Attachment_3_MOJA_Hidden_Hill_mine_EECA_3-7-17.pdf

ENGINEERING EVALUATION/COST ANALYSIS

Hidden Hills Mine

Mojave National Preserve

P15PD02788

EDL# 5PWR2428

March 7, 2017

Prepared for:

Mojave National Preserve

2701 Barstow Road

Barstow, CA 92311

Prepared by:

Resource Environmental Management Consultants, Inc.

8496 S Harrison Street, Suite 102

Midvale, Utah 84047

801-255-2626

Hidden Hills Mine EE/CA Page i

Contents

1.0 EXECUTIVE SUMMARY

2.0 SITE CHARACTERIZATION

2.1 SITE DESCRIPTION AND BACKGROUND INFORMATION

2.2 PREVIOUS INVESTIGATIONS AND SITE ACTIVITIES

2.3 SOURCE, NATURE AND EXTENT OF CONTAMINATION

2.3.1 VSP Soil Sampling and Groundwater Sampling

2.3.2 Soil Sampling Results

2.3.3 Hazardous Waste Characterization Results

2.3.4 Groundwater Sampling Results

2.3.5 Data Validation

2.4 PRELIMINARY REMEDIAL GOALS

2.5 RISK EVALUATION

2.5.1 Human Health Risk Evaluation

2.5.2 Ecological Risk Evaluation

3.0 IDENTIFICATION OF REMOVAL ACTION OBJECTIVES

3.1 STATUTORY LIMITS ON REMOVAL ACTIONS

3.2 SCOPE OF THE REMOVAL FOR THE SITE

3.3 POTENTIAL SCHEDULES FOR THE REMOVAL AT THE SITE

3.4 PLANNED REMOVAL/REMEDIAL ACTIVITIES

3.5 REMOVAL ACTION OBJECTIVES

3.6 APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS

3.6.1 Contaminant-Specific, Location-Specific and Action Specific Requirements

3.6.2 Definitions of “Applicable” and “Relevant and Appropriate”

3.7 SUMMARY OF POTENTIAL ARARS

4.0 IDENTIFICATION AND ANALYSIS OF REMOVAL ACTION ALTERNATIVES

4.1 EVALUATION CRITERIA

4.2 HIDDEN HILLS MINE ALTERNATIVES

4.2.1 Hidden Hills Mine Alternative 1 – No Action

4.2.2 Hidden Hills Mine Alternative 2 – Institutional Controls

4.2.3 Hidden Hills Mine Alternative 3 – Off-Site Disposal of Tailings, Placement of Cover Soils Over Waste

Rock

4.2.4 Hidden Hills Mine Alternative 4 – Full Removal and Off-Site Disposal

5.0 COMPARATIVE ANALYSIS OF REMOVAL ACTION ALTERNATIVES

5.1 EFFECTIVENESS CRITERIA

5.2 IMPLEMENTABILITY CRITERIA

5.3 COSTS CRITERIA

5.4 COMPARISONS

6. 0 RECOMMENDED REMOVAL ACTION ALTERNATIVE

7.0 REFERENCES

Hidden Hills Mine EE/CA Page ii

FIGURES

Figure 1 Regional Map

Figure 2 Site Location Map

Figure 3 Sample Location Map (Sheets 1 through 3)

Figure 4 Conceptual Site Model

Figure 5 Alternative 2 Map (Sheets 1 and 2)

Figure 6 Alternative 3 Map (Sheets 1 and 2)

Figure 7 Alternative 4 Map (Sheet 1 and 2)

Tables

Table 1 Site Characterization Soil Sampling Results

Table 2 Site Characterization STLC and TCLP Sampling Results

Table 3 Groundwater Sampling Results

Table 4 Groundwater Sampling Field Parameters

Table 5 Soil and Groundwater Samples included in Quality Assurance Review

Table 6 Field Duplicate Soil Sample Results and RPDs

Table 7 Field Duplicate Groundwater Sample Results and RPDs

Table 8 Waste Rock and Tailings Pile Dimensions

Table 9 Applicable or Relevant and Appropriate Requirements (ARARs)

Table 10 Hidden Hills Mine Alternative 2 Costs

Table 11 Hidden Hills Mine Alternative 3 Costs

Table 12 Hidden Hills Mine Alternative 4 Costs

Table 13 Hidden Hills Mine Comparison of Alternatives

Table 14 Human Health Risk Evaluation Screening Criteria

Table 15 Ecological Risk Evaluation Screening Criteria

Table 16 Soil COPCs

Table 17 Area Use Factors

Table 18 Site Specific TRVs and SLs

Table 19 Total Cancer Risk to Humans

Table 20 Proposed Soil Preliminary Remedial Goals (PRGs)

APPENDICES

Appendix A Analytical Laboratory Reports

Appendix B Data Validation Report

Hidden Hills Mine EE/CA Page 1

1.0 EXECUTIVE SUMMARY

This Engineering Evaluation/Cost Analysis (EE/CA) has been prepared by Resource

Environmental Management Consultants, Inc. (RMC) to address metals contamination at the

Hidden Hills Mine located in Mojave National Preserve (MOJA, Site).

This EE/CA provides supporting documentation for a non-time critical removal action (NTCRA) at the Site. The EE/CA is being conducted in accordance with Guidance on Conducting Non-

Time Critical Removal Actions under CERCLA (EPA, 1993).

The EE/CA is a streamlined focused document that provides site characterization data, assesses ecological and human health risks, evaluates various response alternatives, recommends a preferred response alternative and provides a vehicle for public involvement. This EE/CA was conducted in accordance with the MOJA-approved EE/CA Sampling and Analysis Plan (RMC, 2016).

Regional and Site Location Maps for the Hidden Hills Mine are presented in Figures 1 and 2.

The environmental threat at the Site consists of metals-impacted soils from historic mining operations. One source of contamination has been identified in the EE/CA:

Arsenic, barium, cadmium, copper, lead, silver, vanadium and zinc in tailings, waste rock, and soil.

One Removal Action Objective (RAO) has been established for the Site:

Eliminate the potential for metals impacts to human health and the environment.

The EE/CA analyzed the following removal action alternatives for the Site:

Alternative 1 – No Action;

Alternative 2 – Institutional Controls;

Alternative 3 – Off-Site Disposal of Tailings, Placement of Cover Soils Over Waste

Rock; and

Alternative 4 – Full Removal and Off-Site Disposal.

