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.
1

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.
3/2011

2

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

Sgw

2 +Ssw2 + Ss2 + Sa2

 XXXXXXX 4,010.6889

(Sgw

2 +Ssw2 + Ss2 + Sa2

) / 4 XXXXXXX 1,002.6722

SQRT ((Sgw

2 +Ssw2 + Ss2 + Sa2

) / 4) XXXXXXX 31.66
3/2011

3

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 
3/2011

4

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 

(lines 19a + 19b + 19c)

b 0 

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 
3/2011

5

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, 

subject to a maximum of 100) 100

63.33

 

SURFACE WATER OVERLAND/FLOOD MIGRATION COMPONENT SCORE

30. Component Score (Sof)

c

, 

(highest score from line 29 for all watersheds evaluated, 

subject to a maximum of 100)

100 63.33 a

Maximum value applies to waste characteristics category.

b

Maximum value not applicable.

c

Do not round to nearest integer.
3/2011

6

REFERENCES

Reference 

Number Description of the Reference

1 Hazard Ranking System; Final Rule, 14 December 1990, Vol. 55, No. 241, 138 pages.

Available at: http://www.epa.gov/superfund/sites/npl/hrsres/index.htm#HRS%20Rule.

2

U.S. Environmental Protection Agency (EPA), Superfund Chemical Data Matrix 

(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.

3 Weston Solutions, Inc., New Idria Mercury Mine, Expanded Site Inspection, Idria, 

San Benito County, California, Sampling and Analysis Plan, April 2010, 48 pages.

4 Ecology & Environment, Inc., New Idria Mercury Mine, Preliminary Assessment/Site 

Inspection, March 31, 1998, 68 pages.

5 Dames & Moore, Application for Facility Permit/Waste Discharge, New Idria Mine, 

Idria, California 95021, April 21, 1989, 22 pages.

6 Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, 

San Benito County, California, October 2010, 1200 pages.

7

Bureau of Land Management, Draft Clear Creek Management Area Resource 

Management Plan & Environmental Impact Statement, Hollister, California, 

November 2009, 735 pages.

8 University of California Santa Cruz, Mercury Speciation in Acid Mine Drainage: 

New Idria Quicksilver Mine, California, September 1998, 41 pages.

9 California Regional Water Quality Control Board, Central Valley Region, New Idria 

Mine, San Benito County – Complaint Investigation, June 2, 1975, 4 pages.

10 California Regional Water Quality Control Board, Central Valley Region, Inspection 

of New Idria Mine, San Benito County, July 22, 1988, 6 pages.

11 California Regional Water Quality Control Board, Central Valley Region, New Idria 

Mine, San Benito County, March 7, 2003, 15 pages.

12

TechLaw, Inc., Potentially Responsible Party and Insurance Archeology Search 

Report, New Idria Mercury Mine Site, San Benito County, California, June 5, 2006, 

205 pages.

13

U.S. Environmental Protection Agency, Region IX, Wetlands Regulatory Office 

(WTR-8), Summary of Site Inspection to Verify the Presence of Jurisdictional 

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.

14 Weston Solutions, Inc., Expanded Site Inspection. Logbook, New Idria Mercury 

Mine, July 6, 2010 through July 23, 2010, 8 pages.

15 U.S. Bureau of Mines, Mercury Potential of the United States, Information Circular 

8252, 1965, 34 pages.

16

U.S. Environmental Protection Agency, Solid Waste and Emergency Response, Using 

Qualified Data to Document an Observed Release and Observed Contamination, 

November 1996, 18 pages.
3/2011

7

17

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 

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.

19 National Operations Center, Sampling and Analysis Plan, Archer Mine, April 15, 

2009, 12 pages.

20 Weston Solutions, Inc., Surface Water Pathway and Target Distance Limit Map, New 

Idria Mercury Mine, December 13, 2010, 1 page.

21 Weston Solutions, Inc., Site Latitude and Longitude and Source Area, New Idria 

Mercury Mine, December 13, 2010, 1 page.

22 Meierer, Bob, CompuChem, Memorandum to Stan Kott, ESAT/ICF-LDC, Case#: 

40334, SDG#: MY71L3, October 5, 2010, 339 pages.

23 Meierer, Bob, CompuChem, Memorandum to Stan Kott, ESAT/ICF-LDC, Case#: 

40334, SDG#: MY71M9, October 11, 2010, 264 pages.

24 CompuChem, Memorandum to Garrett Peterson, U.S. Environmental Protection 

Agency, Case# 40334, SDG#: MY71N3, undated, 271 pages.

25 CompuChem, Memorandum to Garrett Peterson, U.S. Environmental Protection 

Agency, Case# 40334, SDG#: MY71S3, undated, 341 pages.

26 CompuChem, Memorandum to Mary O’Donnell, U.S. Environmental Protection 

Agency, Case# 40334, SDG#: MY71S4, undated, 268 pages.

27 CompuChem, Memorandum to Mary O’Donnell, U.S. Environmental Protection 

Agency, Case# 40334, SDG#: MY71S5, undated, 510 pages.

28 CompuChem, Memorandum to Mary O’Donnell, U.S. Environmental Protection 

Agency, Case# 40334, SDG#: MY71S6, undated, 509 pages.

29 Reilly, Amanda K.C., Weston Solutions, Inc., Memorandum Re: Sample Quantitation 

Limits, January 6, 2011, 5 pages.

30

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 

Service Review of Expanded Site Investigation Report on the New Idria Mercury

Mine, San Benito County, California, January 26, 2011, 9 pages.

31 Western Regional Climate Center, Period of Record Monthly Climate Summary, 

Idria, California, www.wrcc.dri.edu, data extracted January 26, 2011, 1 pages.

32

U.S. Fish and Wildlife Service, Species Report, Environmental Conservation Online 

System, Listed Animals, http://ecos.fws.gov/tess_public/pub/listedAnimals.jsp, data 

extracted January 26, 2011, 77 pages.

33

U.S. Fish and Wildlife Service, Species Report, Environmental Conservation Online 

System, Listed Plants, http://ecos.fws.gov/tess_public/pub/listedPlants.jsp, data 

extracted January 26, 2011, 42 pages.
3/2011

8

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
3/2011

9

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
3/2011

10

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 
3/2011

11

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).
12 Source Characterization

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).
Source No: 1

13 Source Characterization

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).

Calcine tailings samples were collected to document the presence of a hazardous substancesource to 

surface water. WESTON collected 10 soil samples to establish a source of metals at theSite. Soil 

samples NIMM-TL-3 through NIMM-TL-5 were collected from the southern calcinetailings pile, 

and soil samples NIMM-TL-6 through NIMM-TL-10 were collected from the northern calcine 

tailings pile. Soil samples NIMM-SS-1 and NIMM-SS-2 were collected frombackground 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.
Source No: 1

14 Source Characterization

Sample ID

Total

Weight

(grams)

Sieved 

Weight - fines

(grams)

Remaining -

coarse

(grams)

Percentage 

of fines (%) References

NIMM-TL-3

(southern calcine 

tailings pile)

348 199 149 57.18 Ref. 6, p. 885

NIMM-TL-4

(southern calcine 

tailings pile)

214 124 90 57.94 Ref. 6, p. 886

NIMM-TL-5 

(southern calcine 

tailings pile)

265 79 186 29.81 Ref. 6, p. 887

NIMM-TL-6

(southern calcine 

tailings pile)

258 135 123 52.33 Ref. 6, p. 888

NIMM-TL-7

(northern calcine 

tailings pile)

683 161 522 23.57 Ref. 6, p. 889

NIMM-TL-8

(northern calcine 

tailings pile)

367 75 292 20.44 Ref. 6, p. 890

NIMM-TL-9

(northern calcine 

tailings pile)

755 118 637 15.63 Ref. 6, p. 891

NIMM-TL-10

(northern calcine 

tailings pile)

848 197 651 23.23 Ref. 6, p. 892

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).
Source No: 1

15 Source Characterization

Sample ID Date Hazardous 

Substance

Hazardous Substance 

Concentration

(mg/kg fines)

SQL

(mg/kg) References

NIMM-TL-3 

(southern calcine 

tailings pile)

