Andesite,
therefore EPA is selecting the PMB value of 35 mg/kg as the cleanup level for off-mine
residential soils in this area.
For antimony, the risk-based target considering cumulative risk is 7.5 mg/kg, which is slightly
higher than the PMB value for Andesite (7.1 mg/kg). In the interest of conservatism and in light
of uncertainty about the potential toxicity of thallium (below), EPA is selecting the PMB value
of 7.1 mg/kg as the cleanup level in this area.
For thallium, the risk-based target of 0.4 mg/kg is higher than the HHRA EPC background value
of 0.18 mg/kg (UCL); therefore, EPA is selecting the risk-based target of 0.4 mg/kg as the
cleanup level in this area.
Sulphur Bank Mine Road Residential Soils
For arsenic, the risk-based target concentrations for traditional tribal child use are calculated to
be 0.4 mg/kg for non-carcinogenic hazard and 0.14 mg/kg for carcinogenic risk. The PMB value
for the off-mine soils, where the parent geologic material is Franciscan, is 18 mg/kg. Since the
PMB value is higher than the risk-based target concentration, EPA is selecting the PMB of 18
mg/kg as the cleanup level for arsenic for off-mine residential soils in this area.
For mercury, the cumulative risk-based soils target for the off-mine residential soils for the
residential user child exposure scenario is 62.5 mg/kg for total mercury (assuming 10 percent
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mercuric chloride). This value is lower than the PMB value for Franciscan of 98 mg/kg. Because
the Sulphur Bank Mine Road neighborhood sits entirely on Franciscan geology, the PMB value
of 98 mg/kg is being selected as the cleanup level for off-mine residential soils in this area.
For antimony, the cumulative risk-based target is 15.5 mg/kg, which is higher than the PMB
value for both the Andesite (7.1 mg/kg) and Franciscan (5 mg/kg); therefore, EPA is selecting
the risk-based target of 15.5 mg/kg as the cleanup level in this area.
For thallium, the risk-based target of 0.4 mg/kg is higher than the HHRA EPC background value
of 0.18 mg/kg (UCL); therefore, EPA is selecting the risk-based target of 0.4 mg/kg as the
cleanup level in this area.
Groundwater—The cleanup level for mercury in groundwater entering Clear Lake from the Site
is the TMDL for mercury of 0.5 kg/year. An additional remediation goal is to minimize leachate
generation to avoid degradation of groundwater underlying the EIC and North Wetlands.
Stormwater Runoff—The performance standard for storm water is to meet the substantive
requirements of the California Stormwater General Permit under the National Pollutant
Discharge Elimination System (NPDES); to control stormwater runoff to avoid contact with
contaminated soil within the source areas; and to reduce inflow to Herman Impoundment to
reduce groundwater flow out of Herman Impoundment.
Surface Water—The only requirement for surface water applicable to this decision for OU1 is
to meet the Clear Lake TMDL for mercury indicated above.
Fugitive Dust—The emissions due to fugitive dust from contaminated soil will be mitigated
during construction of the remedial action and in the long term. The allowable dust
concentrations will be calculated during the RD, and the appropriate mitigation and monitoring
processes will be identified for implementation during the remedy.
2.8.2 Benefits of OU-1 Remedy on OU-2 and OU-4
• The sediments in the bottom of Clear Lake (OU-2) contain elevated levels of mercury
that are above the criteria allowed by the RWQCB's Basin Plan. OU-1 is a continuing
source of mercury to Clear Lake, and remediating OU-1 is necessary prior to remediation
of lake bottom sediments, otherwise the sediments will be re-contaminated.
• The State of California maintains a fish consumption advisory recommending the
limitation of consumption of certain fish species taken from Clear Lake due to the
presence of mercury in the tissues of these fish in excess of safe levels. The mercury in
the fish tissue is attributed primarily to methylmercury dissolved in Clear Lake as
opposed to the total mercury present in the sediments or water in Clear Lake. The
RWQCB has evaluated potential sources of mercury to Clear Lake and determined
SBMM to be a primary contributor. OU-1 remediation will assist in the mitigation of
methylmercury impacts by reducing the amount of mercury entering Clear Lake.
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• The residents and recreational users of Clear Lake experience negative impacts from
SBMM due to the mercury contamination and subsequent fish consumption advisory as
well as potential health impacts from consumption of fish and waterfowl. Remedial
action on OU-1 to meet the TMDL will support the OU-2 remedy by removing a primary
source of mercury contamination.
• OU-4 is potentially receiving groundwater and dust-related contributions of COCs from
OU-1. The remediation of OU-1 will address loading associated with dust emissions and
limit migration from groundwater into OU-4.
2.9 DESCRIPTION OF ALTERNATIVES
This ROD relies on two FSs for the range of alternatives. The first OU-1 FS was published by
EPA in 2006 and, following feedback from key stakeholder agencies, additional alternatives
were evaluated in 2018 and 2019 and incorporated into the 2021 Final OU-1 FFS (E2, 2021).
Descriptions of the alternatives considered in both FSs are summarized herein.
2.9.1 2006 OU-1 Feasibility Study
The 2006 FS evaluated 9 General Response Actions, 25 Remedial Technologies, and 54 Process
Options. This FS developed and evaluated waste system specific remedial alternatives.
Waste System Specific Remedial Alternatives
• Herman Impoundment System: evaluated alternatives such as no action, capping the
WRD, soil cover for the Satellite Waste Rock Pile, physical/chemical treatment of
surface water for hydraulic control, and containment of surface water using a
groundwater barrier wall.
• Northwest Pit System: evaluated alternatives such as no action, developing a sludge
repository, and containment of shallow groundwater and surface water using an
extraction well.
• Ore Piles System: evaluated alternatives such as no action, on-mine repository, and wet
cover disposal in Herman Impoundment.
• Tailings System: evaluated alternatives such as no action, earthen cover, and paving
Bradley Mine Road.
• Waste Rock System: evaluated alternatives such as no action, earthen cover or earthen
cap with geomembrane liner, northwest waste rock disposal in on-mine repository,
interceptor trench for groundwater containment, and ex situ treatment.
• Disturbed Rock System: evaluated alternatives such as no action and earthen cover.
• Consolidated disposal of all mine wastes and site restoration (solid mine wastes and
surface water).
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Different components of the waste system alternatives were combined into six sitewide remedial
action alternatives. Components of each alternative are provided in Table 2-4, Table 2-5, and
Table 2-6.
2.9.2 Changes in the Site and Conceptual Site Model After 2006 Feasibility Study
Since the 2006 OU-1 FS, the following changes have occurred at the Site resulting in changes to
the CSM. These changes were incorporated into the 2021 FFS (E2, 2021).
• It was hypothesized that the low pH water of Herman Impoundment resulted in loading
of mercury from the WRD into Clear Lake. The pH of water in Herman Impoundment
has risen from less than 3 in the early 2000s to nearly 6 on several occasions in 2021. The
revised CSM now reflects that the pH of groundwater within the WRD is the result of the
acid generating capacity of the material that composes the WRD, rather than the pH of
the water within Herman Impoundment.
• Stormwater monitoring and geochemical modeling indicate the surface water runoff
contribution of acid to Herman Impoundment is very significant, and the reduction of this
source by capping would allow Herman Impoundment water to continue to neutralize.
The prior hypothesis included the possibility that the low pH in Herman Impoundment
was caused by the geothermal water entering the bottom of Herman Impoundment. The
geothermal water contains hydrogen sulfide, and it was hypothesized that the hydrogen
sulfide combined with oxygen to form sulfuric acid, which reduced the pH of the water.
• Algae within Herman Impoundment was first noticed in 2013. Studies prior to 2006
reported that Herman Impoundment was "sterile" and did not support life forms in the
acidic water.
• The CA Department of Toxic Substances Control performed a detailed analysis of
existing well logs, water quality data, lithologic information, and other relevant data to
estimate the portion of the Waste Rock Dam material playing a controlling role in the
loading of site contaminants to Clear Lake. Consideration of this portion of the WRD,
termed the "saturated waste rock of significance," led to refinements in the CSM.
• Annual Clear Lake water level fluctuations cause a wetting and drying of the Clear Lake-
WRD interface. This can result in ARD formation in the waste rock of WRD, which
could dissolve mercury if it is present in the waste rock. As the water level in Clear Lake
drops, the ARD drains into Clear Lake. This wetting and drying cycle was not included in
the prior CSM.
The 2006 selected remedial program alternative was evaluated by the Multi-Agency Team in
light of the changing Site conditions and the updated CSM and was determined to be insufficient.
This led to the formulation of new alternatives for each type of media instead of by waste
system.
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2.9.3 2021 OU-1 Focused Feasibility Study Alternatives
Remedial alternatives were identified in the 2021 FFS to address contamination in the different
AOCs at SBMM (E2, 2021). To allow a simpler presentation of the alternatives, the elements (or
performance requirements) that are common across the alternatives are presented first, and the
individual alternative descriptions are focused on the unique attributes of the alternative.
2.9.4 Off-Mine Residential Soil
Remedial alternatives were developed and evaluated for the residential soil areas known to
require remediation as follows:
• Alternative 1, No Action—In this alternative, no action would be taken to address the
contaminated soil in the off-mine residential areas. The soil would be left in place
without implementing any land use controls, containment, removal, treatment, or other
mitigating actions.
• Alternative 2, Excavation, Soil Cover, and/or ICs—This alternative includes either
excavation, placement of soil cover, and/or ICs to continue to protect residents from
relatively small amounts of contaminated soil known to remain within the residential
areas. The residential soil areas recommended for remedial action would be delineated
during the pre-RD/RD stage to determine the estimated excavation extents. Soil would be
excavated from areas where concentrations of COCs above cleanup levels are found at
less than 2 ft bgs. Excavated soil would be consolidated under the nearest capped source
area within OU-1. Excavated areas would be backfilled with clean topsoil to prevent
future human contact with remaining contaminated material. Areas that are not
appropriate to excavate, potentially due to their location relative to a building, would be
covered with a soil cover. EPA would place informational ICs or ECs in these areas to
prevent future excavation and exposure to mining waste below clean soil. EPA would
also place informational ICs or ECs on BIA 120 and the paved roads in EIC, where there
is known mining waste below a paved road or its soil shoulder. These ICs would restrict
future excavation of these areas so that the mining waste remains beneath the clean
topsoil and pavement such that there is no exposure pathway to residents. O&M would
consist of periodic inspection of the soil covers and pavement to confirm they have not
been disturbed and compromised, and if necessary, they would be repaired.
Since completion of the 2021 Focused Feasibility Study, the expected preliminary
remediation goals for off-mine residential soil were lowered to better align with the findings
of the Sitewide Human Health Risk Assessment (E2, 2020) and applicable EPA guidance. As
described in the 2023 Focused Feasibility Study Errata, these revised cleanup levels affect
the extent of remediation expected in residential areas. There are now three conditions for the
soil contamination within the EIC and residential soil south of the site:
• Soil that was remediated to the prior remediation goals through excavation, but the new
lower goals require further remediation;
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• Soil that was not completely remediated in the prior removal actions; and
• Soil where the prior investigations indicated no action was necessary, but the new, lower,
and more comprehensive cleanup levels indicate action is now likely necessary.
Additional data collection during pre-RD and RD will refine the extent of necessary action for
residential soil, but the nature of the remedial action is unchanged from the alternatives analyzed
in the 2021 Focused Feasibility Study.
2.9.5 Source Areas
Four remedial alternatives were developed for the Source Areas. Each of the Source Area
alternatives, other than No Action, had the following common elements:
• Each waste pile would be evaluated to establish its classification under Title 27 Section
22490 - Mining Unit Siting and Construction Standards. The current data for the source
areas indicate the waste is Group B waste. However, this would be confirmed by a
phased step-by-step investigation.
• Determination of the extent of contamination above the PRGs.
• Installation or confirmation of sufficient groundwater monitoring wells to evaluate the
performance of the remedy.
• Waste would be consolidated into locations where it will not come into direct contact
with groundwater. Stormwater diversion and detention ponds will be designed so the
groundwater levels will not be raised to the bottom of the waste piles due to any
infiltration of diverted or retained stormwater.
• Stormwater monitoring.
• Stormwater routing and control, including detention basins as needed for sediment
removal.
• Topsoil and vegetation (although different thicknesses may be instituted based on the
alternative).
• Air and dust monitoring during construction.
• Site security.
• Consolidation of the East and West Ore Piles on to the South Waste Rock Pile.
• Consolidation of the Northwest Waste Rock Pile into the North Waste Rock Pile.
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• Optional consolidation of portions of the West Waste Rock Pile in the South Waste Rock
Pile and the entirety of the Satellite Waste Rock Pile in the Northwest Pit or North Waste
Rock Pile.
• The side slopes of the contoured waste, before capping, will be 4:1 (horizontal to
vertical), which is a shallower slope than recommended in Title 27 for mining waste
units.
• The cuts and fills would be balanced within each of the source areas when they are
contoured (e.g., North Waste Rock Pile) to avoid moving material longer distances across
the Site.
• The extent of the capping or excavation would prevent human health and ecological
contact with soil that is above PRGs, prevent contaminated stormwater runoff to Herman
Impoundment and/or Clear Lake, and prevent infiltration of contamination into
groundwater.
• Capping would comply with the Final Cover Requirements discussed in Title 27 Section
21090(a)(l-3).
• Pilot testing would be determined as part of the pre-RD investigations and could be
necessary for selecting between a Resource Conservation and Recovery Act (RCRA) cap
or an alternative cover system.
• ICs would be needed for capping alternatives to prevent future disturbance of the cap,
which could expose contaminated soil or allow excess infiltration of precipitation.
• Depending on the findings of the pre-RD investigation relative to the mining structures
and soil, their remediation could include any of the following for each of the structures or
adjacent soil: no action, clearing debris, demolishing concrete pads within the excavation
or capping, hauling debris to a landfill (the uncontaminated material will be characterized
and sent to the appropriate landfill), demolishing and consolidating the mining operations
retaining wall, or excavating soil.
• The Northwest Pit was identified as a location for waste to be placed for any of the
source area alternatives, and possibly for material from the WRD. Alternatives assumed
the waste would be placed on top of a clean fill of gravel so there is a minimum 5-ft
clearance between the waste and the projected high groundwater level. Geothermal gas
venting would be provided by either: (1) placing a collection gallery below any
permanent cap (as all the caps are being vented), or (2) placing a collection gallery in the
clean fill at the base of the Northwest Pit. Lysimeters would be placed in the waste to
monitor production of leachate, and monitoring wells would be placed within the
footprint of the pit before capping, and along the perimeter of the pit to monitor lateral
movement of groundwater. The pit would be capped with an RCRA-style cap that would
extend a sufficient distance beyond the pit walls so infiltration along the pit walls would
be prevented.
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The O&M activities for the source area alternatives will include cap inspection and maintenance
and monitoring of groundwater and stormwater.
• Alternative 1, No Action—The no-action alternative is included to provide a basis for
comparison to the other alternatives. In this alternative, the Site would be fenced and
monitored but not actively maintained.
• Alternative 2, On-mine Consolidation and Capping with a RCRA Cap—This
alternative would include (1) excavation of the East and West Ore Piles and consolidation
of the excavated material into the South Waste Rock Pile, (2) excavation of the northern
portion of the Northwest Waste Rock Pile and consolidation of the excavated material
into the Northwest Pit or North Waste Rock Pile, and (3) consolidation of excavated
material as much as practical to minimize the area of capping. The excavated areas would
be graded, compacted, and revegetated. All the other source areas would be capped
(including the Northwest Pit), which would include the placement of an impermeable
liner in a cap. Two options are considered, with the basic difference being the liner. The
two options are either a HDPE or a geosynthetic clay liner. This alternative also includes
cleanup of the remains of the former mine operations buildings and the soil around them
(clearing debris, demolishing concrete pads within the excavation or capping, hauling
debris to a landfill, demolishing and consolidating the mining operations retaining wall,
or excavating soil). A contingency action of a groundwater collection trench along the
north side of the mine was evaluated with this alternative, but was not carried forward.
ICs would be implemented to ensure that capped areas remain undisturbed after capping.
ICs may consist of barriers to prevent vehicle access and deed restrictions to prohibit
ground penetrating activities where caps have been placed.
• Alternative 3, On-mine Consolidation and Capping - Alternative Cover System—
This alternative would include all of the components described above, with the exception
of the covering (e.g., capping) method to be used. This option utilizes alternative covers
to achieve design parameters similar to an RCRA cap. One example of an alternative
cover system is an evapotranspiration (ET) cover that includes placement of 3 ft of
locally sourced topsoil and the planting of deep-rooted perennial plants, shrubs, and trees.
This alternative also includes cleanup of the former remains of the mine operations
buildings and the soil around them (clearing debris, demolishing concrete pads within the
excavation or capping, hauling debris to a landfill, demolishing and consolidating the
mining operations retaining wall, or excavating soil).
• Alternative 4, Complete Excavation and Off-Site Disposal—This alternative entails
the complete removal of contaminated source area waste material that poses unacceptable
risk to future users from the Site. The excavated material would be transported off-site
for disposal at an appropriately permitted landfill facility. This alternative would
permanently remove the source area risks at the SBMM Site, but would include
substantial impacts to infrastructure and public welfare during its implementation,
especially impacts associated with the off-site transport.