Alternative 4 is recommended as the preferred Removal Action Alternative. Alternative 4 consists of full removal and off-site disposal of soils exceeding Preliminary Remedial Goals

(PRGs).

Hidden Hills Mine EE/CA Page 2

2.0 SITE CHARACTERIZATION

This section describes Site background, characterization of impacts, risk evaluation and the development of Preliminary Remedial Goals (PRGs).

2.1 Site Description and Background Information

The Site is a relatively small and remote abandoned mine in southern Mojave National Preserve, totaling approximately five acres. The Site is located on the southeast slope of the Providence

Mountains, approximately 4 miles north of Interstate 40. The geographic coordinates of the Site are 34.80613 degrees north latitude and -115.545029 degrees west longitude. The Site is located in the southeast and southwest ¼’s of Section 6, Township 8 North, Range 14 East of the San

Bernardino Meridian. The elevation of the Site is approximately 3,400 feet above sea level. The

Site’s Environmental Disposal and Liabilities (EDL) number is 5PWR2428.

The Site has low visitation. The Site consists of a cabin, a mill foundation, an ore hopper, crumbled rock walls of a possible storage shed, ten open shafts and stopes, three open adits, three bulldozer cuts, ten waste rock piles below the shafts, and one tailings pile behind a small rock dam.

There is an active windmill approximately 0.2 miles south of the mill foundation. The windmill is owned by a local rancher who owns the grazing rights for the area and is being used to supply water for cattle. There are no permanent or perennial surface water bodies in the area. There are no down-gradient water wells that are being used for human consumption within 10 miles of the site. The nearest drinking water well used by humans is located on the California State Parks’

Providence Mountains Recreation Area and is approximately 9 miles from the site. The

Providence Mountains Recreation Area well is up-gradient of the Site and at the higher elevation than the Site.

There are a number of dirt roads that access the Hidden Hills Mine. The most direct route is approximately 5 miles long via Hidden Hills Road.

2.2 Previous Investigations and Site Activities

There have been two previous investigations conducted at the Site as follows:

2001 Preliminary Assessment, Chemical Hazards Associated with the Abandoned Mine

Land Study, Characterization and Prioritization, Mojave National Preserve, CA (BLM, 2001).

Hidden Hills Mine EE/CA Page 3

Park-Wide Inspection of Inactive Historical Mines and Mills at Mojave National

Preserve, San Bernardino County, CA (Tetra Tech, 2005).

The results of both investigations stated that existing data and information indicate a threat to human health, the surrounding environment, or local wildlife (BLM, 2001, and Tetra Tech, 2005). Samples were collected during the completion of the Park-Wide Inspection of Inactive

Historical Mines and Mills at Mojave National Preserve. Laboratory analytical results indicated that concentrations of antimony, arsenic, barium, cadmium, copper, lead, mercury, selenium, silver, vanadium, and zinc exceeded current NPS Ecological Screening Values (NPS, 2014b).

2.3 Source, Nature and Extent of Contamination

This section presents the results of site characterization activities conducted as part of this

EE/CA. All site activities and data analysis were conducted in accordance with the Sampling and Analysis Plan (RMC, 2016).

Soil concentrations are compared to the following action levels:

Human health:

‒ USEPA Region 9 Residential Regional Screening Levels are presented in Table 1.

(USEPA 2015).

Ecological:

‒ NPS Ecological Screening Value (ESV) for terrestrial plants is presented in Table 1

(NPS 2014b);

‒ NPS ESV for soil invertebrates is presented in Table 1 (NPS 2014b);

‒ NPS ESV for mammals is presented in Table 1 (NPS 2014b);

‒ NPS ESV for birds is presented in Table 1 (NPS 2014b);

‒ CalEPA, Office of Environmental Health Hazard Assessment CHHSLs are presented in Table 15 (http://oehha.ca.gov/risk/chhsltable.html);

‒ CalEPA State Water Board Environmental Screening Levels are presented in Table

15;

(http://www.waterboards.ca.gov/sanfranciscobay/water_issues/programs/esl.shtml)

Under CERCLA, concentrations of contaminants of concern below the naturally occurring background levels are not generally subject to a removal action. Contaminants that exceeded the site-specific background concentrations and/or recommended action levels (EPA RSLs, NPS

ESVs, or Site Specific Screening Levels) were considered COPCs and used for the preparation of the Risk Assessment. Laboratory detection limits were below all of the screening levels presented above.

http://oehha.ca.gov/risk/chhsltable.html http://www.waterboards.ca.gov/sanfranciscobay/water_issues/programs/esl.shtml

Hidden Hills Mine EE/CA Page 4

2.3.1 VSP Soil Sampling and Groundwater Sampling

Site characterization utilized the VSP (Visual Sample Plan) soil sampling program which determined the number of samples and sample locations. Sample locations were adjusted in the field based on real-time XRF screening for silver, arsenic, cadmium, chromium, copper, nickel, lead, antimony, selenium, zinc, and mercury and based on field observations of waste rock and tailings pile locations and sizes. Soil samples consisted of five- to ten-point composites collected between 0- to 2-inch soil depths. The number of composites per sample was based on the size of each mine feature. Dimensions of waste rock and tailings piles were measured, as outlined in the approved Sampling and Analysis Plan, during sample collection. There was no evidence that tailings and waste rock piles extended below the native soil interface. Thus, the depth of tailings and waste rock piles could be readily measured without the use of borings or test pit excavations.

Sample locations are presented in Figure 3, Sheets 1-3. Multiple sample locations were adjusted based on field reconnaissance.

Waste Rock piles 1 and 2 were determined to be one feature and samples were combined to be one 10-point composite sample named MOJA-WR-1/2 (see Figure 3, Sheet 2). Three distinct waste rock piles were located at MOJA-WR-9 (see Figure 3, Sheet 3). The sample was collected as a composite of the three waste rock piles. The original MOJA-SO-15 sample was located in an area of the wash consisting of bedrock. The sample was moved downstream to the depositional area near the confluence with the wash below the mill area. MOJA-WR-5 is a large complex consisting of five waste rock piles. A composite sample was collected from the largest and most contaminated waste rock pile. Contamination was determined through XRF analysis. MOJA-

WR-4 is a complex of three piles in a line down slope. Upper piles appeared to have resulted from a dozer cut. The middle pile contained the greatest amount of contamination based on XRF analysis and was where the sample was collected. MOJA-SO-17 was moved upstream to the

MOJA-SO-18 sample location. MOJA-SO-18 was deleted. Figure 3 presents sample locations following all changes made in the field.