7/19/10

Arsenic 22.7 1.01 Ref. 6, pp. 20-21, 

1133, 1135-1136, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 39, 

67, 103, 104, 108, 

115; Ref. 29

Cadmium 1.2 J

1 (0.85) 0.504

Copper 59.9 2.52

Lead 18.1 1.01

Mercury 191 2.016

Nickel 31.3 4.032

Zinc 56.2 6.048

NIMM-TL-4 

(southern calcine 

tailings pile)

7/19/10

Arsenic 17.7 0.995 Ref. 6, pp. 20-21, 

1133, 1135-1136, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 40, 

67, 103, 104, 108, 

115; Ref. 29

Cadmium 0.51 J

1 (0.36) 0.497

Copper 39.2 2.49

Lead 9.0 0.995

Mercury 134 5.03

Nickel 20.4 3.98

Zinc 39.3 5.97

NIMM-TL-5 

(southern calcine 

tailings pile)

7/19/10

Arsenic 16.2 0.964 Ref. 6, pp. 20-21, 

1133, 1135-1136, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 41, 

67, 103, 104, 108, 

115; Ref. 29

Cadmium 1.3 J

1 (0.92) 0.482

Copper 48.5 2.41

Lead 49.5 0.964

Mercury 446 10.14

Nickel 67.7 3.86

Zinc 396 5.78

NIMM-TL-6 

(southern calcine 

tailings pile)

7/19/10

Arsenic 17.2 0.965 Ref. 6, pp. 20-21, 

1133, 1135-1136, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 42, 

67, 103, 104, 108, 

115; Ref. 29

Cadmium 0.99 J

1 (0.7) 0.483

Copper 47.1 2.41

Lead 61.5 0.965

Mercury 38.7 2.01

Nickel 159 3.86

Zinc 104 5.79
Source No: 1

16 Source Characterization

Sample ID Date Hazardous 

Substance

Hazardous Substance 

Concentration

(mg/kg fines)

SQL

(mg/kg) References

NIMM-TL-7 

(northern calcine 

tailings pile)

7/19/10

Arsenic 23.6 0.978 Ref. 6, pp. 20-21, 

1133, 1135-1137, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 43, 

67, 103, 104, 108, 

115; Ref. 29

Cadmium 1.5 J

1 (1.06) 0.489

Copper 69.0 2.45

Lead 23.5 0.978

Mercury 3.8 0.096

Nickel 66.1 3.91

Zinc 50.8 5.87

NIMM-TL-8 

(northern calcine 

tailings pile)

7/19/10

Arsenic 18.8 0.99 Ref. 6, pp. 20-21, 

1133, 1135-1137, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 45, 

67, 103, 104, 108, 

115; Ref. 29

Cadmium 1.3 J

1 (0.92) 0.495

Copper 60.3 2.48

Lead 18.2 0.99

Mercury 91.6 5.05

Nickel 99.8 3.96

Zinc 98.7 5.94

NIMM-TL-9 

(northern calcine 

tailings pile)

7/19/10

Arsenic 27.7 1.01 Ref. 6, pp. 20-21, 

1133, 1135-1137, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 46, 

67, 103, 105, 108, 

115; Ref. 29

Cadmium 1.6 J

1 (1.13) 0.505

Copper 79.8 2.53

Lead 22.5 1.01

Mercury 72.3 2.01

Nickel 203 4.04

Zinc 71.3 6.06

NIMM-TL-10 

(northern calcine 

tailings pile)

7/19/10

Arsenic 26.0 0.974 Ref. 6, pp. 20-21, 

1133, 1135-1137, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 3, 47, 

67, 103, 105, 108, 

115; Ref. 29

Cadmium 1.8 J

1 (1.28) 0.487

Copper 73.7 2.44

Lead 26.2 0.974

Mercury 28.9 2.03

Nickel 228 3.90

Zinc 88.4 5.84
Source No: 1

17 Source Characterization

Sample ID Date Hazardous 

Substance

Hazardous Substance 

Concentration

(mg/kg fines)

SQL

(mg/kg) References

NIMM-TL-11 

(duplicate of 

NIMM-TL-10)

7/19/10

Arsenic 26.7 1.0 Ref. 6, pp. 20-21, 

1133, 1135-1137, 

1139; Ref. 14, p. 2; 

Ref. 16, pp. 8, 18; 

Ref. 22, pp. 2, 48, 

67, 103, 105, 108, 

115; Ref. 29

Cadmium 1.8 J

1 (1.28) 0.5

Copper 74.2 2.5

Lead 25.0 1.0

Mercury 31.5 2.02

Nickel 231 4.0

Zinc 90.3 6.0

mg/kg: milligrams analyte per kilogram tailings

J

1

: 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:

 x DF

1kg

1000 g x 1000 mL

1L x sample weight (g wet wgt) x %S

final volume (mL) x 1000 μg

1mg x L

μg CRDL

kg dry wgt

mg SQL 



 



 =















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
Source No: 1

18 Source Characterization

2.2.3 HAZARDOUS SUBSTANCES AVAILABLE TO A PATHWAY

Containment Description Containment 

Factor Value Reference

Release via overland migration and/or flood: The Calcine Tailings 

Piles are unlined and were observed to be in direct contact with San 

Carlos Creek. In addition, there was no engineered cover or retaining 

wall supporting the waste rock from coming into contact with surface 

water.

10

Ref. 1, Table 4-2; 

Ref, 6, pp. 55, 

Photos 7, 8, 26, 27, 

28, 29, 30, 31, 32, 

34.
19 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
20 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).
Source No: 2 

21 Source Characterization

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 AMDimpacted 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.
Source No: 2 

22 Source Characterization

- Source Samples:

AMD Source Concentrations – Total Metals (μg/l)

Sample ID Date Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

NIMM-SW-4

(Level 10 Adit) 7/22/2010

Arsenic 72.4 1.0 Ref. 6, pp. 23, 1170, 

1171, 1184-1187, 

1190; Ref. 14, p. 6; 

Ref. 16, pp. 8, 18; 

Ref. 25, p. 2; Ref. 

27, pp. 34, 54, 70, 

71; Ref. 29

Cadmium 1.7 1.0

Selenium 11.2 J

1 (9.82) 5.0

Zinc 2,500 J

2 2.0

μg/l: micrograms analyte per liter water

J

1

: 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.

J

2

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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 
Source No: 2 

23 Source Characterization

AMD Source Concentrations – Dissolved Metals (μg/l)

Sample ID Date

Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

Arsenic 66.1 1.0

NIMM-SW-4

(Level 10 Adit) 7/22/2010

Ref. 6, pp. 23, 1178, 

1195, 1197-1198; Ref. 

14, p. 6; Ref. 16, pp. 8, 

18; Ref. 26, p. 2; Ref. 

28, pp. 34, 52, 68, 70; 

Ref. 29

Cadmium 1.4 1.0

Copper 3.2 2.0

Nickel 1,440 1.0

Selenium 10.6 J

1 (9.3) 5.0

Zinc 2,350 2.0

μg/l: micrograms analyte per liter water

J

1

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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
Source No: 2 

24 Source Characterization

2.2.3 HAZARDOUS SUBSTANCES AVAILABLE TO A PATHWAY

Containment Description Containment 

Factor Value Reference

Release via overland migration and/or flood: The AMD 

was observed to be flowing directly into the unlined and 

unnamed tributary that flows into San Carlos Creek with no 

containment. There was no engineered cover or retaining 

wall to aid in the containment of the AMD.

10

Ref. 1, Table 

4-2; Ref. 6, 

pp. 6-8, 41, 

54, Photos 5, 

6, 22, 24
Source No: 2 

25 Source Characterization

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
Source No: 2 

26 Source Characterization

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
27

SUMMARY OF SOURCE DESCRIPTIONS

Source 

No.

Source 

Hazardous

Waste 

Quantity

Value

Source 

Hazardous 

Constituent 

Quantity 

Complete? 

(Y/N)

Containment Factor Value by Pathway

Ground 

Water 

(GW)

(Ref. 1, 

Table 3-

2)

Surface Water (SW) Air

Overland/

flood

(Ref. 1,

Table 4-2)

GW to 

SW

(Ref. 1,

Table 3-2)

Gas

(Ref. 1,

Table 6-3)

Particulate

(Ref. 1,

Table 6-9)

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
28

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).
29 SWOF-Likelihood of Release

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 includesthe unnamed tributary running through the NIMM property 

to San Carlos Creek adjacent to site sourcesto 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 intermittentstreams 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
30 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).
31 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).