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2.9.6 Waste Rock Dam
Three remedial alternatives were developed for the WRD. The two action alternatives for the
WRD (those other than No Action) were composed with the following common elements.
• Characterization of the extent of mining waste per Title 27 will be performed post-ROD
and prior to design of the remedial action.
• A groundwater monitoring network would be installed in compliance with Title 27
requirements and monitoring of the alternative's performance will be conducted. The
groundwater monitoring network would be installed with the purpose of providing design
information and a system capable of evaluating the performance of the remedy.
• Delineation of the nature and extent of the waste rock that is in contact with Clear Lake
or becomes saturated with groundwater.
• Investigation of the cause of elevated mercury and low pH would be conducted prior to
the RD.
• The two action alternatives require the removal of some portion of the waste rock from
the WRD. The waste rock removed from the WRD would be placed in one of several
consolidation areas on existing waste piles or within the Northwest Pit. Waste
consolidation would be performed in a manner to ensure that waste is placed "high and
dry," defined as a minimum of 5 ft above the historical high groundwater levels.
Stormwater diversion and detention ponds will be designed so the groundwater levels
will not be raised to the bottom of the waste piles due to any infiltration of diverted or
retained stormwater.
• Stormwater monitoring.
• Stormwater routing and control (detention basins as needed).
• Placement of topsoil and establishment of vegetation.
• Turbidity curtains will be placed in Oaks Arm, adjacent to the WRD, to contain turbidity
generated during any on-water construction.
• Air and dust monitoring during construction.
• Site security.
• The side slopes of the contoured waste, before capping, would be approximately 4:1
(horizontal to vertical), which is a shallower slope than recommended in Title 27.
• Pilot testing would be determined as part of the pre-RD investigations and as necessary
for the selection of the capping/cover system.
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• ICs would be needed for capping alternatives to prevent future disturbance of the cap and
exposure of contaminated soil.
• The excavated area would be either backfilled with clean fill or a combination of
limestone gravel and clean fill.
• The O&M activities for the WRD alternatives would include cap inspection and
maintenance, monitoring of groundwater and stormwater.
Informed by these common elements, the following 3 WRD Alternatives were formulated:
• Alternative 1, No Action—The no action alternative is included to provide a basis for
comparison to the other alternatives. In this alternative, the site would be fenced and
monitored but not actively maintained.
• Alternative 2, Excavation of Saturated Waste Rock of Significance and Capping—
This alternative entails excavation of the portion of the WRD that is saturated by
groundwater and has a groundwater flow from Herman Impoundment towards Clear
Lake. The extent of the saturated waste rock of significance was determined by DTSC,
but it would need to be refined during the pre-RD. The excavated area would be either
backfilled with clean fill or a combination of limestone gravel and clean fill. The
excavated material would be consolidated and capped in the North Waste Rock Pile or
placed in the Northwest Pit and capped/covered. A contingency action of groundwater
extraction wells was evaluated in case pockets of elevated mercury caused by the spring
beneath the WRD is not mitigated by caping and continues to exceed water quality
standards.
• Alternative 3, Excavation to Pre-Mining Shoreline and Capping (Includes Saturated
Waste Rock of Significance)—This alternative entails excavation of the portion of the
WRD that is in contact with Clear Lake along the shoreline and the portion that is
saturated by groundwater that is flowing towards Clear Lake (WRD Alternative 2).
Removing additional shoreline would lead to a smaller area of the WRD that requires a
cap because the excavated material would be placed in the North Waste Rock Pile or the
Northwest Pit. The same backfilling and IC requirements as WRD Alternative 2 apply.
The shoreline excavation would be accomplished by placing sheet piling in Clear Lake
outside the shoreline to be excavated then dewatering and excavating the waste rock. The
WRD would be sloped back, and the face protected with the appropriate capping. The
modified shoreline would be protected with boulders and riprap.
2.9.7 Herman Impoundment
Remedial alternatives were developed and evaluated for Herman Impoundment informed by two
key assumptions:
• Herman Impoundment is now filled with a mixture of meteoric and hydrothermal water
and acts as a hydrologic sink for most of the mine site (TtEMI 2006). Water from
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Herman Impoundment flows through the WRD as groundwater, where it encounters the
waste rock and becomes acidic. Absent inputs of ARD from the exposed waste
surrounding it, the water quality of Herman Impoundment would be substantially similar
to the naturally occurring geothermal groundwater at the site.
• The lower pH groundwater flowing laterally accumulates metals, in particular mercury,
as it travels through the waste rock materials before this water enters Clear Lake. In
addition, any precipitation that infiltrates through the WRD is understood to migrate
towards Clear Lake as groundwater.
The remedial alternatives developed for Herman Impoundment based on these assumptions
were:
• Alternative 1, No Action—The no-action alternative is included to provide a basis for
comparison to the other alternatives. In this alternative, no action would be taken to
address the surface water at Herman Impoundment.
• Alternative 2, Actively Maintain and Monitor—This alternative would entail annual
monitoring of the water and sediments within Herman Impoundment and maintaining the
site security and outfalls (monitoring would include water quality, the water levels, and
the condition of the Site security and stormwater controls). The current stormwater
overflows would be put in place to minimize the instances of overflows and prevent
negative impacts to Clear Lake that may result from overflow caused by heavy storms.
• Alternative 3, Construct Hydraulic Barrier Cutoff Wall(s)—This alternative entails
the construction of a slurry cutoff wall (or other method) to isolate the saturated portion
of the WRD from Clear Lake and from groundwater flow. These cutoff walls would
penetrate about 20 ft below the ground on either side of the WRD. The wall between the
WRD and Herman Impoundment would be about 1,000 ft in length to inhibit the flow of
groundwater from Herman Impoundment (or other areas within OU-1), into the WRD.
The second wall would be about 2,500 ft long and aimed at preventing Clear Lake water
from entering the WRD. This alternative would also require the long-term ICs (Site
security) noted above for Alternative 2 for Herman Impoundment.
2.10 SUMMARY OF COMPARATIVE ANALYSIS OF ALTERNATIVES
The comparative analysis of the FFS alternatives, organized by area of contamination, are
discussed in the sections below.
2.10.1 Off-Mine Residential Soil
Remedial alternatives for Off-mine Residential Soil (tribal and non-tribal) are:
• Alternative 1, No Action
• Alternative 2, Excavation, Soil Cover, and/or ICs.
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Alternative 1, No Action, does not meet the RAOs and is not considered a viable alternative.
Alternative 2, Excavation, soil cover, and ICs, is the only alternative that meets the threshold
criteria. Hence, comparative analysis of the off-mine residential soil remedial alternatives was
not conducted. Alternative 2 is the only alternative that meets RAOs and it provides a cost-
effective remedy. Therefore, Alternative 2 is the Selected Remedy for Off-mine Residential Soil.
2.10.2 Source Areas
Remedial alternatives for the Source Areas are:
• Alternative 1, No Action
• Alternative 2, On-mine Consolidation and Capping - RCRA Cap
• Alternative 3, On-mine Consolidation and Capping - Alternative Cover
• Alternative 4, Complete Excavation and Off-site Disposal.
Comparative Analysis of the Source Area Alternatives is provided in Table 2-4. Alternative 1,
No Action, would not meet the RAOs and is not a viable alternative. Alternative 4 is hundreds of
millions of dollars more costly than Alternatives 2 or 3 and presents very substantial
implementation challenges that make state and community acceptance unlikely, and off-site
disposal is only acceptable if other alternatives are not viable.
Alternatives 2 and 3 are implementable, meet the applicable RAOs, and can achieve the
applicable cleanup levels. These two alternatives are similar in the extent of their actions and
protectiveness of human health and the environment. For this reason, these two alternatives are
considered equally viable and components of each may be incorporated into the selected remedy
as location-specific requirements dictate.
Determining where the two cover technologies that distinguish Alternatives 2 and 3 might be
applied requires additional data that would need to be collected during Remedial Design. In
addition, design parameters such as the necessary thickness of soil in the cap or cover would be
based on geotechnical properties of the available cover material(s) as well as location-specific
conditions. The cost estimate in the FFS showed the RCRA Cap (Alternative 2) as being less
costly than an ET Cap (Alternative 3); however, these costs are very sensitive to the thickness of
soil needed for the cap or cover.
In addition, determination of where a RCRA Cap, ET cap, or other engineered alternative cover
is needed depends on the nature of the underlying source materials. CA Title 27 requires mining
wastes to be classified as Group A, Group B, or Group C. The selection of the appropriate cap or
cover will depend on the Title 27 waste classification for the material contained in each unit.
2.10.3 Waste Rock Dam
Remedial Alternatives for the WRD are:
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• Alternative 1, No Action
• Alternative 2, Excavation of Saturated Waste Rock of Significance and Capping
• Alternative 3, Excavation to Pre-Mining Shoreline and Capping (Includes the Saturated
Waste Rock of Significance).
Comparative Analysis of the WRD Alternatives is provided in Table 2-5. Alternative 1, No
Action, will not meet the RAOs and is not a viable alternative.
The selection between Alternatives 2 and 3 requires explaining what is known and what is
uncertain for the two alternatives. The following is a summary of the current CSM for the WRD,
relative to the remedial alternatives:
• Portions of the base of the WRD are below the water table and this saturated waste rock
has contaminated the groundwater.
• Groundwater flows from Herman Impoundment, under the WRD, and into Clear Lake.
Portions of the groundwater flows through the saturated waste rock and portions flow
through native geologic material (ULS or Andesite).
• The WRD overlays the fault line that runs east to west through the Herman Impoundment
and into Clear Lake. Geothermal gases and fluids are associated with the fault line, and
historical documents refer to geothermal springs below the WRD. The precise location of
the springs within the WRD is not known.
• Groundwater containing elevated mercury and low pH occurs discontinuously throughout
the WRD, with some areas of elevated mercury not yet having been clearly attributed to
loading from the overlying WRD material.
• Waste rock was pushed into Clear Lake during the early mining operations, and waste
rock also eroded from the face of the WRD prior to the removal action by EPA to flatten
its slope and provide erosion protection. The western portion of the WRD sits beyond the
natural shoreline of Clear Lake, as mapped by USGS in 1888. A portion of the material
beyond the natural shoreline of the lake is therefore understood to become saturated and
dewatered each year as Clear Lake's water level rises in the winter and falls during the
summer and fall. When the lake level falls, the shoreline waste rock drains into the lake.
If mercury or other contaminants are mobilized when the lake rises and saturates the
shoreline waste rock, those mobile contaminants would flow into Clear Lake when the
lake level declines, though the magnitude mercury flux to Clear Lake resulting from this
process has not yet been quantified.
Alternatives 2 and 3 are similar in that they both involve the capping of the WRD (with either a
RCRA Cap or ET Cap), and the removal of waste rock that is saturated and could be contributing
to mercury in Clear Lake. The principal difference is in the extent of saturated waste rock
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excavation entailed by each alternative. Alternative 2 is focused on the waste rock beneath the
WRD and in the groundwater flow path from the Herman Impoundment to Clear Lake.
Alternative 3 encompasses Alternative 2 and adds excavating the waste rock back to the original
1888 shoreline. Both Alternatives 2 and 3 include capping the final configuration of the WRD
after the excavation of the waste rock.
The implementation of the two alternatives is comparable in cost and protectiveness, but the
Alternative 3 adds the complication of excavation within Clear Lake and the substantive
requirements of Section 404 of the Clean Water Act. Alternative 3 is also considered more likely
to achieve RAO 5 (annual groundwater mercury flux requirement of no more than 0.5 kg/year).
2.10.4 Herman Impoundment
Remedial Alternatives for the Herman Impoundment are:
• Alternative 1, No Action
• Alternative 2, Actively Maintain and Monitor
• Alternative 3, Construct Hydraulic Barrier Cutoff Wall(s).
Herman Impoundment is an integral part of the surface water and groundwater system at SBMM,
and its role is, therefore, considered in the alternatives. While it is not expected at this time that a
remedial action will be needed for the Herman Impoundment, the alternatives listed above are
included to address the need for ongoing monitoring and the possibility that rerouting the flow
from Herman Impoundment to the west and around the WRD may provide some cost-effective
benefits. The consideration of these alternatives assumes that alternatives for the source areas
and WRD are selected for the remedy. The source area and WRD actions will also provide
sufficient protection for the ecological receptors within OU-1 because the cleanup levels for the
COCs are more stringent for public health exposure than for the ecological receptors and
exposures.
Comparative Analysis of Herman Impoundment Alternatives is provided in Table 2-6.
Alternative 1, No Action, is not suitable for Herman Impoundment, even though there are no
specific RAOs for Herman Impoundment. Herman Impoundment is part of a dynamic surface
water and groundwater system within the SBMM Site. The source control remedial actions are
predicted to improve the water quality within Herman Impoundment by removing the ARD
input. Over time, Herman Impoundment is expected to approach the ambient water quality of
hydrothermal groundwater. Because of these anticipated changes and the critical role Herman
Impoundment has in the groundwater flow, Herman Impoundment will be monitored.
Furthermore, Herman Impoundment should be secured from visitors due to its potential physical
hazards.
Alternative 2 would provide the monitoring and maintenance of security for the Herman
Impoundment. The cost is low and it is easy to implement. The monitoring and maintenance are
both essential to the remedy. In conjunction with the source area capping, Alternative 2 is a
monitored natural recovery for the Herman Impoundment water. At this time, the geochemistry
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of the SBMM system is estimated to approach ambient hydrothermal groundwater conditions
after the source area caps are installed and ARD inputs to Herman Impoundment are minimized.
Monitoring Herman Impoundment's water quality is essential to demonstrating progress towards
ambient groundwater conditions. Water quality in Herman Impoundment has been improving
due to prior surface water diversions; therefore, the monitoring should proceed continuously
through the RD and remedial action process. Removal of the waste rock to allow the
groundwater exiting Herman Impoundment to reach Clear Lake without degradation of its water
quality would comply with the RWQCB requirements of protection of high-quality waters (Clear
Lake), because Herman Impoundment water is expected to approach background hydrothermal
groundwater concentrations after the source controls have been implemented.
Alternative 3 would divert groundwater to reduce the flux of mercury to Clear Lake, which is
caused by groundwater flowing beneath the WRD. If the Selected Remedy for the WRD
performs as predicted, the need for a cutoff wall to divert the groundwater is not necessary. Two
potential cutoff wall locations were evaluated: (1) between Herman Impoundment and the WRD
to divert groundwater before it encounters waste rock beneath the WRD, and (2) between the
WRD and Clear Lake to halt the intrusion of Clear Lake water into the waste rock along the
shoreline.
The construction of a cutoff wall between Clear Lake and the WRD to prevent intrusion of Clear
Lake into the shoreline waste rock is best evaluated as part of the OU-2 RI/FS because its
analysis and implementation would have to be performed in conjunction with any alternatives for
addressing waste rock and contaminated sediments that are in Clear Lake.
2.11 PRINCIPAL THREAT WASTE
Principal threat wastes are source materials considered to be highly toxic or highly mobile that
generally cannot be reliably contained or would present a significant risk to human health or the
environment should exposure occur. Low-level threat wastes are those source materials that
generally can be reliably contained and that would present only a low risk in the event of release,
including source materials that exhibit low toxicity, low mobility in the environment, or are near
health-based levels (EPA 1991b).
Apart from Herman Impoundment, all the Areas of Concern (AOCs) within OU-1 are considered
"source material," which includes or contains hazardous substances, pollutants, or contaminants
that act as reservoirs for migration of contaminants to groundwater, surface water, air, or act as
sources for direct exposure. Source material is characterized as either principal threat waste or
low-level threat waste. Principal threat waste is material considered to be highly toxic or highly
mobile and, in general, cannot be reliably contained and/or would present significant risk to
human health or the environment should exposure occur. Low- level threat wastes are those
source materials that can be reliably contained and that present low risk in the event of release.
Solid media within OU-1 that contain contaminants above their respective remedial criteria
constitute source materials because they act as a reservoir for migration of contamination to
groundwater and surface water. Solid media, including mine waste and contaminated soil, at the
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mining-related sources addressed by this remedy are considered low-level threat waste, rather
than principal threat waste, for the following reasons:
• Contaminants in solid media are not highly mobile.
— The contaminants present at this Site are inorganics that are generally bound as part
of mineral assemblages within the solid media and are only mobile when in contact
with acidic water over time.
• Contaminants in solid media can be reliably contained.
— The contaminants present at this Site are inorganics generally bound as part of
mineral assemblages within the solid media. Contaminants so bound are particularly
amenable to containment strategies that isolate them from water and oxygen.
The WRD Area includes a large pile of mine waste that was placed on the shore of Clear Lake
between the Herman Impoundment and the lake. The WRD is not a conventional dam and does
not prevent the Herman Impoundment water from flowing through it as groundwater into Clear
Lake. Although the surface of the WRD Area was covered with clean material placed by EPA
under removal authority, its interior contains areas with high levels of mercury and areas that
produce acidic groundwater. The movement of Herman Impoundment water, as a groundwater
flow, through the WRD is the primary pathway for site-related transport of mercury into Clear
Lake.
The WRD is currently a principal threat wastes since it is an ongoing source for contamination to
Clearlake. However, once it is removed and capped it will no longer be a principal threat wastes
because it can be reliably contained and capped along with other wastes once it is removed from
the groundwater flow path from Herman Impoundment to Clearlake.
