RIDGECREST REMEDIAL ACTION WORK PLAN — FRAMEWORK AND OUTLINE
Prepared Pursuant to Section VII of the Consent Decree in Civil Action No. 9:24-cv-00087-DWM
Site: Ridgecrest Copper-Molybdenum Mine 14200 Ridgecrest Mine Road Drummond, Granite County, Montana 59832
Prepared by: Pinnacle Creek Environmental, LLC 320 Willowbrook Drive, Suite 100 Bozeman, Montana 59715 Engagement Lead: Dr. Astrid Bergmann, P.E., Senior Principal
Legal Counsel: Calderwood & Harkness LLP Responsible Partner: Harrison Wexford Lead Associate: Elliot Nguyen Working Associate: Grace Nakamura Matter No. 1039-00003
Local Counsel: Holcroft, Dane & Weiss LLP 45 North Last Chance Gulch, Suite 600 Helena, Montana 59601 Lead Partner: Russell Dane
On Behalf of: Torchlight Mining Corp. (TSX: TRC / NYSE: TRC) 1800 Bayfield Tower, 401 Bay Street Toronto, Ontario M5H 2Y4, Canada
Torchlight Mining (U.S.) Inc. (EIN: 83-2047156)
Ridgecrest Mine Holdings, LLC (Montana SOS ID: E1042738)
Date: February 10, 2025
CONFIDENTIAL — Subject to Revision Prior to Final Submission
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Section 1: Executive Summary
1.1 Purpose and Scope of This Document
This document constitutes the Framework and Outline for the Ridgecrest Remedial Action Work Plan (the "Work Plan") required under the consent decree entered in United States of America and State of Montana v. Torchlight Mining Corp. et al., Civil Action No. 9:24-cv-00087-DWM, United States District Court for the District of Montana, Missoula Division (the "Consent Decree"). The Consent Decree, substantially final agreed text of which was dated February 3, 2025, and the execution version of which was lodged with the Court on February 19, 2025, establishes comprehensive remedial obligations applicable to the Ridgecrest Copper-Molybdenum Mine located at 14200 Ridgecrest Mine Road, Drummond, Granite County, Montana 59832 (the "Ridgecrest Site" or "Site").
Pursuant to Section VII of the Consent Decree, the Final Work Plan is due for submission to the United States Environmental Protection Agency ("EPA") and the United States Department of Justice ("DOJ") on or before December 14, 2025. This Framework and Outline is being submitted to EPA Region 8, and specifically to the designated EPA Project Coordinator, Dr. Samuel Tesfaye, for preliminary review and comment. The purpose of this early submission is to facilitate timely completion of the Final Work Plan by providing EPA, DOJ, the State of Montana (through the Montana Department of Environmental Quality, "MT DEQ"), and the court-appointed independent environmental monitor, Verdant Analytics, Inc., with a detailed description of the proposed structure, remedial approach, phasing, milestones, and supporting technical elements that will be developed in full in the Final Work Plan.
The Framework and Outline is not intended to constitute the Final Work Plan itself, and it expressly reserves the right of Torchlight Mining Corp. ("TMC") and its subsidiaries to revise, supplement, and refine the technical approaches, schedules, cost allocations, and other elements described herein during the development of the Final Work Plan. Nonetheless, this document reflects the current state of engineering analysis and legal review conducted by Pinnacle Creek Environmental, LLC ("Pinnacle Creek") and Calderwood & Harkness LLP ("C&H"), respectively, and is submitted in good faith as a meaningful step toward timely compliance with the Consent Decree.
This Framework and Outline has been prepared by C&H in coordination with Pinnacle Creek, with input from TMC's internal Environmental Health & Safety team, and has been reviewed for consistency with applicable Consent Decree requirements and Montana regulatory standards. It addresses the Ridgecrest Site only; separate work plans and compliance documents are being developed for the other three sites covered by the Consent Decree (Gallatin Chemical Works, Bonanza Creek Mine, and Kanawha Bend Chemical Plant).
1.2 Summary of Consent Decree Remedial Obligations at Ridgecrest
The Consent Decree requires TMC and its U.S. subsidiaries to perform comprehensive remedial actions at the Ridgecrest Site to address three principal categories of environmental contamination: (1) acid mine drainage ("AMD") from the open-pit mine, waste rock dumps, and associated mine workings; (2) tailings impoundment seepage and its impacts on surface water and groundwater; and (3) contamination of Flint Creek and its tributaries resulting from historical and ongoing discharges of contaminated water from the Site.