Site characterization soil sample results are presented in Table 1. Waste rock, tailings, and contaminated soil dimensions are presented in Table 8.

Soluble Threshold Limit Concentration (STLC) and Toxicity Characteristic Leaching Procedure

(TCLP) sample results are presented in Table 2. STLC and TCLP laboratory analysis was performed on the tailings sample.

One groundwater sample was collected from the active windmill located 0.2 miles south of the

Site. Groundwater sample results are presented in Table 3.

Hidden Hills Mine EE/CA Page 5

Quality Assurance/Quality Control (QA/QC) sampling at each Site consisted of the collection of the following samples:

Three QA/QC field duplicates were collected for laboratory analysis, which accounted for 11% of the total soil sample load. One QA/QC field duplicate was collected for groundwater. QA/QC soil sample results are presented in Table 6. QA/QC groundwater sample results are presented in

Table 7.

2.3.2 Soil Sampling Results

Soil sample locations are presented in Figure 3. Site characterization soil sample results are presented in Table 1. Waste rock and tailings pile dimensions are presented in Table 8.

Twenty-eight soil samples were collected during Site characterization. Background soil and mineralized rock samples were collected by Tetra Tech during the completion of the Park-wide

Inspection (Tetra Tech, 2005). RMC utilized the background soil and mineralized bedrock sample results presented in the Park-wide Inspection during the completion of this EE/CA.

Sample results were compared to current NPS Ecological Screening Values (ESV), USEPA

Region 9 RSLs, and background soil and mineralized rock concentrations. Analytical results are discussed below:

Antimony: Concentrations of antimony ranged from non-detect to 7.2 mg/kg. The background soil sample contains 1.7 mg/kg and the mineralized bedrock sample contains 7.5 mg/kg.

Background soil and mineralized bedrock exceed NPS ESVs for Mammals (0.27 mg/kg). All sample results were below the NPS ESV for Soil Invertebrates (79 mg/kg) and the USEPA

Region 9 Residential Regional Screening Level (31 mg/kg).

Arsenic: Concentrations of arsenic ranged from 3.8 mg/kg to 69.4 mg/kg. The background soil contains 11 mg/kg and the mineralized bedrock contains 11.2 mg/kg. Background soil and mineralized bedrock exceed the USEPA Region 9 Residential Regional Screening Level (0.7 mg/kg). Two soil samples (MOJA-SO-8; MOJA-SO-9), the tailings sample (MOJA-TA-1), and five waste rock samples (MOJA-WR-1/2; MOJA-WR-3; MOJA-WR-4; MOJA-WR-7; MOJA-

WR-8) exceed the NPS ESV for Terrestrial Plants (18 mg/kg). Three waste rock samples

(MOJA-WR-1/2; MOJA-WR-3; MOJA-WR-4) exceed the NPS ESV for Mammals (46 mg/kg) and for Birds (43 mg/kg).

Barium: Concentrations of barium ranged from 148 mg/kg to 1,230 mg/kg. The background soil contains 170 mg/kg and mineralized bedrock contains 588 mg/kg. Background soil and mineralized bedrock exceed the NPS ESV for Soil Invertebrates (40 mg/kg) and Mammals (21

Hidden Hills Mine EE/CA Page 6 mg/kg). All samples were below the USEPA Region 9 Residential Regional Screening Level

(15,000 mg/kg).

Cadmium: Concentrations of cadmium ranged from non-detect to 2.7 mg/kg. The background soil contains 0.63 mg/kg and the mineralized bedrock contains 1.3 mg/kg. Background soil and mineralized bedrock exceed the NPS ESV for Mammals (0.36 mg/kg). All samples were below the USEPA Region 9 Residential Regional Screening Level (71 mg/kg), NPS ESV for Terrestrial

Plants (32 mg/kg), and NPS ESV for Soil Invertebrates (140 mg/kg). One waste rock sample

(MOJA-WR-8) exceeded the NPS ESV for Birds (0.77 mg/kg).

Chromium: Concentrations of chromium ranged from 2.8 to 21.6 mg/kg. The background soil contains 15.4 mg/kg and the mineralized bedrock contains 15 mg/kg. There are no applicable screening values for chromium.

Copper: Concentrations of copper ranged from 39.7 to 5,410 mg/kg. The background soil contains 25.8 mg/kg and the mineralized bedrock contains 159 mg/kg. Background soil and mineralized bedrock exceed the NPS ESV for Terrestrial Plants (70 mg/kg), Soil Invertebrates

(80 mg/kg), Mammals (49 mg/kg), and Birds (28 mg/kg). One tailings sample (MOJA-TA-1) exceeds the USEPA Region 9 Residential Screening Level (3,100 mg/kg).

Lead: Concentrations of lead ranged from 6.3 to 155 mg/kg. The background soil contains 15.6 mg/kg and the mineralized bedrock contains 10.5 mg/kg. Background soil and mineralized bedrock sample results exceed the NPS ESV for Birds (11 mg/kg). All samples are below the

NPS ESV for Soil Invertebrates (1,700 mg/kg) and the USEPA Region 9 Residential Screening

Level (400 mg/kg). One soil sample (MOJA-SO-8), one tailings sample (MOJA-TA-1), and five waste rock samples (MOJA-WR-1/2; MOJA-WR-3; MOJA-WR-6; MOJA-WR-8; MOJA-WR-

9) exceed the NPS ESV for Mammals (56 mg/kg). One tailings sample (MOJA-TA-1) and one waste rock sample (MOJA-WR-9) exceed the NPS ESV for Terrestrial Plants (120 mg/kg).

Nickel: Concentrations of nickel ranged from 5.7 to 26.3 mg/kg. The background soil contains

8.8 mg/kg and the mineralized bedrock contains 110.3 mg/kg. Background soil and mineralized bedrock sample results were below all screening values. All samples are below the NPS ESV for

Terrestrial Plants (38 mg/kg), Soil Invertebrates (280 mg/kg), Mammals (130 mg/kg), Birds (210 mg/kg), and the USEPA Region 9 Residential Regional Screening Level (670 mg/kg).

