HRS DOCUMENTATION RECORD COVER SHEET

Name of Site: New Idria Mercury Mine EPA ID No. CA0001900463

Contact Persons

Site Investigation: Matt Mitguard, EPA Region 9

Documentation Record: Karen Jurist, EPA Region 9

Pathways, Components, or Threats Not Scored

Ground Water Pathway

The ground water pathway was not scored because there are currently no known drinking water wells within four miles of site sources.

Soil Exposure Pathway

The soil exposure pathway was not scored because there are currently no known resident individuals, workers, sensitive environments, or resources on or within 200 feet of sources at the site.

Air Migration Pathway

The air pathway was not scored because there is no documented observed release to the atmosphere, and scoring the potential to release to air would not likely contribute significantly to the overall listing decision. HRS DOCUMENTATION RECORD

Name of Site: New Idria Mercury Mine

EPA Region: 9 Date Prepared: 3/2011

Street Address of Site: Township: 17S, Range: 12E

City, County, State, Zip Code: Idria, San Benito County, CA 95043

Latitude: 36.418392 North Longitude: 120.672511 West (Ref. 18; Ref 21)

Latitude/Longitude Reference Point: The latitude and longitude were measured from the centroid of Source 1, the Calcine Tailings Piles (Ref. 18; Ref. 21).

Scores

Air Pathway = not scored Ground Water Pathway = not scored Soil Exposure Pathway = not scored Surface Water Pathway = 63.33

HRS SITE SCORE = 31.66

*The street address, coordinates, and contaminant locations presented in this HRS documentation record identify the general area the site is located. They represent one or more locations EPA considers to be part of the site based on the screening information EPA used to evaluate the site for NPL listing. EPA lists national priorities among the known "releases or threatened releases" of hazardous substances; thus, the focus is on the release, not precisely delineated boundaries. A site is defined as where a hazardous substance has been "deposited, stored, placed, or otherwise come to be located." Generally, HRS scoring and the subsequent listing of a release merely represent the initial determination that a certain area may need to be addressed under CERCLA. Accordingly, EPA contemplates that the preliminary description of facility boundaries at the time of scoring will be refined as more information is developed as to where the contamination has come to be located.

1 HAZARD RANKING SYSTEM SUMMARY SCORESHEETS

SITE NAME: New Idria Mercury Mine

CITY/COUNTY/STATE: Idria, San Benito County, California

EPA ID #: CA0001900463

EVALUATOR: Christina Marquis DATE: March 2011

LATITUDE: 36.418392 North LONGITUDE: 120.672511 West

S S2

Ground Water Migration Pathway Score (Sgw) Not scored Not scored Surface Water Migration Pathway Score (Ssw) 63.33 4,010.6889 Soil Exposure Pathway Score (Ss) Not scored Not scored Air Migration Pathway Score (Sa) Not scored Not scored 2 2 2 2 Sgw +Ssw + Ss + Sa XXXXXXX 4,010.6889 2 2 2 2 (Sgw +Ssw + Ss + Sa ) / 4 XXXXXXX 1,002.6722 2 2 2 2 SQRT ((Sgw +Ssw + Ss + Sa ) / 4) XXXXXXX 31.66

2 3/2011 SURFACE WATER OVERLAND/FLOOD MIGRATION COMPONENT SCORESHEET REF. 1, TABLE 4-1

Factor Categories and Factors Maximum Value Value Assigned

DRINKING WATER THREAT

Likelihood of Release:

1. Observed Release 550 550

2. Potential to Release by Overland Flow:

2a. Containment 10 NS

2b. Runoff 25 NS

2c. Distance to Surface Water 25 NS

2d. Potential to Release by Overland Flow (lines 2a x [2b + 2c]) 500 NS

3. Potential to Release by Flood:

3a. Containment (Flood) 10 NS

3b. Flood Frequency 50 NS

3c. Potential to Release by Flood (lines 3a x 3b) 500 NS

4. Potential to Release (lines 2d + 3c, subject to a maximum of 500) 500 NS

5. Likelihood of Release (higher of lines 1 and 4) 550 550

Waste Characteristics:

6. Toxicity/Persistence a 10,000

7. Hazardous Waste Quantity a 10,000

8. Waste Characteristics 100 100

Targets:

9. Nearest Intake 50 0

10. Population:

10a. Level I Concentrations b 0

10b. Level II Concentrations b 0

10c. Potential Contamination b 0

10d. Population (lines 10a + 10b + 10c) b 0

11. Resources 5 5

12. Targets (lines 9 + 10d + 11) b 5

3 3/2011

Factor Categories and Factors Maximum Value Value Assigned

DRINKING WATER THREAT (Concluded)

Drinking Water Threat Score:

13. Drinking Water Threat Score ([lines 5 x 8 x 12]/82,500, subject to a maximum of 100) 100 3.33

HUMAN FOOD CHAIN THREAT

Likelihood of Release:

14. Likelihood of Release (same value as line 5) 550 550

Waste Characteristics:

15. Toxicity/Persistence/Bioaccumulation a 500,000,000

16. Hazardous Waste Quantity a 10,000

17. Waste Characteristics 1,000 1,000

Targets:

18. Food Chain Individual 50 0

19. Population:

19a. Level I Concentrations b 0

19b. Level II Concentrations b 0

19c. Potential Human Food Chain Contamination b 0

19d. Population b 0 (lines 19a + 19b + 19c)

20. Targets (lines 18 + 19d) b 0

Human Food Chain Threat Score:

21. Human Food Chain Threat Score ([lines 14 x 17 x 20]/82,500, subject to a maximum of 100) 100 0

4 3/2011

Factor Categories and Factors Maximum Value Value Assigned

ENVIRONMENTAL THREAT

Likelihood of Release:

22. Likelihood of Release (same value as line 5) 550 550

23. Ecosystem Toxicity/Persistence/ Bioaccumulation a 500,000,000

24. Hazardous Waste Quantity a 10,000

25. Waste Characteristics 1,000 1,000

Targets:

26. Sensitive Environments:

26a. Level I Concentrations b 0

26b. Level II Concentrations b 750

26c. Potential Contamination b 0

26d. Sensitive Environments (lines 26a + 26b + 26c) b 750

27. Targets (value from 26d) B 750

Environmental Threat Score:

28. Environmental Threat Score ([lines 22 x 25 x 27]/82,500, subject to a maximum of 60) 60 60

SURFACE WATER OVERLAND/FLOOD MIGRATION COMPONENT SCORE FOR A WATERSHED

29. Watershed Scorec (lines 13 + 21 + 28, 63.33 subject to a maximum of 100) 100

SURFACE WATER OVERLAND/FLOOD MIGRATION COMPONENT SCORE

c 30. Component Score (Sof) , (highest score from line 29 for all watersheds evaluated, subject to a maximum of 100) 100 63.33 aMaximum value applies to waste characteristics category. bMaximum value not applicable. cDo not round to nearest integer.

5 3/2011 REFERENCES

Reference Description of the Reference Number Hazard Ranking System; Final Rule, 14 December 1990, Vol. 55, No. 241, 138 pages. 1 Available at: http://www.epa.gov/superfund/sites/npl/hrsres/index.htm#HRS%20Rule. U.S. Environmental Protection Agency (EPA), Superfund Chemical Data Matrix 2 (SCDM) Methodology, January 2004, 15 pages. A complete copy of SCDM is available at http://www.epa.gov/superfund/sites/npl/hrsres/tools/scdm.htm. Weston Solutions, Inc., New Idria Mercury Mine, Expanded Site Inspection, Idria, 3 San Benito County, California, Sampling and Analysis Plan, April 2010, 48 pages. Ecology & Environment, Inc., New Idria Mercury Mine, Preliminary Assessment/Site 4 Inspection, March 31, 1998, 68 pages. Dames & Moore, Application for Facility Permit/Waste Discharge, New Idria Mine, 5 Idria, California 95021, April 21, 1989, 22 pages. Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, 6 San Benito County, California, October 2010, 1200 pages. Bureau of Land Management, Draft Clear Creek Management Area Resource 7 Management Plan & Environmental Impact Statement, Hollister, California, November 2009, 735 pages. University of California Santa Cruz, Mercury Speciation in Acid Mine Drainage: 8 New Idria Quicksilver Mine, California, September 1998, 41 pages. California Regional Water Quality Control Board, Central Valley Region, New Idria 9 Mine, San Benito County – Complaint Investigation, June 2, 1975, 4 pages. California Regional Water Quality Control Board, Central Valley Region, Inspection 10 of New Idria Mine, San Benito County, July 22, 1988, 6 pages. California Regional Water Quality Control Board, Central Valley Region, New Idria 11 Mine, San Benito County, March 7, 2003, 15 pages. TechLaw, Inc., Potentially Responsible Party and Insurance Archeology Search 12 Report, New Idria Mercury Mine Site, San Benito County, California, June 5, 2006, 205 pages. U.S. Environmental Protection Agency, Region IX, Wetlands Regulatory Office (WTR-8), Summary of Site Inspection to Verify the Presence of Jurisdictional 13 Wetlands for the New Idria Mercury Mine Site, including San Carlos and Silver Creeks, San Benito and Fresno Counties, California, December 16, 2010, 18 pages. Weston Solutions, Inc., Expanded Site Inspection. Logbook, New Idria Mercury 14 Mine, July 6, 2010 through July 23, 2010, 8 pages. U.S. Bureau of Mines, Mercury Potential of the United States, Information Circular 15 8252, 1965, 34 pages. U.S. Environmental Protection Agency, Solid Waste and Emergency Response, Using 16 Qualified Data to Document an Observed Release and Observed Contamination, November 1996, 18 pages. 6 3/2011 County of San Benito, Proposal by the County of San Benito before the Central Valley Regional Water Quality Control Board – Request for Investigation, Enforcement, and 17 Regional Planning necessary to clean up and abate the discharge of pollutants from the New Idria Mercury Mine in San Benito County, undated, 28 pages. 18 U.S. Geological Survey, Idria, California 7.5 Minute quadrangle 1969, 1 page. National Operations Center, Sampling and Analysis Plan, Archer Mine, April 15, 19 2009, 12 pages. Weston Solutions, Inc., Surface Water Pathway and Target Distance Limit Map, New 20 Idria Mercury Mine, December 13, 2010, 1 page. Weston Solutions, Inc., Site Latitude and Longitude and Source Area, New Idria 21 Mercury Mine, December 13, 2010, 1 page. Meierer, Bob, CompuChem, Memorandum to Stan Kott, ESAT/ICF-LDC, Case#: 22 40334, SDG#: MY71L3, October 5, 2010, 339 pages. Meierer, Bob, CompuChem, Memorandum to Stan Kott, ESAT/ICF-LDC, Case#: 23 40334, SDG#: MY71M9, October 11, 2010, 264 pages. CompuChem, Memorandum to Garrett Peterson, U.S. Environmental Protection 24 Agency, Case# 40334, SDG#: MY71N3, undated, 271 pages. CompuChem, Memorandum to Garrett Peterson, U.S. Environmental Protection 25 Agency, Case# 40334, SDG#: MY71S3, undated, 341 pages. CompuChem, Memorandum to Mary O’Donnell, U.S. Environmental Protection 26 Agency, Case# 40334, SDG#: MY71S4, undated, 268 pages. CompuChem, Memorandum to Mary O’Donnell, U.S. Environmental Protection 27 Agency, Case# 40334, SDG#: MY71S5, undated, 510 pages. CompuChem, Memorandum to Mary O’Donnell, U.S. Environmental Protection 28 Agency, Case# 40334, SDG#: MY71S6, undated, 509 pages. Reilly, Amanda K.C., Weston Solutions, Inc., Memorandum Re: Sample Quantitation 29 Limits, January 6, 2011, 5 pages. Susan Moore, U.S. Department of the Interior, Fish and Wildlife Service, letter addressed to Ms. Debbie Schechter, USEPA, Subject: United States Fish and Wildlife 30 Service Review of Expanded Site Investigation Report on the New Idria Mercury Mine, San Benito County, California, January 26, 2011, 9 pages. Western Regional Climate Center, Period of Record Monthly Climate Summary, 31 Idria, California, www.wrcc.dri.edu, data extracted January 26, 2011, 1 pages. U.S. Fish and Wildlife Service, Species Report, Environmental Conservation Online 32 System, Listed Animals, http://ecos.fws.gov/tess_public/pub/listedAnimals.jsp, data extracted January 26, 2011, 77 pages. U.S. Fish and Wildlife Service, Species Report, Environmental Conservation Online 33 System, Listed Plants, http://ecos.fws.gov/tess_public/pub/listedPlants.jsp, data extracted January 26, 2011, 42 pages.

7 3/2011 ACRONYM LIST

AMD Acid Mine Drainage BLM Bureau of Land Management CERCLA Comprehensive Environmental Response, Compensation, and Liability Act CLP Contract Laboratory Program CLPAS Contract Laboratory Program Analytical Services EPA U. S. Environmental Protection Agency ESI Expanded Site Inspection gpm gallons per minute HRS Hazard Ranking System mg/kg milligrams per kilogram µg/l micrograms per liter NIMM New Idria Mercury Mine PPE Probable Point of Entry ppm parts per million SAP Sampling and Analysis Plan USFW U.S. Fish and Wildlife Service XRF X-ray Fluorescence

8 3/2011 NOTES TO THE READER

1. Attachment contains the following maps:

Figure 1: New Idria Mercury Mine Site Location Map

Figure 2: Sample Locations – Map 1A

Figure 3: Historic Sample Locations – Map 1B

Figure 4: Sample Locations – Map 1C

Figure 5: Historic Sample Locations – Map 1D

Figure 6: Sample Locations – Map 2A

Figure 7: Historic Sample Locations – Map 2B

Figure 8: Sample Locations – Map 3

Figure 9: Sample Locations – Map 4A

Figure 10: Historic Sample Locations – Map 4B

Figure 11: Sample Locations – Map 5A

Figure 12: Historic Sample Locations – Map 5B

9 3/2011 SITE DESCRIPTION

The New Idria Mercury Mine (NIMM) is located at the abandoned town of Idria in San Benito County, California, approximately 135 miles southeast of San Francisco within the New Idria Mining District, Sections 28, 29, and 32, Township 17 South, Range 12 East of the Mount Diablo Meridian. The facility property covers approximately 880 acres and is located 70 miles southeast of Hollister, California (Ref. 6, p. 7; Ref. 12, p. 2).

The New Idria Mercury Mine operated from approximately 1854 to 1972. Estimates ranging from 0.5 to 2 million cubic yards of waste rock and calcines (processed mercury ore) cover 48.677 acres throughout the facility and along San Carlos Creek. The abandoned Town of Idria is comprised of dozens of dilapidated buildings. A large furnace and process area and other mine working features remain at the property. Acid mine drainage (AMD) discharges from the main adit at the facility, known as the Level 10 Adit. The AMD then enters an unnamed tributary that flows between and is in direct contact with a waste rock pile and calcine tailings pile. It discharges to San Carlos Creek, approximately 2,000 feet away from the Level 10 adit. The AMD channel bypasses the former settling basin (AMD pond), located near the discharge point (Ref. 5, p. 7; Ref. 6, p. 7-8, Photo 23, Photo 26; Ref. 7, p. 263; Ref. 12, pp. 2-3; Ref. 21).