Sample ID Date Hazardous 

Substance

Hazardous 

Substance 

Concentration

(mg/kg fines)

SQL

(mg/kg) References

NIMM-TL-4 

(southern calcine 

tailings pile)

7/19/10

Arsenic 17.7 0.995 Ref. 6, pp. 20-

21, 1133, 1135-

1136, 1139; Ref. 

14, p. 4; Ref. 22, 

pp. 2,40, 67, 

103, 104, 108, 

112, 115; Ref. 

29

Cadmium 0.51J1 (0.36) 0.497

Copper 39.2 2.49

Lead 9.0 0.995

Mercury 134 5.03

Nickel 20.4 3.98

Zinc 39.3 5.97

mg/kg: milligrams analyte per kilogram

J

1

: 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:

 x DF

1kg

1000 g x 1000 mL

1L x sample weight (g wet wgt) x %S

final volume (mL) x 1000 μg

1mg x L

μg CRDL

kg dry wgt

mg SQL 



 



 =















Where:

Final volume (mL) is the final extract volume following sample preparation
32 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)

Sample ID Date Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

NIMM-SW-4

(Level 10 Adit) 7/22/2010

Arsenic 72.4 1.0 Ref. 6, pp. 23, 1170, 

1171, 1184-1187, 

1190; Ref. 14, p. 6; 

Ref. 16, pp. 8, 18; 

Ref. 25, p. 2; Ref. 

27, pp. 34, 54, 70, 

71; Ref. 29

Cadmium 1.7 1.0

Selenium 11.2 J1 (9.82) 5.0

Zinc 2,500 J2 2.0

μg/l: micrograms analyte per liter water

J

1

: 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.

J

2

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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

AMD Source Concentrations – Dissolved Metals (μg/l)

Sample ID Date Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

Arsenic 66.1 1.0

NIMM-SW-4

(Level 10 Adit) 7/22/2010

Ref. 6, pp. 23, 1178, 

1195, 1197-1198; Ref. 

14, p. 6; Ref. 16, p. 8, 

18; Ref. 26, p. 2; Ref. 

28, pp. 34, 52, 68, 70; 

Ref. 29

Cadmium 1.4 1.0

Copper 3.2 2.0

Nickel 1,440 1.0

Selenium 10.6 J1 (9.3) 5.0

Zinc 2,350 2.0

μg/l: micrograms analyte per liter water

J

1

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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 
34 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. Foursediment 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-SD32) 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.
35 SWOF-Likelihood of Release

Sample ID

Total

Weight

(grams)

Sieved 

Weight - fines

(grams)

Remaining -

coarse

(grams)

Percentage 

of fines (%) References

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).
36 SWOF-Likelihood of Release

Table A

Sample ID Sample 

Date

Sample Depth

(below 

surface)

Sample Description References

Background Samples

NIMM-SD-1 7/22/2010 2 inches Dark yellowish brown fine to course sand, no 

gravel Ref. 6, p. 893

NIMM-SD-11 7/22/2010 2 inches Dark yellowish brown fine to medium sand, 

angular gravel Ref. 6, p. 907

Comparative Contaminated Samples

NIMM-SD-3 7/21/2010 Not noted Light olive brown, fine to course sand, fine to 

course rounded gravel Ref. 6, p. 899

NIMM-SD-8 7/21/2010 2 inches Grayish brown fine to medium sand, fine to 

medium rounded and subrounded gravel Ref. 6, p. 904

NIMM-SD-9 7/21/2010 2 inches Olive brown fine to medium sand, angular 

gravel Ref. 6, p. 905

NIMM-SD-10 7/22/2010 2 inches Light olive brown fine to medium sand, fine to 

medium subrounded to angular gravel Ref. 6, p. 906

NIMM-SD-12 7/22/2010 2 inches Light yellowish brown fine to course sand, fine 

to course rounded gravel Ref. 6, p. 908

NIMM-SD-13 7/22/2010 2 inches Light yellowish brown very fine sand, fine 

gravel Ref. 6, p. 909

NIMM-SD-15 7/22/2010 Not noted Light yellowish brown fine to medium sand, 

very fine to medium angular gravel Ref. 6, p. 911

Table B

Sample ID Sample 

Date

Sample Depth

(below 

surface)

Sample Description References

Background Sample

NIMM-SD-14 7/21/2010 2 inches Olive brown very fine to medium sand, fine to 

medium rounded gravel Ref. 6, p. 910

Comparative Contaminated Samples

NIMM-SD-16 7/21/2010 2 inches Light olive brown fine to course sand, some 

fine gravel Ref. 6, p. 912

NIMM-SD-17 7/22/2010 Not noted Light yellowish brown very fine sand, rounded 

gravel Ref. 6, p. 913

NIMM-SD-18 7/22/2010 Not noted Light olive brown fine to medium sand, fine to 

medium gravel Ref. 6, p. 914

NIMM-SD-20 7/20/2010 2 inches Light olive brown fine to course sand, fine 

rounded gravel Ref. 6, p. 916

NIMM-SD-21 7/20/2010 2 inches Light olive brown very fine to medium sand, 

fine to medium rounded gravel Ref. 6, p. 917

NIMM-SD-22 7/20/2010 2 inches Light yellowish brown very fine to medium 

sand, fine to course rounded gravel Ref. 6, p. 918

NIMM-SD-23 7/20/2010 2 inches Grayish brown fine to course sand, fine 

rounded gravel Ref. 6, p. 919
37 SWOF-Likelihood of Release

NIMM-SD-24 7/20/2010 2 inches Light olive brown fine sand, no gravel Ref. 6, p. 920

NIMM-SD-25 7/20/2010 2 inches Grayish brown fine to medium sand, fine to 

medium rounded gravel Ref. 6, p. 921

NIMM-SD-26 7/20/2010 2 inches Light yellowish brown fine to medium sand Ref. 6, p. 922

NIMM-SD-27 7/20/2010 2 inches Light brownish gray fine to course sand, 

rounded gravel Ref. 6, p. 923

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 ID Sample 

Date

Sample Depth

(below 

surface)

Sample Description References

Background Samples

NIMM-SD-33 7/23/2010 2 inches Light brownish gray fine to course sand, some 

gravel Ref. 6, p. 929

NIMM-SD-61

(Duplicate of 

NIMM-SD-33)

7/23/2010 2 inches Light brownish gray fine to course sand, some 

gravel Ref. 6, p. 929

Comparative Contaminated Sample

NIMM-SD-34 7/23/2010 2 inches Sandy Ref. 6, p. 930
38 SWOF-Likelihood of Release

- Background Concentrations:

Total Metals in Sediments (mg/kg)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

(mg/kg)

SQL

(mg/kg) References

NIMM-SD-1

(San Carlos Creek, upstream of 

all contaminated samples)

Arsenic 1.0 U 1.01 Ref. 6, pp. 26, 30, 

1137, 1139; Ref. 14, 

p. 5; Ref. 22, pp. 3, 

49, 67, 105, 108, 

112; Ref. 29

Copper 6.2 2.53

Lead 3.9 1.01

Mercury 0.19 0.101

NIMM-SD-11

(Tributary to East Fork San 

Carlos Creek – upstream of all 

contaminated samples)

Arsenic 1.1 U 1.05 Ref. 6, pp. 26, 30, 

1154-1155, 1160, 

1163; Ref. 14, p. 5; 

Ref. 24, pp. 2, 33, 

60, 91, 92, 95; Ref. 

29

Copper 8.6 2.63

Lead 4.1 1.05

NIMM-SD-14

(Larious Creek – upstream of 

contaminated samples NIMMSD-15 through -32, -34)

Arsenic 6.6 1.06 Ref. 6, pp. 27, 30, 

1155, 1160, 1163; 

Ref. 14, p. 4; Ref. 

16, pp. 8, 18; Ref. 

24, pp. 2, 36, 60, 91, 

92, 95, 100; Ref. 29

Copper 19.4 2.65

Lead 7.3 1.06

Mercury 0.081 J

1 (0.15) 0.101

NIMM-SD-33

(Panoche Creek – upstream of 

contaminated sample NIMMSD-34)

Arsenic 3.9 0.975 Ref. 6, pp. 29-30, 

1145, 1148, 1150; 

Ref. 14, p. 6; Ref. 

