Attachment_2_MOJA_Hidden_Hills_NTCRAWP.pdf

Non-Time Critical Removal Action Work Plan

Mojave National Preserve Hidden Hills Mine

EDL 5PWR2428

Prepared by

Applied Intellect, LLC

Paul Hunter P.G.

Project Manager

05/01/2024 Date

Katherine Swords, P.E.

California Registered Professional Engineer License # C70071

05/10/2024 Date

Mojave National Preserve, Hidden Hills Mine May, 2024 Page | i

Table of Contents Table of Contents .................................................................................................................. i

List of Figures ....................................................................................................................... ii

List of Tables ........................................................................................................................ ii

List of Appendices ................................................................................................................ ii

List of Abbreviations and Acronyms ..................................................................................... iii

1 Introduction .................................................................................................................. 1-1

2 Regulatory Framework .................................................................................................. 2-1

3 Purpose......................................................................................................................... 3-1

4 Physical and Environmental Setting ............................................................................... 4-1

4.1 Site Location .............................................................................................................. 4-1

4.2 Geologic Setting ........................................................................................................ 4-1

4.3 Hydrogeology ............................................................................................................ 4-1

4.4 Ecology ...................................................................................................................... 4-2

4.5 Site History ................................................................................................................ 4-2

4.6 Site Features ............................................................................................................. 4-3

4.7 Current Land Use ...................................................................................................... 4-4

5 Site Conditions and Background .................................................................................... 5-1

5.1 Historical Investigations ............................................................................................ 5-1

5.1.1 Preliminary Assessment (2001) ............................................................................ 5-1

5.1.2 Park-Wide Site Inspection (2005) ......................................................................... 5-1

5.1.3 Engineering Evaluation/Cost Analysis (2017)....................................................... 5-2

5.1.4 Action Memorandum (2018) ................................................................................ 5-4

5.2 Environmental Site Conditions ................................................................................. 5-6

5.2.1 Soil ........................................................................................................................ 5-6

5.2.2 Waste Rock ........................................................................................................... 5-6

5.2.3 Tailings .................................................................................................................. 5-6

5.2.4 Groundwater ........................................................................................................ 5-7

6 Removal Action Objectives and Removal Action Levels .................................................. 6-1

7 Applicable or Relevant and Appropriate Requirements .................................................. 7-1

8 Removal Action Alternative ........................................................................................... 8-1

9 Removal Action Plan ..................................................................................................... 9-1

9.1 Protection of Threatened and Endangered Species ................................................. 9-1

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9.2 Preservation of Historical Site Features ................................................................... 9-2

9.3 Removal Action Activities ......................................................................................... 9-2

9.3.1 Site Preparation .................................................................................................... 9-2

9.3.2 Tailings Excavation and Removal ......................................................................... 9-3

9.3.3 Delineation of Extent ............................................................................................ 9-4

9.3.4 Waste Characterization and Disposal .................................................................. 9-4

9.3.5 Site Restoration .................................................................................................... 9-5

9.3.6 Institutional Controls ............................................................................................ 9-5

10 References .................................................................................................................. 10-1

List of Figures

Figure 1 – Site Location Map Figure 2 – Site Access Map Figure 3 – Site Features Figure 4 – Historic Soil Sampling Locations Figure 5 – Removal Action Area Figure 6 – Proposed ISM Sample Units

List of Tables

Table 1 – Historical Soil Sample Analytical Results Table 2 – Applicable or Relevant and Appropriate Requirements Table 3 – Preliminary Remediation Goals

List of Appendices

Appendix A – Site Visit Photographs (February 1, 2022)

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List of Abbreviations and Acronyms

AI Applied Intellect, LLC

AM Action Memorandum

ARAR Applicable or Relevant and Appropriate Requirements bgs below ground surface

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act

CFR Code of Federal Regulations

COC constituent of concern

COPC Contaminant of Potential Concern

COPEC Contaminant of Potential Ecological Concern

CY cubic yards

DOI Department of the Interior

ECM Environmental Compliance Memorandum

EDL Environmental Disposal and Liabilities

EE/CA Engineering Evaluation / Cost Analysis

EPA United States Environmental Protection Agency

ERE Ecological Risk Evaluation

ESV Ecological Screening Values

GPS Global Positioning System

HHRE Human Health Risk Evaluation

HI Hazard Index

HQ Hazard Quotient

HRMC human risk management criteria

ISM Incremental Sampling Methodology

MCLs Maximum Contaminant Levels mg/kg milligrams per kilogram

MOJA Mojave National Preserve

NCP National Oil and Hazardous Substances Pollution Contingency Plan (also known as, National Contingency Plan)