The NCP states a preference for treatment alternatives to reduce toxicity or mobility, where
practicable, and to use engineering controls, such as containment, for wastes that pose a
relatively low long-term threat or where treatment is impracticable. At SBMM, treatment of the
large volumes of source material is impracticable and therefore this treatment preference cannot
be met. The source materials at SBMM are particularly amendable to containment strategies that
isolate the contaminants from contact with water, leaching, and migration. The selected
containment remedy described below is achievable and effective.
2.12 SELECTED REMEDY
The selected remedy described below was developed based upon the alternatives evaluated in the
Focused Feasibility Study. The remedy for each area of contamination is informed by the
components and requirements of one or more FFS alternative, customized in a manner intended
to provide the best compliance with the CERCLA 9 Criteria stipulated in the NCP.
The selected OU-1 remedy is the permanent remedy for the mine site and the residential areas
north and south of the mine. However, EPA is not selecting a remedy for groundwater as part of
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this ROD. EPA will evaluate groundwater as part of a future remedial decision for the Site. The
source control measures selected will protect groundwater from further release of contaminants
and will protect Clear Lake by preventing mercury flux in groundwater above the performance
standard of 0.5 kg/year.
In addition, the OU-1 remedy does not address Clear Lake (OU-2) or OU-4 (including the North
Wetlands) which is expected to be addressed in future RODs. The remedy described within this
ROD is not intended to foreclose on any potential actions for the other OUs.
Long-term monitoring of the groundwater and surface water will be conducted to evaluate the
effectiveness of the remedy, including monitoring water quality, water elevations, and flow.
The selected remedy also includes common elements such as conducting pre- and post-
construction surveys, installation and monitoring of temporary and permanent erosion and
sediment control measures, dust suppression, access road improvements (as necessary),
generation of uncontaminated borrow for construction of remedial components and access roads,
and implementation and monitoring of engineering and institutional controls.
2.12.1 Summary of the Rationale for the Selected Remedy
Based on information currently available, EPA (the lead agency) determined the selected remedy
meets the threshold criteria specified by law (CERCLA). The selected remedy provides the best
balance of trade offs among alternatives presented in the FFS and takes into consideration the
balancing/modifying criteria.
The selected remedy satisfies the following legal and statutory requirements of CERCLA Section
121(b). The selected remedy meets the requirements of the two mandatory threshold criteria:
(1) protection of human health and the environment, and (2) compliance with ARARs, while
providing the best balance of benefits and tradeoffs among the five balancing criteria. It is
comprised of cost-effective alternatives, and it utilizes permanent solutions (to the maximum
extent practicable).
The selected remedy will protect human health by reducing exposure of local residents or on-
mine users to contaminated soil by direct contact or dust and will protect from exposure to
mercury vapors in the source areas.
The selected remedy reduces environmental risk from the Site contamination that will remain on-
mine (capped or covered) and ensures the long-term protectiveness of the removal actions taken
to date.
The selected remedy will protect the environment by reducing the transport of contaminants
from the mine to downstream surface water receptors such as fish and macroinvertebrates in
Clear Lake. Protection of the environment will be accomplished by reducing the mobilization of
contaminants by surface water runoff, groundwater, and wind. Surface water runoff will be
managed to keep runoff from interacting with on-mine soil containing COCs before it enters
Clear Lake.
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Migration of contaminants to groundwater will be controlled by containing mine waste and
contaminated material sources beneath caps or covers that are design and constructed to reduce
infiltration into the underlying waste and minimize leachate production in the source areas.
The OU-1 remedy described below is necessary because OU-1 contains the vast majority of
contaminant source materials, which influence contamination in OU-2 and OU-4 via fugitive
dust emissions, surface water, stormwater, and groundwater migration. Addressing these source
materials prior to addressing the downstream affected areas helps to ensure that recontamination
does not jeopardize remedies installed in other OUs. Furthermore, the proximity between
exposed waste materials and affected communities is an environmental justice issue that warrants
prompt and permanent solutions.
2.12.2 Description of the Selected Remedy
The selected remedy for OU-1 includes the following actions, as shown on Figure 2-8:
• Off-mine Residential Soil Cleanup
• Excavation of Mine Waste and mine-impacted soils, consolidation of these materials, and
capping/covering of the Source Areas where mine waste remains.
• Development of a soil borrow area(s) for sourcing on-site clean cover material.
• Backfill of the Northwest Pit (with site-waste and/or clean fill)
• Demolition and Removal/Consolidation of Abandoned Structures
• Monitoring of Herman Impoundment and stormwater infrastructure improvements to
reduce water inputs into the impoundment.
• Removal of waste rock in the WRD Area where it lies beyond the 1888 pre-mining
shoreline of Clear Lake and where it is saturated with groundwater between Herman
Impoundment and Clear Lake.
• Capping/Covering of remaining waste rock within the WRD Area.
• Engineering Controls and Institutional Controls
• Operations and Maintenance for Major Remedy Components, including Mercury Flux
and Remedial Effectiveness Monitoring
There are five remedial action objectives (RAOs) for this remedy. Remedial Design, including
pre-design and design investigations, will provide data necessary to inform many specific details
of the planned remedial action.
Due to the risk posed by consumption of fish from Clear Lake, RAO 5 sets a limit on the total
flux of mercury from OU-1 to Clear Lake via groundwater. This RAO informs the WRD
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components of the remedy. The WRD components of this remedy will be implemented as early
in the Remedial Action process as practicable so OU-1 contributions to risk in OU-2 are
addressed early.
Common amongst all components of the selected remedy is a need for appropriate construction
controls, such as dust management, traffic mitigation, stormwater management, and safety-
related protective measures, as well as closure and post-closure planning documents. Specific
plans will be developed for such construction controls as determined necessary during Remedial
Design.
Historic properties and tribal-cultural/archaeological resources potentially impacted by the
remedial action will be avoided or mitigated in accordance with Section 106 of the National
Historic Preservation Act. EPA will consult with the State Office of Historic Preservation and
the Elem Indian Colony. EPA is not required to develop an NHPA Memorandum of Agreement
(MOA) under CERCLA for the remedy because it is a procedural requirement. For site-specific
reasons, EPA intends to negotiate an MOA to document how EPA plans to avoid, minimize or
mitigate any adverse effects on historic properties. If EPA is unable to negotiate an MOA, EPA
may proceed with the remedy after documenting the actions EPA will take to avoid, minimize or
mitigate any adverse effects on historic properties.
2.12.3 Components of the Selected Remedy
The remedial action is composed of the following major components, broken down by action
area:
Off-mine Residential Soil Cleanup (based upon Residential Soils Alternative 2 of the FFS).
Remediation of residential soils is needed on the Elem Indian Colony and in the southern
neighborhood along Sulphur Bank Mine Road. In these areas, residential lots where soils exceed
cleanup levels will be excavated and/or capped to provide 12-24 inches of clean cover material
with the following exceptions and clarifications:
• The residential soil cleanup levels for mercury, arsenic, and antimony are based upon an
estimate of background concentrations for these constituents. These cleanup levels may
change if data demonstrate that background levels of mercury and arsenic differ
significantly from those prescribed in Table 2-3. Such a modification will be documented
in an appropriate change to the remedy.
•
• Exceedance of the cleanup levels set to background will be assessed by a statistical test
that compares the distribution of sample results for each residential lot to the distribution
of data used to calculate background concentrations for each COC.
• Along roadways a paved roadway cap may be substituted.
• In areas where cleanup levels are not met at the maximum excavation depth,
institutional/engineering controls will be implemented to ensure the long-term
protectiveness of the remedy. These controls may include elements such as visual
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barriers, ongoing coordination with tribal leadership/community, posting of notices, more
frequent cap inspections, etc.
• Excavated materials will be returned to its source on the mine site. Excavation areas will
be backfilled with the clean soil and will be revegetated.
• Soil imported for use as cover in the residential areas will be sampled and analyzed for
COCs to verify it does not exceed cleanup levels and to verify it meets the criteria for
clean fill, established by the California DTSC clean fill guidance (DTSC 2001).
• Some areas may require additional armoring with coarse material beyond the target cap
thickness to ensure the long-term integrity of the cap.
• There may be limited areas, such as potential "clean corridors," where a clean soil cap of
greater than 24 inches in total thickness would be necessary to provide long term
protectiveness. Such considerations will be evaluated during Remedial Design.
• For areas previously remediated under removal authority, the protectiveness of the caps
installed will be evaluated with consideration to revised cleanup levels and design
requirements. Additional remedial work in these areas will be performed as necessary.
Specifically, the addition of thallium as a COC may require additional remediation in lots
that were previously subject to removal action.
Non-residential areas of the EIC: Limited data is available regarding COC concentrations in
common areas of the EIC (those areas that have not be subdivided into residential lots). If Pre-
Design and Design-related sampling identifies COC concentrations in excess of the residential
soil cleanup levels in these areas, the appropriate remedial decision unit will be determined in
coordination with the EIC government and community based upon current and expected future
uses. Remedial action will then be implemented consistent with the residential soil cleanup for
those decision units where COCs exceed cleanup levels. Informational ICs and ECs will be
implemented as necessary to ensure community awareness of potential contamination beneath
any caps or in areas with less than 24 inches of clean cover.
Though the HHRA demonstrated that acorns from trees growing in highly contaminated areas
represent unacceptable risk for the traditional tribal use exposure scenario, additional
investigation work to be performed in Operable Unit 4 will inform the question of whether risk is
present in areas where COC concentrations are lower. EPA anticipates that the soil cleanup on
the EIC would address risk due to site contamination from foraging of traditional plants,
however if the planned work in OU4 indicates otherwise, EPA intends to coordinate with the
EIC to provide institutional controls or further remedial action. An appropriate change to this
decision document may be needed to accommodate this.
Consistent with the wishes of the EIC, no investigation or cleanup activities will be performed
within the EIC cemetery. Informational ICs and ECs will be implemented to ensure community
awareness of potential contamination in this area so that appropriate precautions can be taken.
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Sulphur Bank Mine Road Improvements for Safe Access: Currently, residents living in homes
south of the SB MM mine along Sulphur Bank Mine Road (Southern Residences) access their
residences via the northern section of Sulphur Bank Mine Road, which crosses the South Waste
Rock Pile, Tailings Pile and West Waste Rock Pile. To provide protectiveness and improve
implementability of the selected remedy, EPA anticipates closing this easement through the mine
during remedy implementation. For this reason, improvements to Sulphur Bank Mine Road south
of the residences are needed. These improvements may include road widening, paving,
installation of gates, and other associated infrastructure improvements.
Removal of Mine Waste, Consolidation, and Capping/Covering of Source Areas (based
upon Source Areas Alternative 2/3 of the FFS). This component of the remedy includes
removal/consolidation of numerous mine waste piles/areas, expected to include, but not
necessarily restricted to: the Northwest Waste Rock Pile, the Satellite Waste Rock Pile, the West
Ore Pile, the East Ore Pile, the Tailings Pile, and waste placed into Herman Impoundment during
early actions at the site. Multiple lines of evidence will be used to inform when excavation is
complete and/or capping is appropriate in a particular area excavation area (e.g. material color,
texture, lithology changes, minerology, XRF-based chemical concentrations, etc). Further
consolidation/recontouring of waste is also expected around and atop the North Waste Rock Pile,
the South Waste Rock Pile and within the Northwest Pit.
Final caps or covers will be constructed over the consolidated waste units at the North Waste
Rock Pile, the South Waste Rock Pile and/or the Northwest Pit source areas. These caps will be
designed and constructed in accordance with the requirements of Title 27 according to the
classification(s) of the waste (Group A, B, or C) being capped, as defined during Pre-Design
and/or Design investigation(s).
Either a RCRA-Subtitle C prescriptive cap or an "engineered alternative" cover will be placed
over the mining wastes described in this section. Alternative covers will be evaluated during
design (to include, but not limited to ET covers, soil covers with a capillary barrier, a soil cover
with a compacted barrier layer, a soil cover with a clay barrier, or other combinations).
Determination of whether a RCRA Subtitle C multilayer cap or an alternative cover is
appropriate will be made during RD, depending on an evaluation of the underlying waste
classification per Title 27, site-specific climate conditions (e.g., precipitation, evaporation, and
transpiration), depth to groundwater, evaluation of drain down/leachate quality and quantity,
operations and maintenance requirements for capped areas and suitability of available
construction materials to reduce infiltration of meteoric water into the underlying waste.
Treatability/Pilot studies may also be needed to demonstrate the performance of alternative
cover(s).
Capping material is expected to come from one or more nearby sources including:
• Soil borrow areas constructed on-site and/or on lands adjacent/near to the mine site
(Figure 2-8). The sourcing of on-site borrow soil will entail a number of related
activities/processes, including vegetation clearing, construction of support facilities,
stormwater controls, excavation, crushing, sorting, blending, and revegetation.
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• Offsite fill soil and/or topsoil.
• Clean sediments potentially processed/blended on-site (this may include dewatering and
material rehandling)
Prior to importation, EPA will ensure that appropriate characterization is performed to
demonstrate that these materials are suitable for their intended use.
Inclusion of irrigation systems for capped areas is not assumed, but the potential need for such
will be evaluated during remedial design and included as necessary.
In areas where mining waste cannot be identified, but COC concentrations exceed identified
cleanup levels due to suspected impacts from the mine operations (e.g. fugitive dust emissions or
mineralized areas exposed by mine operations), remediation may involve the placement of clean
cover material, growth media and/or an engineered cap, as necessary to provide protectiveness
for the expected future uses of the subject area. The use of alternative or supplemental cover
materials such as biosolids, compost, and wood mulch will be evaluated during RD for use in
these areas. These alternative materials will also be evaluated for potential use to speed
revegetation/recovery for capped areas and in areas of thin or infertile soil, such as areas where
overlying mining waste has been removed.
The expected future land use for on-mine areas is limited traditional tribal use. This use is
expected to include hunting of big game or upland species, transit to reach off-mine foraging
areas and other activities that entail pedestrian use of the area. EPA intends to coordinate closely
with the EIC during Remedial Design to tailor revegetation efforts to best align with the tribe's
needs in terms of traditional practice on the property, however foraging of traditional edible
plants is not a traditional practice that is anticipated due to the requirement that deep rooted
vegetation be excluded from capped areas and the relatively thin cap thickness in areas that do
not contain mine waste. Pedestrian corridors may be constructed to protect capped areas.
Informational ICs and ECs will be implemented that will indicate this limitation on future use.
ICs and ECs will be implemented to monitor and maintain the integrity of the engineered cap or
cover. Long-term cap/cover monitoring and maintenance will be conducted to ensure the
integrity of these remedy structures is protective.
Backfill of the Northwest Pit - Existing waste is present at the Northwest Pit that requires
remediation in a manner consistent with Title 27 in order to comply with RAO 4 (pertaining to
the prevention of ARD generation). EPA will determine if the NW Pit will be used as a waste
management unit, after coordination with the State of California. If, during remedial design and
after coordinating with the State, EPA determines that the NW Pit waste management unit
cannot be constructed and operated in a manner that achieves RAO 4, the NW Pit will be
backfilled with clean fill.
The evaluation of potential waste backfill will include consideration of whether sufficient
capacity exists in other waste management units on site to accommodate the waste requiring
consolidation and capping, the risks/impacts of importing a larger volume of clean fill, the
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protective measures available for placement of site-derived waste within the pit, and the potential
risk to groundwater of a WMU without a bottom-liner. In addition, technical evaluations of
several key uncertainties are expected, such as whether geothermal gases that emanate from the
bottom of the NW pit can be reliably diverted for venting without generating leachate with the
potential to impact groundwater and if the potential presence of a fault(s) beneath the NW Pit
would endanger the long-term performance of the waste management unit in a manner that its
design could not be adapted to mitigate.
If, during remedial design and after coordination with the State, EPA determines that a NW Pit
waste management unit can be constructed and operated in a manner that achieves RAO 4, the
LCRS exemption(s) under Title 27 sections 22470(b) and/or (c) would apply to the NW pit area
and therefore a Title 22 compliant Leachate Control and Removal System (LCRS) would not be
included. In this case, the Dry Unit Liner/LCRS Exemption monitoring procedures and contingency
requirements would apply. This backfill would be designed in a manner to prevent leachate
production and migration, therefore additional protective measures to ensure leachate control
may include elements such an engineered alternative/modified LCRS design.
If the Northwest Pit will be backfilled with site-derived waste, it will be performed in the
following manner:
• Where required by 20240(c) of Title 27, mining waste and mine-influenced sediments in
the bottom of the Northwest Pit will be removed to provide at least 5 feet of separation
between the highest anticipated groundwater elevation and any mining waste within the
pit.
• Where required by 20240(c) of Title 27 uncontaminated fill will be placed in the lowest
portions of the pit as necessary to provide at least 5 feet of separation from the highest
anticipated groundwater elevation and the bottom of waste backfill.
• The pit will then be backfilled with mine waste consolidated from elsewhere onsite in a
manner that provides a final backfilled surface that is mounded to passively shed
stormwater.
• Gas venting and leachate monitoring systems will be incorporated into the construction of
the backfill.
• The backfilled pit will then be capped with a Title 22 RCRA-Subtitle C cap.