23, pp. 4, 41, 58, 89, 

90, 92, 98; Ref. 29

Copper 8.1 2.44

Lead 4.0J

2 (5.76) 0.975

Mercury 0.10 U 0.1

NIMM-SD-61

(duplicate of NIMM-SD-33)

Arsenic 3.6 0.986 Ref. 6, pp. 29-30, 

1145, 1149-1150; 

Ref. 14, p. 6; Ref. 

23, pp. 4, 46, 58, 89, 

90, 92, 98; Ref. 29

Copper 7.9 2.47

Lead 4.6J

2 (6.6) 0.986

Mercury 0.050 J

3 0.101

mg/kg: milligrams analyte per kilogram sediment

J

1

: 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.

J

2

: 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.

J

3

: 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:

 x DF

1kg

1000 g x 1000 mL

1L x sample weight (g wet wgt) x %S

final volume (mL) x 1000 μg

1mg x L

μg CRDL

kg dry wgt

mg SQL 



 



 =















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 
39 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)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

(mg/kg)

SQL

(mg/kg) Reference

NIMM-SD-3

(San Carlos Creek adjacent 

to southern calcine pile)

Arsenic 9.0 1.01 Ref. 6, pp. 27, 30, 1138-1139; 

Ref. 14, p. 4; Ref. 22, pp. 3, 

51, 67, 103, 105, 109, 115; 

Ref. 29

Copper 44.6 2.53

Mercury 4.9 9.05

NIMM-SD-8

(San Carlos Creek adjacent 

to southern calcine pile)

Arsenic 10 1.02 Ref. 6, pp. 27, 30, 1138-1139; 

Ref. 14, p. 4; Ref. 22, pp. 3, 

55, 67, 103, 105, 109, 115; 

Ref. 29

Copper 40.3 2.55

Mercury 4.7 0.195

NIMM-SD-9

(San Carlos Creek adjacent 

to northern calcine pile)

Arsenic 14.1 1.03

Ref. 6, pp. 27, 30, 1139; Ref. 

14, p. 4; Ref. 22, pp. 4, 56, 67, 

103, 105, 109, 115; Ref. 29

Copper 55.9 2.58

Lead 16.0 1.03

Mercury 4.9 0.196

NIMM-SD-10

(San Carlos Creek 0.6 mile 

from PPE)

Arsenic 8.7 1.01 Ref. 6, pp. 27, 30, 1156, 1160, 

1163; Ref. 14, p. 5; Ref. 16, p. 

8; Ref. 24, pp. 2, 32, 60, 91, 

92, 95, 102; Ref. 29

Copper 30.7 2.54

Mercury 32.6 J1 2.03

NIMM-SD-12

(San Carlos Creek 2.4 miles

from PPE)

Arsenic 8.3 1.02 Ref. 6, pp. 27, 30, 1156, 1160, 

1163; Ref. 14, p. 5; Ref. 16, p. 

8; Ref. 24, pp. 2, 34, 60, 91, 

92, 95, 102; Ref. 29 Mercury 8.9 J1 0.412

NIMM-SD-13

(San Carlos Creek 4.1 miles

from PPE)

Arsenic 11.9 1.14 Ref. 6, pp. 27, 30, 1156, 1160, 

1163; Ref. 14, p. 5; Ref. 16, p. 

8; Ref. 24, pp. 2, 35, 60, 91, 

92, 95, 100; Ref. 29 Mercury 2.0 J1 0.114

NIMM-SD-15

(San Carlos Creek 4.7 miles

from PPE)

Mercury 4.6 J1 0.194

Ref. 6, pp. 27, 30, 1156, 1160, 

1163; Ref. 14, p. 5; Ref. 16, p. 

8; Ref. 24, pp. 2, 37, 60, 92, 

102; Ref. 29

NIMM-SD-59

(duplicate of NIMM-SD15)Mercury 4.0 0.194

Ref. 6, pp. 27, 30, 1149-1150; 

Ref. 14, p. 5; Ref. 23, pp. 4, 

44, 58, 90, 100; Ref. 29

NIMM-SD-15

(unsieved sample) Mercury 4.4 0.194

Ref. 6, pp. 27, 30, 1149-1150; 

Ref. 14, p. 5; Ref. 24, pp. 2, 

37, 60, 92, 102; Ref. 29

NIMM-SD-16

(Silver Creek 5.4 miles Mercury 1.8 J1 0.107 Ref. 6, pp. 28, 30, 1156, 1161, 1163; Ref. 14, p. 4; Ref. 16, p. 
40 SWOF-Likelihood of Release

Total Metals in Sediments Showing an Observed Release (mg/kg)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

(mg/kg)

SQL

(mg/kg) Reference

from PPE) 8; Ref. 24, pp. 2, 38, 60, 92, 

100; Ref. 29

NIMM-SD-17

(Silver Creek 6.4 miles 

from PPE)

Mercury 3.9 J1 0.204

Ref. 6, pp. 28, 30, 1156, 1161, 

1163; Ref. 14, p. 5; Ref. 16, p. 

8; Ref. 24, pp. 2, 39, 60, 92, 

102; Ref. 29

NIMM-SD-18

(Silver Creek 7.4 miles 

from PPE)

Mercury 2.7 J1 0.101

Ref. 6, pp. 28, 30, 1156, 1161, 

1163; Ref. 14, p. 5; Ref. 16, p. 

8; Ref. 24, pp. 2, 40, 60, 93, 

100; Ref. 29

NIMM-SD-20

(Silver Creek 9.2 miles 

from PPE)

Mercury 11.2 J1 0.491

Ref. 6, pp. 28, 30, 1156, 1161, 

1163; Ref. 14, p. 3; Ref. 16, p. 

8; Ref. 24, pp. 3, 42, 60, 93, 

102; Ref. 29

NIMM-SD-21

(Silver Creek 10.2 miles 

from PPE)

Mercury 1.3 J1 0.101

Ref. 6, pp. 28, 30, 1156, 1161, 

1163; Ref. 14, p. 3; Ref. 24, 

pp. 3, 43, 60, 93, 100; Ref. 29

NIMM-SD-22

(Silver Creek 11.2 miles 

from PPE)

Mercury 0.60 J1 0.101

Ref. 6, pp. 28, 30, 1156, 1162-

1163; Ref. 14, p. 3; Ref. 16, p. 

8; Ref. 24, pp. 3, 44, 60, 93, 

100; Ref. 29

NIMM-SD-24

(Silver Creek 13.0 miles 

from PPE)

Mercury 4.2 J1 0.206

Ref. 6, pp. 28, 30, 1156, 1162-

1163; Ref. 14, p. 3; Ref. 16, p. 

8; Ref. 24, pp. 3, 46, 60, 93, 

102; Ref. 29

NIMM-SD-25

(Silver Creek 14.0 miles 

from PPE)

Mercury 2.4 J

1 0.103

Ref. 6, pp. 28, 30, 1156, 1162-

1163; Ref. 14, p. 3; Ref. 16, p. 

8; Ref. 24, pp. 3, 47, 60, 93, 

100; Ref. 29

NIMM-SD-60

(duplicate of NIMM-SD25)Mercury 1.4 0.098

Ref. 6, pp. 28, 30, 1146, 1149-

1150; Ref. 14, p. 3; Ref. 23, 

pp. 4, 45, 59, 90, 98; Ref. 29

NIMM-SD-26

(Silver Creek 15.0 miles 

from PPE)

Mercury 1.7 J1 0.101

Ref. 6, pp. 29, 30 1156, 1162-

1163; Ref. 14, p. 3; Ref. 16, p. 

8; Ref. 24, pp. 3, 48, 60, 93, 

99; Ref. 29

NIMM-SD-27

(Silver Creek 15.8 miles 

from PPE)

Mercury 1.5 J1 0.101

Ref. 6, pp. 29, 30 1156, 1162-

1163; Ref. 14, p. 3; Ref. 16, p. 