NPS National Park Service

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NTC Non-Time Critical

NTCRA non-time-critical removal action

NTCRAWP Non-Time Critical Removal Action Work Plan

PA Preliminary Assessment pH Hydrogen Potential

PPL Priority Pollutant List ppm parts per million

PRG preliminary removal goal

RAO removal action objectives

RCRA Resource Conservation and Recovery Act

RSL Regional Screening Level

SI (Park-Wide) Site Inspection sf square foot

SOW Statement of Work

SSSL Site Specific Screening Level

STLC Soluble Threshold Limit Concentration

SU sampling unit

TBC to be considered

TCLP Toxicity Characteristic Leaching Procedure

TRV Toxicity Reference Value

TTLC Total Threshold Limit Concentration

T&E Threatened and Endangered

USACE U.S. Army Corps of Engineers

USC United States Code

USDOI U.S. Department of the Interior

USEPA United States Environmental Protection Agency

USGS United States Geological Survey µg/L Micrograms per Liter

WLRMC wildlife and livestock risk management criteria

XRF x-ray fluorescence

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1 Introduction Applied Intellect, LLC (AI) has prepared this Non-Time Critical Removal Action Work Plan (NTCRAWP) to summarize planned removal action activities to be conducted at the Hidden Hills Mine Site located in Mojave National Preserve (MOJA) in San Bernadino County, California. AI was contracted by the United States Department of the Interior (DOI), National Park Service (NPS) to prepare this NTCRAWP under Order Number 140P8121F0043 of Contract GS-10F-026BA. The Site has an NPS Environmental Disposal and Liabilities (EDL) number of 5PWR2428.

This NTCRAWP has been prepared to address the Removal Action Objectives (RAOs) and facilitate the implementation of the selected removal alternative for the Site. In accordance with the contract performance Statement of Work (SOW), this NTCRAWP has been organized into the following sections:

Section 1 – Introduction This section describes the contractual requirements for preparing this NTCRAWP, and organization of this document.

Section 2 – Regulatory Framework This section summarizes the regulatory authority, process, and basis for preparation of this

NTCRAWP.

Section 3 – Purpose This section presents the purpose of this NTCRAWP.

Section 4 – Physical and Environmental Setting This section provides a physical description of the Site, including Site location and access, geological setting, Site hydrology and ecology, historical use of the Site, predominant Site features and current land use.

Section 5 – Site Conditions and Background This section describes the historical investigations that have been prepared for the Site, a summary of environmental Site conditions, and characterization information associated with the contamination.

Section 6 – Removal Action Objectives and Removal Action Levels This section describes the RAOs and Removal Goals (RGs) that have been developed for the Site, as presented in the Engineering Evaluation/Cost Analysis Report (EE/CA).

Section 7 – Applicable or Relevant and Appropriate Requirements This section describes Applicable or Relevant and Appropriate Requirements (ARARs) introduced in the EE/CA (2017), as adopted by the Action Memorandum (AM) (2018).

Section 8 – Removal Action Alternative This section describes the Removal Action that was recommended in the EE/CA and approved in the AM.

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Section 9 – Removal Action Plan This section presents the plan for implementing the Removal Action Alternative, including the protection of endangered species, preservation of historical Site features, removal action alternatives, waste characterizations, confirmation sampling, and institutional controls.

Section 10 – References This section provides a list of references and documents used in preparation of this NTCRAWP.

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2 Regulatory Framework Section 104 of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) authorizes the President to take any response action not inconsistent with the National Oil and Hazardous Substances Pollution Contingency Plan, commonly called the National Contingency Plan (NCP), 40 Code of Federal Regulations (CFR) Part 300, which the President deems necessary to protect the public health or welfare or the environment from threats posed by the release or threatened release of hazardous substances into the environment. This response authority has been delegated to the Department of the Interior (DOI) and, with respect to any release on or from land under NPS jurisdiction, further delegated to NPS by DOI Departmental Manual Part 207 Chapter 7. The NPS Director has redelegated these authorities to ECCD within the Park Planning, Facilities and Lands Directorate.

CERCLA’s implementing regulations, codified in the NCP, establish the framework for responding to releases and threatened releases of hazardous substances. The NCP prescribes two primary processes for responding to releases: (1) a removal action process and (2) a remedial action process (see NCP Sections

300.400 through 300.440). Following a preliminary assessment and site investigation, NPS determined that additional Site investigations were warranted. Pursuant to Section 300.415(b) of the NCP, NPS initiated a non-time-critical removal action by conducting an Engineering Evaluation/Cost Analysis (EE/CA) to better characterize the nature and extent of contamination at the Site. The EE/CA Report was issued on March 7, 2017, and documented the presence of hazardous substances posing threats to human health and the environment at the Site. An Action Memorandum was issued in 2018 selecting the removal action alternative for the Site.

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3 Purpose The purpose of this NTCRAWP is to describe the plan for performing the Removal Action at the Site.

The purpose of the Removal Action is to remove the tailings pile and contaminated soils that pose a threat to human health and the environment and to dispose of the contaminated material offsite at an appropriate disposal facility.

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4 Physical and Environmental Setting This section provides a physical description of the Site, including Site location and access, geological setting, Site hydrology and ecology, historical use of the Site, predominant Site features and current land use.

4.1 Site Location

The Site is a relatively small and remote abandoned mine located on federal lands on the southeast slope of the Providence Mountains in the southern portion of the MOJA (Figure 1). The Site is situated in the southeast and southwest ¼’s of Section 6, Township 8 North, Range 14 East of the USGS 7.5-minute Van Winkle Spring topographic quadrangle. The geographic coordinates of the Site are 34.80613 degrees north latitude and -115.545029 degrees west longitude. The Site totals approximately five acres and is at an elevation of approximately 3,400 feet above sea level.