Performance monitoring for a backfilled NW Pit would be expected to include multi-depth
lysimeters, wells installed at the base of the Pit at first encountered groundwater level, along with
monitoring wells outside of the Pit that are sufficient to determine flow direction and potential
migration of leachate release. The monitoring should include a sampling protocol that defines
baseline, pre-construction geochemical conditions in the groundwater beneath the Pit.
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If performance monitoring within, beneath or outside of the waste management unit indicates
that disposal procedures/containment measures were to be inadequate to comply with RAO 4,
EPA will develop and implement contingencies pursuant to Title 27, 22470.
If, during remedial design and after coordinating with the State, EPA determines that the NW Pit
waste management unit cannot be constructed and operated in a manner that achieves RAO 4,
the NW Pit will be backfilled with clean fill. To ensure consistency with RAO 4, clean backfill
of the NW Pit is expected to be performed in a manner that allows passive surface water runoff.
Clean backfill would provide additional protection for area groundwater but would limit options
for mining waste placement elsewhere. In the clean backfill scenario, additional design elements
and/or institutional controls would be needed to protect users of the site from the physical
hazards posed by the remaining pit highwalls. These may include, backfill sloping, fencing
and/or signage.
Demolition and removal of abandoned structures (based upon common element of source
area alternatives). Abandoned mine operations buildings and structures on site will be
demolished and disposed of on and off-site according to the materials encountered based on the
following:
• Concrete elements and non-concrete components determined to be non-hazardous waste
will be consolidated into one or more of the waste piles to be capped.
• Non-concrete elements that meet the definition of hazardous waste will be disposed of
off-site in an appropriately permitted facility to be identified during Remedial Design.
Herman Impoundment (based upon Herman Impoundment Alternative 2 of the FFS). Herman
Impoundment will be secured to restrict access due to physical hazards. The collection and
diversion pipelines will be repaired or replaced as necessary. Contouring of waste piles and wall
rock areas surrounding Herman Impoundment will be performed in coordination with capping of
these areas in a manner intended to move as much clean non-contact stormwater away from
Herman Impoundment as practicable with the aim of reducing the water level within the
impoundment. Monitoring will include analyzing water quality, measuring water elevation, and
maintaining site security.
After implementation of the remedy described above and below, Herman Impoundment will
have hydrothermal groundwater (from springs at the bottom of the pit and minor lateral
contributions through the sidewalls of the pit) and rainfall as its two sources of water.
Geochemical modeling indicates that in the long-term, the Herman Impoundment water will
approach the background threshold values for hydrothermal groundwater. If the future water
quality of Herman Impoundment is found to represent a risk to human or ecological receptors,
then the need for further action will be evaluated and a modification to the Record of Decision
would be made, if necessary.
Waste Rock Dam Excavation and Capping (based upon WRD Alternatives 2 and 3 of the FFS
and modified to ensure full protectiveness and ARARs compliance).
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The WRD will be excavated back to the 1888 pre-mining shoreline. Saturated waste within the
"saturated waste rock of significance" will also be excavated to minimize contact between
Herman Impoundment water and waste within the WRD. The exact contours of the material that
will need to be excavated for this remedy component will be further refined during pre-design
and remedial design.
Excavated waste rock dam material will be removed and consolidated into one of the Source
Areas described above before the permanent engineered cap or cover is installed. Capping
material will be sourced as described above. Clean fill will be placed in the excavated area to
promote site drainage, ensure slope stability, create a stable shoreline, and to provide access for
long-term cap/cover monitoring and maintenance. Vegetation restoration may also be included to
provide habitat improvement along the modified post-remedy lake shoreline. The portion of the
WRD left in place will likewise be capped as necessary according to its Title 27 waste
classification. Long-term cap/cover monitoring and maintenance will be conducted to ensure the
integrity of these remedy structures is protective.
The WRD components of the remedy are intended to address the mercury contamination
entering Clear Lake through groundwater. The removal of WRD back to the 1888 pre-mining
shoreline is being selected to provide the greatest likelihood of remedial success across a range
of future conditions, to address community concerns, and to minimize the volume of capping
material that needs to be imported to the site. The capping of mining waste and installation of
stormwater controls described in this section and those above are expected to limit rainfall
infiltrating to groundwater and runoff entering Herman Impoundment. This is expected to reduce
the water level of Herman Impoundment, thereby significantly changing the movement of
groundwater through the WRD. These changes in the hydrology of the site, along with the
removal and capping of waste with the potential to introduce COCs to the groundwater between
Herman Impoundment and Clear Lake, are expected to provide compliance with RAO 5.
A monitoring program will be designed to evaluate the selected remedy's effectiveness with
regard to the 0.5 kg/year total mercury load requirement in RAO 5. Mercury flux from the site to
Clear Lake will be calculated using the high-resolution groundwater modeling developed as part
of an ongoing flux monitoring effort that has been initiated as part of Pre-RD work and is
expected to continue through Remedial Action and into O&M.
Monitoring data would be used to develop future response actions if necessary. Monitoring of
the flux of mercury to Clear Lake will commence as soon as feasible after the WRD and Herman
Impoundment components of the remedy have been completed and the post-excavation flux
monitoring network has been installed. After 8 quarters of such monitoring, if the gathered data
related to mercury flux do not show compliance with RAO 5, EPA intends to evaluate whether
further action is appropriate in a Focused Feasibility Study.
RAO compliance analysis will be based on the best available assessment methodology as
informed by modeling and/or data including water level, pH, water chemistry, isotopes for both
Herman Impoundment and groundwater within the WRD. RAO compliance will also be
considered in relationship to the probability of compliance over the anticipated range of climate
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conditions and the potential for an asymptotic end point above the TMDL-based performance
target.
2.12.4 Remedial Design Investigation and Key Design Decisions
As described in prior sections, focused investigations will need to be conducted during
development of the RD. These design investigations will address the requirements of Title 27
regulations, identified areas of uncertainty, and obtain data needed to develop design details.
Table 2-7 summarizes existing estimates of mine waste pile metrics that serve as initial
information for development of the RD. Data obtained from the design investigation will not
alter the remedy selection but are required to meet the RAOs by refine design details of the
remedy for constructability, in accordance with standard engineering practices. The key
investigations that will be needed to complete the RD are listed below. As the RD is developed,
additional specific design investigations may be needed.
• Consolidation of waste versus capping/covering waste in place. The extent of the
source area caps/covers will depend on a range of factors that will be evaluated during
RD, likely including, the extent and concentrations of COCs in soil, the availability of
repository capacity, the thickness of existing waste piles, and the cost effectiveness of
consolidation vs capping where either would provide equivalent protectiveness.
Decisions on the cap/cover geometry will include consideration of the EIC desire to have
more area of the Site to be less restricted in its future use. Cap/cover configuration will
also be designed to maximize runoff, minimize ponding, and to include surface water
drainage features to protect the caps/covers from erosion. The economic tradeoff of
consolidating waste and capping/covering the consolidation areas versus capping or
covering waste in place will be evaluated in the RD.
• Northwest Pit. As noted above, the Northwest Pit will have field investigations and
engineering analyses performed to evaluate its suitability to receive additional waste to
backfill it. This evaluation will include consideration of whether sufficient capacity exists
in other waste management units on site to accommodate the waste requiring
consolidation and capping, the risks/impacts of importing a larger volume of clean fill,
the protective measures available for placement of site-derived waste within the pit, and
the potential risk to groundwater of a WMU without a bottom-liner. In addition, technical
evaluations of several key uncertainties are expected, such as whether geothermal gases
that emanate from the bottom of the NW pit can be reliably diverted for venting without
generating leachate with the potential to impact groundwater and if the potential presence
of a fault(s) beneath the NW Pit would endanger the long-term performance of the waste
management unit in a manner that its design could not be adapted to mitigate. The data
will be used to establish design parameters for the pit as either a waste repository or to
define how clean backfill will be performed. Alternative fill materials may also be
considered, including those materials that may not be soil or soil-like.
• RCRA-Style Caps versus Alternative Caps/Covers. Either a RCRA-Subtitle C
prescriptive cap or an "engineered alternative" cover will be placed over the mining
wastes described in this section. Alternative covers will be evaluated during design (to
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include, but not limited to ET covers, soil covers with a capillary barrier, a soil cover
with a compacted barrier layer, a soil cover with a clay barrier, or other combinations).
Determination of whether a RCRA Subtitle C multilayer cap or an alternative cover is
appropriate will be made during RD, depending on an evaluation of the underlying waste
classification per Title 27, site-specific climate conditions (e.g., precipitation,
evaporation, and transpiration), depth to groundwater, evaluation of drain down/leachate
quality and quantity, operations and maintenance requirements for capped areas and
suitability of available construction materials to reduce infiltration of meteoric water into
the underlying waste. Treatability/Pilot studies may also be needed to demonstrate the
performance of alternative cover(s).
• Sources of capping material. As indicated above, capping material needed for this
remedy is expected to come from one or more nearby sources including:
— Soil borrow areas constructed on-site and/or on lands adjacent/near to the mine site
(Figure 2-8). The sourcing of on-site borrow soil will entail a number of related
activities/processes, including vegetation clearing, construction of support facilities,
stormwater controls, excavation, crushing, sorting, blending, and revegetation.
— Offsite fill soil and/or topsoil.
— Clean sediments potentially processed/blended on-site (this may include dewatering
and material rehandling)
Identification of the specific sources within these general categories will be evaluated
during pre-design and Remedial Design. The relative amounts of cap material that might
come from these various sources will be evaluated during design and may be subject to
change according to the needs of the remedy throughout remedy implementation.
Capping material evaluation during RD will also include consideration of the
circumstances in which alternative cover materials might be appropriate, including
biosolids, compost, and wood mulch.
2.12.5 Remedy Sequencing
The remedial actions described in this section will be implemented according to a logical order
based on risk, implementability, and availability of resources. Final schedule and sequencing
plans will be determined during remedial design in coordination with site stakeholders, but are
expected to occur over staggered parallel timelines in the approximate sequence below. Care
will be needed to ensure that areas disturbed during each construction season (spring to fall) can
be excluded from contact with precipitation until the subsequent construction season. Ongoing
monitoring and site maintenance activities are expected across all of the indicated phases of
work.
• Residential Cleanup - The impacted communities on the EIC and in the Southern
Residential area have been affected by site contamination for over a century. Final
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remedial action to address remaining sources of risk to these communities are to be
prioritized in light of the density of sensitive receptors and Environmental Justice related
issues. This work may be further segmented into two parts - northern and southern,
pending the availability of resources.
• Source Areas that drain towards Herman Impoundment - Remediation of those
source areas that contribute stormwater and shallow groundwater flow towards Herman
Impoundment would then be remediated, either prior to or in conjunction with the Waste
Rock Dam component of the remedy.
• Waste Rock Dam - There is a preference for implementation of the Waste Rock Dam
component of the remedy as early in the remedial sequencing as practicable. This work
will require that an acceptable repository location has been identified during design
and/or prior phases of work.
• Remaining Source Areas - Remaining mining wastes would then be remediated as
necessary to provide ARAR compliance and ensure achievement of all RAO.
2.12.6 Institutional Controls and Engineering Controls
Institutional Controls (ICs) and Engineering Controls (ECs) may consist of non-engineered
instruments, such as administrative or legal controls and engineered physical barriers, such as
fences and signs. The selected remedy includes ICs and ECs to protect the remedy structures
(e.g., caps or covers), maintain protectiveness of the constructed remedy, and inform the public
of use restrictions within specific areas.
In residential areas, IC will be implemented where cap/covers have been installed over waste or
soil exceeding cleanup levels. ICs or ECs to prevent tilling, excavation, grading, construction, or
any other intrusive activity that can damage the cap or cover or expose underlying contaminated
soil will be implemented. These ICs or ECs will also restrict activities that might interfere with
or adversely affect the implementation, integrity, or protectiveness of the remedial action at the
Site. These controls may include elements such as visual barriers, ongoing coordination with
tribal leadership and the community, posting of notices, and cap inspections.
For on-mine areas ICs or ECs will be designed to prohibit activities that:
• Disturb or damage caps/covers, revegetated areas, or constructed stormwater drainages
• Expose buried tailings, waste material, or contaminated soil
• Disturb any monitoring systems, including groundwater monitoring wells, stormwater
monitoring devices, and water level monitoring devices
• Disrupt or impede the free flow of stormwater through the collection and control
structures (drainage ditches or other surface water diversions)
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To accomplish the above, institutional controls will be developed with the State and Elem Indian
Colony during Remedial Design and are expected to include, but are not necessarily limited to:
• Signage
• Informational fliers
• Public meetings
• Fencing
• Periodic inspections
• Land use restrictions
Land use covenants will be recorded on the mine site by the Redevelopment Trust as the current
landowner as referenced in Table A-l "Applicable Relevant and Appropriate Requirements".
Land use covenants will not be recorded on private residential properties south of the mine site
or the EIC.
2.12.7 Long-Term Operation, Maintenance, and Monitoring
The selected remedial action components will be maintained to ensure the remedy remains
protective of human health and the environment. Long-term operations, maintenance, and
monitoring will include regular inspection of constructed remedy structures, maintenance of the
caps/covers, maintenance of surface water channels and diversions, maintenance of engineered
structures, and monitoring and maintenance of revegetated areas to ensure the vegetative cover is
adequate to maintain protectiveness, control erosion, and effective evapotranspiration if ET
covers are constructed. Monitoring will also include ongoing sampling of Herman
Impoundment and a sufficient number of wells to inform the analysis of flux of mercury from the
mine to Clear Lake.
An OU-1 O&M Plan(s) will be developed during preparation of the RD, then finalized after
installation of the remedy is complete. The O&M Plan will define ongoing operation,
maintenance, and monitoring requirements for all remedy components.
O&M for the residential soil components of the remedy is expected to include ongoing
education/maintenance of ECs/ICs for subject properties, inspection/self-reporting by
residents/owners, and occasional repair such as pavement resurfacing/sealcoating, etc.
Routine and non-routine repair is anticipated. Non-routine repair is expected to include relatively
infrequent, but predictable replacement of system components such as gas venting systems,
revegetation/replacement of eroded cover material after heavy erosional events, sediment
removal from stormwater basins, and possible replacement of impermeable cap material in
damaged areas.
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The initial O&M will be performed by EPA, and the State of California will assume O&M at the
time required by the NCP.
2.12.8 Compliance with ARARs
The selected remedy will comply with the ARARs listed in Appendix A Table A-l.
2.12.9 Summary of the Estimated Remedy Costs
As shown in Table 2-9, the estimated cost of the Selected Remedy is $94,507,640 based upon the
selected remedy components as above. This reflects estimated capital costs plus operations and
maintenance costs at a 7% discount rate over a 30 year period. The calculation of net present
value (NPV) at the 7% discount rate is in accordance with A Guide to Developing and
Documenting Cost Estimates During the Feasibility Study (EPA, 2000).
The information in this cost estimate summary table is based on the best available information
regarding the anticipated scope of the Selected Remedy. Changes in the cost elements are likely
to occur as a result of new information and data collected during the engineering design of the
Selected Remedy. This is an order-of-magnitude engineering cost estimate that is expected to be
within +50 to -30 percent of the actual project cost. EPA anticipates additional refinements in the
cost estimate for each remedy component as work progresses through pre-design, design, and
preparation for construction.
The details of the cost estimate are provided in Table 2-9. The values provided have been
adjusted for inflation according to the Consumer Price Index adjustment between 2016 and 2023.
Calculations that support the cost summary provided in Table 2-9 are presented in the FFS (E2
2021).
2.13 STATUTORY DETERMINATIONS
The remedial action selected for implementation at the SBMM Site is consistent with CERCLA,
and, to the extent practicable, the NCP. The Selected Remedy is protective of human health and
the environment, will comply with ARARs, and is cost-effective. In addition, the Selected
Remedy utilizes permanent solutions and alternate treatment technologies or resource recovery
technologies to the maximum extent practicable.
2.13.1 Protection of Human and the Environment
The Selected Remedy will adequately protect human health and the environment by reducing
exposures of human and environmental receptors to Site contaminants through engineering
controls, treatment, and ICs. The Selected Remedy will reduce environmental risk to protective
ARARs, HHRA, or background (PMB) levels. Specifically, the remedy will reduce metal
concentrations in exposed surface soil by consolidating (or excavating) and capping mine waste
above the PMB concentrations for each COC. Lastly, the remedy will permanently address the
waste rock that causes groundwater mercury flux to Clear Lake from the mine site by removing
from potential contact with groundwater and Clear Lake water.
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Implementation of the Selected Remedy will not pose any unacceptable short-term risks or cause
cross-media impacts.
2.13.2 Compliance with Applicable or Relevant and Appropriate Requirements
The Selected Remedy will comply with all ARARs identified in Appendix A, Table A-l.
2.13.3 Cost Effectiveness
EPA has determined the Selected Remedy is cost-effective. In making this determination, the
following definition set forth in the NCP was used: "A remedy shall be cost-effective if its costs
are proportional to its overall effectiveness" (40 CFR 430[f][l][ii][D]). Of those alternatives that
are protective of human health and the environment and comply with ARARs, the Selected
Remedy provides "overall effectiveness" in terms of balancing the long-term effectiveness and
permanence, short-term effectiveness and reduction in toxicity, mobility, and volume. The
"overall effectiveness" of the Selected Remedy was then compared to costs to determine cost
effectiveness. Cost effectiveness was also evaluated with regard to potential costs that may be
incurred in other areas of the Site if the selected remedy were not implemented. For instance, this
remedy targets the control of sources of contamination, thereby reducing or avoiding costs that
may otherwise later be incurred in the treatment of receiving waters.