8; Ref. 24, pp. 3, 49, 60, 93, 

100; Ref. 29

NIMM-SD-28

(Silver Creek 16.2 miles

from PPE)

Mercury 1.1 J1 0.101

Ref. 6, pp. 29, 30 1156, 1163; 

Ref. 14, p. 3; Ref. 16, p. 8; 

Ref. 24, pp. 3, 50, 60, 93, 100; 

Ref. 29
41 SWOF-Likelihood of Release

Total Metals in Sediments Showing an Observed Release (mg/kg)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

(mg/kg)

SQL

(mg/kg) Reference

NIMM-SD-29

(Silver Creek 16.9 miles 

from PPE)

Mercury 2.3 J1 0.101

Ref. 6, pp. 29, 30 1156, 1163; 

Ref. 14, p. 6; Ref. 16, p. 8; 

Ref. 24, pp. 3, 51, 60, 93, 100; 

Ref. 29

NIMM-SD-30

(Silver Creek 17.8 miles 

from PPE)

Mercury 1.2 0.107

Ref. 6, pp. 29, 30 1148, 1150; 

Ref. 14, p. 3; Ref. 23, pp. 4, 

38, 58, 90, 98; Ref. 29

NIMM-SD-31

(Silver Creek 18.7 miles 

from PPE)

Mercury 1.0 0.101

Ref. 6, pp. 29, 30 1148, 1150; 

Ref. 14, p. 3; Ref. 23, pp. 4, 

39, 58, 90, 98; Ref. 29

NIMM-SD-32

(Silver Creek 19.5 miles 

from PPE)

Mercury 1.0 0.096

Ref. 6, pp. 29, 30 1148, 1150; 

Ref. 14, p. 6; Ref. 23, pp. 4, 

40, 58, 90, 98; Ref. 29

NIMM-SD-34

(Panoche Creek 19.9 miles 

from PPE)

Mercury 1.1 0.103

Ref. 6, pp. 29, 30 1148, 1150; 

Ref. 14, p. 6; Ref. 23, pp. 4, 

42, 58, 90, 98; Ref. 29

mg/kg: milligrams analyte per kilogram sediment

J

1

: 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:

 x DF

1kg

1000 g x 1000 mL

1L x sample weight (g wet wgt) x %Sfinal volume (mL) x 1000 μg

1mg x L

μg CRDL

kg dry wgt

mg SQL 



 



 =















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
42 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, NIMMSW-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 NIMMSW-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 NIMMSW-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).
43 SWOF-Likelihood of Release

Table D

Sample ID Sample 

Date Sample Description References

Background Samples

NIMM-SW-1 7/22/2010 Some green algae, 2-foot wide drainage with 

low to moderate flow Ref. 6, p. 894

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

BLM-SW-2 7/22/2010 Sandy banks, 2.5-foot wide drainage with low 

to moderate flow Ref. 6, p. 896

BLM-SW-3 7/22/2010 Grassy banks, 3-foot wide drainage with low to 

moderate flow Ref. 6, p. 897

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

NIMM-SW-8 7/22/2010 Cloudy water with orange-rust colored 

sediments Ref. 6, p. 906

NIMM-SW-10 7/22/2010 Salt deposits on banks, orange-rust colored 

stream bed Ref. 6, p. 908

- Background Concentrations:

Total Metals in Surface Water (μg/l)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

NIMM-SW-1

(Upper San Carlos Creek –

upstream of all contaminated 

samples)

Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1166-1167, 

1170-1171, 1182-1183, 1187, 

1190; Ref. 14, p. 5; Ref. 16, 

pp. 8, 18; Ref. 25, pp. 2, 33, 

62, 96, 104; Ref. 27, pp. 31, 

54, 70, 71; Ref. 29

Cadmium 1.0 U 1.0

Copper 2.0 U 2.0

Mercury 0.084 J

1 0.20

Zinc 1.4 J

1,2 (1.81) 2.0

NIMM-SW-9

(Tributary leading to East Fork 

San Carlos Creek – upstream 

of all contaminated samples)

Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1166, 1167, 

1170-1171, 1182, 1183, 1185, 

1188, 1190; Ref. 14, p. 5; Ref. 

16, pp. 8, 18; Ref. 25, pp. 3, 

41, 62, 96, 104; Ref. 27, pp. 

39, 54, 70, 71; Ref. 29

Cadmium 1.0 U 1.0

Copper 4.0 2.0

Mercury 0.10 J

1 0.20

Zinc 9.5 J2 (12.32) 2.0

BLM-SW-1

Upper San Carlos Creek, 

Aurora Mine background –

upstream of all contaminated 

samples)

Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1171, 1182, 

1183, 1189-1190; Ref. 14, p. 

6; Ref. 16, pp. 8, 18; Ref. 25, 

pp. 4, 48, 62, 97, 104; Ref. 27, 

pp. 46, 54, 70, 72; Ref. 29

Cadmium 1.0 U 1.0

Copper 2.0 U 2.0

Mercury 0.20 U 0.20

Zinc 1.8 J

1,2 (2.32) 2.0
44 SWOF-Likelihood of Release

Total Metals in Surface Water (μg/l)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

BLM-SW-2

Upper San Carlos Creek

adjacent to Aurora Mine –

upstream of all contaminated 

samples)

Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1171, 1182, 

1183, 1189-1190; Ref. 14, p. 

6; Ref. 16, pp. 8, 18; Ref. 25, 

pp. 5, 49, 62, 97, 105; Ref. 27, 

pp. 47, 54, 70, 72; Ref. 29

Cadmium 1.0 U 1.0

Copper 2.0 U 2.0

Mercury 0.20 U 0.20

Zinc 1.4J J

1,2 (1.81) 2.0

BLM-SW-3

(Upper San Carlos Creek

downstream of Aurora Mine –

upstream of all contaminated 

samples)

Arsenic 1.0 U 1.0 Ref. 6, pp. 34, 1166, 1167, 

1171, 1182, 1183, 1189-1190; 

Ref. 14, p. 6; Ref. 16, pp. 8, 

18; Ref. 25, pp. 5, 50, 62, 97, 

105; Ref. 27, pp. 48, 70, 72; 

Ref. 29

Cadmium 1.0 U 1.0

Copper 2.0 U 2.0

Mercury 0.20 U 0.20

Zinc 1.3 J

1,2 (1.68) 2.0

μg/l : micrograms analyte per liter water

CRQL: Contract Required Quantitation Limit

J

1

: Analyte detected at a level less than the CRQL. 

J

2

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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

- Contaminated Samples:

Total Metals in Surface Water Showing an Observed Release (μg/l)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL Reference

NIMM-SW-3

(San Carlos Creek adjacent to 

southern calcine pile)

Mercury 0.28 0.20

Ref. 6, pp. 35, 1170-1171; Ref. 

14, p. 4; Ref. 25, pp. 2, 35, 62, 

96, 104; Ref. 27, pp. 54, 70, 

71; Ref. 29

NIMM-SW-6

(San Carlos Creek adjacent to 

southern calcine pile)

Arsenic 1.3 1.0 Ref. 6, pp. 35, 1170-1171, 

1185, 1187, 1190; Ref. 14, p. 

4; Ref. 16, p. 8; Ref. 25, pp. 3, 

38, 62, 96, 104; Ref. 27, pp. 

36, 54, 70, 71; Ref. 29

Mercury 4.9 0.20

Zinc 396 J

1 2.0

NIMM-SW-7

(San Carlos Creek northern 

calcine pile)

Arsenic 1.9 1.0 Ref. 6, pp. 35, 1170-1171, 

1185, 1188, 1190; Ref. 14, p. 

4; Ref. 16, p. 8; Ref. 25, pp. 3, 

39, 62, 104; Ref. 27, pp. 54, 

37, 70, 71; Ref. 29

Cadmium 1.2 1.0

Copper 17.2 2.0

Mercury 4.8 0.20

Zinc 1,150 J

1 2.0

NIMM-SW-15

(duplicate of NIMM-SW-7)

Arsenic 1.3 1.0 Ref. 6, pp. 35, 1171, 1185, 

1189-1190; Ref. 14, p. 4; Ref. 

16, p. 8; Ref. 25, pp. 4, 45, 62, 

97, 104; Ref. 27, pp. 43, 54, 

70, 71; Ref. 29

Copper 18.0 2.0

Mercury 4.7 0.20

Zinc 1,270 J

1 2.0

NIMM-SW-8

(San Carlos Creek 0.6 miles 

from Site)

Arsenic 1.8 1.0 Ref. 6, pp. 35, 1170-1171, 

1185, 1188, 1190; Ref. 14, p. 

5; Ref. 16, p. 8; Ref. 25, pp. 3, 

40, 62, 96, 104; Ref. 27, pp. 