Selenium: Concentrations of selenium were all below the detection limit. The background soil contains 0.75 mg/kg and the mineralized bedrock contains 0.74 mg/kg. All samples are below the NPS ESV for Terrestrial Plants (0.52 mg/kg), Soil Invertebrates (4.1 mg/kg), Mammals (0.6 mg/kg), Birds (1.2 mg/kg), and the USEPA Region 9 Residential Regional Screening Level (390 mg/kg).

Hidden Hills Mine EE/CA Page 7

Silver: Concentrations of silver ranged from 0.26 to 14.5 mg/kg. The background soil contains

0.83 mg/kg and the mineralized bedrock contains 1.0 mg/kg. All samples are below the NPS

ESV for Terrestrial Plants (560 mg/kg), Mammals (14 mg/kg), Birds (4.2 mg/kg), and the

USEPA Region 9 Residential Regional Screening Level (390 mg/kg).

Vanadium: Concentrations of vanadium ranged from 18.1 to 64.7 mg/kg. The background soil contains 43.3 mg/kg and the mineralized bedrock contains 34.2 mg/kg. Background and mineralized bedrock sample results exceed the NPS ESV for Birds (7.8 mg/kg) and Terrestrial

Plants (2 mg/kg). All samples are below the NPS ESV for Mammals (280 mg/kg), and the

USEPA Region 9 Residential Regional Screening Level (390 mg/kg).

Zinc: Concentrations of zinc ranged from 37.6 to 356 mg/kg. The background soil contains 42.3 mg/kg and the mineralized bedrock sample contains 48.3 mg/kg. Background soil and mineralized bedrock sample results exceed the NPS ESV for Birds (46 mg/kg). All samples are below the USEPA Region 9 Residential Regional Screening Value (23,000 mg/kg). Two soil samples (MOJA-SO-14; MOJA-SO-9), and three waste rock samples (MOJA-WR-7; MOJA-

WR-8; MOJA-WR-9) exceed the NPS ESV for Mammals (79 mg/kg). One waste rock sample

(MOJA-WR-8) exceeds the NPS ESV for Terrestrial Plants (160 mg/kg) and Soil Invertebrates

(120 mg/kg).

Mercury: Concentrations of mercury ranged from 0.061 to 1.9 mg/kg. The background soil contains 0.03 mg/kg and the mineralized bedrock sample contains 0.16 mg/kg. All samples were below the USEPA Residential Regional Screening Level.

2.3.3 Hazardous Waste Characterization Results

STLC and TCLP sample results are presented in Table 2. Sample results were compared to State of California (Title 22 TTLC & STLC) and Federal (RCRA-TCLP) Hazardous Waste Criteria.

STLC sample results are below detection limits for antimony, cadmium, selenium, silver, and mercury. TCLP sample results are below detection limits for antimony, arsenic, cadmium, chromium, nickel, selenium, silver, vanadium, and mercury. All other results are discussed below.

Arsenic: STLC results for arsenic were 0.228 mg/L, which is below the State of California Title

22 STLC Hazardous Waste Criteria (5 mg/L). TCLP results for arsenic were below detection limits.

Hidden Hills Mine EE/CA Page 8

Barium: STLC results for barium were 12.1 mg/L, which is below the State of California Title

22 STLC Hazardous Waste Criteria (100 mg/L). TCLP results for barium were 1.7 mg/L, which is below the Federal RCRA TCLP Hazardous Waste Criteria (100 mg/L).

Chromium: STLC results for chromium were 0.215 mg/L, which is below the State of California

Title 22 STLC Hazardous Waste Criteria (560 mg/L). TCLP results for chromium were below detection limits.

Copper: STLC results for copper were 305 mg/L, which exceeds the State of California Title 22

STLC Hazardous Waste Criteria (25 mg/L). TCLP results for copper were 37.9 mg/L. Federal

RCRA TCLP Hazardous Waste Criteria do not have a standard for copper.

Lead: STLC results for lead were 3.34 mg/L, which is below the State of California Title 22

STLC Hazardous Waste Criteria (5 mg/L). TCLP results for lead were 0.34 mg/L, which is below the Federal RCRA TCLP Hazardous Waste Criteria (5 mg/L).

Nickel: STLC results for nickel were 0.126 mg/L, which is below the State of California Title 22

STLC Hazardous Waste Criteria (20 mg/L). TCLP results for nickel were below detection limits.

Vanadium: STLC results for vanadium were 0.204 mg/L, which is below the State of California

Title 22 STLC Hazardous Waste Criteria (24 mg/L). TCLP results for vanadium were below detection limits.

Zinc: STLC results for zinc were 0.705 mg/L, which is below the State of California Title 22

STLC Hazardous Waste Criteria (250 mg/L). TCLP results for zinc were 0.154 mg/L. Federal

RCRA TCLP Hazardous Waste Criteria do not have a standard for zinc.

2.3.4 Groundwater Sampling Results

One groundwater sample was collected from the windmill located 0.2 miles south of the Site.

Groundwater sample results are presented in Table 3. Field parameters of the groundwater sample are presented in Table 4.

Sample results were compared to current chronic and acute NPS Freshwater ESVs for Aquatic

Receptors and EPA drinking water Maximum Contaminant Levels (MCLs). Neither the NPS

Freshwater ESVs nor EPA MCLs are considered applicable to the Site, but are presented for completeness and comparison purposes. Groundwater concentrations are below detection limits for antimony, arsenic, cadmium, chromium, nickel, selenium, silver, and vanadium. All other metals are discussed below. Low metals concentrations in the groundwater sample and the reported depth to groundwater (Tetra Tech, 2005) indicates that the Site is not impacting

Hidden Hills Mine EE/CA Page 9 groundwater and the metals present are the result of the naturally mineralized nature of bedrock at the Site.

Barium: Concentration of barium is 21.8 µg/L, exceeding the chronic Freshwater ESV for

Aquatic Receptors (4 µg/L). The barium concentration is below the acute Freshwater ESV for

Aquatic Receptors (110 µg/L) and the EPA drinking water MCL (2,000 µg/L).