The relationship of the overall effectiveness of this remedial alternative was determined to be
proportional to its costs and hence this Selected Remedy represents a reasonable value for the
money spent.
2.13.4 Utilization of Permanent Solutions and Alternative Treatment (or Resource
Recovery) Technologies to the Maximum Extent Practicable
The Selected Remedy consists of permanent solutions to reduce the flux of mercury reaching
Clear Lake from the mine, thus reducing contaminant mobility, the volume of contaminated
water emanating from the mine, and impacts to downstream waters. The selected source control
alternatives are expected to require standard maintenance with the result of permanently reducing
the generation of ARD and reducing public exposure to fugitive dust or direct contact with the
mine waste. Treatment of the large volumes of waste is not practicable; however, the wastes can
be contained cost effectively using conventional capping technology. If the groundwater
containment contingencies are necessary, treatment of the extracted water will be completed
on-site, instead of transporting contaminated water off-site, thereby conforming with the
preference for treatment instead of off-site disposal.
2.13.5 Five-Year Review Requirements
The remedy will result in hazardous substances remaining on-mine above levels that would
allow for unlimited use and unrestricted exposure; therefore, statutory reviews are required, no
less often than each 5 years after initiation of remedial action, to ensure that the remedial action
being implemented protects human health and the environment. The first review will be
conducted within 5 years after initiation of the remedial action. The review described in Section
2.12 may be incorporated into the remedy review process.
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2.14 DOCUMENTATION OF SIGNIFICANT CHANGES TO THE REMEDY
The PP for the OU-1 remedy was issued for public review on January 10, 2023. Comments were
received after a 90-day public comment period. EPA has reviewed all written and
verbal comments submitted during that comment period. It was determined that several
significant changes to the remedy, as originally identified in the proposed plan, were necessary.
These include:
• The addition of Thallium as a Contaminant of Concern for the site/remedy - This
adjustment resulted from reevaluation of the risk assessment. It does not affect the
comparative analysis of alternatives because it would apply to all alternatives for the
residential component of the remedy. This addition will however influence the design and
implementation of the residential area clean up, as well as the cost estimate. This revised
cost has been incorporated into the overall remedy cost throughout this document.
• Several of the numeric cleanup levels presented in the Proposed Plan have been revised
following evaluation of supporting documents in the Administrative Record. These
adjustments are intended to improve protectiveness, bring identified cleanup levels into
closer alignment with EPA guidance recommendations, and/or address
community/stakeholder concerns.
• The two-phased structure for the Source Areas/WRD components of the remedy
described in the Proposed Plan has been removed. The Selected Remedy instead selects
the high-end of the range of possible WRD excavation described in the Proposed Plan in
order to provide greater confidence in the long-term performance and protectiveness of
the remedy.
• As a result of eliminating the two-phased structure of the remedy, the management and
monitoring actions described in the remedy are substantially similar to Herman
Impoundment Alternative 2 from the FFS.
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November 2023
. 1990. National Oil and Hazardous Substances Pollution Contingency Plan. 40 Code of
Federal Regulations, Part 300. .
. 1991b. A Guide to Principal Threat and Low Level Threat Wastes. November.
. 1994a. Baseline Public Health Evaluation, Sulphur Bank Mine Site. January.
. 1994b. Remedial Investigation/Feasibility Study SBMM Superfund Site Clear lake Oaks,
California. Operable Unit 1 and Operable Unit 2.
. 1994c. Cultural Resource Survey, Sulphur Bank Superfund Site, Lake County,
California. April
. 1995. Determination of Site-Specific Cleanup Goals for Soils at Sulphur Bank Mine.
October.
—. 1997a. Human Health Effects Assessment Summary Tables (HEAST). Office of Health
and Environmental Assessment, Environmental Assessment and Criteria Office, Cincinnati,
Ohio. Prepared for Office of Solid Waste and Emergency Response, Office of Emergency
and Remedial Response, Washington, D.C.
. 1997b. Mercury Study Report to Congress, Volume III: Fate and Transport of Mercury
in the Environment. Office of Air Quality Planning and Standards and Office of Research
and Development.
. 2000a. Draft Guidance for Ecological Soil Screening Levels (Eco-SSLs). 10 July.
. 2000b. Water Quality Standards; Establishment of Numeric Criteria for Priority Toxic
Pollutants for the State of California. 40 CFR Part 131. May.
. 2002. National Recommended Water Quality Criteria. November.
. 2003. Human Health Toxicity Values in Superfund Risk Assessments. OSWER Directive 9285.7-
53. Memo from Michael B. Cook. December 5, 2003.
. 2004. Clean up Goals for Mercury in Soils at Sulphur Bank Mercury Mine - Some
Additional Considerations. February.
. 2007. Traditional Tribal Subsistence Exposure Scenario and Risk Assessment Guidance
Manual. Grant No. EPA-STAR-J1-R831046. August.
. 2011. Exposure Factors Handbook: 2011 Edition, Office of Research and Development,
EPA/600/R- 090/052F. September.
. 2012. Office of Solid Waste and Emergency Response Directive No. 9200.1-113.
December.
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, California
Operable Unit 1 Record of Decision
-------
Version: FINAL
Page 4-6
November 2023
. 2014. Human Health Evaluation Manual, Supplemental Guidance: Update of Standard
Default Exposure Factors. OSWER 9200.1-120, February 6.
. 2019a. Meeting between EPA, the EIC, and their consultants. September.
. 2019b. Regional Screening Levels (RSL) Table, https://www.epa.gov/risk/regional-
screening-levels-rsls-generic-tables. Accessed November 18, 2019.
. 2019b. Provisional Peer-Reviewed Toxicity Values Database. http://hhpprtv.ornl.gov.
. 2019d. Integrated Risk Information System (IRIS).
http://cfpub.epa.gov/ncea/iris/index.cfm. Accessed December 2019.
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, California
Operable Unit 1 Record of Decision
-------
Tables
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Version: FINAL
Table 2-1. Page 1 ofl
November 2023
Table 2-1. Mining Facility Building Description
1 ll>
lfiiil Vimr
iK'sci'ipliun .iimI llislorir 1 m1
1
Block Factory
This feature is the structural remains and waste products associated with a factor) that produced
blocks made of aggregate. The factory was operating from the 1940s through the 1960s. The concrete
pad remains as well as large bins used in block production. There are large amounts of debris piled
up.
2
Wooden Frame
This feature is a wooden frame apparently built to support a hoist of some sort. It was assumed to be a
part of a tramway to move ore from the open pit to milling and processing equipment on the hill to the
east.
3
Metal Water Tanks
This feature consists of remains of two large metal tanks set on a concrete pad against a concrete
retaining wall on the hillside of the east ore pile. These tanks were assumed to provide water for the
block factory.
4
Concrete Retaining Wall
This feature is concrete retaining wall and concrete pad with some piers or machinery mounts. Use of
this feature is unknown.
5
1940s Mine Office
This feature is the structural remains of an old mine office which operated from the 1940s. It has a
concrete foundation extending up as a retaining wall against the hillside. Some of the debris seen in
the area includes metal debris, a propane cooking stove, propane water heater, oil burning space
heater, a drawer/cabinet, standing furnace, and pieces of bedspring.
6
Metal Water Tanks
This feature consists of two metal water tanks. These are situated near the truck maintenance garage
and assumed to provide water for washing trucks and other activities.
7
Truck Maintenance Garage
This feature is the structural remains of a truck maintenance garage where mining vehicles were
washed and serviced. A building pad, retaining wall against the hillside on the southeast and
Northeast and the pits from which mechanics worked are observed.
8
Mercury Processing Area
This feature is the remains of mine processing units. This feature is multilevel and one of the biggest
structural features remaining on site. It is believed that the ore was moved from the lower levels of the
feature to the top; ore was heated through the furnace and then went through a series of facilities that
provided for cooling and condensation of mercury vapors and the collection of liquid mercury.
Currently, the area encompasses concrete pads for a two-level building, remains of concrete tanks and
a tunnel connecting them, segments of vitrified sewer pipes, series of concrete piers and a
considerable amount of fallen debris.
9
Meat house
This feature is a concrete walled building without a roof that housed a compressor to cool a meat
locker. The building is filled with wooden and metal debris.
10
Soil Fertilizer Plant
This feature is reported to be remains of a soil fertilizer plant that operated in the 1950s or 1 960s.
There are two concrete pads with debris.
11
Explosives Shack
This feature is documented to be an intact concrete-walled building set into a hillside near the
northwest pit.
12
Truck Garage
This feature is a structural remain of a truck garage and has a concrete pad.
13
Generator House
This feature is a fallen building of wooden frame and sheet metal roofing containing some metal
machinery debris. This building is assumed to have housed a generator to provide electricity for the
mine.
14
Old Mine Office
This feature is the structural remains of an old mine office before 1940s that consists of two levels of
concrete pads cut into the hillside. There is a shed at the entrance to the office area.
15
Fuel Tanks and Storage
Shed
This feature has a sunken shed (5 ft below the earthen ramp) t hat housed two large empty fuel tanks.
16
Grease Pit and Storage
Shed
This feature includes a concrete pad with a grease pit for mechanics used. Next to it is a sheet-metal-
walled shed full of 55-gallon drums.
17
Old Pump House and Truck
Garage.
This feature is a 60-meter-long building that was used as a truck garage and housed an old pump
house.
18
Old Ward House
This feature is the structural remains of an old residence. Remnants of a retaining wall are currently
visible.
19
Miners' Shacks
Two structures remain on-mine from the miners' shacks.
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California Operable Unit 1 Record of Decision
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Table 2-2. Page 1 of 1
November 2023
Table 2-2. Summary of Current Contaminants of Concern and Medium-Specific Exposure Point Concentrations
( niiriiiii.iimii Dciiriiil
Mrililllll
1 \|HlMllf I'llilll
ll.|X|x
( Iii'IiikmI "1 ( "iiivi ii
Mm
M.i\
1 nil\
1 H'«|iii'iK-v "1 hi'lirliiHi
1 \|HiMlll- I'lUIII < nlK'i llll.lllnll
1 \|himiii- 1'iniii c iiiui-iiii:iiiiiii 1 mix
Sl.ilixlir.il Mimxiiic
On-Mine Soil
Arsenic
0.53
316
mg/kg
100
73.84
mg/kg
95% Approximate Gamma UCL
Lakeside residents and SBMM
Road
Arsenic
0.53
222
mg/kg
95.47
19.03
mg/kg
95% KM (Chebyshev) UCL
Antimony
0.49
679
mg/kg
61.25
56.48
mg/kg
Thallium
1.35
1.6
mg/kg
0
1.6
mg/kg
Maximum concentration
Elem Indian Colony
Arsenic
0.27
7.6
mg/kg
80.22
3.166
mg/kg
95% KM (Chebyshev) UCL
(unexcavated areas)
Thallium
0.87
3.1
mg/kg
21.33
1.309
mg/kg
95% KM (t) UCL
Soil
Elem Indian Colony (fill areas)
Arsenic
0.54
55.3
mg/kg
94.44
4.494
mg/kg
KM H-UCL
Thallium
0.46
7.1
mg/kg
46.88
2.013
mg/kg
95% KM (t) UCL
PMB - Andesite
Arsenic
1.2
30
mg/kg
76.74
8.714
mg/kg
Gamma Adjusted KM-UCL (use when
k<=l and 15 < n <50 but k <=1)
Antimony
0.07
7.9
mg/kg
58.14
1.803
mg/kg
KM H-UCL
Thallium
0.057
0.336
mg/kg
75
0.184
mg/kg
95% KM Adjusted Gamma UCL
Franciscan (PMB)
Arsenic
1.59
78
mg/kg
73.96
9.852
mg/kg
95% KM (Chedbyshev) UCL
Thallium
0.04
0.117
mg/kg
76.92
0.0724
mg/kg
95% KM (t) UCL
Lakeside Residents
Dissolved
Arsenic
0.78
6.7
mg/kg
90.54
3.804
mg/kg
95% KM (Chebyshev) UCL
Drinking Water
Total
Arsenic
0.74
6.8
mg/kg
90.54
3.845
mg/kg
95% KM (Chebyshev) UCL
Tribal
Dissolved
Arsenic
0.56
6.5
mg/kg
78.57
2.762
mg/kg
95% KM (Chebyshev) UCL
Total
Arsenic
0.5
12.1
mg/kg
72.5
3.056
mg/kg
95% KM (Chebyshev) UCL
Acorns
Wet
Arsenic
0.0426
0.0769
mg/kg
60
0.0606
mg/kg
95% KM (t) UCL
Wet
Thallium
0.00522
0.446
mg/kg
75
0.185
mg/kg
95% KM (t) UCL
Cattail Stalks
Wet
Arsenic
0.036
0.045
mg/kg
20
0.045
mg/kg
Maximum concentration
Wet
Thallium
0.000263
0.000263
mg/kg
10
0.000263
mg/kg
Maximum concentration
Plants
Cattail Roots
Wet
Arsenic
0.0432
0.083
mg/kg
10
0.0703
mg/kg
95% KM (t) UCL
Wet
Thallium
0.000298
0.0117
mg/kg
70
0.00683
mg/kg
95% KM (Chebyshev) UCL
Tule Stalks
Wet
Arsenic
0.054
0.054
mg/kg
10
0.054
mg/kg
Maximum concentration
Wet
Thallium
0.000394
0.00408
mg/kg
50
0.00224
mg/kg
95% KM (t) UCL
Tule Roots
Wet
Arsenic
0.0471
0.0559
mg/kg
20
0.0559
mg/kg
Maximum concentration
Wet
Thallium
0.00086
0.0029
mg/kg
40
0.00158
mg/kg
95% KM (t) UCL
Clear Lake
Total
Arsenic
0.41
27.3
mg/kg
54.55
2.454
mg/kg
95% KM (Chebyshev) UCL
FLT
Arsenic
0.36
6.9
mg/kg
59.2
1.473
mg/kg
95% KM (Chebyshev) UCL
Surface Water
Herman Impoundment
Total
Arsenic
2.2
7.6
mg/kg
75
4.717
mg/kg
95% KM (t) UCL
FLT
Arsenic
0.91
4.4
mg/kg
77.78
3.372
mg/kg
95% KM (t) UCL
Ambient
Total
Arsenic
3.1
3.6
mg/kg
100
3.449
mg/kg
95% Student's-t UCL
FLT
Arsenic
3.1
3.5
mg/kg
100
3.318
mg/kg
95% Student's-t UCL
Sediment
Sediment
Dry
Arsenic
0.68
190
mg/kg
72.57
29.11
mg/kg
95% KM (Chebyshev) UCL
Ambient Sediment
Dry
Arsenic
1.09
31.7
mg/kg
100
17.35
mg/kg
95% Student's-t UCL
Fish
Game Fishers
Wet
Mercury
0.03
1.15
mg/kg
100
0.322
mg/kg
95% Approximate Gamma UCL
Traditional Tribal
Wet
Mercury
0.027
1.15
mg/kg
100
0.287
mg/kg
95% H-UCL
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California
Operable Unit 1 Record of Decision
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Table 2-3. Page 1 of 1
November 2023
Table 2-3. Media Specific Risk-Based Target Soil Concentrations and Remediation Goals
Mnli.i
1- \|)HSl|| i- .Hill Sl'i ll.U lHS
Klslx IJ.IM-ll 1 .lllii-l < H|K'Milr;il|H||s
< k ;iiiii|> 1 k \ l is
On-Mine Soils
Exposure to site-related carcinogenic and non-
carcinogenic risk at the on-mine site for unauthorized
tribal users
Mercury'1' = 619 mg/kg
Mercury'1' = 204 mg/kg
Arsenic = 2 mg/kg (0.36 mg/kg for
carcinoaenic risk")
Arsenic'2' = 22 mg/kg (PMB)
Thallium = 4 mg/kg
Thallium = 1.3 mg/kg
Antimony = 155 mg/kg
Antimony = 51 mg/kg
Exposure to carcinogenic and non-carcinogenic risk at
the on-mine site for trespassers
Mercury(1) = 4890 mg/kg
On-Mine Soils Remediation Goals
based on tribal use (above)
Arsenic = 13 mg/kg (2.27 mg/kg for
carcinogenic risk)
On-Mine Soils Remediation Goals
based on tribal use (above)
Thallium = 31 mg/kg
On-Mine Soils Remediation Goals
based on tribal use (above)
Antimony = 1244 mg/kg
On-Mine Soils Remediation Goals
based on tribal use (above)
Exposure to ecological risk for terrestrial species
Mercury = 805 mg/kg
On-Mine Soils Remediation Goals
based on tribal use (above)
Off-mine Residential
Soils
Exposure to carcinogenic and non-carcinogenic risk at
the off-mine residential area for the traditional tribal use
by tribal residents3 (based on child risk)
Mercury'1' = 60 mg/kg
Mercury'1' = 35 mg/kg (PMB)
Arsenic = 0.2 mg/kg (0.03 mg/kg for
carcinoaenic risk")
Arsenic'2' = 18 mg/kg (PMB)
Thallium = 0.4 mg/kg
Thallium = 0.4 mg/kg
Antimony = 15 mg/kg
Antimony = 7.1 mg/kg (PMB)
Exposure to carcinogenic and non-carcinogenic risk
the off-mine residential area (Sulphur Bank Mine Road
area) for the residential use by the lakeside residents
(based on child risk)
Mercury'1' = 60 mg/kg
Mercury'1' = 98 mg/kg (PMB)
Arsenic = 0.4 mg/kg (0.14 mg/kg for
carcinogenic risk)
Arsenic'2' = 18 mg/kg (PMB)
Thallium =0.8 mg/kg
Thallium = 0.4 mg/kg
Antimony = 31 mg/kg
Antimony = 15.5 mg/kg
Groundwater flowing
from OU-1 to Clear
Lake
Transport of contaminated groundwater and ARD
impacted groundwater from OU-1 (the on-mine area)
to Clear Lake (preventing unacceptably degrading Clear
Lake water quality)
There is not a risk-based target
concentration because the groundwater
within OU-1 is unsuitable for domestic
or municipal potable use due to naturally-
occurring constituents. Therefore,
groundwater is not a complete human
exposure pathway so risk could not be
calculated in the HHRA
Groundwater entering Clear Lake
shall be comparable to naturally
occurring hydrothermal
groundwater, as approximated by
calculated background threshold
values (BTVs).