The Site is approximately 4.4 miles due north of Interstate 40 and can be accessed by traveling north 6.6 miles on Kelbaker Road (paved) from Exit 78 on Interstate 40, then East for 4.8 miles along Hidden Hill Road (dirt road requires four-wheel drive and high clearance vehicles; Figure 2).

4.2 Geologic Setting

The geology at MOJA is complex and varied, and each mine and mill site is unique. The major mountain ranges (Clark, Ivanpah, New York, and Providence) are partly composed of carbonate rocks. The eastern regions of the preserve contain Paleozoic crystalline rocks, predominantly gneissic granites.

4.3 Hydrogeology

Most surface water at MOJA comes from the ~200 seeps and springs located along geologic features such as faults and fissures. The majority of the springs (78 percent) occur between elevations of 4,000 and 6,000 feet; discharge is generally less than 5 gallons per minute. The only two significant surface water bodies within the Preserve are Piute Creek and Cornfield Spring, which are not in proximity to the Hidden Hills Mine Site. There are no permanent or perennial surface water bodies in the area of the Site.

There is an ephemeral wash in the area of the Site.

Groundwater at MOJA is primarily recharged by precipitation and runoff from the mountains. The depth to groundwater at MOJA can be in excess of 700 feet below ground surface (bgs). Limited available data indicates that groundwater in the basins may contain elevated concentrations of metals and nitrate.

There are no downgradient 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, is approximately 9 miles from the Site, and most likely not hydraulically connected to the Site A well that provides water for livestock is located approximately 0.2 miles south and topographically downgradient of the Site (NPS, 2018).

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4.4 Ecology

MOJA contains elements from three of the four major desert ecosystems in North America, including the Mojave, Sonoran, and Great Basin. The Site is located in the southern portion of MOJA and is in a transition zone into the Sonoran Desert. This ecosystem lies below an elevation of 3,000 feet and is dominated by creosote bush, ocotillo, cholla cactus, and spring annuals.

MOJA is characterized by a desert climate, with hot summers and mild winters. Summer temperatures exceed 105 degrees Fahrenheit (°F); winter months are cooler, with temperatures near or below freezing.

Average annual precipitation ranges from less than 4 inches at lower elevations to as much as 9 inches at higher elevations within the park.

Wildlife at MOJA includes typical desert inhabitants. Lizards, small rodents, and numerous species of birds are most likely to be seen because they are active largely during the day. However, desert animals come out to roam at night. Mostly nocturnal animals include snakes, bighorn sheep, kangaroo rats, coyotes, and black-tailed jackrabbits. MOJA is also home to the threatened Desert Tortoise. The Site is located in Desert Tortoise Critical Habitat and tortoises are regularly observed in the area.

4.5 Site History

MOJA is in southeastern California within San Bernardino County and encompasses 1.6 million acres.

Historical mining and processing left potentially hazardous materials at the numerous abandoned mine and mill sites throughout MOJA, with the potential for both human and ecological risks associated with contamination at the sites.

Each of the mine and mill sites in MOJA is unique, with physical and chemical features that depend on the site’s geology and operational history. Mining-related geological materials that may remain at MOJA mine sites include:

Ore – Ore is the metals-bearing rock mined and processed for metals extraction. Although ore piles may contain the highest concentrations of metals, the generally large size of the rocks means that exposure to human or ecological receptors is very unlikely.

Waste Rock – Waste rock is material that was moved during mining operations to access the ore and has not been processed in any manner. Waste rock may contain elevated concentrations of metals, but at concentrations that were insufficient for profitable extraction. Most mine sites contain piles of waste rock which were commonly used to build roads or benches. Concentrations of metals in waste rock piles can be highly variable and difficult to sample, with the smaller grain sizes (fine material) potentially having higher concentrations and the mostly larger material not resulting in exposure due to the larger particle sizes (sand, gravel, pebbles).

Tailings – Fine-grained material (dominantly silt-sized particles) produced as a result of processing metallic mineral ore to extract metals. Tailings material may have been subjected to different processing methods and was usually left at the sites where the ore was processed.

Tailings are generally more mobile and more likely to result in exposure than the mixed material in waste rock piles.

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The type of material that remains at abandoned mines and mill sites depends on what mining activities were conducted.

The Hidden Hills Mine became active around 1882. The mine consisted of five claims, of which the Golden Queen and Hidden Hill were most important. Mine workings at the Hidden Hills Mine included shafts, tunnels, and surface cuts, which followed thin quartz veins. Gold was the primary focus of the mine but the mine also produced silver, copper, iron, lead, zinc, tin, and rare earth minerals. Most shafts were 50 to 100 feet deep, but the main inclined shaft was 240 feet deep. A cross-cut tunnel was reportedly 350 feet long. The Golden Queen claim produced more than $6,000 before 1907. One ore pocket encountered in 1913 produced $13,000 from 300 pounds of rock. An estimated mass of 3,000 to 4,000 tons of ore was processed at the Hidden Hills Mine to produce gold, silver, and copper (BLM, 2001).