Copper: Concentration of copper is 17.8 µg/L, exceeding the chronic Freshwater ESV for

Aquatic Receptors (1.6 µg/L). The copper concentration is below the EPA drinking water MCL

(1,300 µg/L).

Lead: Concentration of lead is 5.8 µg/L, exceeding the chronic Freshwater ESV for Aquatic

Receptors (1 µg/L). The lead concentration is below the acute Freshwater ESV for Aquatic

Receptors (24 µg/L) and the EPA drinking water MCL (15 µg/L).

Zinc: Concentration of zinc is 35.7 µg/L. The zinc concentration is below the chronic Freshwater

ESV for Aquatic Receptors (54 µg/L) and the acute Freshwater ESV for Aquatic Receptors (54 µg/L).

Mercury: Concentration of mercury is 0.53 µg/L. The zinc concentration is below the chronic

Freshwater ESV for Aquatic Receptors (0.77 µg/L), the acute Freshwater ESV for Aquatic

Receptors (1.4 µg/L), and the EPA drinking water MCL (2 µg/L).

Field water quality parameters are presented in Table 4 and are typical of groundwater.

2.3.5 Data Validation

A Data Validation Review is presented in Appendix B. The results of the Data Validation

Review indicate that overall, the analytical data are of good quality and acceptable for use.

2.4 Preliminary Remedial Goals

Preliminary Remedial Goals (PRGs) are based on screening levels established by the NPS (for ecological receptors) and the EPA (for human receptors) and site specific screening levels calculated based on Toxicity Reference Values. Toxicity Reference Values and Site Specific

Screening Levels (SSSLs) are presented in Table 18. PRGs were selected only for Contaminants of Potential Concern (COPCs). COPCs were selected based on hazard quotients presented in

Table 16. Hazard quotients and COPCs are further discussed in Section 2.5. These levels are general values that have been determined to be protective of human health and the environment as discussed below.

Hidden Hills Mine EE/CA Page 10

The soil PRGs proposed for the Site are presented below. Screening values were compared to mineralized bedrock and background soil concentrations. Under CERCLA, concentrations of contaminants of concern below the naturally occurring background levels are not generally subject to a removal action. Contaminants that exceeded the site-specific background concentration and/or recommended action levels (EPA RSLs, NPS ESVs, SSSLs) were considered COPCs and used for the preparation of the Risk Assessment. The proposed soil PRGs are the most restrictive applicable screening value, background soil, or mineralized bedrock concentration. Proposed soil PRGs are presented in Table 20.

Antimony: Proposed soil PRG for antimony is 7.5 mg/kg based on the mineralized bedrock concentration. The background soil contains 1.7 mg/kg. Therefore, the NPS ESV for Mammals (0.27 mg/kg) is not applicable to the Site. The Eco-SSSL for Mammals (7.2 mg/kg) is below the mineralized bedrock concentration and is not applicable. The background soil and mineralized bedrock concentrations are below the USEPA Region 9

Residential Screening Level (31 mg/kg) and the NPS ESV for Soil Invertebrates (78 mg/kg).

Arsenic: Proposed soil PRG for arsenic is 11.2 mg/kg based on the mineralized bedrock concentration. The background soil contains 11 mg/kg. Therefore, the USEPA Region 9

Residential Regional Screening Level (0.7 mg/kg) and the Eco-SSSL for Mammals (2.83 mg/kg) are not applicable to the Site. The background soil and mineralized bedrock concentrations are below the NPS ESV for Terrestrial Plants (18 mg/kg), for Mammals

(46 mg/kg), for Birds (43 mg/kg) and the Eco-SSSL for Birds (14 mg/kg).

Barium: Proposed soil PRG for barium is 588 mg/kg based on the mineralized bedrock concentration. The background soil contains 170 mg/kg. Therefore, the NPS ESVs for

Soil Invertebrates (40 mg/kg) and Mammals (21 mg/kg), and the Eco-SSSL for Mammals

(305 mg/kg) are not applicable to the Site. The background soil and mineralized bedrock concentrations are below the USEPA Region 9 Residential Screening Level (15,000 mg/kg) and the Eco-SSSL for Birds (2,036 mg/kg).

Cadmium: Proposed soil PRG for cadmium is 1.3 mg/kg based on the mineralized bedrock concentration. The background soil contains 0.63 mg/kg. Therefore, the NPS

ESVs for Birds (0.77 mg/kg) and Mammals (0.36 mg/kg), and the Eco-SSSL for

Mammals (0.1 mg/kg) are not applicable to the Site.

Copper: Proposed soil PRG for copper is 159 mg/kg based on the mineralized bedrock concentration. The background soil contains 25.8 mg/kg. Therefore, the NPS ESVs for

Terrestrial Plants (70 mg/kg), Soil Invertebrates (80 mg/kg), Mammals (49 mg/kg), and

Birds (28 mg/kg) are not applicable to the Site. The mineralized bedrock concentration is

Hidden Hills Mine EE/CA Page 11 below the Eco-SSSL for Birds (12,993 mg/kg), the Eco-SSSL for Mammals (298 mg/kg), and the USEPA Region 9 Residential Screening Level (3,100 mg/kg).

Lead: Proposed soil PRG for lead is 15.6 mg/kg based on the background soil concentration. The mineralized bedrock contains 10.5 mg/kg. Therefore, the NPS ESV for Birds (11 mg/kg) and the Eco-SSSL for Mammals (5.6 mg/kg) are not applicable to the Site. The mineralized bedrock and background soil concentrations are below the

USEPA Region 9 Residential Screening Level (400 mg/kg), the NPS ESVs for Terrestrial

Plants (120 mg/kg), Soil Invertebrates (1,700 mg/kg) and Mammals (56 mg/kg), and the

Eco-SSSL for Birds (117 mg/kg).

Silver: Proposed soil PRG for silver is 1.0 mg/kg based on the mineralized bedrock concentration. The mineralized bedrock and background soil concentrations are below all

NPS ESVs and the Eco-SSSL for Birds. The Eco-SSSL for Mammals (0.21 mg/kg) is below the mineralized bedrock and background soil (0.83 mg/kg) concentrations and is not applicable to the site.