The negotiated remediation cost for the Ridgecrest Site is $198.6 million over a 30-year remediation obligation period, expressed in nominal dollars. The net present value of this obligation, calculated at a 3% discount rate, is approximately $134.9 million. This negotiated figure represents the resolution between TMC's cost estimate of $187.5 million (NPV: $127.4 million) and EPA's cost estimate of $224.0 million. The negotiated amount reflects technical discussions and compromise regarding treatment system scale, cover system design specifications, and the duration and intensity of long-term operations and maintenance activities. EPA's cost estimation was supported by the independent technical review of Cascade Geosciences, Inc., under the direction of Dr. Patricia Voss, Ph.D., Hydrogeology.
The remediation obligation period is 30 years, commencing on the Effective Date of the Consent Decree, which is targeted for June 14, 2025 (30 days after entry of the Consent Decree by the Court). Remedy construction at the Ridgecrest Site is required to commence no later than June 14, 2027, and the 30-year remediation obligation is scheduled to conclude on June 14, 2055.
In addition to the forward-looking remediation costs, the Consent Decree resolves several categories of retrospective liability. EPA past-cost reimbursement for removal actions previously conducted at the Ridgecrest Site totals $14.3 million, which represents a component of the overall $18.1 million in CERCLA § 107 cost recovery across all four sites. The civil penalty attributable to the Ridgecrest Site is $14.2 million, which forms part of the total civil penalty of $31.5 million assessed across all four sites in the Consent Decree. Finally, the Consent Decree requires TMC to fund a Supplemental Environmental Project ("SEP") in the form of the Flint Creek Watershed Restoration Project, budgeted at $5.5 million. This SEP encompasses stream-bank stabilization, riparian replanting, and fish-passage improvements along approximately 12 miles of Flint Creek downstream of the Ridgecrest Site. The SEP will be coordinated with, but is contractually and operationally separate from, the remedial action obligations addressed in this Work Plan.
1.3 Site Overview
The Ridgecrest Copper-Molybdenum Mine is located at 14200 Ridgecrest Mine Road, Drummond, Granite County, Montana 59832. The total controlled property encompasses approximately 8,700 acres, including buffer lands, with the mine footprint itself covering approximately 3,200 acres. The property is situated in the Flint Creek Valley of west-central Montana, within the Clark Fork River basin.
The Ridgecrest Site is an open-pit copper-molybdenum mine that has been operational since 1978. Active mining operations at the Site ceased on December 31, 2019, and the facility has since been in a closure and reclamation phase. During its operational life, the mine employed conventional open-pit extraction methods, with copper recovery conducted through a solvent-extraction/electrowinning ("SX/EW") circuit. The mine produced significant quantities of both copper and molybdenum concentrates over its approximately 41-year operational history.
Real property and mineral rights at the Ridgecrest Site are held by Ridgecrest Mine Holdings, LLC ("RMH"), a Montana limited liability company (Montana Secretary of State ID: E1042738). RMH is wholly owned by Torchlight Mining (U.S.) Inc. ("TMC-US"), a Delaware corporation (EIN: 83-2047156), which in turn is wholly owned by Torchlight Mining Corp. ("TMC"), a Canadian public company listed on the Toronto Stock Exchange (TSX: TRC) and the New York Stock Exchange (NYSE: TRC). All three entities are defendants in Civil Action No. 9:24-cv-00087-DWM and are signatories to the Consent Decree.
The Ridgecrest Site has been proposed for listing on the National Priorities List ("NPL") under CERCLA. Prior to the lodging of the Consent Decree, the Site was addressed under an Administrative Order on Consent ("AOC"), EPA Docket No. CERCLA-08-2022-0039, executed September 15, 2022. The Consent Decree supersedes and expands upon the remedial obligations previously established under the AOC, and sets forth a comprehensive remediation program that is the subject of this Work Plan.
1.4 Regulatory Framework
The remedial obligations at the Ridgecrest Site arise from alleged violations of three principal federal environmental statutes:
Clean Water Act ("CWA") § 301. The United States alleges that TMC and its subsidiaries discharged pollutants, including acid mine drainage and contaminated seepage from the tailings impoundment, into Flint Creek and its tributaries without authorization under a National Pollutant Discharge Elimination System ("NPDES") permit or a Montana Pollutant Discharge Elimination System ("MPDES") permit, in violation of CWA § 301, 33 U.S.C. § 1311.