38, 54, 70, 71; Ref. 29

Copper 13.6 2.0

Mercury 4.6 0.20

Zinc 987 J

1 2.0

NIMM-SW-10

(San Carlos Creek 2.4 miles 

from Site)

Mercury 0.79 0.20

Ref. 6, pp. 35, 1170-1171, 

1185, 1188, 1190; Ref. 14, p. 

5; Ref. 16, p. 8; Ref. 25, pp. 3, 

42, 62, 96, 104; Ref. 27, pp. 

40, 54, 70, 71; Ref. 29 Zinc 176 J

1 2.0

μg/l : micrograms analyte per liter water

CRQL: Contract Required Quantitation Limit

J

1

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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

- Background Concentrations:

Dissolved Metals in Surface Water (μg/l)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL References

NIMM-SW-1

(Upper San Carlos Creek –

upstream of all contaminated 

samples)

Copper 0.4J1 2.0 Ref. 6, pp. 34, 1178-

1179; Ref. 14, p. 5; Ref. 

26, pp. 2, 32, 56, 82, 88; 

Ref. 28, pp. 31, 52, 68, 

70; Ref. 29

Mercury 0.20 U 0.20

Nickel 4.5 1.0

Selenium 5.0 U 5.0

Zinc 2.9 2.0

NIMM-SW-9

(Tributary leading to East Fork 

San Carlos Creek – upstream 

of all contaminated samples)

Copper 2.0 U 2.0 Ref. 6, pp. 34, 1178-

1179; Ref. 14, p. 5; Ref. 

26, pp. 3, 40, 56, 82, 88; 

Ref. 28, pp. 39, 52, 68, 

70; Ref. 29

Mercury 0.20 U 0.20

Nickel 5.1 1.0

Selenium 5.0 U 5.0

Zinc 2.4 2.0

BLM-SW-1

(Upper San Carlos Creek, 

Aurora Mine background –

upstream of all contaminated 

samples)

Copper 2.0 U 2.0 Ref. 6, pp. 34, 1179; Ref. 

14, p. 6; Ref. 16, p. 8, 18; 

Ref. 26, pp. 5, 45, 56, 82, 

89; Ref. 28, pp. 44, 52, 

68, 71; Ref. 29

Mercury 0.20 U 0.20

Nickel 3.7 1.0

Selenium 5.0 U 5.0

Zinc 1.5 J

1,2 (1.94) 2.0

BLM-SW-2

Upper San Carlos Creek 

adjacent to Aurora Mine –

upstream of all contaminated 

samples)

Copper 0.38 J

1 2.0

Ref. 6, pp. 34, 1175, 

1179; Ref. 14, p. 6; Ref. 

26, p. 5; Ref. 28, pp. 45, 

52, 68, 71; Ref. 29

Nickel 3.5 1.0

Selenium 5.0 U 5.0

Zinc 2.3 2.0

μg/l : micrograms analyte per liter water

CRQL: Contract Required Quantitation Limit

J

1

: Analyte detected at a level less than the CRQL. 

J

2

: 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.

J

3

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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

- Contaminated Samples:

Dissolved Metals in Surface Water Showing an Observed Release (μg/l)

Sample ID Hazardous 

Substance

Hazardous 

Substance 

Concentration

SQL Reference

NIMM-SW-3

(San Carlos Creek adjacent to southern 

calcine pile)

Mercury 0.30 0.20

Ref. 6, pp. 35, 1178-

1179; Ref. 14, p. 4; Ref. 

26, pp. 2, 34, 56, 82, 88; 

Ref. 29

NIMM-SW-6

(San Carlos Creek adjacent to southern 

calcine pile)

Nickel 231 1.0 Ref. 6, pp. 35, 1178-

1179, 1198, 1200; Ref. 

14, p. 4; Ref. 26, p. 3; 

Ref. 28, pp. 36, 52, 68, 

70; Ref. 29

Zinc 61.6 2.0

NIMM-SW-7

(San Carlos Creek northern calcine 

pile)

Copper 3.2 2.0 Ref. 6, pp. 35, 1178-

1179, 1195, 1199-1200; 

Ref. 14, p. 4: Ref. 16, p. 

8, 18; Ref. 26, p. 3; Ref. 

28, pp. 37, 52, 68, 70; 

Ref. 29

Nickel 175 1.0

Selenium 6.6 J

1 (5.79) 5.0

Zinc 66.1 2.0

NIMM-SW-15

(duplicate of NIMM-SW-7)

Nickel 172 1.0 Ref. 6, pp. 35, 1179, 

1195, 1200; Ref. 14, p. 4; 

Ref. 16, p. 8, 18; Ref. 26, 

p. 4; Ref. 28, pp. 43, 52, 

68, 70; Ref. 29

Selenium 6.3 J

1 (5.53) 5.0

Zinc 54.9 2.0

NIMM-SW-8

(San Carlos Creek 0.6 miles from Site)

Nickel 73.2 1.0 Ref. 6, pp. 35, 1178-

1179, 1199-1200; Ref. 

14, p. 5; Ref. 26, p. 3; 

Ref. 28, pp. 38, 52, 68, 

70; Ref. 29

Zinc 13.5 2.0

NIMM-SW-10

(San Carlos Creek 2.4 miles from Site)

Copper 2.1 2.0 Ref. 6, pp. 35, 1178-

1179, 1199-1200; Ref. 

14, p. 5; Ref. 26, p. 3; 

Ref. 28, pp. 40, 52, 68, 

70; Ref. 29

Nickel 33.5 1.0

Zinc 9.1 2.0

μg/l : micrograms analyte per liter water

CRQL: Contract Required Quantitation Limit

J

1

: 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:

( ) x DF

initial volume (mL)

final volume (mL) x Lμg SQL / CRDL 



 





μg L =

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
48 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 NIMMSD-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).
49 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-SW6, 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).
50 SWOF-Likelihood of Release

Hazardous Substances Released

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

Surface Water Observed Release Factor Value: 550
51 SWOF-Likelihood of Release

4.1.2.1.2 Potential to Release

Potential to Release was not scored, because an observed release was documented.
52 SWOF/DW-Waste Characteristics

4.1.2.2 Drinking Water Threat Waste Characteristics

4.1.2.2.1 Toxicity/Persistence

Hazardous 

Substance

Source 

No.

Toxicity

Factor

Value

Persistence

Factor

Value*

Toxicity/

Persistence

Factor Value 

(Ref. 1, Table 4-12)

Reference

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)
53 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)
54 SWOF/DW-Targets

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)
55 SWOF/HFC–Waste Characteristics

4.1.3.2 Human Food Chain Threat Waste Characteristics

4.1.3.2.1 Toxicity/Persistence/Bioaccumulation

Hazardous 

Substance

Source

No.

Toxicity 

Factor 

Value

Persistence 

Factor 

Value*

Bioaccumulation

Value**

Toxicity/

Persistence/

Bioaccumulation

Factor Value

(Ref. 1, Table 4-16)

Reference

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
56 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)
57 SWOF/HFC–Targets

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
 58 SWOF/ENV-Waste Characteristics

4.1.4.2 Environmental Threat Waste Characteristics

4.1.4.2.1 Ecosystem Toxicity/Persistence/Bioaccumulation

Hazardous 

Substance

Source

No.

Ecosystem

Toxicity Factor 

Value

Persistence 

Factor

Value*

Environmental

Bioaccumulation

Value**

Ecosystem Toxicity/

Persistence/Env.

Bioaccumulation

Factor Value

(Ref. 1, Table 4-21)

Reference

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
59 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) 
60 SWOF/ENV-Targets

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 
61 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).

Sensitive Environment

Distance from 

PPE to Nearest 

Sensitive 

Environment

References

Sensitive 

Environment

Value (Ref. 1,

Table 4-23)

Particular areas, relatively 

small in size, important to 

maintenance of unique biotic 

communities.