The New Idria mining claim was declared in 1854 by prospectors and investors. In 1857 the first brick furnace to roast cinnabar ore was built at the Facility. The mine operations expanded to include San Carlos, Aurora, and Molina mines and miles of tunnels, shafts and drifts were used to access the cinnabar. In the 1920s, the overburden was stripped down to form pits in order to access cinnabar. The mining continued nearly uninterrupted until the early 1970s with a few idle and low productivity periods due to economic lows in mercury values and land owner disputes. Several furnaces were built over the years including four large furnaces still located at the facility adjacent to the Level 10 adit. Mercury was extracted from the cinnabar ore by crushing the ore and roasting it to release elemental mercury vapor which was cooled and condensed for bottling. The roasting process is called calcination and the roasted ore is known as calcines, which still contain some soluble mercury. Some of the older tailings piles were reprocessed as mercury extraction methods improved. The furnaces at the Facility were also used to process mercury ore from the San Carlos and Aurora Mines which was transported by a 2-mile aerial tram or through the Level 10 adit. Ore was also trucked in from the Alpine, Anita, North Star, and other mines to be processed at the New Idria furnace. The New Idria Mine was reported to be the second largest mercury mine in the country and produced over 500,000 flasks (38,250,000 pounds) of mercury. One flask contains 76.5 pounds of mercury (Ref. 6, p. 7; Ref. 12, pp. 2-3, 20, 23-25, 28).

Over 30 miles of tunnels and 20 levels were constructed at the facility. The Level 10 adit was used as the main haulage level and is connected to other levels by shafts and raises. The extensive mine levels have flooded with ground water, or possibly surface water, which reacts with the high iron and sulfur content of the bedrock to form an acidic solution. The ground water drains from the Level 10 Adit as AMD and flows into an unnamed tributary. The AMD and unnamed tributary are presently not treated and flow through tailings piles and discharge to San Carlos Creek. San Carlos Creek

10 3/2011 flows along the base of massive calcine tailings piles for a length of approximately 2,500 feet (Ref. 6, p. 8; Ref. 10, p. 1; Ref. 11, p. 6; Ref. 15, pp. 29-30).

Surface water from the site drains to San Carlos Creek which flows to Silver Creek and then to Panoche Creek. Panoche Creek flows to the Mendota Pool and San Joaquin River during periods of heavy precipitation and flood events. The Mendota Pool and San Joaquin River are recreational fisheries and are located approximately 45 river miles downstream from the Site. The San Joaquin River flows to the San Francisco Bay, which is a commercial fishery. Sensitive habitats and wetlands are found along the surface water pathway between the Site and the San Joaquin River. The entire lengths of San Carlos Creek and Silver Creek downstream of the site sources are documented wetlands (Ref. 6, p. 6; Ref. 13, p. 6).

Numerous complaints regarding degradation of San Carlos Creek by AMD have been made by property owners downstream of the New Idria Mercury Mine (Ref. 6, pp. 9-10; Ref. 9, pp. 1-4; Ref. 10, pp. 1-6; Ref. 11, p. 8). Previous investigations of surface water downstream of the site have demonstrated impacts to surface water bodies by mercury and other metals from site sources (Ref. 4, p. 28; Ref. 6, p. 9; Ref. 8, p. 19; Ref. 9, pp. 1-4; Ref. 10, pp. 1-6; Ref. 11, p. 8).

In July 2010, EPA tasked Weston Solutions, Inc. (WESTON) to conduct an Expanded Site Inspection (ESI) of the New Idria Mercury Mine facility, including the collection of environmental samples. The specific sampling objectives were to collect data that could be used to document whether a release of mercury or other metals had occurred to San Carlos and Silver Creeks that could be attributed to the Site. An observed release of arsenic, cadmium, copper, lead, mercury, nickel, selenium, and zinc was documented by direct observation and by chemical analysis. These hazardous substances were documented in hazardous substance sources at the site, including the Calcine Tailings Piles and the Level 10 Adit AMD. The Calcine Tailings Piles and the Level 10 Adit AMD were observed to be in direct contact with San Carlos Creek. Mercury attributable to the site was detected at concentrations significantly above background to a distance of 19.9 miles downstream of the site sources (Ref. 6, pp. 5, 42-43).

11 3/2011

2.2 SOURCE CHARACTERIZATION

2.2.1 SOURCE IDENTIFICATION

Name of source: Calcine Tailings Piles Number of source: 1

Source Type: Pile

Description and Location of Source (see Reference 6, page 55 for map of Source 1):

Several furnaces were built over the years at the New Idria Mercury Mine facility, including four large furnaces still located at the facility adjacent to the Level 10 Adit. Mercury was extracted from the cinnabar ore by crushing the ore and roasting it in the furnaces to release elemental mercury vapor which was cooled and condensed for bottling. The roasting process is called calcination and the roasted ore is known as calcines, which still contain some soluble mercury. Some of the older tailings piles were reprocessed as mercury extraction methods improved. Extensive waste rock and calcine tailings piles are still present and cover a large portion of the property. The Southern and Northern Calcine Tailings Piles are estimated to cover 48.677 acres. The calcine tailings area was calculated by creating a polygon of observable tailings from aerial images and filed observations and processing data (Ref. 6, p. 7; Ref. 12, p. 17; Ref. 21).

The furnaces at the facility were also used to process mercury ore from the San Carlos and Aurora Mines which were transported by a 2-mile aerial tram or through the Level 10 Adit. Ore was also trucked in from the Alpine, Anita, North Star, and other mines to be processed at the New Idria furnace. The New Idria Mine was reported to be the second most productive mercury mine in North America and produced over 500,000 flasks (38,250,000 pounds) of mercury. One flask contains 76.5 pounds of mercury (Ref. 6, p. 7; Ref. 12, p. 20-21, 23, 28).

12 Source Characterization Source No: 1

2.2.2 HAZARDOUS SUBSTANCES ASSOCIATED WITH THE SOURCE

In July, 2010, EPA tasked WESTON to conduct an ESI of the New Idria Mercury Mine, including the collection of environmental samples. The specific sampling objectives were to collect data that could be used to document whether a release of mercury or other metals had occurred to San Carlos and Silver Creeks that could be attributed to the Site. The field work was conducted in accordance with WESTON’s Sampling and Analysis Plan (SAP), approved by the EPA in April 2010 (Ref. 3, p. 1; Ref. 6, pp. 15-16).

WESTON implemented the SAP during July 19-23, 2010. WESTON collected samples from the Calcine Tailings Piles. Background soil samples were collected from locations south of the Facility at areas upgradient from mercury mining activities. Samples were collected from 10 soil/tailings locations. Samples were submitted to Liberty Analytical under the EPA Contract Laboratory Program (CLP) for metal analysis by EPA CLPAS ILM05.4. The analytical data were validated by the EPA (Ref. 6, p. 16).

Cal ci ne tai l i ngs sampl es were col l ected to document the presence of a hazardous substance source to surf ace water. WESTON collected 10 soil samples to establish a source of metals at the Si te. Soil sampl es NI M M -TL-3 through NIMM-TL-5 were collected from the southern calcine tailings pile, and soil samples NIMM-TL-6 through NIMM-TL-10 were collected from the northern cal ci ne tailings pile. Soil samples NIMM-SS-1 and NIMM-SS-2 were col l ected f rom background locations (Ref. 6, p. 19). Sample locations are shown on Figure 2 and Figure 4.

Three samples were collected at each sampling location. Based on the BLM standard procedure for mercury mine sampling, the samples were sifted using a disposable sieve to obtain a grain size of less than 1.3 millimeters (Ref. 6, p. 19; Ref. 19, p. 10-11). The sieved samples were field screened for mercury concentrations using an x-ray florescence (XRF) metal analyzer. The sieved sample exhibiting the highest mercury concentration was selected for laboratory analyses. The coarse fraction of the sample and the sieved portion of the sample were weighed to determine the percentage of fines in the matrix of each sample submitted to the laboratory. The XRF mercury measurements ranged from 35 to 785 parts per million (ppm) for the calcine tailings samples. Samples for laboratory analysis were contained in 4-ounce glass containers and placed on ice for preservation (Ref. 6, p. 19; Ref. 14, p. 2).

The weighted fractions of the sample used to determine the percentages of fines are presented in the table below.

13 Source Characterization Source No: 1

Total Sieved Remaining - Percentage Sample ID Weight Weight - fines coarse References of fines (%) (grams) (grams) (grams) NIMM-TL-3 (southern calcine 348 199 149 57.18 Ref. 6, p. 885 tailings pile) NIMM-TL-4 (southern calcine 214 124 90 57.94 Ref. 6, p. 886 tailings pile) NIMM-TL-5 (southern calcine 265 79 186 29.81 Ref. 6, p. 887 tailings pile) NIMM-TL-6 (southern calcine 258 135 123 52.33 Ref. 6, p. 888 tailings pile) NIMM-TL-7 (northern calcine 683 161 522 23.57 Ref. 6, p. 889 tailings pile) NIMM-TL-8 (northern calcine 367 75 292 20.44 Ref. 6, p. 890 tailings pile) NIMM-TL-9 (northern calcine 755 118 637 15.63 Ref. 6, p. 891 tailings pile) NIMM-TL-10 (northern calcine 848 197 651 23.23 Ref. 6, p. 892 tailings pile)

The tailings samples presented in the table below are representative of New Idria Mercury Mine calcine tailings pile materials. The source samples are not compared to background levels because the media sampled is waste tailings (Ref. 21).

14 Source Characterization Source No: 1

Hazardous Substance Hazardous SQL Sample ID Date Concentration References Substance (mg/kg) (mg/kg fines) Arsenic 22.7 1.01 Ref. 6, pp. 20-21, Cadmium 1.2 J1 (0.85) 0.504 1133, 1135-1136, NIMM-TL-3 Copper 59.9 2.52 1139; Ref. 14, p. 2; (southern calcine 7/19/10 Lead 18.1 1.01 Ref. 16, pp. 8, 18; tailings pile) Mercury 191 2.016 Ref. 22, pp. 2, 39, Nickel 31.3 4.032 67, 103, 104, 108, Zinc 56.2 6.048 115; Ref. 29 Arsenic 17.7 0.995 Ref. 6, pp. 20-21, Cadmium 0.51 J1 (0.36) 0.497 1133, 1135-1136, NIMM-TL-4 Copper 39.2 2.49 1139; Ref. 14, p. 2; (southern calcine 7/19/10 Lead 9.0 0.995 Ref. 16, pp. 8, 18; tailings pile) Mercury 134 5.03 Ref. 22, pp. 2, 40, Nickel 20.4 3.98 67, 103, 104, 108, Zinc 39.3 5.97 115; Ref. 29 Arsenic 16.2 0.964 Ref. 6, pp. 20-21, Cadmium 1.3 J1 (0.92) 0.482 1133, 1135-1136, NIMM-TL-5 Copper 48.5 2.41 1139; Ref. 14, p. 2; (southern calcine 7/19/10 Lead 49.5 0.964 Ref. 16, pp. 8, 18; tailings pile) Mercury 446 10.14 Ref. 22, pp. 2, 41, Nickel 67.7 3.86 67, 103, 104, 108, Zinc 396 5.78 115; Ref. 29 Arsenic 17.2 0.965 Ref. 6, pp. 20-21, Cadmium 0.99 J1 (0.7) 0.483 1133, 1135-1136, NIMM-TL-6 Copper 47.1 2.41 1139; Ref. 14, p. 2; (southern calcine 7/19/10 Lead 61.5 0.965 Ref. 16, pp. 8, 18; tailings pile) Mercury 38.7 2.01 Ref. 22, pp. 2, 42, Nickel 159 3.86 67, 103, 104, 108, Zinc 104 5.79 115; Ref. 29

15 Source Characterization Source No: 1

Hazardous Substance Hazardous SQL Sample ID Date Concentration References Substance (mg/kg) (mg/kg fines) Arsenic 23.6 0.978 Ref. 6, pp. 20-21, Cadmium 1.5 J1 (1.06) 0.489 1133, 1135-1137, NIMM-TL-7 Copper 69.0 2.45 1139; Ref. 14, p. 2; (northern calcine 7/19/10 Lead 23.5 0.978 Ref. 16, pp. 8, 18; tailings pile) Mercury 3.8 0.096 Ref. 22, pp. 2, 43, Nickel 66.1 3.91 67, 103, 104, 108, Zinc 50.8 5.87 115; Ref. 29 Arsenic 18.8 0.99 Ref. 6, pp. 20-21, Cadmium 1.3 J1 (0.92) 0.495 1133, 1135-1137, NIMM-TL-8 Copper 60.3 2.48 1139; Ref. 14, p. 2; (northern calcine 7/19/10 Lead 18.2 0.99 Ref. 16, pp. 8, 18; tailings pile) Mercury 91.6 5.05 Ref. 22, pp. 2, 45, Nickel 99.8 3.96 67, 103, 104, 108, Zinc 98.7 5.94 115; Ref. 29 Arsenic 27.7 1.01 Ref. 6, pp. 20-21, Cadmium 1.6 J1 (1.13) 0.505 1133, 1135-1137, NIMM-TL-9 Copper 79.8 2.53 1139; Ref. 14, p. 2; (northern calcine 7/19/10 Lead 22.5 1.01 Ref. 16, pp. 8, 18; tailings pile) Mercury 72.3 2.01 Ref. 22, pp. 2, 46, Nickel 203 4.04 67, 103, 105, 108, Zinc 71.3 6.06 115; Ref. 29 Arsenic 26.0 0.974 Ref. 6, pp. 20-21, 1 Cadmium 1.8 J (1.28) 0.487 1133, 1135-1137, NIMM-TL-10 Copper 73.7 2.44 1139; Ref. 14, p. 2; (northern calcine 7/19/10 Lead 26.2 0.974 Ref. 16, pp. 8, 18; tailings pile) Mercury 28.9 2.03 Ref. 22, pp. 3, 47, Nickel 228 3.90 67, 103, 105, 108, 115; Ref. 29 Zinc 88.4 5.84

16 Source Characterization Source No: 1

Hazardous Substance Hazardous SQL Sample ID Date Concentration References Substance (mg/kg) (mg/kg fines) Arsenic 26.7 1.0 Ref. 6, pp. 20-21, Cadmium 1.8 J1 (1.28) 0.5 1133, 1135-1137, NIMM-TL-11 Copper 74.2 2.5 1139; Ref. 14, p. 2; (duplicate of 7/19/10 Lead 25.0 1.0 Ref. 16, pp. 8, 18; NIMM-TL-10) Mercury 31.5 2.02 Ref. 22, pp. 2, 48, Nickel 231 4.0 67, 103, 105, 108, Zinc 90.3 6.0 115; Ref. 29 mg/kg: milligrams analyte per kilogram tailings

J1: The analyte was positively identified; however, the reported value is an estimate due to ICP inter-element interference problems. The result was reported from undiluted analyses that contained iron concentrations above the level present in the interference check sample (ICS) solution, therefore the applied inter-element correction (IEC) factors may not sufficiently compensate.

For cadmium the value reported may be biased high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 mg   μg  1 mg final volume (mL) 1L 1000 g SQL   = CRDL   x x x x x DF  kg dry wgt   L  1000 μg sample weight (g wet wgt) x %S 1000 mL 1 kg

Where:

Final volume (mL) is the final extract volume following sample preparation Sample weight (g wet wgt) is the weight of the sample used for analysis % S is % solids value divided by 100; % solids is determined as follows: (g dry weight of sample used for analysis)/(g wet weight of sample used for analysis) x100% DF is the dilution factor for the instrumental analysis

List of Hazardous Substances Associated with Source

Arsenic, Cadmium, Copper, Lead, Mercury, Nickel, Zinc

17 Source Characterization Source No: 1

2.2.3 HAZARDOUS SUBSTANCES AVAILABLE TO A PATHWAY

Containment Containment Description Factor Value Reference Release via overland migration and/or flood: The Calcine Tailings Ref. 1, Table 4-2; Piles are unlined and were observed to be in direct contact with San Ref, 6, pp. 55, Carlos Creek. In addition, there was no engineered cover or retaining 10 Photos 7, 8, 26, 27, wall supporting the waste rock from coming into contact with surface 28, 29, 30, 31, 32, water. 34.

18 Source Characterization

2.4.2 HAZARDOUS WASTE QUANTITY

2.4.2.1.1 Tier A: Hazardous Constituent Quantity - Not Evaluated (NE)

The information available is not sufficient to meaningfully evaluate Tier A, hazardous constituent quantity; therefore it is not possible to adequately determine a hazardous constituent quantity for the Calcine Tailings Piles (Ref. 1, Section 2.4.2.1.1). Scoring proceeds to the evaluation of Tier B, hazardous wastestream quantity (Ref. 1, Section 2.4.2.1.2).