Resource Conservation and Recovery Act ("RCRA") § 3004. The United States alleges that TMC and its subsidiaries mismanaged hazardous waste generated by the SX/EW circuit at the Ridgecrest Site, including spent electrolyte solutions and organic solvent residuals, in violation of RCRA § 3004, 42 U.S.C. § 6924. Specifically, the government alleges that certain waste streams from the SX/EW process constituted characteristically hazardous waste (for toxicity and/or ignitability) and were stored and disposed of without compliance with applicable RCRA permitting and management standards.
Comprehensive Environmental Response, Compensation, and Liability Act ("CERCLA") § 107. The United States seeks recovery of response costs incurred by EPA in connection with removal actions conducted at the Ridgecrest Site between January 2021 and May 2023, totaling $14.3 million, pursuant to CERCLA § 107(a), 42 U.S.C. § 9607(a).
The State of Montana is a co-plaintiff in Civil Action No. 9:24-cv-00087-DWM. The State's interests are represented by Annika Hedstrom, Assistant Attorney General, Montana Attorney General's Office, Environmental Bureau. The State's parallel regulatory involvement is primarily through MT DEQ, which administers the MPDES permit program and the hard rock mining reclamation permit program applicable to the Ridgecrest Site. Coordination with MT DEQ on state permitting and parallel state compliance obligations is being managed through local counsel Holcroft, Dane & Weiss LLP (Russell Dane, Lead Partner).
The case is pending before the Honorable Donald W. Molloy, Senior Judge, United States District Court for the District of Montana, Missoula Division.
1.5 Independent Monitor
Pursuant to the Consent Decree, Verdant Analytics, Inc. has been appointed as the court-appointed independent environmental monitor for the remedial actions at the Ridgecrest Site and the three other sites covered by the Consent Decree. The Senior Managing Director responsible for the Ridgecrest monitoring engagement is Dr. Kamal Osei. Verdant Analytics maintains its principal office at 1550 NW Couch Street, Suite 400, Portland, Oregon 97209.
Verdant Analytics will perform quarterly compliance audits, assessing TMC's adherence to the remedial milestones, performance standards, and reporting requirements established in the Consent Decree and the Final Work Plan. In addition, Verdant will prepare annual public reports summarizing the status of remediation at each covered site, including the Ridgecrest Site, and will provide independent assessments of remedy effectiveness, environmental performance, and compliance trends.
The estimated costs for Verdant Analytics' monitoring services over the initial five-year monitoring period are approximately $4.8 million, reflecting annual costs of approximately $960,000 per year. These monitoring costs are borne by TMC pursuant to the terms of the Consent Decree and the Independent Monitor Agreement (execution version dated February 15, 2025).
The Work Plan must be designed to facilitate Verdant's monitoring and audit activities. Specifically, the Work Plan must establish sampling protocols, data management systems, reporting formats, and access provisions that enable Verdant to perform its oversight functions efficiently and without undue reliance on TMC's own reporting. The Final Work Plan will include provisions for independent split sampling, data verification procedures, and access to all relevant site areas and records by Verdant personnel.
Section 2: Site Description and Background
2.1 Site Location and Physical Setting
The Ridgecrest Copper-Molybdenum Mine is located in Granite County, Montana, approximately 12 miles northwest of the town of Drummond. The Site is accessed via Ridgecrest Mine Road, which connects to Montana State Highway 1 approximately 7 miles south of the mine gate. The property is situated in the upper Flint Creek Valley, within the broader Clark Fork River drainage basin. The nearest incorporated community is Drummond (population approximately 300), located along Interstate 90 at the confluence of Flint Creek and the Clark Fork River.
Topography and Elevation. The Site occupies a complex topographic setting characterized by moderately steep terrain typical of the Northern Rocky Mountain physiographic province. Elevations within the mine footprint range from approximately 5,200 feet above mean sea level at the floor of the open pit to approximately 6,800 feet at the crest of the surrounding ridgelines. The tailings impoundment occupies a broad, gently sloping valley at an average elevation of approximately 5,600 feet. The natural drainage pattern is generally south-southwest toward Flint Creek, with several ephemeral and intermittent tributaries crossing or originating within the mine property.