Vanadium: Proposed soil PRG for vanadium is 43.3 mg/kg based on the background soil concentration. The mineralized bedrock contains 34.2 mg/kg. The NPS ESVs for Birds

(7.8 mg/kg) and Terrestrial Plants (2.0 mg/kg), and the Eco-SSSL for Mammals (27 mg/kg) are below the mineralized bedrock and background soil concentrations and not applicable to the Site.

Zinc: Proposed soil PRG for zinc is 48.3 mg/kg based on the mineralized bedrock concentration. The background soil contains 42.3 mg/kg. The NPS ESV for Birds (46 mg/kg) and the Eco-SSSL for Mammals (30 mg/kg) are below the mineralized bedrock concentration and not applicable to the Site.

Groundwater PRGs were not determined. Low metals concentrations in the groundwater sample and the reported depth to groundwater (Tetra Tech, 2005) indicates that the Site is not impacting groundwater and the metals present are the result of the naturally mineralized nature of bedrock at the Site.

Threatened and Endangered (T&E) species that may occur at the Site are listed below. T&E species occurring in MOJA were determined based on the National Park Service Mojave

National Preserve website (http://www.nps.gov/moja/learn/education/classrooms/upload/MDD-

Unit-VIII-Endangered-Species.pdf). The T&E list is as follows:

Hidden Hills Mine EE/CA Page 12

Endangered Species:

Least Bell’s Vireo (Vireo bellii pusillus) Listed as State Endangered Species

Threatened Species:

Desert Tortoise (Gopherus agassizii)

Candidate Species:

Mountain Lion (Felis concolor) No State Listing

Townsend’s Big-Eared Bat (Plecotus townsendii)

Species of Concern:

Desert Bighorn Sheep (Ovis canadensis nelson) No State Listing

State T&E status was determined using the California Department of Fish and Wildlife

Threatened and Endangered Species Website

(https://www.dfg.ca.gov/wildlife/nongame/t_e_spp/).

Further information on screening levels and how they relate to PRGs are provided in Section 2.3.

Compliance with other applicable or relevant and appropriate requirements (ARARs) is addressed in Section 3.6.

2.5 Risk Evaluation

The risk evaluation uses sampling data from the Site to identify the Contaminant(s) of Potential

Concern (COPCs), provide an estimate of how and to what extent people and the environment might be exposed to the COPC(s), and provide an assessment of effects associated with the

COPC(s). This section presents the results of the Human Health Risk Evaluation (HHRE) and the Ecological Risk Evaluation (ERE).

2.5.1 Human Health Risk Evaluation

As described in the approved Sampling and Analysis Plan, the HHRE considered the following published screening values:

USEPA Region 9, Residential RSLs (USEPA 2015; http://www.epa.gov/region9/superfund/prg/)

USEPA Region 9 Residential RSLs are presented in the Table 14. The HHRE compared on-Site concentrations of metals in soil to the values presented in Table 14. Hazard Quotients were calculated for all metals and are presented in Table 16. Hazard Quotients are a ratio of the

Hidden Hills Mine EE/CA Page 13 maximum contaminant concentration to individual screening vales and are used to determine

Contaminants of Potential Concern (COPCs). Hazard Quotients exceeded a value of one for arsenic and copper. Therefore, arsenic and copper are Contaminants of Potential Concern for human health. The Hazard Index is 102 (Table 16). The Hazard Index indicates a potential cancer risk to humans primarily for arsenic. The Hazard Index is the summation of Hazard

Quotients for all chemicals to which an individual is exposed. A Hazard Index of less than one indicates no adverse effects are expected to occur to the exposed individual. Total cancer risk is presented in Table 19. Arsenic is the only Class A Human Carcinogen present on site. The total cancer risk is 5.7x10 -6 indicating the risk of developing cancer is approximately 6 in 1,000,000.

For reference, USEPA’s generally acceptable risk level for cancer is 1 in 1,000,000. The total cancer risk was calculated based on the Chronic Daily Intake and Slope Factor for arsenic. The

Chronic Daily Intake was calculated using assumptions for adult hiker scenarios allowing the calculation to be as applicable as possible to the Site. The adult hiker scenario is determined by the EPA and assumes an adult recreator visits the site for 2 days per month, 8 months per year for 20 years. The adult hiker scenario is a conservative estimate of risk associated with the

Hidden Hills Mine. Complete details of assumptions used to calculate the total cancer risk are detailed in Table 19. Arsenic exposure can cause liver, kidney, lung, bladder, and skin cancers.

Exposure pathways for humans only apply to recreational visitors. Human residential exposure is not applicable to the site based on the lack of residential development and low likelihood of future residential development. Exposure pathways are possible through direct contact with soil through incidental ingestion. The complete Conceptual Site Model presenting possible exposure pathways is presented in Figure 4.

2.5.2 Ecological Risk Evaluation

The ERE considered the following published screening values:

National Park service (NPS) Ecological Screening Values (ESVs) (NPS 2014b)

CalEPA, Office of Environmental Health Hazard Assessment CHHSLs

(http://oehha.ca.gov/risk/chhsltable.html)

CalEPA State Water Board Environmental Screening Levels

(http://www.waterboards.ca.gov/sanfranciscobay/water_issues/programs/esl.shtml)

Ecological risk-related criteria for metals are presented in Table 15. The ERE compared on-Site concentrations of metals in soils and groundwater to the values presented in Table 15.

The maximum soil concentrations of arsenic, barium, copper, lead, silver, vanadium and zinc exceeded one or more of the applicable screening criteria described above as shown in Table 1.

http://oehha.ca.gov/risk/chhsltable.html http://www.waterboards.ca.gov/sanfranciscobay/water_issues/programs/esl.shtml

Hidden Hills Mine EE/CA Page 14

The maximum groundwater concentrations of barium, copper, and lead exceeded one or more of the applicable screening criteria described above. However, low metals concentrations in the groundwater sample indicate that the Site is not impacting groundwater and the metals present are the result of the naturally mineralized nature of bedrock at the Site. Thus, groundwater remediation is not considered in the Removal

Action Alternatives described in Section 4.

Hazard Quotients were calculated for all metals and are presented in Table 16. Hazard

Quotients for terrestrial plants exceed a value of one for arsenic, copper, lead, vanadium, and zinc. Therefore, arsenic, copper, lead, vanadium, and zinc are Contaminants of

Potential Concern to Terrestrial Plants. The Hazard Index is 118 (Table 16). The Hazard

Index indicates a potential risk to terrestrial plants.