Hazardous Constituent Quantity Value: NE

2.4.2.1.2 Tier B: Hazardous Wastestream Quantity - NE

The information available is not sufficient to meaningfully evaluate Tier B, hazardous wastestream quantity; therefore, it is not possible to adequately determine a hazardous waste quantity for the Calcine Tailings Piles (Ref. 1, Section 2.4.2.1.2). Scoring proceeds to the evaluation of Tier C, volume (Ref. 1, Section 2.4.2.1.3).

Hazardous Wastestream Quantity Value: NE

Tier C: Volume - NE

The information available is not sufficient to meaningfully evaluate Tier C, volume; therefore it is not possible to adequately determine a volume for the Calcine Tailings Piles (Ref. 1, Section 2.4.2.1.3). Scoring proceeds to the evaluation of Tier D, area (Ref. 1, Section 2.4.2.1.4).

Volume Assigned Value: 0

Tier D: Area

The Southern and Northern Calcine Tailings Piles are estimated to cover 48.677 acres. The calcine tailings area was calculated by creating a polygon of observable tailings from aerial images and field observations and processing data using mapping tools (Ref. 21). Therefore:

48.677 acres x 43,560 square feet = 2,120,370.12 square feet 2,120,370.12 / 13 = 163,105.3938 (Ref. 1, Table 2-5)

Area Assigned Value: 163,105.3938

2.4.2.1.5 Source Hazardous Waste Quantity Value

Highest assigned value assigned from Ref. 1, Table 2-5: 163,105.3938

19 Source Characterization

2.2.1 SOURCE IDENTIFICATION

Name of source: Level 10 Adit AMD Number of source: 2

Source Type: Other

Description and Location of Source (see Reference 6, page 55 for map of Source 2):

Over 30 miles of tunnels and 20 levels were constructed as part of mining operations at the New Idria Mercury Mine. The Level 10 Adit was used as the main haulage level and is connected to other levels by shafts and raises. The extensive mine levels have flooded with ground water, or possibly surface water, which reacts with the high iron and sulfur content of the bedrock to form an acidic solution. The ground water drains from the Level 10 Adit as AMD. Once the ground water discharges from the Level 10 Adit, it enters an unnamed tributary that bypasses the former settling pond (AMD pond) and runs between and is in direct contact with a waste rock pile and the southern calcine tailings pile (Source 1). It discharges to San Carlos Creek, approximately 2,000 feet away from the Level 10 adit. The AMD is presently not treated prior to discharging (Ref. 6, p. 7-8, Photo 23, Photo 26; Ref. 10, p. 1; Ref. 11, p. 6; Ref. 15, pp. 29-30).

In an attempt to control the flow of water from the mine, the Level 10 Adit was originally sealed with a concrete bulkhead. However, as water backed up in the mine, it eventually broke through the bulkhead. Discharge to San Carlos Creek has been observed to be approximately 8 to 50 gallons per minute (gpm) (Ref. 4, Photo 6; Ref. 5, p. 9; Ref. 10, p. 1; Ref. 11, pp. 6, 12).

20 Source Characterization Source No: 2

2.2.2 HAZARDOUS SUBSTANCES ASSOCIATED WITH THE SOURCE

In July 2010, EPA tasked WESTON to conduct an ESI of the New Idria Mercury Mine, including the collection of environmental samples. The specific sampling objectives were to collect data that could be used to document whether a release of mercury or other metals had occurred to San Carlos and Silver Creeks that could be attributed to the Site. The field work was conducted in accordance with WESTON’s SAP, approved by the EPA in April 2010 (Ref. 3, p. 7; Ref. 6, pp. 15-16).

WESTON implemented the SAP during July 19-23, 2010. WESTON collected samples from the Calcine Tailings Piles and AMD at the Site and stream samples (surface water and sediments) from the surface water pathway. Background soil and stream samples were collected from locations south of the Site at areas upgradient from mercury mining activities. Samples were collected from 10 soil/tailing locations, 34 sediment locations, and 15 surface water locations. Samples were submitted to Liberty Analytical under the EPA CLP for metal analysis by EPA CLPAS ILM05.4. The analytical data were validated by EPA (Ref. 6, p. 16).

AMD samples were collected to document the presence of hazardous substance sources to surface water. Filtered and un-filtered surface water samples NIMM-SW-4 were collected from AMD- impacted surface water approximately 6 to 8 feet from the Level 10 Adit (Ref. 6, p. 22, Photo 21). Sample locations are shown on Reference 6, page 55.

21 Source Characterization Source No: 2

- Source Samples:

AMD Source Concentrations – Total Metals (µg/l) Hazardous Hazardous Sample ID Date Substance SQL References Substance Concentration Arsenic 72.4 1.0 Ref. 6, pp. 23, 1170, 1171, 1184-1187, Cadmium 1.7 1.0 1190; Ref. 14, p. 6; NIMM-SW-4 7/22/2010 Ref. 16, pp. 8, 18; (Level 10 Adit) 1 Selenium 11.2 J (9.82) 5.0 Ref. 25, p. 2; Ref.

2 27, pp. 34, 54, 70, Zinc 2,500 J 2.0 71; Ref. 29 µg/l: micrograms analyte per liter water

J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses.

J2: The analyte was positively identified; however, the reported value is an estimate due to the serial dilution percent differences being outside the method quantitation control limits. The value reported may be biased low. However, the presence of the analyte in the sample is not in doubt.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

22 Source Characterization Source No: 2

AMD Source Concentrations – Dissolved Metals (µg/l) Hazardous Hazardous Sample ID Date Substance SQL References Substance Concentration

Arsenic 66.1 1.0 Ref. 6, pp. 23, 1178, Cadmium 1.4 1.0 1195, 1197-1198; Ref. NIMM-SW-4 Copper 3.2 2.0 14, p. 6; Ref. 16, pp. 8, 7/22/2010 (Level 10 Adit) Nickel 1,440 1.0 18; Ref. 26, p. 2; Ref. Selenium 10.6 J1 (9.3) 5.0 28, pp. 34, 52, 68, 70; Zinc 2,350 2.0 Ref. 29 µg/l: micrograms analyte per liter water

J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

List of Hazardous Substances Associated with Source

Arsenic, Cadmium, Copper, Nickel, Selenium, Zinc

23 Source Characterization Source No: 2

2.2.3 HAZARDOUS SUBSTANCES AVAILABLE TO A PATHWAY

Containment Containment Description Reference Factor Value Release via overland migration and/or flood: The AMD Ref. 1, Table was observed to be flowing directly into the unlined and 4-2; Ref. 6, unnamed tributary that flows into San Carlos Creek with no 10 pp. 6-8, 41, containment. There was no engineered cover or retaining 54, Photos 5, wall to aid in the containment of the AMD. 6, 22, 24

24 Source Characterization Source No: 2

2.4.2 HAZARDOUS WASTE QUANTITY

2.4.2.1.1 Tier A: Hazardous Constituent Quantity

The information available is not sufficient to meaningfully evaluate Tier A, hazardous constituent quantity; therefore it is not possible to adequately determine a hazardous constituent quantity for the Level 10 Adit AMD (Ref. 1, Section 2.4.2.1.1). Scoring proceeds to the evaluation of Tier B, hazardous wastestream quantity (Ref. 1, Section 2.4.2.1.2).

Hazardous Constituent Quantity Value: NE

2.4.2.1.2 Tier B: Hazardous Wastestream Quantity

The rate of AMD discharging from the Level 10 adit has been measured over numerous years and investigations. In April 1988, the RWQCB conducted an inspection on the NIMM and observed the discharge from the Level 10 adit to be approximately 50 gpm (Ref. 10, p. 1). In 1989, the discharge was reported to have a range of 20 to 50 gpm, depending on year and season (Ref. 5, p. 9). In June 1997, the discharge from the Level 10 adit was observed to be 8 gpm (Ref. 4, Photo 6). In 2003, the RWQCB conducted Site Investigation on the NIMM property the discharge rate was observed to be greater than 10 gpm (Ref. 11, p. 6). Using a conservative flow rate estimate based on these measurements, the following hazardous wastestream quantity is calculated:

For one year of discharge:

8 gpm = 4,204,800 gallons per year 4,204,800 gallons = 42,048,000 pounds 42,048,000 pounds / 5,000 = 8,409.6 (Ref. 1, Table 2-5)

Hazardous Wastestream Quantity Value: 8,409.6

Tier C: Volume

The information available is not sufficient to meaningfully evaluate Tier C, volume; therefore it is not possible to adequately determine a volume for the Level 10 Adit AMD (Ref. 1, Section 2.4.2.1.3). Scoring proceeds to the evaluation of Tier D, area (Ref. 1, Section 2.4.2.1.4).

Volume Assigned Value: NE

25 Source Characterization Source No: 2

Tier D: Area

The information available is not sufficient to meaningfully evaluate Tier D, area; therefore it is not possible to adequately determine an area for the Level 10 Adit AMD (Ref. 1, Section 2.4.2.1.4).

Area Assigned Value: NE

2.4.2.1.5 Source Hazardous Waste Quantity Value

Highest assigned value assigned from Ref. 1, Table 2-5 (Ref. 1, Section 2.4.2.1.5): 8,409.6

26 Source Characterization

SUMMARY OF SOURCE DESCRIPTIONS

Containment Factor Value by Pathway Source Source Ground Surface Water (SW) Air Hazardous Hazardous Water Source Constituent Overland/ GW to Waste (GW) Gas Particulate No. Quantity flood SW Quantity (Ref. 1, (Ref. 1, (Ref. 1, Complete? (Ref. 1, (Ref. 1, Value Table 3- Table 6-3) Table 6-9) (Y/N) 2) Table 4-2) Table 3-2) 1 163,105.3938 N NE 10 NE NE NE 2 8,409.6 N NE 10 NE NE NE Total: 171,514.9938

NE: Not evaluated

27

POSSIBLE SOURCES NOT SCORED

• AMD Pond. Historically, drainage from the Level 10 adit flowed through a shallow ponded depression, referred to in previous reports as the “upper pond.” The water then flowed down the steep face of the dump into a channel incised in the calcines past a small holding pond (AMD Pond), referred to in precious reports as the “lower pond.” During a 1989 investigation, the lower pond was breached and dry. However, the pond was historically used as a settling pond for drainage from the Level 10 adit (Ref. 4, Figure 2-4, Photo 2; Ref. 5, p. 9).

In 1997, a surface water sample and a tailings sample were collected adjacent to the AMD pond. The surface water sample, NIM-5 exhibited a mercury concentration of 0.4 ug/L, and the tailings sample, NIM-16, exhibited a mercury concentration of 42.2 mg/kg (Ref. 4, p. 21-22, Figure 2-4).

• Waste Rock Pile. Immediately north of the Level 10 adit, is a graded pile of waste rock removed from the underground workings (Ref. 5, p. 8; Ref. 6, Fig. 4). The waste rock pile is a separate pile from the calcine tailings piles, which can distinctly be seen due to the different colors of the piles (Ref. 6, Photo 27).

In 1997, a waste rock pile sample was collected and exhibited a mercury concentration of 27.1 mg/kg (Ref. 4, p. 22; Ref. 6, Figure 5).

28

4.0 SURFACE WATER MIGRATION PATHWAY

4.1 OVERLAND/FLOOD MIGRATION COMPONENT

4.1.1.1 Definition of Hazardous Substance Migration Path for Overland/flood Component

The HRS surface water pathway includes the unnamed tributary running through the NIMM property to San Carlos Creek adjacent to site sources to Silver Creek and Silver Creek to Panoche Creek. The majority of San Carlos Creek is a perennial stream and becomes an intermittent stream before it reaches the confluence of Larious Creek. Larious and Silver Creeks are also intermittent streams as well as Panoche Creek at the Silver Creek Confluence. Although the streams are intermittent, they are still evaluated under the HRS as an eligible surface water bodies as the drainage area receives less than 20 inches of mean annual precipitation (Ref. 6, pp. 40; Ref. 12, p. 14).

Ground water drains from the Level 10 Adit as AMD (Source 2). Once the ground water discharges from the Level 10 Adit, it enters an unnamed tributary that bypasses the former settling pond (AMD pond) and runs between and is in direct contact with a waste rock pile and the southern calcine tailings pile (Source 1). It discharges to San Carlos Creek, approximately 2,000 feet away from the Level 10 adit. (Ref. 6, p. 7-8, Photo 23, Photo 26; Ref. 10, p. 1; Ref. 11, p. 6; Ref. 15, pp. 29-30).

San Carlos Creek is mostly perennial, with only the lowermost reaches becoming intermittent by mid-summer. San Carlos Creek is bordered on both banks by wetlands along its entire length from the New Idria Mercury Mine facility downstream to its confluence with Larious Creek (the beginning of Silver Creek) (Ref. 13, p. 4). Silver Creek is formed by the confluences of San Carlos and Larious creeks. Silver Creek is mostly intermittent, with seasonal flows typically forming a series of isolated bedrock pools interrupted by long reaches of dry stream channel by mid-summer. Silver Creek supports a mosaic riverine riparian, palustrine emergent, and palustrine shrub-scrub wetlands that form a continuous border of varying widths on both banks for its entire length (Ref. 13, p. 5).

The remote Panoche-Silver-San Carlos Creek watershed is one of only a few large watersheds characterized by intermittent to perennial flow conditions flowing east to the San Joaquin Valley from the 200 mile long Diablo Range. San Carlos and Silver creeks are characterized by extensive wetlands in a predominantly arid, upland environment. These wetlands are continuous, running the entire length of both streams. As such, the aquatic and wetland habitats associated with San Carlos and Silver creeks, and their dependent animal species, are of regional ecological significance. The presence of the Pacific chorus frog and western pond turtle within this watershed is one measure of the importance of the aquatic and wetland features to regionally rare aquatic species. In addition, wetlands associated San Carlos and Silver Creeks perform other important ecological functions including surface and subsurface storage of water, energy dissipation, moderation of ground water flow or discharge, nutrient cycling, removal of imported elements and compounds, retention of particulates, organic carbon storage, the support of characteristic plant communities and detrital biomass, the maintenance of the spatial structure of animal habitat, the provision of habitat

29 SWOF-Likelihood of Release

interspersion and connectivity, and habitat support for the distribution and abundance of invertebrates and vertebrates (Ref. 13, p. 6).

The Level 10 Adit AMD (Source 2) flows overland through calcine and waste rock piles and discharges into San Carlos Creek south of the crossing of New Idria Road. The AMD-impacted San Carlos Creek flows northward along the base of the Calcine Tailings Piles (Source 1) for a distance of approximately 2,500 feet resulting in additional mercury contamination. The AMD impact to San Carlos Creek is observed by the deposition of orange iron flocculants deposited on the stream bed and is apparent downstream to Silver Creek. San Carlos Creek becomes Silver Creek at the confluence of Larious Creek, approximately 4.7 miles downstream of the site sources. Silver Creek flows northward between the Griswold Hills and Tumey Hills and enters Panoche Creek approximately 19.5 river miles downstream from the Calcine Tailings Piles (Ref. 6, pp. 41; Ref. 20).

Site sources are in direct contact with and are a continuous probable point of entry (PPE) in San Carlos Creek from approximately the location of samples NIMM-SW-3 to NIMM-SW-7. The furthest downstream PPE is at the location of sample NIMM-SW-7 (Ref. 6, p. 55, Photo 23, Photo 26; Ref. 20). Sampling shows the presence of hazardous substances meeting the criteria for an observed release attributable to the site to a distance of 19.9 miles downstream of the furthest downstream PPE (Ref. 6, p. 29; Ref. 20). Although other mines existed in the vicinity of the site, the NIMM facility was the central processing area for the surrounding mines. The furnaces on the NIMM facility were used to process mercury ore from the San Carlos and Aurora mines which were transported by a 2-mile aerial tram and through the Level 10 adit (Source 2). In addition, ore was transported from the Alpine, Anita, North Star, and other mines to be processed at the NIMM furnace (Ref. 6, pp. 7, 64; Ref. 12, p. 20, 23-25, 28). The target distance limit for the New Idria Mercury Mine site is 19.9 miles (Ref. 20).