Lower-grade quartz ore was crushed in a two-stamp mill on the property. The mill was powered by a gasoline engine and was capable of processing 3 tons of ore in a 24-hour period. The Site was evaluated and found eligible for the National Register of Historic Places.

4.6 Site Features

The following Site features were documented in the Preliminary Assessment (PA) (BLM, 2001) and confirmed during the Park-Wide Site Inspection (SI) (TetraTech, 2005):

• cabin;

• mill foundation;

• ore hopper (collapsed since 2005);

• crumbled rock walls;

• open shafts and stopes (ten total; most are now mitigated);

• open adits (three total; most are now mitigated);

• bulldozer cuts (three total);

• waste rock piles (ten total) below the shafts, adits, and cuts; and

• tailings pile behind a small rock dam.

Bedrock exposed at the Hidden Hills Mine is primarily granitic, though carbonate rock was also observed.

Bedrock appears hydrothermally altered near the mine workings. Bedrock along the central bulldozer cut is argillically altered and mineral enriched, with secondary iron oxide and copper carbonate minerals.

Waste rock piles contain yellowish brown to brown sand, gravel, and cobbles. The waste rock piles are fairly homogeneous, though some piles show a greater degree of mineral enrichment than others.

The small tailings pile contained a loosely consolidated light brown rock flour with a few gray gravel clasts. The shape and margins of the pile suggest that tailings have been transported from the Site by wind or surface runoff. Based on the size of the mine and mill, the volume remaining in the small tailings pile is probably only a fraction of the material generated during operations. At the time of the 2005 EE/CA, the surface area of the tailings pile was reported to be 600 square feet (sf), with an estimated volume of 6 cubic yards (CY). At the time of the 2022 Site visit, the tailings were distributed over an area of approximately 1,850 sf, and 69 CY of comingled tailings and potentially affected underlying material (assuming a thickness of approximately one foot).

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Site features at the Hidden Hills Mine are shown on Figure 3. Site photographs collected during a Site visit on February 1, 2022 are presented in Appendix A.

4.7 Current Land Use

MOJA hosts more than 500,000 recreational visitors annually. The preserve is used for many recreational activities, including camping, hiking, mountain biking, bird watching, horseback riding, stargazing, rock climbing, and hunting.

The Hidden Hills Mine is currently abandoned and previously produced gold, silver and copper. The Site is open to the public but has relatively low visitation. There is a windmill that is approximately 0.2 miles south of the mill foundation (down topographic gradient).

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5 Site Conditions and Background This section provides a description of historical environmental investigations that occurred at the Site, and a summary of current environmental Site conditions.

5.1 Historical Investigations

The major milestones in the CERCLA process that have been completed for this Site include a PA, SI, an EE/CA, and a subsequent AM. These Hidden Hills documents are summarized below.

5.1.1 Preliminary Assessment (2001)

In 2001, BLM conducted PAs on 42 sites, including 37 abandoned mine sites within MOJA (BLM, 2001). The PA identified and described site features and present maps and photographs of the sites. Data collected for the PA was limited to site visits and review of available historical information; neither solid matrix nor fluid samples were collected. The PA identified 15 priority sites to be considered for remediation, including the Hidden Hills Mine site.

The Hidden Hills Mine Site was reported to include a mill and a minor amount of tailings. It was recommended that the tailings and soil from the slab floor of the assay lab be sampled and analyzed for metals and chemical content. Open shafts, cuts, and adits were identified at the Site, and classified as moderate to high physical hazards.

5.1.2 Park-Wide Site Inspection (2005)

In 2005, an SI was conducted for the USACE of 21 inactive mine sites in MOJA, including the Hidden Hills Mine. The SI was conducted to assess whether hazardous substances from past mining activities were released and potentially threaten human health or the environment at each of the 21 sites.

Soil samples collected at the Hidden Hills Mine Site included one waste rock sample, one tailings sample, two soil samples, and two background samples. One background sample was "mineralized bedrock" and the second was a background soil sample. Analytical results from these samples are presented in Table 1 and were compared against screening benchmarks selected by NPS, including BLM human risk management criteria (HRMC) for a camper scenario, EPA residential preliminary remediation goals (PRGs), and BLM wildlife and livestock risk management criteria (WLRMC). Results indicated metals were at concentrations below human health and ecological benchmarks, except for arsenic, copper, and mercury. Samples collected from the tailings material and soil from beneath the tailings pile and alongside the assay lab slab exceeded the arsenic HRMC for a camper scenario. The copper concentration in the soil sample collected below the tailings pile exceeded WLRMC and HRMC. The mercury concentration in the soil sample collected down slope from the tailings pile exceeded the WLRMC.

Results of paste pH analysis showed a range of 8.10 to 8.48 standard units, indicating the material does not generate acidity.

The SI recommended that NPS consider two options:

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• Conduct a baseline human health and ecological risk assessment to estimate risks associated with exposure to concentrations of metals that remain in mining-related material, or

• Remove mining-related material that may result in significant adverse effects to human or ecological receptors. At the Hidden Hills Mine site, this includes the soil below (downslope) the tailings pile that contains an elevated copper concentration (reference sample MOJA-HH-S-01).