Hazard Quotients for soil invertebrates exceed a value of one for antimony, arsenic, barium, cadmium, copper, lead, silver, and zinc. Therefore, antimony, arsenic, cadmium, copper, lead, silver, and zinc are Contaminants of Potential Concern to Soil Invertebrates.

The Hazard Index is 102 (Table 16). The Hazard Index indicates a potential risk to soil invertebrates.

Hazard Quotients for mammals exceed a value of one for antimony, arsenic, barium, cadmium, copper, lead, silver, and zinc. Therefore, antimony, arsenic, cadmium, copper, lead, silver, and zinc are Contaminants of Potential Concern to mammals. The Hazard

Index is 213 (Table 16). The Hazard Index indicates a potential risk to mammals.

Hazard Quotients for birds exceed a value of one for arsenic, cadmium, copper, lead, silver, vanadium and zinc. Therefore, arsenic, cadmium, copper, lead, silver, vanadium and zinc are Contaminants of Potential Concern to birds. The Hazard Index is 232 (Table

16). The Hazard Index indicates a potential risk to birds.

Exposure pathways for all ecological receptors are possibly complete through direct contact with soils through both incidental ingestion and food chain exposure. The complete Conceptual Site

Model presenting possible exposure pathways is presented in Section 2.4 of the Sampling and

Analysis Plan (RMC, 2016).

3.0 IDENTIFICATION OF REMOVAL ACTION OBJECTIVES

At any release where the lead agency determines there is a threat to public health, welfare, or the environment, a removal action may be taken to abate, prevent, minimize, stabilize, mitigate, or eliminate the release. This Section describes the objectives of the Removal Action.

3.1 Statutory Limits on Removal Actions

Authority for responding to releases or threat of releases from an impacted site is addressed in

Section 104(a) of CERCLA, 42 U.S.C. § 9604(a). CERCLA, Section 104 and Section 300.415

Hidden Hills Mine EE/CA Page 15 of the NCP, 40 C.F.R. § 300.415, specifically address non time-critical removal actions. Section

104(c)(1) of CERCLA generally restricts the timing of removal actions funded with Superfund monies to one year and the cost to $2 million; however, CERCLA does not impose these limitations on a removal action funded by a federal agency at a federal facility with dedicated monies appropriated to that agency for that purpose apart from Superfund. Therefore, no statutory limits apply to these removal actions.

3.2 Scope of the Removal for the Site

The scope of the Removal Action will be limited to remediation of metals impacts from historic mining activities to comply with Site PRGs. Activities performed at the Site will be sufficient to achieve the Removal Action Objective established in Section 3.5.

3.3 Potential Schedules for the Removal at the Site

The schedule for the Removal Action will be determined by MOJA and will be designed within a reasonable time frame that ensures adequate protection of public health and the environment.

3.4 Planned Removal/Remedial Activities

Planned removal activities at the Site will consist of tasks to reduce soil metals impacts to concentrations that minimize impacts to human health and the environment. Planned removal activities may include but are not limited to:

No action;

Land use restrictions;

Excavation of impacted soils;

Covering of impacted soils;

Disposal of impacted soils; and

Site reclamation.

Potential removal activities are detailed further in Section 4.0.

3.5 Removal Action Objectives

Removal Action Objectives (RAOs) were developed based on the nature and extent of contamination as documented in Section 2.1.

One source of contamination has been identified in the EE/CA:

Hidden Hills Mine EE/CA Page 16

Arsenic, barium, cadmium, copper, lead, silver, vanadium and zinc in tailings, waste rock and soils.

One RAO has been established for the Site:

3.6 Applicable or Relevant and Appropriate Requirements

This section presents a summary of applicable or relevant and appropriate requirements

(ARARs) for the Site. The National Contingency Plan (NCP) requires that fund-financed removal actions under CERCLA Section 104 and removal actions pursuant to CERCLA Section

106 attain ARARs under Federal environmental, State environmental or facility siting laws "to the extent practicable" considering the urgency of the situation and the scope of the removal action (See 40 C.F.R. Part 300.415(j)).

3.6.1 Contaminant-Specific, Location-Specific and Action Specific Requirements

ARARs are divided into contaminant-specific, location-specific and action-specific requirements.

Contaminant-specific ARARs govern the release of material containing specific contaminants.

In the case of the Hidden Hills Mine, contaminants are limited to arsenic, barium, cadmium copper, lead, silver, vanadium and zinc.

Location-specific ARARs relate to the geographic or physical location of the site, rather than the nature of contaminants. These ARARs place restrictions, such as the concentration of hazardous substances or the conduct of cleanup activities, due to their location in the environment at the site.

Action-specific ARARs are usually technology- or activity-based requirements on actions taken with respect to hazardous substances. A particular remedial activity will trigger an action-specific ARAR. Unlike contaminant- or location-specific ARARs, action-specific ARARs do not determine the remedial alternative to be used, but rather how the selected remedy must be achieved.

The removal alternatives presented in this EE/CA were selected based on a combination of contaminant-specific, location-specific and action-specific ARARs.

Hidden Hills Mine EE/CA Page 17

3.6.2 Definitions of “Applicable” and “Relevant and Appropriate”

Applicable

Applicable requirements are those cleanup standards, standards of control, and other substantive requirements, criteria, or limitations promulgated under Federal environmental, State environmental or facility siting laws that specifically address a hazardous substance, pollutant, contaminant, remedial action, location, or other circumstance found at a CERCLA site. Only those State standards that are identified by the State in a timely manner and are more stringent than Federal requirements may be applicable.

Relevant and Appropriate

Relevant and appropriate requirements means those cleanup standards, standards of control, and other substantive requirements, criteria, or limitations promulgated under Federal environmental, State environmental or facility siting laws that, while not “applicable” to a hazardous substance, pollutant, contaminant, remedial action, location, or other circumstance at a CERCLA site, address problems or situations sufficiently similar to those encountered at a CERCLA site that their use is well suited to the particular site. Only those State standards that are identified in a timely manner and that are more stringent than Federal requirements may be relevant and appropriate.

3.7 Summary of Potential ARARs

A detailed list of ARARs applicable to the Site is presented in Table 9. These ARARs were developed to encompass all potentially relevant activities conducted on-Site.