0 miles Ref. 13, pp. 5-6; 

Ref. 30, p. 1, 2 25

San Joaquin kit fox (Vulpes 

macrotis mutica) 0 miles Ref. 30, p. 2, 3; Ref. 32, p. 22 75

Giant kangaroo rat 

(Dipodomys ingens), 0 miles Ref. 30, p. 3; Ref. 32, p. 28 75

Blunt-nosed leopard lizard 

(Gambilia sila) 0 miles Ref. 30, p. 3; Ref. 32, p. 31 75

Sum of Level II Sensitive Environments Value: 250

Level II Wetland Frontages
62 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

San Carlos Creek 9.4 miles Ref. 6, p. 27; Ref. 13, pp. 4, 6, Figures 

1-9; Ref. 20

Silver Creek 30.4 miles Ref. 6, p. 29; Ref. 13, pp. 5, 6, Figures 

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
63

Attachments
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Map 1

Map 2

Map 3

Map 4

Map 5

Belmont Ave

Panoche Creek

Panoche

Creek

Silver Creek

Larious

Creek

San Carlos

Creek

Silver Creek

East Fork

San Carlos Creek

S 

urface Water Pathway

Flow Direction

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

2

Sample Locations - Map 1A

New Idria Mercury Mine

San Benito County, California

i[ Mercury Mines

!)i Other Mines

Property Boundary

Site Boundary (Inset)

Surface Water Pathway

Streams 

0 2,000

Feet

Reservoir

ppb = parts per billion

ppm = parts per million

ppt = parts per trillion

Hg = mercury

MMHg = methylmercury

Spanish

Mines

Aurora

Mine

North Star

Mine

San Carlos

Mine

Ranchito

Mine

Creek Pit Mine

Sulphur Spring

Mine

New Idria

Mine

Molina Mine

NIMM-SW-9/SD-11

Hg in sediment: <0.11R ppm

Hg in water: 0.10J ppb (total)

Hg in water: <0.20 ppb (dissolved)

NIMM-SS-2

Hg in soil: <0.10 ppm

NIMM-SS-1

Hg in soil: 0.19 ppm

NIMM-SD-1

Hg in sediment: 0.19 ppm

NIMM-SW-1

Hg in water: 0.084J ppb (total)

Hg in water: <0.20 ppb (dissolved)

BLM-SW-1

Hg in water: <0.20 ppb (total)

Hg in water: <0.20 ppb (dissolved)

BLM-SW-2

Hg in water: <020 ppb (total)

Hg in water: <0.20 ppb (dissolved)

BLM-SW-3

Hg in water: <0.20 ppb (total)

Hg in water: <0.20R ppb (dissolved)

NIMM-SW-2/SD-2 (Reservoir Water Sample)

Hg in sediment: 5.5 ppm

Hg in water: <0.20 ppb (total)

Hg in water: <0.20 ppb (dissolved)

MMHg in water: 0.245 ppt (dissolved)

Tributary

to

East Fork

San

Carlos 

Creek

po Surface Water Sample

po

!H Sediment & Surface

Water Samples

Legend

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Samples

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

3

Historic Sample Locations - Map 1B

New Idria Mercury Mine

San Benito County, California

Legend

"/ 1997 EPA Samples

![( USGS Samples

#0 RWQCB Samples

i[ Mercury Mines

!)i Other Mines

Property Boundary

Site Boundary (Inset)

Surface Water Pathway

Streams 

0 2,000

Feet

1997 EPA Sample SW-1

Hg in sediment: 1.5 ppm

Hg in water: 0.1 ppb

1999 USGS Sample 99AU-5

Hg in sediment: 2.13 ppm

Hg in water: 0.0224 ppb

1997 EPA Sample SW-2

Hg in sediment: 1.6 ppm

Hg in water: 0.1 ppb

1997 EPA Sample NIM-19

Hg in soil: 3.9 ppm

1999 USGS Sample 99AU-3

Hg in sediment: 0.83 ppm

Hg in water: 0.00084 ppb

1999 USGS Sample 99AU-4

Hg in water: 0.876 ppb

1999 USGS Sample 99AU-1

Hg in sediment: 12.07 ppm

Hg in water: 0.0364 ppb

1999 USGS Sample 99AU-2

Hg in sediment: >100 ppm

Hg in water: 0.00826 ppb

1999 USGS Sample 99AU-6

Hg in sediment: 61.4 ppm

Hg in water: 0.0215 ppb

1999 USGS Sample 99AU-7

Hg in sediment: 52.4 ppm

Hg in water: 0.0139 ppb

1997 EPA Sample SW-3

Hg in sediment: 2.6 ppm

Hg in water: 0.1 ppb

1997 EPA Sample NIM-18

Hg in soil: 1.4 ppm

1988 RWQCB Sample NIM6

Hg in water: ND <0.3 ppb

2003 RWQCB Sample 30115-5

Hg in water: ND <0.2 ppb

Reservoir

ppb = parts per billion

ppm = parts per million

Hg = mercury

Tributary

to

East Fork

San

Carlos 

Creek

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

4

Sample Locations - Map-1C

New Idria Mercury Mine

San Benito County, California

New Idria

Road

Clear Creek Road

Furnace

San

Carlos Creek

VT

Level 10 Adit

Site Boundary

Calcine Piles

Waste Rock Pile

Acid Mine Drainage

San Carlos Creek

0 400

Feet

ppb = parts per billion

ppm = parts per million

ppt = parts per trillion

Hg = mercury

MMHg = Methyl mercury

School Parcel

Northern

Tailing

Piles

Southern

Tailing

Piles

Waste

Rock

Pile

NIMM-SW-4/SD-4

Hg in sediment: 14.4 ppm

Hg in water: <0.20 ppb (total)

Hg in water: <0.20 ppb (dissolved) NIMM-SW-3/SD-3

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)

NIMM-SW-5/SD-7

Hg in sediment: 25.0 ppm

Hg in water: 21.2 ppb (total)

Hg in water: 11.2 ppb (dissolved)

MMHg in water:2.30 ppt (dissolved)

NIMM-SD-5

Hg in sediment: 12.1 ppm

NIMM-SD-6

Hg in sediment: 41.3 ppm NIMM-SW-6/SD-8

Hg in sediment: 4.7 ppm

Hg in water: 4.9 ppb (total)

Hg in water: 0.16J ppb (dissolved)

NIMM-TL-5

Hg in tailings: 446 ppm

NIMM-TL-4

Hg in tailings: 134 ppm

NIMM-TL-3

Hg in tailings: 191 ppm

NIMM-TL-6

Hg in tailings: 38.7 ppm

NIMM-TL-7

Hg in tailings: 3.8 ppm

NIMM-TL-8

Hg in tailings: 91.6 ppm

NIMM-TL-9

Hg in tailings: 72.3 ppm

NIMM-TL-10

Hg in tailings: 31.5 ppm

NIMM-SW-7/SD-9

Hg in sediment: 4.9 ppm

Hg in water: 4.8 ppb (total)

Hg in water: 0.14J ppb (dissolved)

MMHg in water: 0.210 ppt (dissolved)

Legend

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po

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Samples

Water Samples

Former

Unpermitted

Landfill

Level 10 Adit

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010.
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5