30 SWOF-Likelihood of Release

4.1.2.1 Likelihood of Release

4.1.2.1.1 Observed Release

Direct Observation

- Basis for Direct Observation:

Both the Southern and Northern Calcine Tailings Piles (Source 1) were observed to be in direct contact with San Carlos Creek. San Carlos Creek flows along the base of the Northern and Southern Calcine Tailings Piles (Ref. 6, pp. Photo 26, Photo 34). In addition, calcines attributable to the NIMM site were observed in San Carlos Creek and Silver Creek to the target distance limit (Ref. 6, p. 36). The PPE for the Level 10 Adit (Source 2) is the point at which the AMD leaves the mine. Once the ground water is discharged from the Level 10 adit, it was observed to be flowing uncontrolled over waste rock and calcine tailings before entering San Carlos Creek (Ref. 4, pp. 50 and 51; Ref. 6, pp. 6-8, 39, 41, 55, 84, 90).

- Hazardous Substances in Release:

Sample NIMM-TL-4 was collected from the southern calcine tailings pile which was in direct contact with San Carlos Creek (Ref. 6, Photo 26).

Hazardous Hazardous Substance SQL Sample ID Date References Substance Concentration (mg/kg) (mg/kg fines) Arsenic 17.7 0.995 Ref. 6, pp. 20- Cadmium 0.51J1 (0.36) 0.497 21, 1133, 1135- 1136, 1139; Ref. NIMM-TL-4 Copper 39.2 2.49 14, p. 4; Ref. 22, (southern calcine 7/19/10 Lead 9.0 0.995 pp. 2,40, 67, tailings pile) Mercury 134 5.03 103, 104, 108, Nickel 20.4 3.98 112, 115; Ref. Zinc 39.3 5.97 29 mg/kg: milligrams analyte per kilogram J1: The analyte was positively identified; however, the reported value is an estimate due to ICP inter-element interference problems. The result was reported from undiluted analyses that contained iron concentrations above the level present in the interference check sample (ICS) solution, therefore the applied inter-element correction (IEC) factors may not sufficiently compensate. For cadmium the value reported may be biased high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 mg   μg  1 mg final volume (mL) 1L 1000 g SQL   = CRDL   x x x x x DF  kg dry wgt   L  1000 μg sample weight (g wet wgt) x %S 1000 mL 1 kg

Where:

Final volume (mL) is the final extract volume following sample preparation

31 SWOF-Likelihood of Release

Sample weight (g wet wgt) is the weight of the sample used for analysis % S is % solids value divided by 100; % solids is determined as follows: (g dry weight of sample used for analysis)/(g wet weight of sample used for analysis) x100% DF is the dilution factor for the instrumental analysis

Sample NIMM-SW-4 was collected from ground water discharging from the Level 10 adit, which was observed to be in direct contact with waste rock and calcine tailings piles prior to entering San Carlos Creek (Ref. 6, pp. 6-8, 39, 41, 55, 84, 90).

AMD Source Concentrations – Total Metals (µg/l) Hazardous Hazardous Sample ID Date Substance SQL References Substance Concentration Arsenic 72.4 1.0 Ref. 6, pp. 23, 1170, 1171, 1184-1187, Cadmium 1.7 1.0 1190; Ref. 14, p. 6; NIMM-SW-4 7/22/2010 Ref. 16, pp. 8, 18; (Level 10 Adit) 1 Selenium 11.2 J (9.82) 5.0 Ref. 25, p. 2; Ref. 2 27, pp. 34, 54, 70, Zinc 2,500 J 2.0 71; Ref. 29 µg/l: micrograms analyte per liter water

J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses.

J2: The analyte was positively identified; however, the reported value is an estimate due to the serial dilution percent differences being outside the method quantitation control limits. The value reported may be biased low. However, the presence of the analyte in the sample is not in doubt.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

32 SWOF-Likelihood of Release

AMD Source Concentrations – Dissolved Metals (µg/l) Hazardous Hazardous Sample ID Date Substance SQL References Substance Concentration

Arsenic 66.1 1.0 Ref. 6, pp. 23, 1178, Cadmium 1.4 1.0 1195, 1197-1198; Ref. NIMM-SW-4 Copper 3.2 2.0 14, p. 6; Ref. 16, p. 8, 7/22/2010 (Level 10 Adit) Nickel 1,440 1.0 18; Ref. 26, p. 2; Ref. Selenium 10.6 J1 (9.3) 5.0 28, pp. 34, 52, 68, 70; Zinc 2,350 2.0 Ref. 29 µg/l: micrograms analyte per liter water

J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

33 SWOF-Likelihood of Release

Chemical Analysis

Sediment Sampling

WESTON collected a total of 34 sediment samples (NIMM-SD-1 through NIMM-SD-34) from the facility and from upstream and downstream locations of the Site to establish whether a release to surface water has occurred. Background sediment sample NIMM-SD-1 was collected from the headwaters of San Carlos Creek approximately 3.9 miles upstream from the AMD and calcine tailings piles, and background sediment sample NIMM-SD-11 was collected from a tributary leading to East Fork San Carlos Creek located approximately 2.7 miles south-southeast of the Site. Three sediment samples (NIMM-SD-3, NIMM-SD-8 and NIMM-SD-9) were collected from San Carlos Creek adjacent to the Calcine Tailings Piles. Four sediment samples (NIMM-SD-10, NIMM-SD-12, NIMM-SD-13, and NIMM-SD-15) were collected from San Carlos Creek from locations 0.6, 2.4, 4.1, and 4.7 miles, respectively, downstream from the Calcine Tailings Piles (Ref. 6, p. 24).

One sediment sample (NIMM-SD-14) was collected from Larious Creek, upstream from its confluence with San Carlos and Silver Creeks, approximately 4.5 miles downstream from the calcine tailings piles, to assess attribution from mercury mines in the New Idria Mining District draining to Sampson and Larious Creeks. A total of 17 sediment samples (NIMM-SD-16 through NIMM-SD- 32) were collected at intervals of approximately one mile along Silver Creek to Panoche Creek. Two sediment samples (NIMM-SD-33 and NIMM-SD-34) were collected from Panoche Creek at locations upstream and downstream of Silver Creek, respectively (Ref. 6, p. 24). Sampling locations are shown on Reference 6, pages 53, 55, 57, 59-60, and 62.

Three sediment sub-samples were collected at each location and were sieved and field screened for mercury concentrations with the XRF. Samples for laboratory analysis were contained in 4-ounce glass containers and placed on ice for preservation (Ref. 6, p. 24).

The weighted fractions of the sample used to determine the percentages of fines are presented in the table below.

34 SWOF-Likelihood of Release

Total Sieved Remaining - Percentage Sample ID Weight Weight - fines coarse References of fines (%) (grams) (grams) (grams) NIMM-SD-1 343 262 81 76.38 Ref. 6, p. 893 NIMM-SD-3 407 80 327 19.66 Ref. 6, p. 899 NIMM-SD-8 445 111 334 24.94 Ref. 6, p. 904 NIMM-SD-9 707 437 270 61.81 Ref. 6, p. 905 NIMM-SD-10 403 188 215 46.65 Ref. 6, p. 906 NIMM-SD-11 262 119 143 45.42 Ref. 6, p. 907 NIMM-SD-12 400 111 289 27.75 Ref. 6, p. 908 NIMM-SD-13 248 54 194 21.77 Ref. 6, p. 909 NIMM-SD-14 425 166 259 39.06 Ref. 6, p. 910 NIMM-SD-15 690 255 435 36.96 Ref. 6, p. 911 NIMM-SD-16 280 238 42 85.0 Ref. 6, p. 912 NIMM-SD-17 289 92 197 31.83 Ref. 6, p. 913 NIMM-SD-18 486 256 230 52.67 Ref. 6, p. 914 NIMM-SD-20 229 201 28 87.77 Ref. 6, p. 916 NIMM-SD-21 431 316 115 73.32 Ref. 6, p. 917 NIMM-SD-22 649 217 432 33.44 Ref. 6, p. 918 NIMM-SD-23 365 207 158 56.71 Ref. 6, p. 919 NIMM-SD-24 Not noted Not noted Not noted Not noted Ref. 6, p. 920 NIMM-SD-25 556 435 121 78.24 Ref. 6, p. 921 NIMM-SD-26 792 647 145 81.69 Ref. 6, p. 922 NIMM-SD-27 262 144 118 54.96 Ref. 6, p. 923 NIMM-SD-28 295 261 34 88.47 Ref. 6, p. 924 NIMM-SD-29 269 100 169 37.17 Ref. 6, p. 925 NIMM-SD-30 344 286 58 83.14 Ref. 6, p. 926 NIMM-SD-31 261 61 200 23.37 Ref. 6, p. 927 NIMM-SD-32 532 234 298 43.98 Ref. 6, p. 928 NIMM-SD-33 376 285 91 75.80 Ref. 6, p. 929 NIMM-SD-34 213 142 71 66.67 Ref. 6, p. 930

The tables presented below are sample description tables with the background samples corresponding with the downstream comparative contaminated samples. Background samples NIMM-SD-1 and NIMM-SD-11 were collected upstream of all contaminated samples (Table A). Background sample NIMM-SD-14 was collected in Larious Creek, upstream of contaminated samples NIMM-SD-15 through NIMM-SD-32 and contaminated sample NIMM-SD-34 (Table B). Background samples NIMM-SD-33 and NIMM-SD-61 were collected in Panoche Creek, upstream of sample NIMM-SD-34 (Table C) (Figures 2, 4, 6, 8, 9, 11 of this HRS documentation record).

35 SWOF-Likelihood of Release

Table A Sample Depth Sample Sample ID (below Sample Description References Date surface) Background Samples NIMM-SD-1 Dark yellowish brown fine to course sand, no 7/22/2010 2 inches Ref. 6, p. 893 gravel NIMM-SD-11 Dark yellowish brown fine to medium sand, 7/22/2010 2 inches Ref. 6, p. 907 angular gravel Comparative Contaminated Samples Light olive brown, fine to course sand, fine to NIMM-SD-3 7/21/2010 Not noted Ref. 6, p. 899 course rounded gravel Grayish brown fine to medium sand, fine to NIMM-SD-8 7/21/2010 2 inches Ref. 6, p. 904 medium rounded and subrounded gravel Olive brown fine to medium sand, angular NIMM-SD-9 7/21/2010 2 inches Ref. 6, p. 905 gravel Light olive brown fine to medium sand, fine to NIMM-SD-10 7/22/2010 2 inches Ref. 6, p. 906 medium subrounded to angular gravel Light yellowish brown fine to course sand, fine NIMM-SD-12 7/22/2010 2 inches Ref. 6, p. 908 to course rounded gravel Light yellowish brown very fine sand, fine NIMM-SD-13 7/22/2010 2 inches Ref. 6, p. 909 gravel Light yellowish brown fine to medium sand, NIMM-SD-15 7/22/2010 Not noted Ref. 6, p. 911 very fine to medium angular gravel

Table B Sample Depth Sample Sample ID (below Sample Description References Date surface) Background Sample NIMM-SD-14 Olive brown very fine to medium sand, fine to 7/21/2010 2 inches Ref. 6, p. 910 medium rounded gravel Comparative Contaminated Samples

Light olive brown fine to course sand, some NIMM-SD-16 7/21/2010 2 inches Ref. 6, p. 912 fine gravel Light yellowish brown very fine sand, rounded NIMM-SD-17 7/22/2010 Not noted Ref. 6, p. 913 gravel Light olive brown fine to medium sand, fine to NIMM-SD-18 7/22/2010 Not noted Ref. 6, p. 914 medium gravel Light olive brown fine to course sand, fine NIMM-SD-20 7/20/2010 2 inches Ref. 6, p. 916 rounded gravel Light olive brown very fine to medium sand, NIMM-SD-21 7/20/2010 2 inches Ref. 6, p. 917 fine to medium rounded gravel Light yellowish brown very fine to medium NIMM-SD-22 7/20/2010 2 inches Ref. 6, p. 918 sand, fine to course rounded gravel Grayish brown fine to course sand, fine NIMM-SD-23 7/20/2010 2 inches Ref. 6, p. 919 rounded gravel

36 SWOF-Likelihood of Release

NIMM-SD-24 7/20/2010 2 inches Light olive brown fine sand, no gravel Ref. 6, p. 920 Grayish brown fine to medium sand, fine to NIMM-SD-25 7/20/2010 2 inches Ref. 6, p. 921 medium rounded gravel NIMM-SD-26 7/20/2010 2 inches Light yellowish brown fine to medium sand Ref. 6, p. 922 Light brownish gray fine to course sand, NIMM-SD-27 7/20/2010 2 inches Ref. 6, p. 923 rounded gravel NIMM-SD-28 7/20/2010 2 inches Light brownish gray sand, some gravel Ref. 6, p. 924 NIMM-SD-29 7/23/2010 2 inches Light brownish gray sand, rounded gravel Ref. 6, p. 925 NIMM-SD-30 7/20/2010 2 inches Light brownish gray sand, rounded gravel Ref. 6, p. 926 NIMM-SD-31 7/20/2010 2 inches Light brownish gray sand with rounded gravel Ref. 6, p. 927 NIMM-SD-32 7/23/2010 2 inches Light brownish gray rounded gravel Ref. 6, p. 928

Table C Sample Depth Sample Sample ID (below Sample Description References Date surface) Background Samples Light brownish gray fine to course sand, some NIMM-SD-33 7/23/2010 2 inches Ref. 6, p. 929 gravel NIMM-SD-61 Light brownish gray fine to course sand, some (Duplicate of 7/23/2010 2 inches Ref. 6, p. 929 gravel NIMM-SD-33) Comparative Contaminated Sample NIMM-SD-34 7/23/2010 2 inches Sandy Ref. 6, p. 930

37 SWOF-Likelihood of Release

- Background Concentrations: Total Metals in Sediments (mg/kg) Hazardous Hazardous Substance SQL Sample ID References Substance Concentration (mg/kg) (mg/kg) Arsenic 1.0 U 1.01 Ref. 6, pp. 26, 30, NIMM-SD-1 Copper 6.2 2.53 1137, 1139; Ref. 14, (San Carlos Creek, upstream of p. 5; Ref. 22, pp. 3, all contaminated samples) Lead 3.9 1.01 49, 67, 105, 108, Mercury 0.19 0.101 112; Ref. 29 Arsenic 1.1 U 1.05 Ref. 6, pp. 26, 30, NIMM-SD-11 1154-1155, 1160, (Tributary to East Fork San Copper 8.6 2.63 1163; Ref. 14, p. 5; Carlos Creek – upstream of all Ref. 24, pp. 2, 33, contaminated samples) Lead 4.1 1.05 60, 91, 92, 95; Ref. 29 Arsenic 6.6 1.06 Ref. 6, pp. 27, 30, NIMM-SD-14 1155, 1160, 1163; (Larious Creek – upstream of Copper 19.4 2.65 Ref. 14, p. 4; Ref. contaminated samples NIMM- 16, pp. 8, 18; Ref. Lead 7.3 1.06 SD-15 through -32, -34) 24, pp. 2, 36, 60, 91, 1 Mercury 0.081 J (0.15) 0.101 92, 95, 100; Ref. 29 Arsenic 3.9 0.975 Ref. 6, pp. 29-30, NIMM-SD-33 1145, 1148, 1150; (Panoche Creek – upstream of Copper 8.1 2.44 Ref. 14, p. 6; Ref. contaminated sample NIMM- Lead 4.0J2 (5.76) 0.975 23, pp. 4, 41, 58, 89, SD-34) Mercury 0.10 U 0.1 90, 92, 98; Ref. 29 Arsenic 3.6 0.986 Ref. 6, pp. 29-30, 1145, 1149-1150; NIMM-SD-61 Copper 7.9 2.47 Ref. 14, p. 6; Ref. (duplicate of NIMM-SD-33) 2 Lead 4.6J (6.6) 0.986 23, pp. 4, 46, 58, 89, Mercury 0.050 J3 0.101 90, 92, 98; Ref. 29 mg/kg: milligrams analyte per kilogram sediment J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias low. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses. J2: The analyte was positively identified; however, the reported value is an estimate due to ICP serial dilution results being outside method quantitation control limits. The results may be bias low. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses. J3: Analyte detected at a level less than the CRQL. U The analyte was analyzed for but was not detected above the level of he reported sample quantitation limit.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 mg   μg  1 mg final volume (mL) 1L 1000 g SQL   = CRDL   x x x x x DF  kg dry wgt   L  1000 μg sample weight (g wet wgt) x %S 1000 mL 1 kg