4.0 IDENTIFICATION AND ANALYSIS OF REMOVAL ACTION ALTERNATIVES

This section presents removal action alternatives proposed to achieve the RAO identified in

Section 3.5. Removal Action Alternatives were evaluated on their ability to meet the RAO, their ability to comply with ARARs, as well as their effect on the threatened Desert Tortoise, site visitors, and grazing permitees. Both long-term and short-term effects were considered.

The proposed removal action alternatives for the Hidden Hills Mine include the following:

Alternative 1 – No Action;

Alternative 2 – Institutional Controls;

Alternative 3 – Off-Site Disposal of Hazardous Mine Wastes, Placement of Cover Soils

Over Non-Hazardous Mine Wastes; and

Hidden Hills Mine EE/CA Page 18

Alternative 4 – Full Removal and Offsite Disposal

4.1 Evaluation Criteria

As specified by EPA guidance (USEPA, 1993), each response alternative is evaluated in terms of three criteria: Effectiveness, Implementability and Cost. These three criteria encompass the elements required to meet National Oil and Hazardous Substances Pollution Contingency Plan

(NCP) removal criteria. The criteria are described below:

Effectiveness: The effectiveness of a proposed alternative refers to the ability to meet the removal action objective, and to the degree of protectiveness of the environment as well as public and site worker health, both in the short and long term. The RAO for the Site are:

Eliminate the potential for impacts to human health and the environment from metals contamination.

Effectiveness also includes the degree of compliance with ARARs (Table 9).

Implementability: Implementability addresses the technical and administrative feasibility of implementing an alternative. Technical feasibility includes the difficulty of conducting the proposed removal action, including future operations and maintenance requirements.

Administrative feasibility includes issues such as statutory limits, permitting, availability of services and disposal sites and the likelihood of public and regulatory acceptance.

Cost: The cost of each proposed alternative includes direct and indirect capital costs as well as operations and maintenance (O&M) costs. There are no costs associated with Alternative 1.

Estimated costs are presented in Tables 10, 11, and 12 for Alternatives 2, 3, and 4.

4.2 Hidden Hills Mine Alternatives

The alternatives for the Hidden Hills Mine are presented in Sections 4.2.1 through 4.2.4 below.

4.2.1 Hidden Hills Mine Alternative 1 – No Action

Alternative 1 is a baseline alternative by which other alternatives may be compared. No Action involves not taking any further actions to manage environmental concerns at the Site.

Effectiveness: The Site would remain as-is. Implementation of the No Action alternative would not achieve the RAO. The No Action alternative would achieve ARARs associated with Historic

Preservation and Historic Sites.

Hidden Hills Mine EE/CA Page 19

Implementability: The No Action alternative is technically feasible to implement.

Cost: As this alternative does not involve taking any actions at the Site, there are no associated costs.

4.2.2 Hidden Hills Mine Alternative 2 – Institutional Controls

Alternative 2 involves implementing institutional controls to control and warn users of hazards that they may encounter while using the Site. Institutional controls would include a set of written agreements for contractors working in impacted areas and deed restrictions. Institutional controls for recreational users would include the posting of contamination hazard warnings. “No

Trespassing” signs and site closure is not necessary as the Site does not contain metals concentrations high enough to be dangerous to the occasional recreational visitor if they are informed of the hazards that are present. Fencing would be installed and/or repaired surrounding open shafts and adits which pose a physical hazard to the public. Fencing locations are presented on Figure 5. Figure 5 Sheet 1 presents fencing locations for the former mill area. Figure 5 Sheet

2 presents fencing locations for the upper mine areas. In the event that construction is considered at the Site, construction workers would be trained in proper health and safety protocols as well as construction Best Management Practices (BMPs). Deed restrictions would stay with the land as long as metals-impacted soils remain on-Site. A five-year review would be conducted to evaluate the implementation and performance of the remedy in order to determine if the remedy was or would be protective of human health and the environment.

Effectiveness: Implementation of institutional controls would not fully achieve the RAO or comply with all ARARs. This alternative would only achieve protection of human health and would not provide protection of the environment. Metals concentrations on site are not high enough to be a danger to properly informed occasional recreational site visitors. Physical hazards to humans would be minimized through fencing installation around open shafts and adits. .

Construction worker exposure, although unlikely, would be limited by following health and safety protocols. Risks to the environment would be unabated with this alternative. ARARs related to Historical Preservation and Historic Site would be achieved by leaving the site intact.

Implementability: Institutional controls are technically feasible to implement. A five-year review program would be conducted to evaluate the implementation and performance of the remedy in order to determine if the remedy was or would be protective of human health and the environment.

Cost: Costs for implementation of Alternative 2 are presented in Table 10. The estimated total cost for this alternative is $ 79,620.

Hidden Hills Mine EE/CA Page 20

4.2.3 Hidden Hills Mine Alternative 3 – Off-Site Disposal of Tailings, Placement of

Cover Soils Over Waste Rock

Alternative 3 involves the off-site disposal of tailings (sample MOJA-TA-1), followed by grading and covering of remaining waste rock and soils that exceed PRGs. Leachability tests performed on the tailings sample indicate that the tailings materials qualify as hazardous waste per California Code of Regulations, Title 22, Chapter 11, Article 3 (Table 2). Tailings would be excavated by an excavation contractor and then transported to an appropriate off-site disposal facility. Waste characterization samples (TCLP and/or STLC) would be collected to confirm that the tailings classify as hazardous waste prior to offsite shipment. Confirmation samples would be collected to determine that all tailings had been removed. Remaining waste rock piles that exceed PRGs would be graded as needed to provide side slopes and surfaces suitable for cover soil placement. Following grading, a minimum of two feet of clean cover soils would be placed. Clean cover soils would be generated from an onsite borrow area or other nearby location. Following cover soil placement cover soils would be revegetated or otherwise stabilized. Cover soil integrity would be monitored indefinitely.

The former mill building at the Site will be left intact to comply with ARARs associated with

Historic Preservation and Historic Sites. The former mill building will be evaluated in consultation with the State Historic Preservation Officer (SHPO) and appropriate mitigations will be implemented with concurrence from SHPO for structures determined to be eligible for listing on the National Register of Historic Places.