Mercury = 0.5 kg/year'4'
Prevent contaminated groundwater and ARD from
degrading groundwater underlying the EIC and North
Wetlands
Groundwater-specific remedial
action will be addressed in a future
decision document
Surface Water
Prevent contaminated surface water from limiting the
ability of Clear Lake to meet surface water quality
standards, or from unacceptably degrading Clear Lake
water aualitv
Surface water is not a complete pathway
so risk could not be calculated in the
HHRA.
No numeric water quality standards
apply to non-stormwater surface
waters present in OU-1.
Stormwater Runoff
Prevent contaminated stormwater from limiting the
ability of Clear Lake to meet surface water quality
standards, or from unacceptably degrading Clear Lake
water quality
Stormwater runoff is not a complete
pathway so risk could not be calculated
m the HHRA
Stormwater quality requirements
are construction standards that will
be evaluated during remedy design
and incorporated into the remedial
action as appropriate
Air Dust
Control air emissions of mercury from mining wastes
during construction and long term from the waste piles
NA
Dust control requirements are
construction standards that will be
evaluated during remedy design
and incorporated into the remedial
action as aDDrooriate
Notes:
(a) Based on the sitewide human health risk assessment, risk-based target concentrations were developed for soil for different exposure scenarios. The
acceptable risk range for carcinogenic risk is 104 to 106. The acceptable risk range for non-carcinogenic risk is less than a HI of 1. Concentrations shown are
for single-contaminant HI=1.
(b) Cleanup levels for each COC were selected weighing cumulative risk such that the total cumulative HI is less than or equal to 1 when the HQs for all
COCs are summed.
(1) Mercury RBT Concentrations were adjusted using the mercury speciation of 10% in soil (EPA 2012).
(2) Based on the HHRA, the risk-based target soil concentration for arsenic for all exposure scenarios was found to be below the naturally occurring arsenic
concentration (22 mg/kg for Andesite and 18 mg/kg for Franciscan). As CERCLA limits the cleanup of sites to no less than naturally occurring levels, the
arsenic remedial action goal will be reflective the estimated pre-mining baseline condition. Pre-mining baseline concentrations are estimated using the UTL
95/95 for the Andesite formation and Franciscan Complex (E2 2021a). Since Franciscan arsenic concentrations are lower than Andesite, the more stringent
Franciscan concentration will be used for arsenic in the off-mine areas.
(3) The site-specific EPCs for arsenic in off-mine residential soils for tribal residents were lower than the EPC for pre-mining baseline levels. The arsenic
remedial action goal is therefore reflective of the estimated pre-mining baseline condition.
(4) The limit of mercury entering Clear Lake through groundwater from the mine site is obtained from the Regional Board, Basin Plan (Regional Board
(5) State of California MCL/California Primary Drinking Water Standards.
"PMB"= Pre-Mining Baseline estimate of background concentration in soil based upon the parent geologic formation underlying the remedial area
indicated (Andesite, Franciscan or Upper Lake Sediments), with the exception of arsenic on the EIC, where the Franciscan PMB is used to be conservative.
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California
Operable Unit 1 Record of Decision
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Version: FINAL
Table 2-4 Page 1 of 1
November 2023
T able 2-4. Comparative Analysis Summary of Source Area Alternatives
ki'iiii'diiil Mlrniiilhr
1- a silusiiinii < riii'i'iii
Mlrriiiiln r 1
\ll(Tii;iliM' 2
\IUTii;ilhc
Mlrniiilhr 4
I'l'i'liTCrd ki'liii'd}
\n \i'i i<>ii
2 \ ( >ii iiiinc
rniisnlidiiiimi mid
r;i|»|»iii^ |{< |< \ < ;i|»
willi < ;< 1.
21* < >n iniiir i-niisnlidiiiimi
mid » ;i|>|>in^ l<< l< \ < ii|t
u iili 1II >l'l
< >n iniiir i'
-------
Version: FINAL
Table 2-5, Page 1 of 1
November 2023
Table 2-5. Waste Rock Dam Removal Alternatives Comparison
I-aiilii;iiiiui ( I'iici iii
Urmrdhil Allrni;ili\r
Allrni;ili\r 1
AlU-riiiilnr 2
Alu-rn;ilin- 3
I'lvfrnvd Krmril\
Nil Ailimi
iilinn nl'siiliM-iilcd "iMr rock <>l'
vi;jiilii ;iiKr ;ind nipping
K\r;i\ si 1 ion In pri--miiiiiiu vhiurlinr.
including llir v;ilm ;ilril Hiivlc l in k <> 1'
viiiiiilii uiHr (All 2) iind nipping
( iimliiiiiiliiin nl' N\ Kl) 2 ;ind 3
Overall Protection of
Human Health and the
Environment
No
Yes
Yes
Yes
Compliance with ARARs
Not Applicable
Possibly. There is unavoidable
uncertainty regarding the effect of Phase 1
on TMDL compliance. The portion of the
waste rock dam that needs to be removed
to meet clean-up levels will be determinec
between Phase 1 and 2.
Possibly. Thre is unavoidable uncertainty
regarding the effect of Phase 1 on TMDL
compliance. The portion of the wast rock dam
that nees to be removed to meet clean-up
levels will be determined between Phase 1 and
2.
Yes. The extent of excavation would be
determined between Phase 1 and Phase with
the purpose of removing enough waste rock
to comply with the TMDL.
Long-Term Effectiveness
and Permanence
Not Included
Medium
High - based on assumption that future
conditions, such as climate change, could
result in unanticipated releases from the
shoreline waste material if left in place.
High
Reduction in Toxicity,
Mobility, and Volume
through Treatment
Not Included
Medium
Medium
Medium
Short-Term Effectiveness
Not Included
Medium
Medium
Medium
Implementability
Not Included
Medium
Medium
Medium
Capital Cost
$0
$20.5M to $26.1M
$17Mto $29.8M
$17M to $29.8M
Annual O&M Cost
$0
$21.8k to $21.9k
$21,8k to $21.9k
$21.8k to $21.9k
Net Present Value
$0
$21Mto $26.6M
$17.5Mto $30.3M
$17.5M to $30.3M
Reducing the footprint of the WRD through consolidation reduces costs due to the high cost of commercially available topsoil fo
capping. Local sources of clean topsoil will be investigated prior to Remedial Design, potentially affecting the cost comparison betwee
Alternatives 2 & 3.
Time to construct and
implement to meet RAOs
None
Completion of RD and RA estimated to
take 3 years, beginning after Source Areat
remedy is completed.
Completion of RD and RA estimated to take 3
years, beginning after Source Areas remedy is
completed.
Completion of RD and RA estimated to take
3 years, beginning after Source Areas
remedy is completed.
Notes:
ARARs = Applicable or Relevant and Appropriate Requirements
k = thousand
M = million
O&M = Operations & Maintenance
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California
Operable Unit 1 Record of Decision
-------
Version: FINAL
Table 2-6, Page 1 of 1
November 2023
Table 2-6. Comparative Analysis Summary of Herman Impoundment Alternatives
l-'\iiliiiiliftn Criteria
Remedial Allernali\e
Allernali\e 1
Allernali\e 2
Alternate e 3
No Action
Acti\ol> Mainlain
and Monitor
11> «l ran lie Barrier CulolTWall
Overall Protection of Human Health
and the Environment
No
Yes
Yes
Compliance with ARARs
Not Applicable
Yes
Yes
Long-Term Effectiveness and
Permanence
Not Included
NA
High
Reduction in Toxicity, Mobility, and
Volume through Treatment
Not Included
Poor
Medium
Short-Term Effectiveness
Not Included
NA
Medium
Implementability
Not Included
High
Medium
Net Present Value
$0
$1,156,281
$26,297,734
Notes:
ARAR = Applicable or Relevant and Appropriate Requirements
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California
Operable Unit 1 Record of Decision
-------
Version: FINAL
Table 2-7, Page 1 of 1
November 2023
Table 2-7. Estimated Area and Volume of Mine-Related Waste Piles and Remedial Alternative Event
Tolal Area1"
( on (on red Area'"'
(ill
l-ill
Tolal
Mining Wasle Pile/Areas
(ft2)
(ft2)
-------
Version: FINAL
Table 2-8, Page 1 of 1
November 2023
Table 2-8. Operable Unit-1 Site Building/Structures Recommended Remedial Actions
I-Oilllliv II)
ISuildinu Niiim-
Recommended Rcmcdi;il Action
1
Llock l acloi'N
Demolish concrete pad and haul debris lo landfill
3
Metal Water Tanks
Demolish water tanks and haul debris to landfill
4
Concrete Retaining Wall
No action
5
1940s Mine Office
Demolish Building remains and haul debris to landfill
6
Metal Water Tanks
Demolish water tanks and haul debris to landfill
7
Truck Maintenance Garage
Demolish retaining wall and concrete pads and haul debris to landfill
8
Mercury Processing Area
Clean and haul debris to landfill, sample beneath the building footing and
nearby soil for elemental mercury
9
Meat house
Demolish Building and haul debris to landfill
10
Soil Fertilizer Plant
Demolish concrete pads and haul debris to landfill
11
Explosives Shack
Demolish Building and haul debris to landfill
12
Truck Garage
Demolish concrete pads and haul debris to landfill
13
Generator House
Clean and haul debris to landfill
14
Old Mine Office
Demolish concrete pads and haul debris to landfill
15
Fuel Tanks and Storage
Shed
Demolish shed, sample the area for petroleum contamination, discard fuel
tank
16
Grease Pit and Storage
Shed
Clean and haul debris to landfill
17
Old Pump House and Truck
Garage
Demolish Building and haul debris to landfill
18
Old Ward House
Clean and haul debris to landfill
Sulphur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California
Operable Unit 1 Record of Decision
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Version: FINAL
Table 2-9, Page 1 of 1
November 2023
Table 2-9. Selected Remedy Cost Estimate
("nsl Ill-Ill
21 lift ( usl
( Il\l
\djusli-d lor Inllalion
Source Areas
Clear and Grub
$341,925.08
$443,100.71
Sitewide Access Roads
$216,935.34
$281,126.51
Sitewide Staging Areas
$111,550.24
$144,557.96
Excavation and on-site consolidation (Alternatives 2 and
3 - excavation of Northwest Waste Rock Pile, East and
West Ore Piles)
$345,823.43
$448,152.58
Grading of consolidated material
$3,118,738.79
$4,041,573.60
Capping Source Areas (including importation/generation of cap material)
South Waste Rock Pile
$10,612,809.19
$13,753,139.43
West Waste Rock Pile
$4,655,313.66
$6,032,820.97
Tailings Pile
$5,383,018.76
$6,975,854.01
Disturbed Rock, Satellite Waste Rock Pile
$7,416,143.91
$9,610,580.89
North Waste Rock Pile
$8,031,109.46
$10,407,514.75
North West Pit
$1,562,670.17
$2,025,064.28
Backfill of the Northwest Pit (assuming WRD and NWWRP derived waste only)
$410,810.78
$532,369.69
Demolition and Removal of abandoned Structures
$530,682.25
$687,711.13
Off-gas system
$427,898.64
$554,513.85
Soil Moisture Monitoring System for Source Areas
$660,000.00
$855,294.00
Surface Water Runoff Collection System
$628,049.71
$813,889.62
Total Costs
$44,453,479.42
$57,607,263.97
Residential
Off-Mine Residential Soil
Off-Mine Remedial Actions
$7,981,267.80
$7,981,267.80
No Inflation Adjustment Required
Subtotal Costs
$7,981,267.80
$7,981,267.80
Sulphur Bank Mine Road Improvements for Safe Access
Infrastructure Improvements
$3,730,675.00
$3,730,675.00
Subtotal Costs
$3,730,675.00
$3,730,675.00
No Inflation Adjustment Required
Total Costs
$11,711,942.80
$11,711,942.80
Herman Impoundment
Infrastructure Improvements at Herman Impoundment
$33,000.00
$42,764.70
Total Costs
$33,000.00
$42,764.70
Waste Rock Dam
Clear and Grub
$171,713.92
$222,524.07
Excavation + Load & Haul
$4,482,397.63
$5,808,739.09
Fill with clean soil
$5,071,561.83
$6,572,236.98
RCRA Cap (w HDPE)
$5,358,230.99
$6,943,731.55
Total Costs
$15,083,904.37
$19,547,231.68
30 year Net Present Value of Annual Operations and Maintenance (O&M) cost (7% Discount Rate)
Source areas with an HDPE Cap O&M, including 5-year review
$3,434,811.20
$4,451,171.83
Waste Rock Dam with an HDPE Cap O&M, including 5-year review
$202,808.77
$262,819.88
Herman Impoundment Monitoring O&M, including 5-year review
$518,696.20
$672,178.41
Residential soil inspection/reporting O&M, including 5-year review
$163,798.80
$212,266.86
Total Costs
$4,320,114.97
$5,598,436.99
Total Cost (NPV)
$71,282,326.59
$94,507,640.14
Notes:
iiie information m tins cost estimate summary table is based on tiie best available mlormation regarding tiie anticipated scope ot tiie remedial alternative. Changes m tiie
cost elements are likely to occur as a result of new information and data collected during the engineering design of the remedial alternative. Major changes may be
documented in the form of a memorandum in the Administrative Record file, an ESD, or a ROD amendment. This is an order-of-magnitude engineering cost estimate
that is expected to be within +50 to -30 percent of the actual project cost.
The NW Pit Backfill Costs indicated are for the waste-backfill scenario, but do not include costs for additional cap enhancements (such as gas venting systems) that may
be needed to comply with the requirements of Title 27 pending further analysis during RD.
Inflation of 29.59% from 2016 to 2023 per Consumer Price Index; RS Means Index National
Sulfur Bank Mercury Mine Superfund Site
Clearlake Oaks, Lake County, California
Operable Unit 1 Record of Decision
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Figures
-------
Pirates Cove
©learlake
.[Mtea
Los Angeles •
Long Beach
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
0 1,000 2,000 4,000
Feet
Map Date; 6/8/2023
Data Sources: Esri, USCB
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Location and Site Map
^ED S7^
m
Figure 1-1
-------
Site ¦
San Francisco,
San Jose*
Si
o
ICS
O
Los Angeles )
Long Beach
San Diego
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
Disturbed Rook
Ore
Overburden and Waste Rock
Tailings
¦ ¦ Dam
Fault
= = =- Sheared Fault Zone
—« Dirt Road
• o Drainage Ditch
nm—» North Wetland Boundary
0000 Riprap (with boulders)
Overflow Diversion System
Above Ground
zmzi Below Ground
m Mining Operation Related Buildings and
Identification Number
II _l Potential Soil/Cover Material Borrow Area
1 1 Approximate Residential Areas to be Evaluated for
1 1 Action During Remedy Design
Map Date: 10/17/2023
Data Sources: E2, EPA, Esri, USCB
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Physical Site Features and Areas of Concern
^ED SJV,^
Figure 1-2
Rattlesnake Island
NORTHWESTjj
WASTE
ROCK PILE",
NORTH WASTE ROCK PILE
Jm
NORTHWEST
PIT
DISTURBED ,
ROCK
SATELLITE
, WASTE
'rockpile
WASTE
ROCK DAM
SOUTH
WASTE I
ROCK PILE
WEST WASTE
ROCK PILE
TAILINGS
PILE
J6 n
westuk
ore
ISHd 'J J
-------
Clear
Lake
Release Mechanisms
Receiving Media
Transport Processes
Exposure Media
Exposure Scenarios
Exposure
Routes
Tribal
Cultural Uses
Lakeside
Residents'"
Recreational
Visitors'3
Trespassers'3
SBMM
Mining
Activity
Surface and
Subsurface Soil
Dust Emission
(from wind and traffic
on unpaved roads)
Residue
(Excavation
and Backfill)3
Off-mine Surface
Soil0
Ore Processing
Emissions (from retort)
Historical
Releases
from SBMM
Herman
Impoundment
Leaching
Dissolution
On-mine
Surface Soilb
Groundwater
Infiltration, Percolation
and Regional
Groundwater Flow
Waste Rock Dam,
WasteRock Piles
(West, Northwest,
North, and South),
West and East
Ore Piles,
Tailings Pile
Mercury in
Sediments and
Suspended Solids
t
Surface Water
Runoff
Dissolved
Metals
Methyl Mercury
in Sediments and
Suspended Solids
Biotic Conversion
to Methyl Mercury
Uptake/Accumulation
Edible Plants
(Wetland and Upland)
Edible Birds
(Wetland and Upland)
Footnotes
a A soil removal action occurred in the mine EIC residential area in 1997 and 2006. The confirmation soil sampling results from the removal action and soil results from areas which were not removed will be used to evaluate the tribal scenario,
b The soil evaluation from the Baseline Public Health Evaluation (dated Jan. 1994) performed for the onsite portion of the site will be used in this HHRA because no new soil data has been collected onsite.
c Lakeside residents are homeowners along Clear Lake Shoreline who may use lake water for household water supply by means of private water intakes and who are also assumed to swim, wade, and fish in Clear Lake.
d Recreational visitors are assumed to swim, wade, and fish in Clear Lake and trespassers wade and fish in Herman Impoundment and are exposed to on-mine surface soil,
e Remedial actions to date included removal of contaminated mine materials and soils from the EIC residential area.
f Currently EIC residents use the municipal system for their drinking water. Potentially, in the future EIC residents may use lake water as their drinking water source. Tribal residents also swim and wade in the lake.