Where: Final volume (mL) is the final extract volume following sample preparation Sample weight (g wet wgt) is the weight of the sample used for analysis % S is % solids value divided by 100; % solids is determined as follows: (g dry weight of sample used for analysis)/(g wet weight of

38 SWOF-Likelihood of Release

sample used for analysis) x100% DF is the dilution factor for the instrumental analysis

- Contaminated Samples: Total Metals in Sediments Showing an Observed Release (mg/kg) Hazardous Hazardous Substance SQL Sample ID Reference Substance Concentration (mg/kg) (mg/kg) Arsenic 9.0 1.01 Ref. 6, pp. 27, 30, 1138-1139; NIMM-SD-3 Copper 44.6 2.53 Ref. 14, p. 4; Ref. 22, pp. 3, (San Carlos Creek adjacent 51, 67, 103, 105, 109, 115; to southern calcine pile) Mercury 4.9 9.05 Ref. 29 Arsenic 10 1.02 Ref. 6, pp. 27, 30, 1138-1139; NIMM-SD-8 Ref. 14, p. 4; Ref. 22, pp. 3, (San Carlos Creek adjacent Copper 40.3 2.55 55, 67, 103, 105, 109, 115; to southern calcine pile) Mercury 4.7 0.195 Ref. 29 Arsenic 14.1 1.03 NIMM-SD-9 Ref. 6, pp. 27, 30, 1139; Ref. Copper 55.9 2.58 (San Carlos Creek adjacent 14, p. 4; Ref. 22, pp. 4, 56, 67, to northern calcine pile) Lead 16.0 1.03 103, 105, 109, 115; Ref. 29 Mercury 4.9 0.196 Arsenic 8.7 1.01 Ref. 6, pp. 27, 30, 1156, 1160, NIMM-SD-10 1163; Ref. 14, p. 5; Ref. 16, p. (San Carlos Creek 0.6 mile Copper 30.7 2.54 8; Ref. 24, pp. 2, 32, 60, 91, from PPE) 1 Mercury 32.6 J 2.03 92, 95, 102; Ref. 29 Ref. 6, pp. 27, 30, 1156, 1160, NIMM-SD-12 Arsenic 8.3 1.02 1163; Ref. 14, p. 5; Ref. 16, p. (San Carlos Creek 2.4 miles 8; Ref. 24, pp. 2, 34, 60, 91, from PPE) Mercury 8.9 J1 0.412 92, 95, 102; Ref. 29 Ref. 6, pp. 27, 30, 1156, 1160, NIMM-SD-13 Arsenic 11.9 1.14 1163; Ref. 14, p. 5; Ref. 16, p. (San Carlos Creek 4.1 miles 8; Ref. 24, pp. 2, 35, 60, 91, from PPE) Mercury 2.0 J1 0.114 92, 95, 100; Ref. 29 Ref. 6, pp. 27, 30, 1156, 1160, NIMM-SD-15 1163; Ref. 14, p. 5; Ref. 16, p. (San Carlos Creek 4.7 miles Mercury 4.6 J1 0.194 8; Ref. 24, pp. 2, 37, 60, 92, from PPE) 102; Ref. 29 NIMM-SD-59 Ref. 6, pp. 27, 30, 1149-1150; (duplicate of NIMM-SD- Mercury 4.0 0.194 Ref. 14, p. 5; Ref. 23, pp. 4, 15) 44, 58, 90, 100; Ref. 29 Ref. 6, pp. 27, 30, 1149-1150; NIMM-SD-15 Mercury 4.4 0.194 Ref. 14, p. 5; Ref. 24, pp. 2, (unsieved sample) 37, 60, 92, 102; Ref. 29 NIMM-SD-16 Ref. 6, pp. 28, 30, 1156, 1161, Mercury 1.8 J1 0.107 (Silver Creek 5.4 miles 1163; Ref. 14, p. 4; Ref. 16, p.

39 SWOF-Likelihood of Release

Total Metals in Sediments Showing an Observed Release (mg/kg) Hazardous Hazardous Substance SQL Sample ID Reference Substance Concentration (mg/kg) (mg/kg) from PPE) 8; Ref. 24, pp. 2, 38, 60, 92, 100; Ref. 29 Ref. 6, pp. 28, 30, 1156, 1161, NIMM-SD-17 1163; Ref. 14, p. 5; Ref. 16, p. (Silver Creek 6.4 miles Mercury 3.9 J1 0.204 8; Ref. 24, pp. 2, 39, 60, 92, from PPE) 102; Ref. 29 Ref. 6, pp. 28, 30, 1156, 1161, NIMM-SD-18 1163; Ref. 14, p. 5; Ref. 16, p. (Silver Creek 7.4 miles Mercury 2.7 J1 0.101 8; Ref. 24, pp. 2, 40, 60, 93, from PPE) 100; Ref. 29 Ref. 6, pp. 28, 30, 1156, 1161, NIMM-SD-20 1163; Ref. 14, p. 3; Ref. 16, p. (Silver Creek 9.2 miles Mercury 11.2 J1 0.491 8; Ref. 24, pp. 3, 42, 60, 93, from PPE) 102; Ref. 29 NIMM-SD-21 Ref. 6, pp. 28, 30, 1156, 1161, (Silver Creek 10.2 miles Mercury 1.3 J1 0.101 1163; Ref. 14, p. 3; Ref. 24, from PPE) pp. 3, 43, 60, 93, 100; Ref. 29 Ref. 6, pp. 28, 30, 1156, 1162- NIMM-SD-22 1163; Ref. 14, p. 3; Ref. 16, p. (Silver Creek 11.2 miles Mercury 0.60 J1 0.101 8; Ref. 24, pp. 3, 44, 60, 93, from PPE) 100; Ref. 29 Ref. 6, pp. 28, 30, 1156, 1162- NIMM-SD-24 1163; Ref. 14, p. 3; Ref. 16, p. (Silver Creek 13.0 miles Mercury 4.2 J1 0.206 8; Ref. 24, pp. 3, 46, 60, 93, from PPE) 102; Ref. 29 Ref. 6, pp. 28, 30, 1156, 1162- NIMM-SD-25 1163; Ref. 14, p. 3; Ref. 16, p. (Silver Creek 14.0 miles Mercury 2.4 J1 0.103 8; Ref. 24, pp. 3, 47, 60, 93, from PPE) 100; Ref. 29 NIMM-SD-60 Ref. 6, pp. 28, 30, 1146, 1149- (duplicate of NIMM-SD- Mercury 1.4 0.098 1150; Ref. 14, p. 3; Ref. 23, 25) pp. 4, 45, 59, 90, 98; Ref. 29 Ref. 6, pp. 29, 30 1156, 1162- NIMM-SD-26 1163; Ref. 14, p. 3; Ref. 16, p. (Silver Creek 15.0 miles Mercury 1.7 J1 0.101 8; Ref. 24, pp. 3, 48, 60, 93, from PPE) 99; Ref. 29 Ref. 6, pp. 29, 30 1156, 1162- NIMM-SD-27 1163; Ref. 14, p. 3; Ref. 16, p. (Silver Creek 15.8 miles Mercury 1.5 J1 0.101 8; Ref. 24, pp. 3, 49, 60, 93, from PPE) 100; Ref. 29 Ref. 6, pp. 29, 30 1156, 1163; NIMM-SD-28 Ref. 14, p. 3; Ref. 16, p. 8; (Silver Creek 16.2 miles Mercury 1.1 J1 0.101 Ref. 24, pp. 3, 50, 60, 93, 100; from PPE) Ref. 29

40 SWOF-Likelihood of Release

Total Metals in Sediments Showing an Observed Release (mg/kg) Hazardous Hazardous Substance SQL Sample ID Reference Substance Concentration (mg/kg) (mg/kg) Ref. 6, pp. 29, 30 1156, 1163; NIMM-SD-29 Ref. 14, p. 6; Ref. 16, p. 8; (Silver Creek 16.9 miles Mercury 2.3 J1 0.101 Ref. 24, pp. 3, 51, 60, 93, 100; from PPE) Ref. 29 NIMM-SD-30 Ref. 6, pp. 29, 30 1148, 1150; (Silver Creek 17.8 miles Mercury 1.2 0.107 Ref. 14, p. 3; Ref. 23, pp. 4, from PPE) 38, 58, 90, 98; Ref. 29 NIMM-SD-31 Ref. 6, pp. 29, 30 1148, 1150; (Silver Creek 18.7 miles Mercury 1.0 0.101 Ref. 14, p. 3; Ref. 23, pp. 4, from PPE) 39, 58, 90, 98; Ref. 29 NIMM-SD-32 Ref. 6, pp. 29, 30 1148, 1150; (Silver Creek 19.5 miles Mercury 1.0 0.096 Ref. 14, p. 6; Ref. 23, pp. 4, from PPE) 40, 58, 90, 98; Ref. 29 NIMM-SD-34 Ref. 6, pp. 29, 30 1148, 1150; (Panoche Creek 19.9 miles Mercury 1.1 0.103 Ref. 14, p. 6; Ref. 23, pp. 4, from PPE) 42, 58, 90, 98; Ref. 29 mg/kg: milligrams analyte per kilogram sediment J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias low. However, the presence of the analyte in the sample is not in doubt.

SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 mg   μg  1 mg final volume (mL) 1L 1000 g SQL   = CRDL   x x x x x DF  kg dry wgt   L  1000 μg sample weight (g wet wgt) x %S 1000 mL 1 kg

Where: Final volume (mL) is the final extract volume following sample preparation Sample weight (g wet wgt) is the weight of the sample used for analysis % S is % solids value divided by 100; % solids is determined as follows: (g dry weight of sample used for analysis)/(g wet weight of sample used for analysis) x100% DF is the dilution factor for the instrumental analysis

41 SWOF-Likelihood of Release

Surface Water Sampling

WESTON collected surface water samples from 15 upstream and downstream locations relative to site sources to establish whether a release to surface water has occurred. Background sample NIMM-SW-1 was collected from the headwaters of San Carlos Creek approximately 3.5 miles upstream from the AMD and Calcine Tailings Piles. Background sample NIMM-SW-9 was collected from a tributary leading to East Fork San Carlos Creek located approximately 2.7 miles south-southeast of the Site. Three surface water samples (BLM-SW-1 through BLM-SW-3) were collected from upper San Carlos Creek at locations upstream, adjacent, and downstream of the reclamated Aurora mercury mine, respectively. Three surface water samples (NIMM-SW-3, NIMM- SW-6 and NIMM-SW-7) were collected from San Carlos Creek adjacent to the Calcine Tailings Piles. Two surface water samples (NIMM-SW-8 and NIMM-SW-10) were collected from San Carlos Creek from locations 0.6 and 2.4 miles downstream from the Calcine Tailings Piles, respectively. One surface water sample (NIMM-SW-12) was collected from Larious Creek, upstream from its confluence with San Carlos and Silver Creeks to assess attribution from mercury mines in the New Idria Mining District draining to Larious Creek. Surface water sample NIMM- SW-14 was collected from a location in Silver Creek approximately 5.4 miles downstream from the calcine tailings piles. At the time of sampling, water in Silver Creek was originating from Larious Creek and San Carlos Creek was dry at the planned upstream surface water sample location NIMM- SW-11 (Ref. 6, p. 31). Sampling locations are shown on Reference 6, pages 53, 55, and 57.

Both filtered and unfiltered water samples were collected at each sampling location by either submerging a clean one-liter plastic bottle beneath the water surface or by using dedicated tubing and a peristaltic pump. The samples were preserved with nitric acid to a pH less than 2; the samples were placed on ice in preparation for shipping to a CLP laboratory for metals analysis by CLP method ILM 5.2 (Ref 6, p. 31).

The tables presented below are sample description tables with the background samples corresponding with the downstream comparative contaminated samples. Background samples NIMM-SW-1, NIMM-SW-9, BLM-SW-1, BLM-SW-2, and BLM-SW-3 were collected upstream of all contaminated samples (Table D) (Figures 2, 4, 6 of this HRS documentation record).

42 SWOF-Likelihood of Release

Table D Sample Sample ID Sample Description References Date Background Samples Some green algae, 2-foot wide drainage with NIMM-SW-1 7/22/2010 Ref. 6, p. 894 low to moderate flow NIMM-SW-9 7/22/2010 Small tributary, serpentine gravel Ref. 6, p. 907 BLM-SW-1 7/22/2010 Grassy banks, moderate flow Ref. 6, p. 895 Sandy banks, 2.5-foot wide drainage with low BLM-SW-2 7/22/2010 Ref. 6, p. 896 to moderate flow Grassy banks, 3-foot wide drainage with low to BLM-SW-3 7/22/2010 Ref. 6, p. 897 moderate flow Comparative Contaminated Samples NIMM-SW-3 7/21/2010 Not noted Ref. 6, p. 899 NIMM-SW-6 7/21/2010 Grassy banks Ref. 6, p. 904 NIMM-SW-7 7/21/2010 Grassy banks, brownish water Ref. 6, p. 905 Cloudy water with orange-rust colored NIMM-SW-8 7/22/2010 Ref. 6, p. 906 sediments Salt deposits on banks, orange-rust colored NIMM-SW-10 7/22/2010 Ref. 6, p. 908 stream bed

- Background Concentrations:

Total Metals in Surface Water (µg/l) Hazardous Hazardous Sample ID Substance SQL References Substance Concentration Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1166-1167, NIMM-SW-1 Cadmium 1.0 U 1.0 1170-1171, 1182-1183, 1187, (Upper San Carlos Creek – Copper 2.0 U 2.0 1190; Ref. 14, p. 5; Ref. 16, upstream of all contaminated 1 pp. 8, 18; Ref. 25, pp. 2, 33, Mercury 0.084 J 0.20 samples) 62, 96, 104; Ref. 27, pp. 31, 1,2 Zinc 1.4 J (1.81) 2.0 54, 70, 71; Ref. 29 Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1166, 1167, NIMM-SW-9 Cadmium 1.0 U 1.0 1170-1171, 1182, 1183, 1185, (Tributary leading to East Fork 1188, 1190; Ref. 14, p. 5; Ref. Copper 4.0 2.0 San Carlos Creek – upstream 1 16, pp. 8, 18; Ref. 25, pp. 3, of all contaminated samples) Mercury 0.10 J 0.20 41, 62, 96, 104; Ref. 27, pp. 2 Zinc 9.5 J (12.32) 2.0 39, 54, 70, 71; Ref. 29 Arsenic 1.0 U 1.0 BLM-SW-1 Ref. 6, pp. 34, 1171, 1182, Upper San Carlos Creek, Cadmium 1.0 U 1.0 1183, 1189-1190; Ref. 14, p. Aurora Mine background – Copper 2.0 U 2.0 6; Ref. 16, pp. 8, 18; Ref. 25, upstream of all contaminated Mercury 0.20 U 0.20 pp. 4, 48, 62, 97, 104; Ref. 27, samples) 1,2 pp. 46, 54, 70, 72; Ref. 29 Zinc 1.8 J (2.32) 2.0