Climate. The Ridgecrest area experiences a continental mountain climate characterized by cold, snowy winters and warm, relatively dry summers. Average annual precipitation at the Site is approximately 14–16 inches, with the majority falling as snow between October and April. Spring snowmelt, typically occurring from mid-April through early June, generates the peak surface water flows and is a primary driver of seasonal AMD production. Average temperatures range from approximately 15°F in January to approximately 70°F in July. The frost-free growing season extends approximately 90–110 days, generally from mid-June through mid-September, which constrains the scheduling of revegetation and outdoor construction activities.
Regional Geologic Setting. The Ridgecrest Site is underlain by a sequence of Cretaceous-age granodiorite and quartz monzonite intrusive rocks of the Boulder Batholith, overlain in places by Tertiary-age volcanic and sedimentary units. The copper-molybdenum mineralization is associated with a porphyry system hosted within the granodiorite. Overburden materials consist of glacial till, alluvial deposits, and colluvial slope wash of variable thickness, ranging from less than 5 feet on ridgelines to more than 60 feet in valley bottoms. Bedrock is moderately to highly fractured, with fracture intensity increasing in the vicinity of the mine workings and open pit, which facilitates groundwater movement and AMD generation.
Surface Water Features. Flint Creek is the principal surface water feature in the vicinity of the Ridgecrest Site. Flint Creek originates approximately 20 miles upstream of the Site, flows in a generally southward direction through the Flint Creek Valley, and ultimately discharges to the Clark Fork River near Drummond. Several unnamed tributaries of Flint Creek traverse or originate within the mine property, including two intermittent drainages that receive direct surface runoff from the waste rock dumps and the tailings impoundment. The proximity of the mine workings and waste management areas to the Flint Creek watershed is a central factor in the contamination pathways that are the subject of this remediation effort. Flint Creek is classified by the Montana Department of Environmental Quality as a B-1 water body, supporting cold-water aquatic life and recreation uses.
Groundwater. Regional groundwater occurs in both the fractured bedrock aquifer system and the overlying alluvial aquifer in the Flint Creek Valley. Depth to the water table varies significantly across the Site, ranging from approximately 15–30 feet in valley-bottom alluvial areas to more than 100 feet beneath ridgelines. Groundwater flow in the alluvial aquifer is generally aligned with surface topography, flowing south-southwest toward Flint Creek. In the fractured bedrock system, flow is controlled by fracture orientation and connectivity. Interaction between groundwater and surface water is a significant concern: Flint Creek alternates between gaining and losing conditions in the vicinity of the Site, and several stream segments downstream of the tailings impoundment are documented gaining reaches, meaning that contaminated groundwater discharges contribute directly to surface water quality degradation. Detailed hydrogeologic characterization of these interactions has been a focus of the Remedial Investigation conducted by Pinnacle Creek Environmental.
2.2 Mining History and Operations
Mine development at the Ridgecrest Site commenced in 1978, following several years of exploration and feasibility studies. The open-pit copper-molybdenum mine was developed and operated initially by Ridgecrest Resources, Inc., which was subsequently acquired by TMC in 1997 through a corporate merger. TMC continued to operate the mine through its U.S. subsidiary, Torchlight Mining (U.S.) Inc., and property ownership was vested in Ridgecrest Mine Holdings, LLC.
Mining Methods. The mine employed conventional open-pit mining methods, utilizing large-scale truck-and-shovel operations to extract copper- and molybdenum-bearing ore from the porphyry deposit. Ore was crushed and processed through a concentrator facility, producing copper and molybdenum concentrates for sale to smelters. Beginning in 1994, TMC supplemented concentrate production with a solvent-extraction/electrowinning ("SX/EW") circuit for recovery of copper from oxide ores and leach solutions. The SX/EW circuit utilized organic solvents (principally kerosene-based diluent and an oxime extractant) and sulfuric acid electrolyte to produce copper cathode on site.
Production History. Over its operational life, the Ridgecrest Mine produced approximately 2.1 billion pounds of copper and 180 million pounds of molybdenum. Peak annual copper production occurred in the 2005–2010 period, when output averaged approximately 75 million pounds per year. Molybdenum production reached its peak in 2008 at approximately 8.5 million pounds. The mine was one of the larger copper producers in the northern Rocky Mountain region and a significant employer in Granite County during its operational years.