The SI also recommended addressing physical hazards at the Hidden Hills Mine site, including closing three adits and at least 10 shafts and stopes. The SI did not include characterization of the lateral extent of the tailings pile, nor did it include an estimated volume of tailings material. NPS followed the first recommendation and conducted an EE/CA which included human health and ecological risk assessments.

5.1.3 Engineering Evaluation/Cost Analysis (2017)

An EE/CA Report was generated in accordance with CERCLA Section 104(b) and NCP Section 300.415(b)(4)(i), the U.S. Environmental Protection Agency (USEPA) Guidance on Conducting Non- Time-Critical Removal Actions Under CERCLA (USEPA, 1993), and the U.S. Department of the Interior (USDOI) Environmental Compliance Memorandum (ECM) 16-3 (USDOI, 2016, superseded by ECM 10- 1, 2018).

The EE/CA included a description of site characterization activities, Screening Level Human Health and Ecological Risk Assessments, analysis of Applicable or Relevant and Appropriate Requirements (ARARs) identification of Removal Action Objectives (RAOs), development and analysis of removal action alternatives, and recommendation of a preferred alternative.

Site Characterization

Site characterization activities included the collection of 29 composite soil samples (including two duplicate samples) collected from 0 – 2-inch depths. Sample locations were adjusted in the field based on field observations and real-time XRF screening for silver, arsenic, cadmium, chromium, copper, nickel, lead, antimony, selenium, zinc, and mercury. The soil samples were analyzed for antimony, arsenic, barium, cadmium, chromium, copper, lead, nickel, selenium, silver, vanadium, zinc, and mercury.

Analytical results from these samples are presented in Table 1. Sample results were compared to current NPS Ecological Screening Values (ESVs), USEPA Region 9 Regional Screening Levels (RSLs), and background soil and mineralized rock concentrations.

Soluble Threshold Limit Concentration (STLC) and Toxicity Characteristic Leaching Procedure (TCLP) analysis was additionally performed on the soil sample collected from the tailings pile. STLC results for copper exceeded the State of California Title 22 STLC Hazardous Waste Criteria. Federal RCRA TCLP Hazardous Waste Criteria do not have a standard for copper.

Preliminary Remediation Goals

PRGs were selected only for Contaminants of Potential Concern (COPCs). Under CERCLA, concentrations of COPCs below the naturally occurring background levels are not generally subject to a removal action. Contaminants that exceeded the site-specific background concentration and/or screening

May, 2024 Page | 5-3 values (EPA RSLs, NPS ESVs, site-specific screening levels [SSSLs]) were considered COPCs and used for the preparation of the Risk Assessment.

The PRGs that were developed in the EE/CA were based on screening level risk assessments and were subsequently revised based on the results of the revised human health and ecological risk assessments presented in the AM (discussed below), and therefore not presented in this NTCRAWP.

Risk Assessment

COPCs and COPECs were determined by performing a Screening Level Human Health and Ecological Risk Assessment.

For human health, a Hazard Quotient (HQ) was calculated based on the concentrations of metals detected in soils compared to the USEPA Region 9 Residential RSLs. Arsenic and copper, having HQs greater than a value of one, were the only two contaminants identified as COPCs. Arsenic is a Class A human carcinogen and had a cancer risk of 5.7x10-6, indicating a slight exceedance of the acceptable USEPA risk level for cancer (e.g., at or below the 1:1,000,000).

For ecological receptors, on-Site concentrations of metals in soils were compared to NPS ecological screening values (ESVs). HQs and Hazard Indexes (HIs) were calculated for metals, and COPECs were determined for the Site.

NOTE: The COPECs that were developed in the EE/CA were subsequently revised by the results of the ecological risk assessment presented in the AM (discussed below), and therefore not presented in this NTCRAWP.

Removal Action Objectives and ARARs

RAOs were developed based on the nature and extent of contamination. The sources of contamination identified in the EE/CA include arsenic, barium, cadmium, copper, lead, silver, vanadium and zinc in tailings, waste rock and soils. There was only one RAO established for the Site which is to eliminate the potential for metals impacts to human health and the environment. The EE/CA did not develop preliminary removal action goals (PRGs); PRGs were identified in the AM based on the ecological risk assessment presented in that document.

ARARs were identified in the EE/CA and summarized in the AM. ARARs were used in conjunction with the PRGs to develop and evaluate removal action alternatives. The ARARs for this Site are presented in Table 2, and further discussed in Section 7.0.

Removal Action Alternatives

The removal action alternatives presented in the EE/CA were developed and evaluated based on a combination of contaminant-specific, location-specific and action-specific ARARs, coupled with the RAO. The following Removal Action Alternatives were developed for this Site:

• Alternative 1 – No Action

• Alternative 2 – Institutional Controls

• Alternative 3 – Excavation and Offsite Disposal of Tailings, Cover Soil on Waste Rock

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• Alternative 4 – Full Removal and Offsite Disposal

Removal Action Alternatives were evaluated on their ability to meet the RAO and their ability to comply with ARARs, as well as their potential effect on the threatened Desert Tortoise, site visitors, and grazing permitees. Both long-term and short-term effects were considered. These alternatives were evaluated against criteria for: effectiveness, implementability, and cost.