43 SWOF-Likelihood of Release

Total Metals in Surface Water (µg/l) Hazardous Hazardous Sample ID Substance SQL References Substance Concentration BLM-SW-2 Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1171, 1182, Upper San Carlos Creek Cadmium 1.0 U 1.0 1183, 1189-1190; Ref. 14, p. adjacent to Aurora Mine – Copper 2.0 U 2.0 6; Ref. 16, pp. 8, 18; Ref. 25, upstream of all contaminated Mercury 0.20 U 0.20 pp. 5, 49, 62, 97, 105; Ref. 27, samples) Zinc 1.4J J1,2 (1.81) 2.0 pp. 47, 54, 70, 72; Ref. 29 Ref. 6, pp. 34, 1166, 1167, BLM-SW-3 Arsenic 1.0 U 1.0 1171, 1182, 1183, 1189-1190; (Upper San Carlos Creek Cadmium 1.0 U 1.0 Ref. 14, p. 6; Ref. 16, pp. 8, downstream of Aurora Mine – Copper 2.0 U 2.0 18; Ref. 25, pp. 5, 50, 62, 97, upstream of all contaminated Mercury 0.20 U 0.20 105; Ref. 27, pp. 48, 70, 72; samples) 1,2 Zinc 1.3 J (1.68) 2.0 Ref. 29 µg/l : micrograms analyte per liter water CRQL: Contract Required Quantitation Limit J1: Analyte detected at a level less than the CRQL. J2: The analyte was positively identified; however, the reported value is an estimate due to ICP serial dilution results being outside method quantitation control limits. The results may be bias low. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses. U: The analyte was analyzed for but was not detected above the level of the reported sample quantitation limit. SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SSQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

44 SWOF-Likelihood of Release

- Contaminated Samples:

Total Metals in Surface Water Showing an Observed Release (µg/l) Hazardous Hazardous Sample ID Substance SQL Reference Substance Concentration Ref. 6, pp. 35, 1170-1171; Ref. NIMM-SW-3 14, p. 4; Ref. 25, pp. 2, 35, 62, (San Carlos Creek adjacent to Mercury 0.28 0.20 96, 104; Ref. 27, pp. 54, 70, southern calcine pile) 71; Ref. 29 Arsenic 1.3 1.0 Ref. 6, pp. 35, 1170-1171, NIMM-SW-6 1185, 1187, 1190; Ref. 14, p. (San Carlos Creek adjacent to Mercury 4.9 0.20 4; Ref. 16, p. 8; Ref. 25, pp. 3, southern calcine pile) 38, 62, 96, 104; Ref. 27, pp. 1 Zinc 396 J 2.0 36, 54, 70, 71; Ref. 29 Arsenic 1.9 1.0 Ref. 6, pp. 35, 1170-1171, NIMM-SW-7 Cadmium 1.2 1.0 1185, 1188, 1190; Ref. 14, p. (San Carlos Creek northern Copper 17.2 2.0 4; Ref. 16, p. 8; Ref. 25, pp. 3, calcine pile) Mercury 4.8 0.20 39, 62, 104; Ref. 27, pp. 54, Zinc 1,150 J1 2.0 37, 70, 71; Ref. 29 Arsenic 1.3 1.0 Ref. 6, pp. 35, 1171, 1185, 1189-1190; Ref. 14, p. 4; Ref. NIMM-SW-15 Copper 18.0 2.0 16, p. 8; Ref. 25, pp. 4, 45, 62, (duplicate of NIMM-SW-7) Mercury 4.7 0.20 97, 104; Ref. 27, pp. 43, 54, Zinc 1,270 J1 2.0 70, 71; Ref. 29 Arsenic 1.8 1.0 Ref. 6, pp. 35, 1170-1171, NIMM-SW-8 Copper 13.6 2.0 1185, 1188, 1190; Ref. 14, p. (San Carlos Creek 0.6 miles 5; Ref. 16, p. 8; Ref. 25, pp. 3, from Site) Mercury 4.6 0.20 40, 62, 96, 104; Ref. 27, pp. 1 38, 54, 70, 71; Ref. 29 Zinc 987 J 2.0 Ref. 6, pp. 35, 1170-1171, NIMM-SW-10 Mercury 0.79 0.20 1185, 1188, 1190; Ref. 14, p. (San Carlos Creek 2.4 miles 5; Ref. 16, p. 8; Ref. 25, pp. 3, from Site) 1 42, 62, 96, 104; Ref. 27, pp. Zinc 176 J 2.0 40, 54, 70, 71; Ref. 29 µg/l : micrograms analyte per liter water CRQL: Contract Required Quantitation Limit J1: The analyte was positively identified; however, the reported value is an estimate due to ICP serial dilution results being outside method quantitation control limits. The results may be bias low. However, the presence of the analyte in the sample is not in doubt. SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

45 SWOF-Likelihood of Release

- Background Concentrations: Dissolved Metals in Surface Water (µg/l) Hazardous Hazardous Sample ID Substance SQL References Substance Concentration 1 Copper 0.4J 2.0 Ref. 6, pp. 34, 1178- NIMM-SW-1 Mercury 0.20 U 0.20 1179; Ref. 14, p. 5; Ref. (Upper San Carlos Creek – Nickel 4.5 1.0 26, pp. 2, 32, 56, 82, 88; upstream of all contaminated Ref. 28, pp. 31, 52, 68, samples) Selenium 5.0 U 5.0 Zinc 2.9 2.0 70; Ref. 29 Copper 2.0 U 2.0 Ref. 6, pp. 34, 1178- NIMM-SW-9 Mercury 0.20 U 0.20 1179; Ref. 14, p. 5; Ref. (Tributary leading to East Fork 26, pp. 3, 40, 56, 82, 88; San Carlos Creek – upstream Nickel 5.1 1.0 Ref. 28, pp. 39, 52, 68, of all contaminated samples) Selenium 5.0 U 5.0 Zinc 2.4 2.0 70; Ref. 29 BLM-SW-1 Copper 2.0 U 2.0 Ref. 6, pp. 34, 1179; Ref. (Upper San Carlos Creek, Mercury 0.20 U 0.20 14, p. 6; Ref. 16, p. 8, 18; Aurora Mine background – Nickel 3.7 1.0 Ref. 26, pp. 5, 45, 56, 82, upstream of all contaminated Selenium 5.0 U 5.0 89; Ref. 28, pp. 44, 52, samples) Zinc 1.5 J1,2 (1.94) 2.0 68, 71; Ref. 29 1 BLM-SW-2 Copper 0.38 J 2.0 Ref. 6, pp. 34, 1175, Upper San Carlos Creek Nickel 3.5 1.0 1179; Ref. 14, p. 6; Ref. adjacent to Aurora Mine – 26, p. 5; Ref. 28, pp. 45, upstream of all contaminated Selenium 5.0 U 5.0 52, 68, 71; Ref. 29 samples) Zinc 2.3 2.0 µg/l : micrograms analyte per liter water CRQL: Contract Required Quantitation Limit J1: Analyte detected at a level less than the CRQL. J2: The analyte was positively identified; however, the reported value is an estimate due to ICP serial dilution results being outside method quantitation control limits. The results may be bias low. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses. J3: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias high. However, the presence of the analyte in the sample is not in doubt. U The analyte was analyzed for but was not detected above the level of the reported sample quantitation limit. SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

46 SWOF-Likelihood of Release

- Contaminated Samples:

Dissolved Metals in Surface Water Showing an Observed Release (µg/l) Hazardous Hazardous Sample ID Substance SQL Reference Substance Concentration Ref. 6, pp. 35, 1178- NIMM-SW-3 1179; Ref. 14, p. 4; Ref. (San Carlos Creek adjacent to southern Mercury 0.30 0.20 26, pp. 2, 34, 56, 82, 88; calcine pile) Ref. 29 Ref. 6, pp. 35, 1178- NIMM-SW-6 Nickel 231 1.0 1179, 1198, 1200; Ref. (San Carlos Creek adjacent to southern 14, p. 4; Ref. 26, p. 3; calcine pile) Zinc 61.6 2.0 Ref. 28, pp. 36, 52, 68, 70; Ref. 29 Copper 3.2 2.0 Ref. 6, pp. 35, 1178- 1179, 1195, 1199-1200; NIMM-SW-7 Nickel 175 1.0 Ref. 14, p. 4: Ref. 16, p. (San Carlos Creek northern calcine 1 Selenium 6.6 J (5.79) 5.0 8, 18; Ref. 26, p. 3; Ref. pile) 28, pp. 37, 52, 68, 70; Zinc 66.1 2.0 Ref. 29 Nickel 172 1.0 Ref. 6, pp. 35, 1179, 1195, 1200; Ref. 14, p. 4; NIMM-SW-15 Selenium 6.3 J1 (5.53) 5.0 Ref. 16, p. 8, 18; Ref. 26, (duplicate of NIMM-SW-7) Zinc 54.9 2.0 p. 4; Ref. 28, pp. 43, 52, 68, 70; Ref. 29 Ref. 6, pp. 35, 1178- Nickel 73.2 1.0 1179, 1199-1200; Ref. NIMM-SW-8 14, p. 5; Ref. 26, p. 3; (San Carlos Creek 0.6 miles from Site) Zinc 13.5 2.0 Ref. 28, pp. 38, 52, 68, 70; Ref. 29 Copper 2.1 2.0 Ref. 6, pp. 35, 1178- 1179, 1199-1200; Ref. NIMM-SW-10 Nickel 33.5 1.0 14, p. 5; Ref. 26, p. 3; (San Carlos Creek 2.4 miles from Site) Zinc 9.1 2.0 Ref. 28, pp. 40, 52, 68, 70; Ref. 29 µg/l : micrograms analyte per liter water CRQL: Contract Required Quantitation Limit J1: The analyte was positively identified; however, the reported value is an estimate due to matrix spike recovery results being outside method quantitation control limits. The reported value may be bias high. However, the presence of the analyte in the sample is not in doubt. The adjusted value is presented in parentheses. SQL: Sample Quantitation Limit. The SQLs provided in this table are CLP contract-required detection limits (CRDLs) for that analyte with any dilution factors, volume adjustments, or percent solids for that analysis taken into account. SQLs were calculated as follows:

 μg  final volume (mL) SQL (µg / L) = CRDL   x x DF  L  initial volume (mL)

Where:

Final volume (mL) is the final extract volume following sample preparation Initial volume (mL) is the volume of sample used DF is the dilution factor

47 SWOF-Likelihood of Release

ATTRIBUTION

During the 1998 PA/SI investigation conducted on the NIMM facility, observed releases of mercury, nickel and zinc from the site to San Carlos Creek and Silver Creek were documented. Elevated downstream mercury concentrations in surface water ranged from 3.1 ug/l to 12.0 ug/l. Nickel had a maximum concentration of 471 ug/l in downstream surface water samples, and zinc had a maximum concentration of 1,920 ug/l (Ref. 4, Table 4-2).

An observed release was documented during the July 2010 sampling investigation. Hazardous substances arsenic, cadmium, copper, lead, mercury, nickel, selenium, and zinc were detected at concentrations significantly above background concentrations in onsite sources and downstream of the New Idria Mercury Mine site (Ref. 6, pp. 26-29, 34-36).

Background sediment samples NIMM-SD-1, NIMM-SD-11 were collected upstream of site sources. Sediment samples NIMM-SD-3, NIMM-SD-8, NIMM-SD-9, NIMM-SD-10, NIMM-SD-12, and NIMM-SD-13 were collected from San Carlos Creek adjacent to or downstream of site sources. Sample locations are presented in Figure 2, Figure 4 and Figure 6 of this HRS documentation record. One or more of the above hazardous substances were detected in all of these samples at concentrations significantly above background (Ref. 6, pp. 26-29, Figures 2, 4, 6). These hazardous substances have been detected in the Calcine Tailings Piles (Source 1) and/or the Level 10 Adit AMD (Source 2) (Ref. 6, pp. 20-21, 23). See tables in Observed Release section (Section 4.1.2.1.1).

Sediment sample NIMM-SD-14 was collected from Larious Creek upstream of the confluence with San Carlos Creek to assess attribution from other mercury mines in the New Idria Mining District draining to Sampson and Larious Creeks. Sediment samples NIMM-SD-15 through NIMM-SD-32 were collected from Silver Creek downstream of the confluence with Larious Creek. Mercury was detected in all of these samples at concentrations significantly above all background samples including NIMM-SD-14 (Ref. 6, pp. 27-29). Sample location NIMM-SD-14 and the surrounding creeks are presented in Figure 6 of this HRS documentation record.

Sediment sample NIMM-SD-33 and its duplicate NIMM-SD-61 were collected from Panoche Creek upstream of the confluence with Silver Creek. Sediment sample NIMM-SD-34 was collected from Panoche Creek immediately downstream of the confluence with Silver Creek. Mercury was detected in NIMM-SD-34 at a concentration significantly above all background samples, including NIMM- SD-33. Sample locations are presented in Figure 11 of this HRS documentation record. Sample NIMM-SD-34 is located 19.9 miles downstream of site sources (Ref. 6, p. 29; Figure 11). Although other mines existed in the vicinity of the site, the NIMM facility was the central processing area for the surrounding mines. The furnaces on the NIMM facility were used to process mercury ore from the San Carlos and Aurora mines which were transported by a 2-mile aerial tram and through the Level 10 adit (Source 2). In addition, ore was transported from the Alpine, Anita, North Star, and other mines to be processed at the NIMM furnace (Ref. 6, p. 7; Ref. 12, p. 20, 23-25, 28). The target distance limit for the New Idria Mercury Mine site is 19.9 miles (Ref. 20).

48 SWOF-Likelihood of Release

Surface water samples NIMM-SW-1, NIMM-SW-9, BLM-SW-1, BLM-SW-2, and BLM-SW-3 were collected upstream of site sources (See Figure 2). Surface water samples NIMM-SW-3, NIMM-SW- 6, NIMM-SW-7 (and its duplicate NIMM-SW-15), NIMM-SW-8, and NIMM-SW-10 were collected from San Carlos Creek adjacent to or downstream of site sources. One or more of the above hazardous substances were detected in all of these samples at concentrations significantly above background (Ref. 6, pp. 34-36). These hazardous substances have been detected in the Calcine Tailings Piles and/or the AMD (Ref. 6, pp. 20-21, 23).

In addition, the Calcine Tailings Piles (Source 1) and the Level 10 Adit AMD (Source 2) are observed to be in direct contact with San Carlos Creek (Ref. 6, pp. 6-8, Photo 23, Photo 26). Arsenic, cadmium, copper, lead, mercury, nickel, selenium, and zinc are present in one or both of these hazardous substance sources (Ref. 6, pp. 20-21, 23).

49 SWOF-Likelihood of Release

Hazardous Substances Released

Arsenic, Cadmium, Copper, Lead, Mercury, Nickel, Selenium, Zinc.

Surface Water Observed Release Factor Value: 550

50 SWOF-Likelihood of Release

4.1.2.1.2 Potential to Release

Potential to Release was not scored, because an observed release was documented.

51 SWOF-Likelihood of Release

4.1.2.2 Drinking Water Threat Waste Characteristics

4.1.2.2.1 Toxicity/Persistence

Toxicity/ Toxicity Persistence Hazardous Source Persistence Factor Factor Reference Substance No. Factor Value Value Value* (Ref. 1, Table 4-12) Arsenic 1,2 10,000 1 10,000 Ref. 2, p. 1 Cadmium 1,2 10,000 1 10,000 Ref. 2, p. 2 Copper 1,2 0 1 0 Ref. 2, p. 3 Lead 1 10,000 1 10,000 Ref. 2, p. 8 Mercury 1 10,000 1 10,000 Ref. 2, p. 8 Nickel 1,2 10,000 1 10,000 Ref. 2, p. 9 Selenium 2 100 1 100 Ref. 2, p. 10 Zinc 1,2 10 1 10 Ref. 2, p. 12 Notes: * The surface water category that includes rivers, oceans, coastal tidal waters, and great lakes was utilized to assign the hazardous substances persistence factor value (Ref. 1, Sect. 4.1.2.2.1.2). A persistence value of 1 was assigned according to the January 2004 Superfund Chemical Data Matrix (SCDM) table (Ref. 2, pp. 1-3, 8-10, 12).