Cessation of Mining. Active mining operations at the Ridgecrest Site ceased on December 31, 2019, following a sustained period of depressed copper and molybdenum prices that rendered continued operations uneconomic. TMC publicly announced the cessation of mining in October 2019 and initiated the transition to a closure and reclamation phase. The SX/EW circuit was placed in a standby condition in January 2020 and subsequently began formal decommissioning in mid-2021.
Current Site Conditions. The principal physical features of the Ridgecrest Site in its current state include: (a) the open pit, which measures approximately 4,000 feet in length, 2,500 feet in width, and 900 feet in depth, with an impounded water body ("pit lake") that has risen approximately 350 feet from the pit floor since cessation of dewatering; (b) approximately 180 million tons of waste rock distributed across three major waste rock dumps covering approximately 650 acres; (c) the tailings impoundment, covering approximately 425 acres and containing approximately 320 million tons of processed tailings; (d) the SX/EW plant, which is currently undergoing decommissioning; (e) an extensive network of haul roads, maintenance facilities, and ancillary infrastructure in various states of decommissioning; and (f) the concentrator and crushing plant, which have been partially dismantled.
2.3 Tailings Impoundment
The Ridgecrest tailings impoundment is located in a natural valley approximately 1.5 miles south of the open pit and covers an area of approximately 425 acres. The impoundment was constructed utilizing the upstream construction method, with the embankment raised progressively over the operational life of the mine using compacted tailings and cycloned sand. The final embankment crest elevation is approximately 5,640 feet above mean sea level, with a maximum embankment height of approximately 280 feet above the original valley floor.
Construction and Liner System. The original embankment was constructed beginning in 1978 and was raised incrementally through 2019. The impoundment is underlain by a compacted clay liner of variable thickness (approximately 12–24 inches), installed over the original valley bottom during initial construction. No synthetic geomembrane liner was installed. The adequacy and integrity of the clay liner have been subjects of technical debate during the Remedial Investigation, with Pinnacle Creek's hydrogeologic analysis suggesting that the liner has experienced localized breaches and degradation over the impoundment's 41-year operational life.
Tailings Characteristics. The tailings consist of finely ground rock (approximately 80% passing 200 mesh) with a mineralogy dominated by quartz, feldspar, biotite, and minor pyrite, chalcopyrite, and molybdenite. Acid-base accounting testing conducted during the Remedial Investigation indicates that the tailings have a net acid-generating potential, with an average net neutralizing potential (NNP) of approximately -45 kg CaCO3/ton. This acid-generating potential is a primary driver of ongoing AMD production from the impoundment.
Seepage. Documented seepage from the tailings impoundment to Flint Creek and its tributaries has been identified at seven locations along the downstream toe of the embankment and at three additional locations where contaminated groundwater from the impoundment discharges to gaining stream segments. Total estimated seepage volume is approximately 150–250 gallons per minute during baseflow conditions, increasing to 400–600 gallons per minute during spring runoff. Seepage water quality is characterized by elevated concentrations of dissolved copper, molybdenum, selenium, and sulfate, consistent with the AMD chemistry described in Section 2.4 below.
Current Condition. The embankment is currently maintained under the existing hard rock mining reclamation permit administered by MT DEQ. Ongoing monitoring includes piezometric measurements, settlement surveys, seepage volume measurements, and periodic geotechnical stability assessments. The most recent stability assessment, completed by Pinnacle Creek in October 2024, concluded that the embankment meets minimum static stability requirements (factor of safety > 1.5 for static conditions) but identified concerns regarding seismic stability and long-term performance under maximum credible earthquake loading. The decant system remains operational to manage surface water accumulation on the impoundment, but the system's long-term adequacy for the post-closure period requires evaluation as part of the remedial design.
2.4 Nature and Extent of Contamination
2.4.1 Acid Mine Drainage (AMD)
Acid mine drainage is the most significant and widespread environmental impact at the Ridgecrest Site. AMD is generated when sulfide minerals (principally pyrite and chalcopyrite) in exposed mine workings, waste rock, and tailings are oxidized upon contact with air and water, producing sulfuric acid and releasing dissolved metals into drainage water.
Sources. The three principal sources of AMD at the Ridgecrest Site are: (1) the open pit and its rising pit lake, which generates acidic water through oxidation of exposed sulfide minerals on the pit walls and interaction with fractured bedrock in the pit shell; (2) the waste rock dumps, which collectively contain approximately 180 million tons of sulfide-bearing material with significant acid-generating potential; and (3) the tailings impoundment, where seepage through the tailings mass leaches dissolved metals and generates acidic drainage.