9 Lakeside residents, recreational visitors and trespassers are potentially exposed to water and sediments during swimming and wading activities.
h Tribal edible plants include acorns, cattails, tules and garden vegetables.
1 Non-tribal edible plants include garden vegetables.
i Edible fish consumed by the tribe include large mouth bass, bluegill, catfish, black crappie, hitch, carp, Tule perch, and Sacramento blackfish. Shellfish will be evaluated qualitatively. Edible fish consumed by recreational fishermen include
large mouth bass, bluegill, catfish, and crappie.
For the toxicological assessment in the HHRA, methyl mercury is assumed in fish and bird tissue, aquatic plants, and sediment; mercuric chloride is assumed in soil, dust, surface water, and terrestrial plants; and elemental mercury is assumed
not present.
Geothermal
Springs
Hydrogen Sulfide
and Mercury Vapors
Outdoor Vapor
Inhalation
O
O
o
•
Ingestion
•
•
O
o
Dermal
Contact
•
•
o
o
Outdoor Dust
Inhalation
•
•
o
o
Ingestion
O
o
o
•
Dermal
Contact
O
o
o
•
Outdoor Dust
Inhalation
o
o
o
•
Ingestion
•
•
O
Dermal
Contact
•
•
•
O
Ingestion
O
o
o
•
Dermal
Contact
O
o
o
•
Ingestion
•
•
o
Dermal
Contact
•
•
•
o
Ingestion
•
•
o
o
Ingestion
•
O
o
o
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
Air
Soil/Sediment
Fish/Birds
# Complete Exposure Pathway
O Incomplete Exposure Pathway
Definitions
EIC: Elem Indian Colony (located off-mine)
SBMM: Sulphur Bank Mercury Mine
Off-mine: Parcels beyond the Operable Unit 1 (OU-1) boundary
On-mine: Parcels under EPA (OU-1 boundary)
Map Date; 10/12/2023
Data Sources: EPA. Esri
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Figure 2-1
Conceptual Site Exposure Model
/ ft *'1
w.
-------
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
-^r Groundwater Monitoring Well
Fault
= = = Sheared Fault Zone
— — 2018 Groundwater Divide
Overflow Diversion System
= Above Ground
Below Ground
Y77A Surface Water
Disturbed Rock
Andesite (Qa)
East Ore Pile
Franciscan Formation (KJf)
North Waste Rock Pile
Northwest Waste Rock Pile
Quaternary Alluvium Sediments (Qal)
Quaternary Lake Sediments (Qs)
South Waste Rock Pile
Tailings Pile
Waste Rock Dam
Waste Rock Pile
West Ore Pile
West Waste Rock Pile
Map Date; 6/8/2023
Data Sources: EPA, Esri
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Surface Geology
Figure 2-2
/ ft *'1
I®,
-------
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
Fault
~ ¦ 2018 Groundwater Divide
a
Herman Impoundment and Pit Runoff
Catchments
£ Groundwater Monitoring Well
Overflow Diversion System
Above Ground
Below Ground
Dissolved Mercury Concentrations
<1 pg/L
1 -10 jjg/L
10 -100 (jg,L
>100 jjg/L
|§||| Areas where Mercury Concentrations are Uncertain
Hydraulic Potential Contour
AMSL - Above Mean Sea Level
ft - Foot (feet)
|jg/L - Microgram(s) per liter
0 100 200 400
1 I I I I
Feet
Map Date; 6/8/2023
Data Sources: EPA, Esri
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Isopleth Map for Mercury in Groundwater
and Hydraulic Potential
^ED STA^
m
Figure 2-3
-------
Clear Lake >
Operable Unit 2)
Nftne Go
NORTHWEST
WASTE ROCK PILE
NORTH WASTE ROCK PILE
NORTHWEST
PIT
SATELLITE
WASTE'ROCK PILE
DISTURBED ROCK
WASTE ROCK
DAM
WEST WASTE
ROCK PILE
TAILINGS PILE
SOUTH WASTE
ROCK PILE
WESTO
PILE
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
Disturbed Rock
Ore
Overburden and Waste Rock
Tailings
Fault
= = = Sheared Fault Zone
¦ I 2018 Groundwater Divide
|i i] Herman Impoundment and Pit Runoff Catchments
Overflow Diversion System
Above Ground
33 Below Ground
200
¦
400
_l_
800
_J
Feet
Map Date; 6/8/2023
Data Sources: EPA, Esri, USCB
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Stormwater Catchments
(ift)
Figure 2-4
-------
~
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4; North Area
Operable Unit 4: Study Area
Project Area
Annual Grassland/California Prairie
Barren
Ditch
Live Oak Woodland
Open Water
Perennial Peppenweed
Pond
Ruderal Species
Rural Residential
MesJC Meadow (Rush/Salt Grassi'Creeping Wild Rye)
Salt Grass
Seasonal Wetland
Shore Zone Riparian
Tule Marsh
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Habitat and Land Cover
Figure 2-5
/ ft *'1
W.
PRO*^
-------
Elem Indian
BIA 120 (Pomo Rd)
NORTHWEST
WASTE
ROCK PILE
NORTHWEST
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
Disturbed Rock
Overburden and Waste Rock
~ Elem Colony Lots
Residential Soil Areas Requiring Institutional
Controls or Sampling to Determine Necessary
Remedial Action
0 50 100 200
Feet
Map Date: 10/17/2023
Data Sources: E2, EPA, Esri
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Elem Indian Colony Residential Lots
^ED SJV,^
6ft)
Figure 2-6
-------
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
Operable Unit 4: North Area
Operable Unit 4: Study Area
~ Parcel Boundary
Residential Soil Areas Requiring Institutional
Controls or Sampling to Determine Necessary
Remedial Action
0 50 100 200
1 I J I I
Feet
Map Date: 10/17/2023
Data Sources: E2, EPA, Esri, Lake County
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Non-Tribal Residential Lots
Figure 2-7
/ ft *'1
W,
-------
NORTHWEST
waste!
ROCK. PILE
NORTH WASTE
ROCK PILE
NORTHWEST PIT
WASTE
RdCK
DAM
SATELLITE
WASTE
ROCK PILE
DISTURBED ROCK
IN-HERMAN
IMPOUNDMENT
DISPOSAL CELL
Clear Lake
Herman Impoundment
WEST WASTE
ROCK PILE
SOUTH WASTE
ROCK PILE
TAILINGS PILE
WEST
ORE PILE
Operable Unit 1: Sulphur Bank Mercury Mine
Superfund Site
a Operable Unit 4: North Area
Operable Unit 4: Study Area
~ Surface Water
¦ Mining Operation Related Buildings to be
Consolidated into Waste Piles or Disposed Offsite
s ) Storm Water Runoff Drain
| 1 Potential Soil/Cover Material Borrow Area
~ Likely Borrow Material Processing Area
Waste Likely to be Excavated for Consolidation
Waste to be Capped by a RCRA Cap or Alternate
~ Cover (After Receiving Material from Consolidation
where Applicable)
~ Waste to be Excavated or Capped In-place
Capped with RCRA Cover After Receiving Waste or
Partially Backfilled with Clean Material
r - -» Approximate Residential Areas to be Evaluated for
1 1 Action During Remedy Design
Possible Consolidation/Placement Location of Waste
if Consolidation Occurs
0 500 1,000
Feet
Map Date: 10/17/2023
Data Sources: EPA, Esri
Sulphur Bank Mercury Mine Superfund Site
Operable Unit 1 Record of Decision
Lake County, California
Operable Unit 1 Selected Remedy
(Conceptual Depiction)
Figure 2-8
-------
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Appendix A
Applicable and Relevant or Appropriate
Requirements Basis and Table A-l
-------
Appendix A - Applicable or Relevant and Appropriate Requirements
Sulphur Bank Mercury Mine Operable Unit 1 Record of Decision
November 2023
-------
Table A-1. Applicable Relevant and Appropriate Requirements (ARARs) and to be Considered (TBCs) for Sulphur Bank Mercury Mine OU-1 Record of Decision
#
Citation
Media/
Action/
Location
Description
Applicability
1
Migratory Bird Treaty
Act
Federal
16 U.S.C. Section 703
et seq., 50 CFR Part
10.13
Action
Specific
Sitewide
Prohibits the take of migratory birds as
provided.
Applicable
Migratory birds are present at the Site. The substantive
provisions are applicable.
Personnel will be trained to avoid taking migratory birds which
may be present at the Site.
2
Bald and Golden
Eagle Protection Act
Federal
16 U.S.C. Sec. 668, et
seq., 50 C.F.R Part 22.
Action
Specific
Sitewide
Prohibits take of eagles or nests as provided.
Applicable
Eagles have been observed at the Site. The substantive
provisions are applicable.
EPA will consult with USFWS prior to remedy implementation
to identify appropriate actions to be incorporated into the
design and/or operations to avoid taking eagles.
Personnel will be trained to take appropriate actions to protect
eagles.
3
Fish and Game Code
Section 3511
Action
Specific
Sitewide
The golden eagle is a fully protected species
and may not be taken or possessed at any
time.
Applicable
Eagles have been observed at the Site. The substantive
provisions are applicable.
EPA will consult with CDFW prior to remedy implementation to
identify appropriate actions to be incorporated into the design
and/or operations to avoid taking eagles.
1
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Table A-1. Applicable Relevant and Appropriate Requirements (ARARs) and to be Considered (TBCs) for Sulphur Bank Mercury Mine OU-1 Record of Decision
#
Citation
Media/
Action/
Location
Description
Applicability
4
National Historic
Preservation Act
Federal
54 U.S.C. Sec.
300101, etseq.,36
C.F.R. Part 800
Action
Specific
Sitewide
Requires federal agencies to take into
account the effects of their undertakings on
historic properties.
Applicable
The substantive provisions are applicable because the remedy
is a federal undertaking that could adversely affect historic
properties which are included in, or eligible for inclusion in, the
National Register of Historic Places. EPA will identify historic
properties potentially affected by the undertaking and seek to
avoid, minimize or mitigate any adverse effects. EPA will
consult with the SHPO and THPO(s) as part of the NHPA
process.
EPA is not required to develop a NHPA Memorandum of
Agreement (MOA) under CERCLA for the remedy because it is a
procedural requirement. However, for site-specific reasons,
EPA intends to negotiate a MOA to document how EPA plans to
avoid, minimize or mitigate any adverse effects on historic
properties. If EPA is unable to negotiate an MOA, EPA may
proceed with the remedy after documenting the actions EPA
will take to avoid, minimize or mitigate any adverse effects on
historic properties.
Personnel shall be trained on NHPA requirements, including any
actions regarding how to avoid, minimize, or mitigate adverse
effects on historic properties.
5
Archaeological
Resources Protection
Act of 1979, as
amended
Location
Specific
Elem Indian
Colony
No person may excavate, remove, damage,
or otherwise alter or deface any
archaeological resource located on public
lands or Indian lands as defined, unless
authorized by a permit or an exception.
Applicable on the Elem Indian Colony
The substantive provisions are applicable if eligible resources
are identified on the Elem Indian Colony.
2
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Table A-1. Applicable Relevant and Appropriate Requirements (ARARs) and to be Considered (TBCs) for Sulphur Bank Mercury Mine OU-1 Record of Decision
#
Citation
Media/
Action/
Location
Description
Applicability
16 U.S.C Section
470aa; 43 C.F.R. Part
7
EPA will consult with the Elem Indian Colony and BIA and
conduct an updated archaeological resources survey prior to
remedy implementation.
The design, construction, operation and maintenance of the
remedy will be conducted with appropriate protections and
procedures for archaeological resources.
Personnel shall be trained for compliance with these
requirements.
6
Native American
Graves Repatriation
Act (NAGPRA)
Federal
25 U.S.C. Section
3001, et seq. and, 43
C.F.R. Part 10 et seq.
California Native
American Graves
Protection and
Repatriation Act of
2001, Health and
Safety Code Section
8010, et seq. (CA
NAGPRA)
Action
Specific
Sitewide
NAGPRA provides a systematic process for
determining the rights of lineal descendants
and Indian tribes to certain Native American
human remains, funerary objects, sacred
objects, or objects of cultural patrimony
discovered on federal or tribal land.
NAGPRA Applicable on the Elem Indian Colony
NAGRPA Relevant and Appropriate for the rest of OU1
CA NAGPRA TBC for land off of the Elem Indian Colony
The substantive requirements of NAGPRA are applicable if
covered Native American human remains or covered objects
are found on the Elem Indian Colony and relevant and
appropriate on the mine site. EPA will also consider the
substantive provisions of CA NAGPRA off of the Elem Indian
Colony.
The identification of potential Native American grave sites
and/or covered objects shall be included in the archaeological
resources survey which shall be updated prior to remedy
implementation.
Per the request of the Elem Indian Colony, EPA does not plan to
investigate or cleanup the known burial ground on the EIC to
3
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Table A-1. Applicable Relevant and Appropriate Requirements (ARARs) and to be Considered (TBCs) for Sulphur Bank Mercury Mine OU-1 Record of Decision
#
Citation
Media/
Action/
Location
Description
Applicability
avoid disturbing human remains. EPA intends to provide
protection for the burial ground through engineering and
institutional controls.
Personnel shall be trained for compliance with these
requirements.
7
American Indian
Religious Freedom
Act
Federal
42 U.S.C. Section
1996 et seq.
Location
Specific
Elem Indian
Colony
The American Indian Religious Freedom Act
provides it is the policy of the United States
to protect and preserve for American
Indians their inherent right of freedom to
believe, express, and exercise their
traditional religions, including but not
limited to access to sites, use and possession
of sacred objects, and the freedom to
worship through ceremonials and traditional
rites.
Applicable on the Elem Indian Colony
Consistent with the American Indian Religious Freedom Act
policy, EPA will evaluate potential Native American religious,
ceremonial, and burial sites prior to remedy implementation.
The design, construction, operation and maintenance of the
remedy will be conducted to protect and preserve EIC's
freedom to practice and believe its traditional religions,
including but not limited to access to sites, use and possession
of sacred objects, and the freedom to worship through
ceremonials and traditional rites.
Personnel shall be trained on any requirements established
through consultation with the EIC.
8
Section 404 of the
Clean Water Act
Federal
33 U.S.C. Section
1251 et seq., 40
Location
Specific
Waste Rock
Dam, and NW
Regulates the discharge of dredged or fill
material into waters of the United States,
including wetlands.
Applicable
Applicable to excavation of portions of the Waste Rock Dam
and Northwest WRP. EPA will consult with the USACE prior to
remedy implementation to ascertain the substantive
requirements under Section 404.
4
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Table A-1. Applicable Relevant and Appropriate Requirements (ARARs) and to be Considered (TBCs) for Sulphur Bank Mercury Mine OU-1 Record of Decision
#
Citation
Media/
Action/
Location
Description
Applicability
C.F.R. Parts 230 and
231.
Waste Rock
Pile
9
Fish and Wildlife
Coordination Act
Federal
16 U.S.C. Section
662a
Location
Specific
Waste rock
dam removal
in and along
the Clear Lake
shoreline
Section 662(a) provides whenever the
waters of any stream or other body of water
are modified for any purpose whatever by
any agency of the United States such agency
first shall consult with the United States Fish
and Wildlife Service, and CDFW with a view
to the conservation of wildlife resources by
preventing loss of and damage to such
resources as well as providing for the
development and improvement thereof.
Applicable
The substantive provisions are applicable to the removal of the
waste rock dam to the extent it requires modifications of Clear
Lake waters. EPA will consult with the USFWS and CDFW prior
to remedy implementation to get input on how to address
impacts to wildlife during remedy construction.
EPA notes that the purpose of this action is to protect human
health and the environment by preventing ARD containing
mercury from flowing into Clear Lake.