Historic Sample Locations - Map-1D

New Idria Mercury Mine

San Benito County, California

Former

Unpermitted

Landfill

New Idria

Road

Clear Creek Road

Furnace

San

Carlos Creek

"/ 1997 EPA Samples

![( USGS Samples

#0 RWQCB Samples

VT

Level 10 Adit

Site Boundary

Calcine Piles

Waste Rock Pile

Acid Mine Drainage

San Carlos Creek

0 400

Feet

1975 RWQCB Sample #2

Hg in AMD: 2.3 ppb

1988 RWQCB Sample NIM1

Hg in AMD: ND <0.3 ppb

2003 RWQCB Sample 30115-7

Hg in AMD: <0.2 ppb

1997 EPA Sample NIM-1

Hg in AMD: 0.2 ppb

1997 EPA Sample NIM-7

Hg in sediment: 1.2 ppm 1997 EPA Sample SW-4

Hg in sediment: 19.7 ppm

Hg in water: 0.54 ppb

1975 RWQCB Sample #1

Hg in water: 0.2 ppb

1988 RWQCB Sample NIM2

Hg in AMD: ND <0.3 ppb

1997 EPA Sample NIM-5

Hg in sediment: 0.4 ppb

1988 RWQCB Sample NIM4

Hg in water: 5.0 ppb

1997 EPA Sample SW-5

Hg in sediment: 8.4 ppm

Hg in water: 5.0 ppb

1975 RWQCB Sample #4

Hg in water: 4 ppb

1988 RWQCB Sample NIM5

Hg in water: ND <0.3 ppm

1997 EPA Sample NIM-6

Hg in sediment 11.7 ppm

1997 EPA Sample NIM-17

Hg in sediment 43 ppm

1997 USGS Sample 97NI-5W

Hg in water: 17.28 ppb

1997 USGS Sample 97NI-6W

Hg in sediment: 0.0211 ppm

Hg in water: 11.985 ppb

1975 RWQCB Sample #3

Hg in water: 2.7 ppb

1988 RWQCB Sample NIM3

Hg in water: 8.0 ppb

1997 USGS Sample 97NI-7W

Hg in water: 4.471 ppb

1999 USGS Sample 99AU-8

Hg in water: 0.372 ppb

1997 EPA Sample NIM-13

Hg in tailings 40.7 ppm

1997 EPA Sample NIM-15

Hg in tailings 27.1 ppm

1997 EPA Sample NIM-14

Hg in tailings 74.6 ppm

1997 EPA Sample NIM-16

Hg in tailings 42.2 ppm

1997 EPA Samples

Hg in Sediment 

NIM-8: 0.22 ppm

NIM-9: 4.2 ppm

NIM-10: 13.7 ppm

NIM-11: 8.5 ppm

NIM-12: 25.7 ppm

Hg in AMD

NIM-2: 0.2 ppb

NIM-3: 0.6 ppb

NIM-4: 16.5 ppb

ppb = parts per billion

ppm = parts per million

Hg = mercury

School Parcel

2003 RWQCB Sample 30115-8

Hg in AMD: ND <0.2 ppb

Northern

Tailing

Piles

Southern

Tailing

Piles

Waste

Rock

Pile

Legend

1997 EPA Samples

Hg in Sediment 

NIM-8: 0.22 ppm

NIM-9: 4.2 ppm

NIM-10: 13.7 ppm

NIM-11: 8.5 ppm

NIM-12: 25.7 ppm

Hg in AMD

NIM-2: 0.2 ppb

NIM-3: 0.6 ppb

NIM-4: 16.5 ppb

Level 10

Adit

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010.
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NIMM-SW-8/SD-10

Hg in sediment: 32.6 ppm

Hg in water: 4.6 ppb (total)

Hg in water: 0.14J ppb (dissolved)

NIMM-SW-10/SD-12

Hg in sediment: 8.9J ppm

Hg in water: 0.79 ppb (total)

Hg in water: <0.20 ppb (dissolved)

NIMM-SD-13

Hg in sediment: 2.0J ppm

NIMM-SW-12/SD-14 (attribution)

Hg in sediment: 0.081J ppm

Hg in water: <0.20 ppb (total)

Hg in water: <0.20 ppb (dissolved)

MMHg in water: 0.086 ppt (dissolved)

NIMM-SW-14/SD-16

Hg in sediment: 1.8J ppm

Hg in water: <0.20 ppb (total)

Hg in water: <0.20 ppb (dissolved)

MMHg in water: 0.072 ppt (dissolved)

NIMM-SD-15

Hg in sediment: 4.6J ppm

Figure

6

Sample Locations - Map 2A

New Idria Mercury Mine

San Benito County, California

i[ Mercury Mines

!)i Other Mines

Surface Water Pathway

Streams

0 2,000

Feet

East Fork San Carlos Creek

ppb = parts per billion

ppm = parts per million

ppt = parts per trillion

Hg = mercury

MMHg = methyl mercury

Larious Creek

Magnetite

Mine

Gypsum

Mine

Legend

!. Sediment Sample

po

!H Sediment & Surface

Water Samples

Citation: Weston Solutions, Inc., Expanded Site Inspection Report, New Idria Mercury Mine, San Benito County, California, October 2010.
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1997 EPA Sample SW-7

Hg in sediment: 1.5 ppm

Hg in water: 0.1 ppb

1999 USGS Sample 99NI-3W

Hg in sediment: 0.0278 ppm

Hg in water: 13.55 ppb

1988 RWQCB Sample NIM7&8

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

Hg in water: 9.6 ppb

1997 EPA Sample SW-8

Hg in sediment: 1.6 ppm

Hg in water: ND

1997 EPA Sample SW-9

Hg in sediment: 2.2 ppm

Hg in water: 12 ppb 1999 USGS Sample 99NI-2W

Hg in sediment: 0.0279 ppm

Hg in water: 10.16 ppb

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-13

Hg in sediment: 5.6 ppm

Hg in water: 6.9 ppb

1997 EPA Sample SW-12

Hg in sediment: 0.11 ppm

1997 EPA Sample SW-10

Hg in sediment: 22.5 ppm

Hg in water: 3.8 ppb

1997 EPA Sample SW-14

Hg in sediment: 9.7 ppm

Hg in water: 0.47 ppb

Figure

7

Historic Sample Locations - Map 2B

New Idria Mercury Mine

San Benito County, California

Legend

"/ 1997 EPA Samples

![( USGS Samples

#0 RWQCB Samples

i[ Mercury Mines

!)i Other Mines

Surface Water Pathway

Streams

0 2,000

Feet

East Fork San Carlos Creek

ppb = parts per billion

ppm = parts per million

Hg = mercury

Larious Creek

Magnetite

Mine

Gypsum

Mine

1997 USGS Sample 97NI-4W

Hg in sediment: 0.0325 ppm

Hg in water: 13.5 ppb

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

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Figure

8

Sample Lcoations - Map 3

New Idria Mercury Mine

San Benito County, California

Legend

!. Sediment Samples

Surface Water Pathway

Streams

0 2,000

Feet

NIMM-SD-17

Hg in sediment: 3.9J ppm

ppb = parts per billion

ppm = parts per million

Hg = mercury

NIMM-SD-18

Hg in sediment: 2.7J ppm

NIMM-SD-19

Hg in sediment: 0.070J ppm

NIMM-SD-20

Hg in sediment: 11.2J ppm

NIMM-SD-21

Hg in sediment: 1.3J ppm

NIMM-SD-22

Hg in sediment: 0.60J ppm

Silver Creek

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

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Figure

9

Sample Locations- Map 4A

New Idria Mercury Mine

San Benito County, California

Legend

!. Sediment Samples

Surface Water Pathway

Streams

0 2,000

Feet

ppm = parts per million

Hg = mercury

NIMM-SD-23

Hg in sediment: 0.38J ppm

NIMM-SD-24

Hg in sediment: 4.2J ppm

NIMM-SD-25

Hg in sediment: 2.4J ppm

NIMM-SD-26

Hg in sediment: 1.7J ppm

NIMM-SD-27

Hg in sediment: 1.5J ppm

NIMM-SD-28

Hg in sediment: 1.1J ppm

NIMM-SD-29

Hg in sediment: 2.3J ppm

NIMM-SD-30

Hg in sediment: 1.2 ppm

San Benito County

Fresno County

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

Figure

10

Historic Sample Locations- Map 4B

New Idria Mercury Mine

San Benito County, California

Legend

"/ 1997 EPA Samples

Surface Water Pathway

Streams

0 2,000

Feet

ppm = parts per million

Hg = mercury

1997 EPA Sample SW-15

Hg in sediment: 1.5 ppm

San Benito County

Fresno County

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

!.

!.

!.

P

Figure

11

Sample Locations - Map 5A

New Idria Mercury Mine

San Benito County, California

Legend

!. Sediment Samples

i[ Mercury Mines

!)i Other Mines

Surface Water Pathway

Streams 

0 2,000

Feet

ppb = parts per billion

ppm = parts per million

Hg = mercury

NIMM-SD-31

Hg in sediment: 1.0 ppm

NIMM-SD-33

Hg in sediment: 0.050J ppm

NIMM-SD-32

Hg in sediment: 1.0 ppm

NIMM-SD-34

Hg in sediment: 1.1 ppm

Diatomite

Mine

Silver Creek

Panoche Creek

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

"/

"/

"/

P

Figure

12

Historic Sample Locations - Map 5B

New Idria Mercury Mine

San Benito County, California

Legend

"/ 1997 EPA Samples

i[ Mercury Mines

!)i Other Mines

Surface Water Pathway

Streams 

0 2,000

Feet

ppb = parts per billion

ppm = parts per million

Hg = mercury

1997 EPA Sample SW-17

Hg in sediment: 0.14 ppm

Hg in water: 0.2 ppb

Diatomite

Mine

Silver Creek

Panoche Creek

1997 EPA Sample SW-16

Hg in sediment: 1.5 ppm

Hg in water: 0.2 ppb

1997 EPA Sample SW-18

Hg in sediment: 0.13 ppm

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