Alternative 4 – Full Removal and Offsite Disposal was determined to be the preferred alternative and recommended for the Removal Action. Alternative 4 involves the removal of all tailings, waste rock and soils exceeding RGs. Tailings, waste rock and soils exceeding RGs would be excavated and transported to an appropriate offsite disposal facility. Waste characterization samples would be collected to determine if the soils classify as hazardous or non-hazardous waste (as determined by TCLP and/or STLC). Tailings are anticipated to be classified as hazardous waste. Waste rock and soils exceeding PRGs are anticipated to be classified as non-hazardous waste. Any tailings, waste rock, or soils deemed hazardous would be transported to an offsite hazardous waste disposal facility. Confirmation samples would be collected to determine that all materials exceeding RGs had been removed. Following remediation, excavated areas would be revegetated or otherwise stabilized.

5.1.4 Action Memorandum (2018)

An AM was subsequently prepared acknowledging that Site conditions meet the NCP Section 300.415(b) criteria for a removal action, and approved the Removal Action Alternative recommended in the EE/CA.

The AM acknowledged that antimony, arsenic, barium, cadmium, copper, lead, mercury, silver, vanadium, and zinc are present at the Site and were identified as COPECs in the EE/CA for soil, tailings and waste rock because the maximum values for each metal exceeded one or more NPS ESV. Arsenic and copper also exceeded USEPA Region 9 Residential RSLs for human health, and therefore identified as a COPC in soil, tailings and waste rock, and copper was identified as a COPC for tailings.

The AM also states:

“For the Hidden Hills waste rock piles, however, several lines of evidence indicate that the release of metals from the waste rock piles is unlikely. These lines of evidence include the following:

• The large particle sizes of the waste rock (gravel size and larger) greatly reduce the bioavailability of metals within the particles.

• A paste pH analysis conducted during the Sl showed a range of 8.10 to 8.48 standard units, indicating that the waste rock does not generate acidity (Tetra Tech 2005). This low acid generating potential means that water contacting the waste rock, primarily from the limited amount of precipitation, would not generate acid and would, therefore, not cause metals to be released from the waste rock during storm water events.

• The waste rock is not in any direct contact with surface water or groundwater, eliminating a migration pathway that typically causes the most risk at mining sites.

• The revised ecological risk assessment in Appendix A shows no evidence that physical erosion from waste rock piles has migrated into the dry washes enough to cause a risk to ecological receptors.

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Based on these lines of evidence, removal of waste rock at the Site is not recommended and is not considered further in this AM.”

The AM reassessed the human health and ecological risk for exposure to only the tailings. As part of the AM, Mojave National Preserve conducted a revised Ecological Risk Evaluation (ERE), which was included as Appendix A to the AM. The basis for the revised ERE was that the EE/CA identified the risk-based EPA soil screening levels (SSLs) as the PRGs, unless the SSLs were less than background concentrations. These screening level PRGs were overly conservative and not based on a Site-specific risk assessment. Furthermore, the risk assessment presented in the EE/CA combined data from the different site features (tailings, soil and waste rock) despite their differences and nature and extent of contamination. The revised ERE presents a “feature-based” analysis of potential ecological risk and developed Site-specific PRGs specific to the mine tailings at the Site.

The HQ and HI were recalculated for a recreational hiker and the ecological receptors. Risk-based PRGs for copper were developed for these receptors. Because CERCLA does not permit remediation goals that are less than background, PRGs need to consider both potential risk and background concentrations. With this consideration, Site-specific, risk-based PRGs were developed and compared to background concentrations in order to determine PRGs for the Site. Based on the above, this analysis determined that a risk-based PRG for copper of 447 mg/kg for the tailings pile would be protective of all ecological receptors. This justified that ‘cleaning up to background’, as proposed in the EE/CA, is not necessary.

Based on the results of the risk assessment and the ARARs (Table 2), excavation and offsite disposal of the tailings and associated soil provides maximum protection to human health and the environment and is the selected alternative for the Site. The selected Removal Action is removal and offsite disposal of the tailings pile and associated soil (the area represented by samples MOJA-TA-1 and MOJA-S0-8) to an appropriate disposal facility. This alternative involves the removal of contaminated tailings and was selected due to effectiveness and implementability. The AM identifies the following work tasks:

1. A staging area will be selected that minimizes ground disturbance;

2. Contaminated tailings in the area represented by samples MOJA-TA-1 and MOJA-S0-8 exceeding the PRG for copper (447 mg/kg) will be excavated;

3. Waste characterization samples (TCLP and/or STLC) will be collected to confirm that the tailings classify as hazardous waste prior to offsite shipment as required by the disposal contractor;

4. Excavated tailings would be transported to an appropriate offsite disposal facility;

5. Confirmation sampling will be conducted (to confirm that all contaminated tailings have been removed); and

6. The Site will be revegetated or otherwise stabilized.

Excavation and offsite disposal involves the removal of the contaminated tailings and waste characterization as hazardous or non-hazardous waste (as determined by TCLP and/or STLC). All tailings and associated soils removed from the Site qualifying as hazardous wastes would be regulated under RCRA Subtitle C, which establishes a system for controlling hazardous waste from the time it is generated until its ultimate disposal (i.e., from “cradle to grave”). Any tailings or soils determined to be non-hazardous would be transported to an appropriate offsite waste disposal facility for disposal as a solid

May, 2024 Page | 5-6 waste as per 40 CFR 261.2. All excavated material will be managed in accordance with all applicable federal, state and local requirements. Confirmation samples will be collected to determine that all contamination has been removed. Removal to an offsite facility would provide the highest level of protection to human health and the environment as all contaminated materials would be removed.