AMD Chemistry. Based on the comprehensive water quality dataset developed during the Remedial Investigation, AMD from the Ridgecrest Site is characterized by the following average concentrations of contaminants of concern ("COCs"):
• Dissolved copper: average 47 mg/L, which exceeds the Montana chronic aquatic life surface-water quality standard of 0.0087 mg/L by a factor of approximately 5,400.
• Molybdenum: average 1.2 mg/L, exceeding applicable standards.
• Selenium: average 0.082 mg/L, exceeding the Montana chronic aquatic life surface-water quality standard.
• Sulfate: average 3,400 mg/L, substantially elevated above background conditions.
Additional constituents of concern include zinc, cadmium, arsenic, manganese, and iron, which are present at elevated concentrations in AMD from one or more source areas.
Volume and Flow Rate. Total AMD discharge from all sources at the Ridgecrest Site is estimated at approximately 600–1,200 gallons per minute during baseflow conditions, increasing to 2,000–3,500 gallons per minute during spring runoff and major storm events. The open pit is the single largest source, contributing approximately 40% of total AMD volume, followed by the tailings impoundment seepage (approximately 35%) and the waste rock dumps (approximately 25%).
Seasonal Variability. AMD production exhibits pronounced seasonal variability, with peak flows and contaminant loads occurring during the spring snowmelt period (April through June) and during significant summer thunderstorm events. During the winter low-flow period (November through March), AMD volumes are reduced, but concentrations of dissolved metals tend to increase due to evaporative concentration and reduced dilution.
2.4.2 Groundwater Contamination
Groundwater contamination associated with the Ridgecrest Site has been delineated through a monitoring well network of 47 wells installed during the Remedial Investigation and previous site characterization efforts. The monitoring well network covers the alluvial aquifer in the Flint Creek Valley, the fractured bedrock aquifer in the vicinity of the mine workings, and the area immediately beneath and downgradient of the tailings impoundment.
Plume Delineation. Dissolved copper, the primary COC in groundwater, has been detected at concentrations exceeding the Montana groundwater quality standard in a plume extending approximately 2.3 miles downgradient of the tailings impoundment in the alluvial aquifer. The maximum detected groundwater copper concentration is 28 mg/L, observed in a monitoring well located 400 feet downgradient of the impoundment toe. Molybdenum and sulfate plumes extend approximately 1.8 miles and 3.1 miles downgradient, respectively, reflecting the greater mobility of sulfate in groundwater systems.
Groundwater–Surface Water Interaction. As noted in Section 2.1, Flint Creek includes several gaining reaches in the vicinity of the Ridgecrest Site where contaminated groundwater discharges directly to the creek. Synoptic flow measurements and mass-balance analyses conducted by Pinnacle Creek indicate that groundwater discharge contributes approximately 15–25% of the dissolved copper loading observed in Flint Creek downstream of the Site during baseflow conditions. This groundwater–surface water interaction pathway is a critical consideration in remedy design, as surface water quality targets in Flint Creek cannot be achieved without addressing the groundwater contamination that feeds these gaining stream segments.
2.4.3 Surface Water Impacts — Flint Creek
The environmental impacts to Flint Creek from the Ridgecrest Site are well documented through an extensive water quality monitoring dataset spanning more than a decade. Surface water quality monitoring has been conducted at multiple stations upstream and downstream of the Site, including stations at the margins of the mine property and at locations extending several miles downstream toward Drummond.
Water Quality Data. upstream of the Ridgecrest Site, Flint Creek water quality generally meets applicable Montana surface-water quality standards, with dissolved copper concentrations typically in the range of 0.002–0.005 mg/L. Immediately downstream of the mine property, dissolved copper concentrations increase sharply, with average concentrations of 0.35 mg/L at the first downstream monitoring station, declining to approximately 0.08 mg/L at a station located 8 miles downstream. These concentrations exceed the Montana chronic aquatic life surface-water quality standard of 0.0087 mg/L at all downstream monitoring stations within at least 12 miles of the Site.