Arsenic, cadmium, lead, mercury and nickel had the highest toxicity/persistence value.

Toxicity/Persistence Factor Value: 10,000

4.1.2.2.2 Hazardous Waste Quantity

Source No. Source Type Source Hazardous Waste Quantity 1 Pile 163,105.3938 2 Other 8,409.6

Sum of Values: 171,514.9938 Hazardous Waste Quantity Factor Value: 10,000 (Ref. 1, Table 2-6)

4.1.2.2.3 Waste Characteristics Factor Category Value

Toxicity/Persistence Factor Value: 10,000 Hazardous Waste Quantity Factor Value: 10,000

Toxicity/Persistence Factor Value x Hazardous Waste Quantity Factor Value: 100,000,000 Subject to a maximum of 1 x 108 (Ref. 1, Section 3.2.3)

52 SWOF/DW-Waste Characteristics

A hazardous waste quantity factor of 10,000 is assigned from Reference 1, Table 2-6 for a source hazardous waste quantity value of 171,514.9938. From Reference 2 and Table 4-12 of the HRS, arsenic, cadmium, lead, mercury and nickel have a toxicity/persistence value of 10,000. The waste characteristics factor category value from Reference 1, Table 2-7 for a waste characteristics product of 1 x 108 is 100.

Waste Characteristics Factor Category Value: 100 (Ref. 1, Table 2-7)

53 SWOF/DW-Waste Characteristics

4.1.2.3 Drinking Water Threat Targets

4.1.2.3.1 Nearest Intake

There are no known drinking water intakes within the target distance limit.

Nearest Intake Factor Value: 0

4.1.2.3.2 Population

There are no known drinking water intakes within the target distance limit.

Population Factor Value: 0

4.1.2.3.3 Resources

Surface water is used for watering livestock within the target distance limit (Ref. 9, p. 1; Ref. 11, p. 1; Ref. 17, pp. 8, 27).

Resources Factor Value: 5 (Ref. 1, Section 4.1.2.3.3)

54 SWOF/DW-Targets

4.1.3.2 Human Food Chain Threat Waste Characteristics

4.1.3.2.1 Toxicity/Persistence/Bioaccumulation

Toxicity/ Toxicity Persistence Persistence/ Hazardous Source Bioaccumulation Factor Factor Bioaccumulation Reference Substance No. Value** Value Value* Factor Value (Ref. 1, Table 4-16) Arsenic 1,2 10,000 1 5 50,000 Ref. 2, p. 1 Cadmium 1,2 10,000 1 5,000 50,000,000 Ref. 2, p. 2 Copper 1,2 0 1 500 0 Ref. 2, p. 3 Lead 1 10,000 1 5 50,000 Ref. 2, p. 8 Mercury 1 10,000 1 50,000 500,000,000 Ref. 2, p. 8 Nickel 1,2 10,000 1 0.5 5,000 Ref. 2, p. 9 Selenium 2 100 1 50 5,000 Ref. 2, p. 10 Zinc 1,2 10 1 5 50 Ref. 2, p. 12 Notes: * The surface water category that includes rivers, oceans, coastal tidal waters, and great lakes was utilized to assign the hazardous substances persistence factor value (Ref. 1, Sect. 4.1.2.2.1.2). A persistence value of 1 was assigned (Ref. 2, pp. 1-3, 8-10, 12).

** Bioaccumulation factor values are assigned from the SCDM (Ref. 2), for the type of water body "Fresh Water", in which the fisheries are located (Ref. 1, Sect. 4.1.3.2.1.3).

Mercury had the highest toxicity/persistence/bioaccumulation factor value.

Toxicity/Persistence/Bioaccumulation Factor Value: 500,000,000

55 SWOF/HFC–Waste Characteristics

4.1.2.2.2 Hazardous Waste Quantity

Source No. Source Type Source Hazardous Waste Quantity 1 Pile 163,105.3938 2 Other 8409.6

Sum of Values: 171,514.9938

Hazardous Waste Quantity Factor Value: 10,000 (Ref. 1, Table 2-6)

4.1.2.2.3 Waste Characteristics Factor Category Value

Toxicity/Persistence Factor Value: 10,000 Hazardous Waste Quantity Factor Value: 10,000 Bioaccumulation Potential Factor Value: 500,000,000

(Toxicity/Persistence Factor Value) x (Hazardous Waste Quantity Factor Value) = 1 x 108 Subject to a maximum of 1 x 108 (Ref. 1, Section 3.2.3)

(Toxicity/Persistence Factor Value x Hazardous Waste Quantity Factor Value) x (Bioaccumulation Potential Factor Value) = 5 x 1016

A hazardous waste quantity factor of 10,000 is assigned from Reference 1, Table 2-6 for a source hazardous waste quantity value of 171,514.9938. From Reference 2 and Table 4-12 of the HRS, mercury has a toxicity/persistence value of 10,000 and a bioaccumulation potential factor of 50,000. The waste characteristics factor category value from Reference 1, Table 2-7 for a waste characteristics product of 5 x 1016 (subject to a maximum product of 1 x 1012) is 1,000.

Waste Characteristics Factor Category Value: 1,000 (Ref. 1, Table 2-7)

56 SWOF/HFC–Waste Characteristics

4.1.3.3 Human Food Chain Threat Targets

4.1.3.3.1 Food Chain Individual

There are no known fisheries within the target distance limit.

Food Chain Individual Factor Value: 0

4.1.3.3.2 Population

There are no known fisheries within the target distance limit.

Human Food Chain Contamination Factor Value: 0

57 SWOF/HFC–Targets

4.1.4.2 Environmental Threat Waste Characteristics

4.1.4.2.1 Ecosystem Toxicity/Persistence/Bioaccumulation

Ecosystem Toxicity/ Ecosystem Persistence Environmental Persistence/Env. Hazardous Source Toxicity Factor Factor Bioaccumulation Bioaccumulation Reference Substance No. Value Value* Value** Factor Value (Ref. 1, Table 4-21) Arsenic 1, 2 10 1 5,000 50,000 Ref. 2, p. 1 Cadmium 1, 2 10,000 1 50,000 500,000,000 Ref. 2, p. 2 Copper 1, 2 1,000 1 5,000 5,000,000 Ref. 2, p. 3 Lead 1 1,000 1 50,000 50,000,000 Ref. 2, p. 8 Mercury 1 10,000 1 50,000 500,000,000 Ref. 2, p. 8 Nickel 1, 2 100 1 500 50,000 Ref. 2, p. 9 Selenium 2 1,000 1 500 500,000 Ref. 2, p. 10 Zinc 1, 2 10 1 50,000 500,000 Ref. 2, p. 12

Notes: * The surface water category that includes rivers, oceans, coastal tidal waters, and great lakes was utilized to assign the hazardous substances persistence factor value (Ref. 1, Sect. 4.1.2.2.1.2). A persistence value of 1 was assigned (Ref. 2, pp. 1-3, 8-10, 12).

** Bioaccumulation factor values are assigned for the type of water body "Fresh Water", in which the targets are located (Ref. 1, Sect. 4.1.3.2.1.3).

Cadmium and mercury had the highest ecosystem toxicity/persistence/environmental bioaccumulation factor value.

Ecosystem Toxicity/Persistence/Environmental Bioaccumulation Factor Value: 500,000,000

58 SWOF/ENV-Waste Characteristics

4.1.4.2.2. Hazardous Waste Quantity

Source No. Source Type Source Hazardous Waste Quantity 1 Pile 163,105.3938 2 Other 8,409.6

Sum of Values: 171,514.9938

Hazardous Waste Quantity Factor Value: 10,000 (Ref. 1, Table 2-6)

4.1.4.2.3. Waste Characteristics Factor Category Value

Ecosystem Toxicity/Persistence Factor Value: 10,000 Hazardous Waste Quantity Factor Value: 10,000

Ecosystem Toxicity/Persistence Factor Value x Hazardous Waste Quantity Factor Value: 100,000,000 Subject to a maximum of 1 x 108 (Ref. 1, Section 3.2.3)

(Ecosystem Toxicity/Persistence Factor Value x Hazardous Waste Quantity Factor Value) x Environmental Bioaccumulation Factor Value: 5,000,000,000,000

A hazardous waste quantity factor of 10,000 is assigned from Reference 1, Table 2-6 for a source hazardous waste quantity value of 171,514.9938. From Reference 2 and Table 4-12 of the HRS, cadmium and mercury have a toxicity/persistence value of 10,000 and an environmental bioaccumulation potential factor of 50,000. The waste characteristics factor category value from Reference 1, Table 2-7 for a waste characteristics product of 5 x 1012 (subject to a maximum product of 1 x 1012) is 1,000.

Waste Characteristics Factor Category Value: 1,000 (Ref. 1, Table 2-7)

59 SWOF/ENV-Waste Characteristics

4.1.4.3 Environmental Threat Targets

Most Distant Level I Sample Samples have not been collected for documentation of Level I Actual Contamination.

Most Distant Level II Sample Sample ID: NIMM-SD-34 Distance from the probable point of entry: 19.9 miles Reference: Ref. 6, p. 29

4.1.4.3.1 Sensitive Environments

4.1.4.3.1.1. Level I Concentrations

Level I Actual Contamination is not documented.

Sum of Level I Sensitive Environments Value: 0

4.1.4.3.1.2 Level II Concentrations

Level II Sensitive Environment Targets

The remote Panoche-Silver-San Carlos Creek watershed is one of only a few large watersheds characterized by intermittent to perennial flow conditions flowing east to the San Joaquin Valley from the 200 mile long Diablo Range. San Carlos and Silver creeks are characterized by extensive wetlands in a predominantly arid, upland environment. The average precipitation measured between 1918 and 1976 in the region is 15.17 inches annually (Ref. 13, pp. 5-6; Ref. 31). These wetlands are continuous, running the entire length of both streams. As such, the aquatic and wetland habitats and associated with San Carlos and Silver Creeks, and their dependent animal species, are of regional ecological significance. The presence of the Pacific chorus frog and western pond turtle within this watershed is one measure of the importance of the aquatic and wetland features to regionally rare aquatic species. In addition, wetlands associated San Carlos and Silver creeks perform other important ecological functions including surface and subsurface storage of water, energy dissipation, moderation of ground water flow or discharge, nutrient cycling, removal of imported elements and compounds, retention of particulates, organic carbon storage, the support of characteristic plant communities and detrital biomass, the maintenance of the spatial structure of animal habitat, the provision of habitat interspersion and connectivity, and habitat support for the distribution and abundance of invertebrates and vertebrates (Ref. 13, pp. 5-6).

The U.S. Fish and Wildlife Service (USFW) reviewed a 4-mile buffer zone area around the site and the wetlands and intermittent pools up to 20 miles downstream of the NIMM site (analysis area) that includes San Carlos, Silver, and Panoche Creeks. USFW stated that the analysis area around the mine and areas of downstream water resources, including wetlands under public ownership

60 SWOF/ENV-Targets constitute very unique habitats to endemic threatened and endangered species which use these resources. The analysis area is comprised of unique biotic communities. In addition, it is in an area that is vitally important for the maintenance of an ecologically valuable biotic community. The combination of a remnant ecosystem, an arid environment, seasonal and perennial wetlands in an important corridor, the northernmost populations of several threatened and endangered species, high biological diversity, and an internationally important area for migratory birds all contribute to the significant ecological uniqueness of the analysis area (Ref. 30, p. 1, 2).

The Panoche Valley is one of only two valleys in California that support arid grassland as uncultivated grassland. This unconverted annual grassland support the endangered San Joaquin kit fox (Vulpes macrotis mutica), giant kangaroo rat (Dipodomys ingens), and blunt-nosed leopard lizard (Gambilia sila) (Ref. 30, p. 3; Ref. 32, p. 22, 28, 31).

The unique soil types and geologic formations of the Idria, Sampson, and North Hill San Carlos peaks has lead to a unique assemblage of endemic plant species, which includes the threatened San Benito evening primrose (Camissonia benitensis) (Ref. 30, p. 4, 5; Ref. 33, p. 7).

Distance from Sensitive PPE to Nearest Environment Sensitive Environment References Sensitive Value (Ref. 1, Environment Table 4-23) Particular areas, relatively small in size, important to Ref. 13, pp. 5-6; 0 miles 25 maintenance of unique biotic Ref. 30, p. 1, 2 communities. San Joaquin kit fox (Vulpes Ref. 30, p. 2, 3; 0 miles 75 macrotis mutica) Ref. 32, p. 22 Giant kangaroo rat Ref. 30, p. 3; Ref. 0 miles 75 (Dipodomys ingens), 32, p. 28 Blunt-nosed leopard lizard Ref. 30, p. 3; Ref. 0 miles 75 (Gambilia sila) 32, p. 31

Sum of Level II Sensitive Environments Value: 250

Level II Wetland Frontages

61 SWOF/ENV-Targets

The EPA conducted a wetland survey of San Carlos Creek at the Site and downstream of the Site, the entire length of Silver Creek, and a portion of Panoche Creek at the confluence of Silver Creek. Field identification of wetlands and other waters of the United States were conducted in accordance with the 1987 Corps of Engineers Wetlands Delineation Manual, National List of Vascular Plant Species that Occur in Wetlands: 1996 National Summary; Arid West Regional Supplement to the Wetlands Delineation Manual; and the Corps Field Guide to the Identification of the Ordinary High Water Mark in the Arid West Region. The Corps’ 1987 Wetlands Delineation manual identifies key diagnostic criteria for determining the presence of wetlands. These include (1) wetland hydrology, (2) hydric soils, and (3) a predominance of wetland vegetation. Wetland sampling sites and boundaries were mapped using GPS. Wetland SAMPLE data sheets were filled out for representative wetland types and sites. The wetland delineation is consistent with guidance (Ref. 13, p. 2).

The wetland survey confirmed the presence of jurisdictional wetlands adjacent to both banks of San Carlos Creek from the New Idria Mercury Mine facility downstream to its confluence with Larious Creek; and adjacent to both banks of Silver Creek from the confluences of San Carlos and Larious creeks downstream to its confluence with Panoche Creek (Ref. 13, p. 6).

Distance from the furthest downstream PPE to the confluence of San Carlos Creek with Larious Creek is 4.7 miles. Distance from the confluence of San Carlos and Larious Creeks to the target distance limit (represented by sample NIMM-SD-34) is 15.2 miles (Ref. 6, pp. 27, 29).

The wetland frontage miles presented below include frontage from both banks of each creek. The length of the wetlands on San Carlos Creek and Silver Creek are presented in Figures 1 through 9 of Reference 13 (Ref. 13).