Following remediation, excavated areas would be revegetated or otherwise stabilized.

NOTE: Subsequent review of the AM revealed a discrepancy in the PRG determination for copper.

Although Section 5.3 of the Revised Ecological Risk Assessment (Appendix A to the AM) states the Copper PRG is 447 mg/kg, Table A-3 and Table A-8 however presents 497 mg/kg as the Toxicity Reference Value (TRV) for plants as referenced in LANL 2017. Clarification was confirmed in August 2022, that the correct PRG for copper should be 497 mg/kg, rather than the specified 447 mg/kg. The PRG for copper at the Site is presented in Table 3.

5.2 Environmental Site Conditions

The environmental conditions at the Site were determined primarily from the results of the 2005 SI and the 2017 EE/CA. Combined, these two investigations provided a depiction of the soil, waste rock, tailings pile and background conditions at the Site. Groundwater was additionally assessed during the EE/CA and is also presented below.

5.2.1 Soil

During the 2005 SI, a total of six composite soil samples were collected at the Site from the tailings, waste rock piles, background soil, and mineralized bedrock. The EE/CA included collection of 29 composite (5-10 point) samples (including two duplicate samples) from tailings, waste rock and soil.

Analytical results from these samples are presented in Table 1.

5.2.2 Waste Rock

COPECs for the Site include arsenic, barium, copper and lead. Analytical results from waste rock samples did not exceed respective plant TRV concentrations of 91 mg/kg, 1,414 mg/kg, 497 mg/kg and 576 mg/kg, except as discussed below. Samples MOJA-WR-1/2, MOJA-WR-5 and MOJA-WR-6 had detected copper concentrations of 1,980 mg/kg, 1,110 mg/kg and 1,970 mg/kg, respectively. Although the detected concentrations for these samples exceeded the 497 mg/kg RBG for copper, the AM determined that “the release of metals from the waste rock piles is unlikely”, and waste rock was therefore removed as a source of concern.

5.2.3 Tailings

Tailings samples MOJA-HH-TA-01, MOJA-SO-8 and MOJA-TA-1 had detected concentrations of 4,700 mg/kg, 2,460 mg/kg and 5,410 mg/kg which were above the 497 mg/kg RG for copper (Plant TRV).

Concentrations of arsenic, barium and lead were below their respective plant TRV concentrations of 91 mg/kg, 1,414 mg/kg and 576 mg/kg.

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5.2.4 Groundwater

One groundwater sample was collected from the windmill located 0.2 miles south of the Site. The water level in this well (Well 08N14E05N001S, Fenner Valley Groundwater Basin) was measured by the United States Geological Survey (USGS) in 1981 at 73.1 feet bgs. The EE/CA compared the results to current chronic and acute NPS Freshwater ESVs for Aquatic Receptors and EPA drinking water Maximum Contaminant Levels (MCLs). Groundwater concentrations are below reporting limits for antimony, arsenic, cadmium, chromium, nickel, selenium, silver, and vanadium. Laboratory reporting limits are 15 µg/L for antimony, arsenic and selenium; 5 µg/L for chromium, nickel, silver, and vanadium; and 1 µg/L for cadmium. Low metals concentrations in the groundwater sample indicate that the Site is not impacting groundwater and that the metals present are likely the result of the naturally mineralized nature of bedrock at the Site.

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6 Removal Action Objectives and Removal Action Levels

This section presents the RAOs for the NTCRA for the Site. The RAOs should be as specific as possible but not so specific that the range of alternatives that can be developed is unduly limited. RAOs define what the Removal Action is intended to accomplish and were developed in the EE/CA based on the nature and extent of contamination. The sources of contamination identified in the EE/CA included arsenic, barium, cadmium, copper, lead, silver, vanadium and zinc in tailings, waste rock and soils.

Consequently, the RAO that has been established for the Site is to eliminate the potential for metals impacts to human health and the environment.

NOTE: The AM (and attached Revised Ecological Risk Assessment) subsequently limited the COPECs to copper, and the media to tailings.

The EE/CA and subsequent AM identified copper as the primary constituent of concern (COC) for all receptors. Risk-based PRGs for copper developed in the EE/CA and as revised in the AM are presented in Table 3. Based on these results, a PRG for copper of 497 mg/kg for the tailings pile would be protective of all ecological receptors. The AM stipulated that cleaning up to background levels, as proposed in the EE/CA, would not be necessary.

The AM further confirmed that low metals concentrations in the groundwater sample indicated that the Site has not impacted groundwater and the metals present are the result of the naturally mineralized nature of bedrock in the area. Thus, groundwater is not considered in the Removal Action Alternatives.