Biological Assessments. Macroinvertebrate community surveys and fish population assessments conducted during the Remedial Investigation document significant ecological impairment of Flint Creek downstream of the Ridgecrest Site. Macroinvertebrate diversity indices (Hilsenhoff Biotic Index) indicate "severely impaired" conditions in the first 4 miles downstream, transitioning to "moderately impaired" conditions for the subsequent 8 miles. Fish surveys document a near-complete absence of westslope cutthroat trout (Oncorhynchus clarkii lewisi), a Montana Species of Concern, in the impaired stream reaches, compared to healthy populations in upstream reference reaches. Bull trout (Salvelinus confluentus), a federally listed threatened species, have been historically documented in Flint Creek but have not been observed in the impacted reaches during recent surveys.
Extent of Impacted Reaches. The Remedial Investigation concludes that approximately 14 miles of Flint Creek and its tributaries are impacted by AMD and contaminated seepage from the Ridgecrest Site, extending from the northern boundary of the mine property downstream to approximately 2 miles upstream of the Clark Fork River confluence. The Final Work Plan will include detailed maps delineating the impacted stream reaches and associated compliance points.
2.4.4 Hazardous Waste from SX/EW Circuit
The SX/EW circuit at the Ridgecrest Site operated from 1994 through 2019 and generated several waste streams that have been identified as hazardous waste under RCRA. The government alleges that these waste streams were managed in a manner inconsistent with RCRA requirements, giving rise to the RCRA § 3004 claims in the Complaint.
Characterization. Waste streams from the SX/EW circuit include: (a) spent organic solvent, consisting of kerosene-based diluent and degraded oxime extractant, which exhibits the characteristic of ignitability (D001); (b) spent electrolyte solutions containing elevated concentrations of copper, lead, and cadmium, which exhibit the characteristic of toxicity for multiple metals; and (c) filter cake and sludge from the electrolyte purification process, which also exhibits toxicity characteristics. RCRA hazardous waste determinations were performed by Pinnacle Creek as part of the Remedial Investigation and confirmed by EPA's sampling during the 2021–2023 removal action.
Storage and Disposal History. The government alleges that TMC stored spent organic solvent in unpermitted above-ground storage tanks and in open-top tanks without adequate secondary containment, and that spent electrolyte and filter cake were disposed of in an unlined surface impoundment adjacent to the SX/EW plant. TMC has asserted defenses to certain of these allegations but has agreed, under the Consent Decree, to conduct RCRA closure of all identified hazardous waste management units and to properly manage, treat, and dispose of all residual hazardous materials at the Site.
Current Inventory. The current inventory of residual hazardous materials on site includes approximately 145,000 gallons of spent organic solvent in storage tanks, approximately 80,000 gallons of spent electrolyte in process tankage, and approximately 3,200 cubic yards of filter cake and sludge in the unlined surface impoundment. Additionally, soil and groundwater contamination associated with releases from the SX/EW area requires characterization and potential remediation.
2.5 Previous Investigations and Removal Actions
EPA Removal Action. EPA commenced a removal action at the Ridgecrest Site on January 15, 2021, in response to an observed fish kill in Flint Creek in the fall of 2020 attributed to a surge in AMD discharge following an unusually heavy autumn rain event. The removal action was completed on May 30, 2023, at a total cost to EPA of $14.3 million. The scope of the removal action included: (a) emergency stabilization of a seepage breakout on the downstream face of the tailings impoundment embankment; (b) installation of a temporary lime-dosing water treatment system to reduce dissolved metals concentrations in the most heavily impacted tributaries; (c) construction of temporary diversion channels to redirect AMD away from Flint Creek; and (d) removal and off-site disposal of approximately 1,500 cubic yards of contaminated sediment from the creek bed immediately downstream of the mine property. The removal action was effective in reducing acute impacts to Flint Creek but was not designed to achieve long-term compliance with water quality standards.
Administrative Order on Consent. EPA and TMC entered into an Administrative Order on Consent ("AOC"), EPA Docket No. CERCLA-08-2022-0039, on September 15, 2022. The AOC required TMC to: (a) continue operation and maintenance of the temporary water treatment system installed during the removal action; (b) conduct additional site characterization necessary to support remedy selection; and (c) submit preliminary remedial design concepts for long-term AMD management. The Consent Decree supersedes and expands upon the obligations established in the AOC, and the AOC will be terminated upon the Effective Date of the Consent Decree.