10
California Fish &
Game code Section
1602(a)
Location
Specific
Waste rock
dam removal
in and along
the Clear Lake
shoreline
Section 1602(a) requires notification to the
CDFW for diversion or obstruction of the
natural flow of, or a substantial change of
the bed or channel of a lake; and requires an
agreement with CDFW where the actions
will substantially affect a wildlife resource.
Relevant and Appropriate
The substantive provisions are relevant and appropriate to the
removal of the waste rock dam to the extent it requires
modifications to Clear Lake. The agreement provisions in
1602(a)(4) are procedural. EPA will comply with the substantive
requirements of 1602(a) by informing CDFW how EPA intends
to protect Clear Lake and by consulting with the CDFW to
obtain input on how to address impacts to Clear Lake resources
during waste rock dam construction. This consultation may be
combined with the consultation under 16 U.S.C. Section 662(a)
referenced above.
5
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Table A-1. Applicable Relevant and Appropriate Requirements (ARARs) and to be Considered (TBCs) for Sulphur Bank Mercury Mine OU-1 Record of Decision
#
Citation
Media/
Action/
Location
Description
Applicability
11
California
Endangered Species
Act(ESA)
State
California Fish and
Game Code Section
2080
Action
Specific
Sitewide
Except as authorized therein, prohibits the
taking of plant and animal species
designated as either threatened or
endangered
Applicable
Substantive provisions are applicable to California listed plant
or animal species. The survey of threatened or endangered
species shall be updated prior to implementation of the
remedy. EPA will consult with CDFW regarding appropriate
mitigation of impacts to any identified threatened or
endangered species.
12
Hazardous Waste
Determination
22 CCR Section
66261.2 &
Section 66261.3
and
27 CCR Section 20200
Action
specific
Applies to off-
site disposal
of hazardous
waste.
Promulgated criteria to determine if a
material is hazardous.
RWQCB criteria for waste characterization
prior to off-site disposal.
Applicable for off-site disposal of hazardous waste.
Any hazardous waste taken off-site for disposal must be
appropriately characterized.
The only hazardous waste anticipated to be taken off-site non-
concrete components of the abandoned mine buildings and
structures that qualify as hazardous waste.
Concrete elements will be consolidated and capped along with
other Bevill exempt materials as the concrete consists largely of
mine waste.
13
22 CCR §67391.1(a),
(d), (e)(2), (f), and (i)
Action
Specific
Requirements
for Land Use
Covenants
Substantive requirements of 22 CCR
§67391.1(a), (d), (e)(2), (f), and (i) apply to
land use restrictions.
Relevant and Appropriate
Land use covenants will be recorded for mine site parcels
impacted by the Site prior to transfer or after the remedial
action is complete (whichever occurs first). The land use
covenants will require non-interference with the caps and other
remedial components and information about any restrictions
on use determined necessary. The land use covenants will
6
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Citation
Media/
Action/
Location
Description
Applicability
comply with the substantive requirements of 22 CCR
§67391.1(a), (d), (e)(2), (f), and (i).
14
22 CCR § 69021,
69022
Toxicity Criteria Rule
Chemical
Specific
The Risk Assessment utilized the table in
Appendix 1 of Section 69021 for
contaminants with more protective numbers
than the criteria established under EPA's
Integrated Risk Information System as a "to
be considered" (TBC) given the unique
circumstances at the Site.
TBC
Risk-based target concentrations were calculated using
Appendix 1 of Section 69021 for those constituents where the
Toxicity Criteria Rule provided more stringent/protective
values. Specifically, this affected the risk-based target
concentrations for arsenic and mercury.
15
22 CCR, Chapter 14,
66264.228(e-q);
66264.310(a) and(b)
Location
Specific
Cover
requirements
for the
Northwest Pit
if it is
backfilled with
waste
Substantive provisions for the construction,
closure, and post closure of the Northwest
Pit if backfilled with waste.
Relevant and appropriate
Relevant and appropriate for the Northwest Pit for site-specific
reasons and any other areas where EPA determines, after
coordination with State, that a Title 22 cap is necessary instead
of a Title 27 cap.
16
Hazardous Waste
Transportation
22 CCR, Ch. 13
Article 1
§66263
Action
Specific
Transportation of hazardous wastes to an
offsite disposal facility would be subject to
hazardous waste transportation
requirements.
Applicable
Any hazardous waste, such as building demolition debris,
shipped off-site for disposal must comply with the applicable
hazardous waste transportation requirements.
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Citation
Media/
Action/
Location
Description
Applicability
17
LCAQMD Rules &
Regulations
Rule 400 Visible
Emissions
Action
Specific
Visible
emission
limits
No emissions as dark, or darker, as that
designated as No. 2 on the Ringelmann
Chart for a period or periods aggregating
more than three minutes in any one hour.
Relevant and appropriate
Visibility standard for material from construction, operation,
and transportation activities.
18
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections, 13172,
13260, and 13263)
Title 27, California
Code of
Regulations (CCR),
Section 22470
Media
Specific
27 CCR 470 contains siting, construction,
monitoring, closure and post closure
standards for Mining Units.
22470(b) provides the following: 1) an
exemption from the liner and leachate
collection and removal system requirements
if it is determined that leachate will not form
in or escape from a Mining Unit; 2) allows
monitoring procedures in lieu of certain
containment features; and 3) requires
contingency plans to be implemented if
monitoring indicates that the disposal
procedures are inadequate.
22470(c) also allows an exemption from
liner requirements for poor groundwater.
Applicable
Substantive requirements of 22470 are applicable for siting,
construction, monitoring, and closure/post closure
requirements.
EPA, in consultation with the State, will determine whether the
Northwest Pit will be filled with mine waste or clean fill during
remedial design based in part on whether the criteria for the
bottom liner exemption in 22470 is met.
19
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections, 13172
and 13260)
Title 27, California
Code of
Media specific
27 CCR 22480 defines classifies mining
wastes into Group A, B, and C.
Applicable
22480(b) requires mining waste to be classified as Group A or B
waste based on 1) whether it would be hazardous waste
pursuant to Chapter 11 of Division 4.5 of Title 22 if it were not
exempt under Bevill and 2) whether it poses a significant
threat to water quality.
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Citation
Media/
Action/
Location
Description
Applicability
Regulations (CCR),
Section 22480
EPA, in consultation with the State, will determine whether
wastes is Group A, B, or C based on the substantive provisions
of 22480.
20
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections,
13172, 13226, and
13263)
Title 27 CCR
Section 22490
Location and
Media
Specific
27 CCR Section 22490
contains Mining Unit siting and construction
standards.
Applicable
Substantive requirements of 22490 are applicable and contain
construction, liner, and operational requirements for mining
units. EPA, in consultation with the State, will determine if
liner exemption criteria are met and whether contingency
requirements are triggered.
21
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections, 13172,
13226, 13263, and
13267)
27 CCR Section
22500
Media specific
27 CCR Section 22500
Contains requirements for water quality
monitoring for Mining Units.
Applicable
Substantive monitoring requirements of 22500 are applicable.
22
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections, 13172,
13226, and 13263 )
Media specific
27 CCR Section 22510(a),
contains closure requirements to protect
groundwater and ensure post closure uses
are protective of covers; 22510(e) requires
Inactive Units Containment structures be
subject to the same standards as apply to
an active Mining Unit under this article; (i)
Applicable
Substantive provisions 22510(a) requires existing Mining Units
be closed so that they no longer pose a threat to water quality
and that no post closure land uses shall be permitted that
might impair the integrity of containment structures.
Substantive provisions of 22510(e) and (i-k) contain applicable
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Citation
Media/
Action/
Location
Description
Applicability
27 CCR Section
22510(a), (e), and
(i-k)
Vegetation for closed Mining Units shall
not impair the integrity of containment
features. Irrigation of vegetation shall be
managed to assure that it does not cause
nor increase the production of leachate;
(j) Group A and B waste piles shall be
closed in accordance with the provisions of
Section 21090(a-c); and
(k) Group A and B surface impoundments
shall be closed in accordance with the
provisions of paragraphs (a) and (b)(1) of
Section 21400. A surface impoundment
can be closed in place if provided with a
cover as in Section 21090(a) and if the liner
(or, in the case of a double liner system,
the outer liner) is clay.
irrigation, vegetation, maintenance, and cover requirements
and include the requirements contained in the cross
references contained therein.
23
Porter-Cologne
Water Quality
Control Act
(California Water
Code Sections 13000,
13050, 13140, 13240
-13248, 13267,
13304, 13307)
Implementation
including Basin Plan,
Chapter 4, Section
4.2.2.1.10 "Policy for
Investigation and
Action
Specific
Basin Plan, Chapter 4, Section 4.2.2.1.10
states "Immediate removal or containment
of the source, to the extent practicable,
should be implemented where necessary
to prevent further spread of pollution as
well as being among the most cost-
effective remediation actions."
TBC
In consideration of 4.2.2.1.10, source control actions will be
designed to minimize leachate generation by surface water or
geothermal water infiltration through mining waste and
control transport of COCs in surface water to minimize impacts
to groundwater and Clear Lake.
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Citation
Media/
Action/
Location
Description
Applicability
Cleanup of
Contaminated Sites."
State Board
Resolution No. 92-49
"Policies and
Procedures for
Investigation and
Cleanup and
Abatement of
Discharges Under
Water Code 13304."
Basin Plan, Chapter
4.
24
Porter-Cologne
Water Quality
Control Act
(California Water
Code Sections 13240,
13241, 13242, and
13243)
Present and
Potential Beneficial
Uses for surface
waters within the
Clear Lake Surface
Water Body, as
prescribed by the
Basin Plan. Basin
Plan, Chapter 2.
Media
Specific
Establishes beneficial uses for surface and
groundwaters in the region.
Clear Lake has the following designated
beneficial uses: MUN (Municipal and
Domestic Supply), AGR (Agricultural
Supply), REC-1 (Water Contact
Recreation), REC-2 (Noncontact Water
Recreation), COMM (Commercial and
Sportfishing), WARM (Warm Freshwater
Habitat), COLD (Cold Freshwater Habitat),
SPWN - WARM (Spawning, Reproduction,
and/or Early Development), WILD
(Wildlife Habitat).
All groundwaters have the MUN, AGR,
IND, AND PRO beneficial use designations
TBC
Specific portions of the Basin Plan include beneficial uses of
affected water bodies.
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Citation
Media/
Action/
Location
Description
Applicability
Present and Potential
Beneficial Uses for
groundwaters within
the Central Valley
Region, as prescribed
by the Basin Plan.
Basin Plan, Chapter
2.
unless otherwise specified by the Basin
Plan.
Unless otherwise specified, beneficial uses
also apply to all tributaries of surface
waters identified in Table 2-1 (i.e., specific
surface waters which are not listed have
the same beneficial uses as the streams,
lakes, wetlands, or reservoirs to which they
are tributary).
25
Porter-Cologne
Water Quality
Control Act
(California Water
Code Sections 13240,
13241, 13242, and
13243)
Narrative and
Numerical Water
Quality Objectives for
all surface and
ground waters of the
Central Valley
Region, as prescribed
by the Basin Plan.
Basin Plan, Chapter
3.
Media
Specific
Establishes water quality objectives,
including narrative and numerical
standards that protect the beneficial uses
and water quality objectives of surface and
groundwaters in the region.
Specific applicable portions of the Basin
Plan include beneficial uses of affected
water bodies and water quality objectives
to protect those uses. Any activity,
including, but not limited to, the discharge
of contaminated waters or in-situ
treatment or containment of contaminated
waters, must not result in actual water
quality exceeding water quality objectives.
TBC
The water quality objectives in Chapter 3 of the Basin Plan
form the basis of the Clear Lake TMDL for Mercury. EPA is
selecting the 0.5 kg/year mercury flux via groundwater from
the mine site into Clear Lake as a performance standard for the
remedy.
26
Porter-Cologne
Water Quality
Control Act
Media
Specific
State Water Resources Control Board
Resolution No. 88-63 ("Sources of
Drinking Water Policy") specifies that,
TBC
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Citation
Media/
Action/
Location
Description
Applicability
(California Water
Code Sections 13000,
13140, and 13240)
13263, 13304
Basin Plan, Chapter
2, Sections 2.1 and
2.2; State Water
Resources Control
Board Resolution No.
88-63 ("Sources of
Drinking Water
Policy")
with certain exceptions, all ground and
surface waters must have the beneficial
use of municipal or domestic water
supply.
This resolution specifies that all
groundwater and surface waters of the
state are considered to be suitable or
potentially suitable for the beneficial use
of MUN. All groundwater and surface
waters should be designated by the
Regional Boards with the MUN beneficial
use except waterbodies that meet the
specified exceptions in the Sources of
Drinking Water Policy. The exceptions
include waters that exceed 3,000 mg/L of
TDS or yield less than 200 gpd. The
Central Valley Regional Board
implemented this resolution by
designating all waters as MUN unless
specifically exempted in the Basin Plan.
Removal of the MUN use requires the
Board to complete a basin plan
amendment.
The naturally occurring TDS in the hydrothermal groundwater
may make site groundwater eligible for beneficial use de-
designation in a Basin Plan Amendment as provided under
Resolution 88-63 and the Basin Plan.
27
Water Code Division
7, Chapter 5.5
(California Water
Code Sections 13370
et seq.)
Action
Specific
The Clean Water Act National Pollutant
Discharge Elimination System (NPDES)
requirements.
Relevant and Appropriate
The substantive requirements of the stormwater Industrial
General Permit and Construction General Permit referred to
below are relevant and appropriate.
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Citation
Media/
Action/
Location
Description
Applicability
33 (JSC section 1251
et seq., 40 CFR
section 123.25.
28
Porter-Cologne
Water Quality
Control Act
(California Water
Code Sections 13172,
13260, 13263, 13267,
and 13304)
Title 27, CCR,
Sections 20090(d),
20200
Media and
Location
Specific
Title 27, CCR, Section 20090(d) provides
that wastes removed from immediate
place of release and discharged to land
must be managed in accordance with
classification and siting requirements of
Title 27 (Section 20200). Wastes contained
or left in place must comply with Title 27 to
the extent feasible.
Relevant and Appropriate
Provides that waste removed from the immediate place of
release must be classified and disposed in accordance with its
Title 27 classification to the extent feasible.
29
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections 13260,
13372, 13373, and
13374)
40 CFR Parts 122,
123, 124, National
Pollution Discharge
Elimination System,
implemented by the
Statewide General
Permit for Storm
Water Discharges
Associated with
Industrial Activities,
Media specific
State Water Resources Control Board
Order No. 2014-0057-DWQ regulates
pollutants in discharge of storm water
("Industrial General Permit") for facilities
covered in Attachment A. Closed mining
sites are not listed in Attachment A, but
EPA determined it is relevant and
appropriate to apply the substantive
provisions identified herein to the
management of stormwater at the Site.
Relevant and Appropriate
The substantive provisions in Section V.a. (BMPs), Section X.
(SWPPPs), Section XI. (Monitoring) of State Water Resources
Control Board Order No. 2014-0057-DWQ are relevant and
appropriate to storm water discharges from the Site. EPA will
determine the relevant and appropriate substantive provisions
of these Sections of the Industrial General Permit in
consultation with the State.
14
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Citation
Media/
Action/
Location
Description
Applicability
State Water
Resources Control
Board Order No.
2014-0057-DWQ, as
amended.
30
Porter-Cologne Water
Quality Control Act
(California Water
Code Sections 13260,
13372, 13373, and
13374)
40 CFR Parts 122,
123, 124, National
Pollution Discharge
Elimination System,
implemented by
State Water
Resources Control
Board Order No.
2009-0009-DWQ
(2009-0009-DWQ
amended by 2010-
0014-DWQ& 2012-
0006-DWQ)
Media specific
General Permit 2009-0009-DWQ, amended
by 2010-0014-DWQ & 2012-0006-DWQ
("Construction General Permit") contains
requirements for discharge of storm water
associated with construction activity
involving the disturbance of 1 acre or
more.
Applicable
The substantive provisions of the Construction General Permit
are applicable to construction activities at the Site. EPA will
determine the applicable substantive provisions of the
Construction General Permit in consultation with the State.
31
Porter-Cologne
Water Quality Control
Act (California Water
Code Section 13000
et seq); California
Media specific
Section 4.5.4.1.2 of the Basin Plan (Sulphur
Bank Mercury Mine) provides that mercury
entering Clear Lake from the mine site via
groundwater should be reduced to 0.5 kg
per year.
TBC
EPA is selecting the 0.5 kg/year mercury flux via groundwater
from the mine site into Clear Lake as a performance standard.
15
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#
Citation
Media/
Action/
Location
Description
Applicability
Water Code Sections
13240 -13243
32
Lake County Code of
Ordinances, Chapter
23-12
Location
Specific
Waste rock
dam removal
in and along
the Clear Lake
shoreline
Chapter 23-12 requires protections for
water quality for construction activities in
or along the shoreline or waters of Clear
Lake.
Relevant and Appropriate
EPA will comply with the substantive requirements of Chapter
23-12 by informing Lake County how EPA intends to protect
Clear Lake and by consulting with the Lake County to obtain
input on how to address impacts to Clear Lake resources
during waste rock dam construction.
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