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7 Applicable or Relevant and Appropriate Requirements The ARARs for the Hidden Hills Mine Site identified in the EE/CA and AM are presented in Table 2.

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8 Removal Action Alternative The EE/CA developed four different removal action alternatives for the Hidden Hills Mine Site. These alternatives included:

Alternative 1 – No Action (Baseline Alternative);

Alternative 2 – Institutional Controls;

Alternative 3 – Offsite Disposal of Hazardous Mine Wastes and Placement of Cover Soils Over Non-Hazardous Mine Wastes; and

Alternative 4 – Full Removal and Offsite Disposal.

These alternatives were each evaluated relative to effectiveness, implementability, and cost. Based on a comparative analysis of these alternatives, the EE/CA recommended Alternative 4 – Full Removal and Offsite Disposal as the preferred alternative.

However, in consideration of the Revised Ecological Risk Assessment (Appendix A to the AM) and determination that the release of metals was unlikely from the Waste Rock at the Site, the AM specified a modified Alternative 3 (i.e., removal of only the tailings piles and associated soil, and offsite disposal) as the recommended alternative for the Site. This proposed Removal Action was selected due to effectiveness and implementability. It addresses releases from the tailings pile and associated soil (the area represented by samples MOJA-TA-1 and MOJA-S0-8). The following work tasks associated with this Removal Action include:

1. A staging area will be selected that minimizes ground disturbance;

2. Contaminated tailings in the area represented by samples MOJA-TA-1 and MOJA-S0-8 (Figure

4) exceeding the PRG for copper (497 mg/kg) will be excavated;

3. Waste characterization samples (TCLP and/or STLC) will be collected to confirm that the tailings classify as hazardous waste prior to offsite shipment as required by the disposal contractor;

4. Excavated tailings will be transported to an appropriate offsite disposal facility;

5. Confirmation sampling will be conducted (to confirm that all contaminated tailings have been removed); and

6. The Site will be revegetated or otherwise stabilized.

The AM further specified that the excavation and offsite disposal will involve the removal of the contaminated tailings and waste characterization as hazardous or non-hazardous waste (as determined by TCLP and/or STLC). All hazardous tailings and associated soils removed from the Site would be regulated under RCRA Subtitle C, which establishes a system for controlling hazardous waste from the time it is generated until its ultimate disposal (i.e., from "cradle to grave”). Any tailings or soils determined to be non-hazardous would be transported to an appropriate offsite waste disposal facility for disposal as a solid waste as per 40 CFR 261.2. All excavated material will be managed in accordance with all applicable federal, state and local requirements. Confirmation samples will be collected to determine that all contamination has been removed. Removal to an offsite facility would provide the highest level of protection to human health and the environment as all contaminated materials would be removed.

Following remediation, removal areas would be revegetated or otherwise stabilized.

The AM acknowledged that a small increase in risk to human health would be encountered during the excavation and transport phase of this work due to the potential for an increase in fugitive dust generation.

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Control of fugitive dusts will be conducted by wetting the tailings as required. Impacts associated with construction activities are considered short term and should not significantly impact human health.

Additionally, short term impacts to the threatened Desert Tortoise could be encountered during construction activities. Therefore, actions associated with Site reclamation work would need to be coordinated with Mojave National Preserve, U.S. Fish and Wildlife Service, and the State Department of Fish and Wildlife. To minimize effects on the threatened Desert Tortoise, appropriate tortoise mitigation will be determined in consultation with the U.S. Fish and Wildlife Service. Mitigations would include tortoise monitoring, biological surveys, and/or working outside of the tortoise active season.

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9 Removal Action Plan This Removal Action Plan (Plan) provides detail for completing the NTCRA activities at the Hidden Hills Mine Site. This Plan includes consideration for protection of Threatened and Endangered (T&E) species, preservation of historical Site features, Site preparation activities, excavation activities, protocol for screening soils and collection of confirmatory samples, waste characterization and disposal, Site restoration, and implementation of institutional controls.

9.1 Protection of Threatened and Endangered Species

The EE/CA identifies the T&E species that may occur at the Site based on the National Park Service Mojave National Preserve website (REMC, 2017). The T&E species include:

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

The Site is in the U.S. Fish & Wildlife Service’s Designated Desert Tortoise Habitat. The Desert Tortoise is listed as “threatened” under the United States federal Endangered Species Act. All work will be conducted in a manner protective of the Desert Tortoise as directed by NPS. Prior to conducting any work at the Site, all on-site personnel shall become familiar with the requirements for tortoise awareness, including watching the following videos:

https://www.nps.gov/media/video/view.htm?id=DBCCFE6F-F66A-FDCE-1909BC5E55030DFF, and https://www.insideedison.com/stories/protecting-threatened-tortoises-part-of-sces-environmental-stewardship

To be protective of all native wildlife, all on-Site personnel will always comply with food-storage and garbage disposal requirements.

In addition to the protection of T&E species, all work must be performed to prevent the introduction and/or spread of non-native plants. Personnel will inspect, remove, and properly dispose of invasive plant seed and plant parts or mud found on clothing, boots, tools, vehicles, and equipment prior to entering the preserve.