Wetland Wetland Frontage (miles) References Ref. 6, p. 27; Ref. 13, pp. 4, 6, Figures San Carlos Creek 9.4 miles 1-9; Ref. 20 Ref. 6, p. 29; Ref. 13, pp. 5, 6, Figures Silver Creek 30.4 miles 1-9; Ref. 20

Sum of Level II Wetland Frontages: 39.8 miles

Wetlands Value (Ref. 1, Table 4-24): 500

Sum of Level II Sensitive Environments Value + Wetlands Value: 750 (Ref. 1, Section 4.1.4.3.1.2)

Level II Concentrations Factor Value: 750

62 SWOF/ENV-Targets

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Site Boundary (Inset) New Idria Mine i[ Surface Water Pathway i[ Ranchito Streams i[Spanish Mine Mines ppb = parts per billion ppm = parts per million ppt = parts per trillion NIMM-SW-2/SD-2 (Reservoir Water Sample) Hg in sediment: 5.5 ppm Hg = mercury Hg in water: <0.20 ppb (total) i[ MMHg = methylmercury Hg in water: <0.20 ppb (dissolved) MMHg in water: 0.245 ppt (dissolved) Creek Pit Mine Feet i[ 02,000 BLM-SW-3 Sulphur Spring Hg in water: <0.20 ppb (total) !.H Mine Hg in water: <0.20R ppb (dissolved) Reservoir po po!. i[i[ Molina Mine BLM-SW-2 Hg in water: <020 ppb (total) !. po rk Hg in water: <0.20 ppb (dissolved) i[i[ o F k

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NIMM-SW-1 Hg in water: 0.084J ppb (total) Hg in water: <0.20 ppb (dissolved)

Sample Locations - Map 1A NIMM-SD-1 New Idria Mercury Mine Hg in sediment: 0.19 ppm San Benito County, California

NIMM-SS-1 Figure Hg in soil: 0.19 ppm 2

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

"/ 1997 EPA Samples

!([ USGS Samples

0

!([ # RWQCB Samples

0

# i[

"/ Mercury Mines

0 #

0 0 #

"/ #

0 "/

# "/"/ !([!([ 0 # i 0 # !) Other Mines i["/"/"/"/"/"/"/ "/ Property Boundary

Site Boundary (Inset) i[ Surface Water Pathway i[ i[ Streams ppb = parts per billion 1988 RWQCB Sample NIM6 Hg in water: ND <0.3 ppb ppm = parts per million 2003 RWQCB Sample 30115-5 Hg = mercury i[ Hg in water: ND <0.2 ppb 1997 EPA Sample NIM-18 i[ Feet Hg in soil: 1.4 ppm 1997 EPA Sample SW-3 02,000

Hg in sediment: 2.6 ppm

1999 USGS Sample 99AU-7 i[ Hg in water: 0.1 ppb 0 Hg in sediment: 52.4 ppm # Hg in water: 0.0139 ppb Reservoir "/ !([ "/ i[ 1999 USGS Sample 99AU-6 i[ Hg in sediment: 61.4 ppm Hg in water: 0.0215 ppb 1999 USGS Sample 99AU-1 !([!([ Hg in sediment: 12.07 ppm Hg in water: 0.0364 ppb rk i[i[ o 1999 USGS Sample 99AU-3 F k

t e Hg in sediment: 0.83 ppm !([ !([ s Hg in water: 0.00084 ppb 1999 USGS Sample 99AU-2 a re E C

Hg in sediment: >100 ppm s o i[i[ t o Hg in water: 0.00826 ppb rl 1999 USGS Sample 99AU-4 !)i ry a a C t i[Hg in water: 0.876 ppb u n b a ri S i[ T

"/ 1997 EPA Sample SW-1 1997 EPA Sample SW-2 "/ Hg in sediment: 1.5 ppm Hg in sediment: 1.6 ppm Hg in water: 0.1 ppb Hg in water: 0.1 ppb 1999 USGS Sample 99AU-5 Hg in sediment: 2.13 ppm 1997 EPA Sample NIM-19 "/!([ Hg in water: 0.0224 ppb Hg in soil: 3.9 ppm

Historic Sample Locations - Map 1B New Idria Mercury Mine San Benito County, California

Figure 3

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend Former !. Soil/Sediment/Tailings Unpermitted Samples Landfill k poH! e Sediment & Surface e NIMM-SW-7/SD-9 r C Water Samples Hg in sediment: 4.9 ppm s o l Hg in water: 4.8 ppb (total) r VT Level 10 Adit a Hg in water: 0.14J ppb (dissolved) C

MMHg in water: 0.210 ppt (dissolved) n Northern a d Site Boundary S a Tailing o R Piles a Calcine Piles i !. r d !. I

Waste Rock Pile po!.H w e

N Acid Mine Drainage !. NIMM-TL-10 San Carlos Creek !. Hg in tailings: 31.5 ppm

ppb = parts per billion NIMM-TL-7 ppm = parts per million Hg in tailings: 3.8 ppm ppt = parts per trillion Hg = mercury NIMM-TL-8 Hg in tailings: 91.6 ppm NIMM-TL-6 MMHg = Methyl mercury Hg in tailings: 38.7 ppm Feet 0400 NIMM-TL-9 Hg in tailings: 72.3 ppm !. NIMM-TL-4 Hg in tailings: 134 ppm

School Parcel

!. NIMM-TL-3 Hg in tailings: 191 ppm Clear Creek Road

Southern Tailing NIMM-SD-6 Furnace NIMM-SW-6/SD-8 Hg in sediment: 41.3 ppm Piles Hg in sediment: 4.7 ppm !.H Hg in water: 4.9 ppb (total) po Hg in water: 0.16J ppb (dissolved) !. NIMM-TL-5 Hg in tailings: 446 ppm

!. !. !.H po NIMM-SW-5/SD-7 po!.H Hg in sediment: 25.0 ppm NIMM-SD-5 Hg in water: 21.2 ppb (total) Hg in sediment: 12.1 ppm Waste Hg in water: 11.2 ppb (dissolved) MMHg in water:2.30 ppt (dissolved) !. Rock Pile NIMM-SW-4/SD-4 Hg in sediment: 14.4 ppm po!.H Hg in water: <0.20 ppb (total) VT NIMM-SW-3/SD-3 Hg in water: <0.20 ppb (dissolved) Hg in sediment: 4.9 ppm Hg in water: 0.28 ppb (total) Hg in water: 0.30 ppb (dissolved) MMHg in water: 0.039J ppt (dissolved) Level 10 Adit

Sample Locations - Map-1C New Idria Mercury Mine San Benito County, California

Figure 4

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend /" 1997 EPA Samples Former Unpermitted [

!( Landfill k USGS Samples e

e

r 0 # C

s o RWQCB Samples l r a C

n VT Level 10 Adit a d S a Northern o R Site Boundary Tailing a i r Piles d I Calcine Piles

w e Waste Rock Pile N

Acid Mine Drainage

San Carlos Creek 1997 USGS Sample 97NI-6W Hg in sediment: 0.0211 ppm ppb = parts per billion Hg in water: 11.985 ppb ppm = parts per million Hg = mercury

Feet 0400 !([

School Parcel

1988 RWQCB Sample NIM4 Hg in water: 5.0 ppb C 1997 EPA Sample SW-5 1997 EPA Sample NIM-14 lear Creek Road Hg in sediment: 8.4 ppm Hg in tailings 74.6 ppm Southern Hg in water: 5.0 ppb Tailing

1997 EPA Sample NIM-16 Piles Hg in tailings 42.2 ppm Furnace 2003 RWQCB Sample 30115-8

Hg in AMD: ND <0.2 ppb

0 /" # 1997 EPA Sample NIM-5 Hg in sediment: 0.4 ppb 1975 RWQCB Sample #4 Hg in water: 4 ppb 1988 RWQCB Sample NIM5 1997 EPA Sample NIM-15 Hg in water: ND <0.3 ppm Hg in tailings 27.1 ppm

/" 1997 EPA Sample NIM-6

Hg in sediment 11.7 ppm

0

1997 EPA Sample NIM-13 /" # 1997 EPA Sample NIM-17

Hg in tailings 40.7 ppm !([ Hg in sediment 43 ppm 0 # 0 /" !([# /" 1997 USGS Sample 97NI-5W 1975 RWQCB Sample #1 Hg in water: 17.28 ppb

Hg in water: 0.2 ppb Waste

0 /" /" # /" Rock 1975 RWQCB Sample #3

/" /" Pile Hg in water: 2.7 ppb

1975 RWQCB Sample #2 /" 1988 RWQCB Sample NIM3

0 " Hg in AMD: 2.3 ppb # / 0 # /" Hg in water: 8.0 ppb 1988 RWQCB Sample NIM1 /" 1997 USGS Sample 97NI-7W Hg in AMD: ND <0.3 ppb VT Hg in water: 4.471 ppb 2003 RWQCB Sample 30115-7 1999 USGS Sample 99AU-8 Hg in AMD: <0.2 ppb Hg in water: 0.372 ppb 1997 EPA Sample NIM-1 Level 10 Hg in AMD: 0.2 ppb 1997 EPA Sample NIM-7 Adit 1997 EPA Samples 1988 RWQCB Sample NIM2 1997 EPA Sample SW-4 Hg in sediment: 1.2 ppm Hg in Sediment Hg in AMD: ND <0.3 ppb Hg in sediment: 19.7 ppm NIM-8: 0.22 ppm Hg in water: 0.54 ppb NIM-9: 4.2 ppm NIM-10: 13.7 ppm NIM-11: 8.5 ppm NIM-12: 25.7 ppm Historic Sample Locations - Map-1D Hg in AMD New Idria Mercury Mine NIM-2: 0.2 ppb San Benito County, California NIM-3: 0.6 ppb NIM-4: 16.5 ppb Figure 5

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

!. Sediment Sample NIMM-SW-14/SD-16 Hg in sediment: 1.8J ppm Hg in water: <0.20 ppb (total) poH! Sediment & Surface Hg in water: <0.20 ppb (dissolved) Water Samples MMHg in water: 0.072 ppt (dissolved) i[ Mercury Mines poH! !)i Other Mines

NIMM-SW-12/SD-14 (attribution) Hg in sediment: 0.081J ppm Surface Water Pathway Hg in water: <0.20 ppb (total) Hg in water: <0.20 ppb (dissolved) Streams MMHg in water: 0.086 ppt (dissolved) ppb = parts per billion ppm = parts per million ppt = parts per trillion poH! !. Hg = mercury ek Cre MMHg = methyl mercury ous Lari Feet 02,000 NIMM-SD-15 Hg in sediment: 4.6J ppm

!.

NIMM-SD-13 Hg in sediment: 2.0J ppm

NIMM-SW-10/SD-12 Hg in sediment: 8.9J ppm Hg in water: 0.79 ppb (total) poH! Hg in water: <0.20 ppb (dissolved)

Magnetite Gypsum Mine !)i !)i Mine

Eas t For k San Car los C reek

NIMM-SW-8/SD-10 Hg in sediment: 32.6 ppm Hg in water: 4.6 ppb (total) Sample Locations - Map 2A Hg in water: 0.14J ppb (dissolved) poH! New Idria Mercury Mine San Benito County, California

Figure 6

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend "/ 1997 EPA Sample SW-14 Hg in sediment: 9.7 ppm "/ 1997 EPA Samples Hg in water: 0.47 ppb

!([ USGS Samples

0 # RWQCB Samples

i[ Mercury Mines

!)i Other Mines

Surface Water Pathway

1997 EPA Sample SW-13 Hg in sediment: 5.6 ppm Streams Hg in water: 6.9 ppb "/ ppb = parts per billion ppm = parts per million ek Hg = mercury Cre ous Lari Feet 02,000

[ 1997 EPA Sample SW-12 "/!( Hg in sediment: 0.11 ppm 1997 EPA Sample SW-12 Hg in sediment: 13.4 ppm "/ Hg in water: 11.4 ppb "/ 1999 USGS Sample 99NI-1W Hg in sediment: 0.0275 ppm Hg in water: 19.6 ppb

1997 EPA Sample SW-10 Hg in sediment: 22.5 ppm Hg in water: 3.8 ppb

!([

1997 EPA Sample SW-9 Hg in sediment: 2.2 ppm 1999 USGS Sample 99NI-2W Hg in water: 12 ppb Hg in sediment: 0.0279 ppm Hg in water: 10.16 ppb

1997 EPA Sample SW-8 Hg in sediment: 1.6 ppm Magnetite Hg in water: ND

Mine )i

! !)i Gypsum

0

"/ # Mine 0 1997 EPA Sample SW-7 # "/ Hg in sediment: 1.5 ppm !([ Hg in water: 0.1 ppb "/ 1999 USGS Sample 99NI-3W Eas t For Hg in sediment: 0.0278 ppm k San Hg in water: 13.55 ppb Car los C 1988 RWQCB Sample NIM7&8 reek Hg in sludge: 1.25 ppm Hg in water: <0.3 ppb 1975 RWQCB Sample #5 Hg in water: 7.8 ppb

1997 EPA Sample SW-6 Hg in sediment: 10 ppm "/ Historic Sample Locations - Map 2B Hg in water: 9.6 ppb New Idria Mercury Mine San Benito County, California 1997 USGS Sample 97NI-4W Hg in sediment: 0.0325 ppm Figure Hg in water: 13.5 ppb !([ 7

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

!. NIMM-SD-22 Sediment Samples Hg in sediment: 0.60J ppm Surface Water Pathway

Streams !. ppb = parts per billion ppm = parts per million Hg = mercury NIMM-SD-21 Hg in sediment: 1.3J ppm Feet 02,000 !.

S i l v e r

C r e e k NIMM-SD-20 !. Hg in sediment: 11.2J ppm

NIMM-SD-19 Hg in sediment: 0.070J ppm !.

NIMM-SD-18 Hg in sediment: 2.7J ppm !.

NIMM-SD-17 Hg in sediment: 3.9J ppm

!. Sample Lcoations - Map 3 New Idria Mercury Mine San Benito County, California

Figure 8

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

!. Sediment Samples

Surface Water Pathway NIMM-SD-30 Hg in sediment: 1.2 ppm !. Streams ppm = parts per million Hg = mercury

Feet 02,000 NIMM-SD-29 Hg in sediment: 2.3J ppm

!.

NIMM-SD-28 Hg in sediment: 1.1J ppm S F a re n s B n e o n C !. i o to u C n o ty u n ty NIMM-SD-27 !. Hg in sediment: 1.5J ppm

NIMM-SD-26 !. Hg in sediment: 1.7J ppm

NIMM-SD-25 Hg in sediment: 2.4J ppm !.

NIMM-SD-24 Hg in sediment: 4.2J ppm

!.

NIMM-SD-23 Hg in sediment: 0.38J ppm

Sample Locations- Map 4A !. New Idria Mercury Mine San Benito County, California

Figure 9

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

/" 1997 EPA Samples

Surface Water Pathway

Streams ppm = parts per million Hg = mercury

Feet 02,000

/" 1997 EPA Sample SW-15 Hg in sediment: 1.5 ppm

S F a re n s B n e o n C i o to u C n o ty u n ty

Historic Sample Locations- Map 4B New Idria Mercury Mine San Benito County, California

Figure 10

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

!. Sediment Samples

i[ Mercury Mines

!)i Other Mines P

Surface Water Pathway

Streams ppb = parts per billion ppm = parts per million Hg = mercury

Feet 02,000

Diatomite P NIMM-SD-33 a Hg in sediment: 0.050J ppm n !)i Mine o c h e C r e e k !. !. NIMM-SD-34 Hg in sediment: 1.1 ppm

NIMM-SD-32 Hg in sediment: 1.0 ppm !.

k

e

e

r

C

r

e Sample Locations - Map 5A

v

NIMM-SD-31 l i New Idria Mercury Mine Hg in sediment: 1.0 ppm S San Benito County, California !. Figure 11

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010. Legend

/" 1997 EPA Samples

i[ Mercury Mines

!)i Other Mines P

Surface Water Pathway

Streams ppb = parts per billion ppm = parts per million Hg = mercury

Feet 02,000

1997 EPA Sample SW-18 Diatomite P Hg in sediment: 0.13 ppm a n !)i Mine o c h e C r e e k /" /" 1997 EPA Sample SW-17 Hg in sediment: 0.14 ppm /" Hg in water: 0.2 ppb

1997 EPA Sample SW-16 Hg in sediment: 1.5 ppm Hg in water: 0.2 ppb

k

e

e

r

C

r

e Historic Sample Locations - Map 5B

v

l i New Idria Mercury Mine S San Benito County, California

Figure 12

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010.