Remedial Investigation / Feasibility Study. Pinnacle Creek Environmental, LLC, under the direction of Dr. Astrid Bergmann, P.E., has served as the lead environmental consultant for the Remedial Investigation and Feasibility Study ("RI/FS") at the Ridgecrest Site. The RI/FS was initiated in January 2023 and the Remedial Investigation report was substantially complete as of November 2024. The Feasibility Study, which evaluated remedy alternatives and supported the remedy selection reflected in the Consent Decree, was completed in December 2024. The RI/FS findings provide the technical foundation for the remedial approach described in this Framework and Outline and will be incorporated by reference in the Final Work Plan.
Government Technical Review. The government's independent technical review of the RI/FS and cost estimation was performed by Cascade Geosciences, Inc., under the direction of Dr. Patricia Voss, Ph.D., Hydrogeology. Cascade Geosciences' review contributed to EPA's cost estimation for remedy selection and identified several areas of technical disagreement regarding treatment system capacity, cover system performance, and long-term O&M requirements. These disagreements were addressed during Consent Decree negotiations and resulted in the negotiated cost of $198.6 million.
Data Gaps. The Remedial Investigation identified several data gaps that will require additional characterization work during Phase 1 of the remedial action. These include: (a) additional hydrogeologic testing to refine the understanding of groundwater–surface water interaction in key gaining reaches of Flint Creek; (b) geotechnical investigation of foundation conditions for the proposed AMD treatment plant; (c) additional tailings characterization to support detailed design of the impoundment cover system; and (d) further delineation of soil and groundwater contamination in the SX/EW area. Plans for addressing these data gaps will be developed in the Final Work Plan.
2.6 Surrounding Land Uses and Receptors
Nearby Communities and Residences. The nearest residential properties to the Ridgecrest Site are located along Flint Creek approximately 3 miles south of the mine property boundary. The town of Drummond, located approximately 12 miles downstream, represents the nearest incorporated community. Several ranching operations are located in the Flint Creek Valley between the mine property and Drummond, with both residential buildings and agricultural operations in proximity to the creek. Federal public lands managed by the Bureau of Land Management and the U.S. Forest Service (Beaverhead-Deerlodge National Forest) adjoin the mine property to the north and west.
Ecological Receptors. Flint Creek supports a cold-water fishery that includes westslope cutthroat trout, a Montana Species of Concern, and historically included bull trout, which is listed as threatened under the Endangered Species Act. Riparian habitat along Flint Creek and its tributaries supports diverse communities of birds, mammals, and amphibians. Wetlands associated with Flint Creek and its tributaries in the vicinity of the mine provide important ecological functions, including flood attenuation, sediment trapping, and wildlife habitat. These ecological receptors are primary drivers of the remedial action objectives for the Ridgecrest Site.
Downstream Water Uses. Water from Flint Creek is appropriated for irrigation and livestock watering by several ranching operations in the Flint Creek Valley downstream of the mine. While no municipal water supplies draw directly from Flint Creek between the mine and Drummond, the creek contributes flow to the Clark Fork River, which is a source of municipal water for downstream communities. Protection of these water uses is a consideration in remedy design and performance standard development.
Flint Creek Watershed Restoration Project (SEP). The $5.5 million SEP required by the Consent Decree encompasses stream-bank stabilization, riparian replanting, and fish-passage improvements along approximately 12 miles of Flint Creek downstream of the Ridgecrest Site. The SEP is designed to address physical habitat degradation that has compounded the effects of water quality impairment from the mine, and to accelerate ecological recovery once water quality improvements are achieved through the remedial action. The SEP is contractually separate from the remedial action Work Plan but will be coordinated to maximize ecological benefit and avoid redundancy of effort.
Section 3: Remedial Action Objectives (RAOs)
3.1 Overall Remedial Goals
The remedial action objectives for the Ridgecrest Site, as established in the Consent Decree and further developed through the RI/FS process, are as follows:
1. Eliminate unpermitted discharges. Eliminate all unpermitted discharges of pollutants from the Ridgecrest Site to Flint Creek, its tributaries, and all other waters of the United States, thereby achieving compliance with CWA § 301 and applicable MPDES permit conditions.
2. Achieve surface water quality standards. Achieve and maintain compliance with applicable Montana surface-water quality standards for all COCs at designated compliance points on Flint Creek and its tributaries, with particular emphasis on achieving the chronic aquatic life standard for copper of 0.0087 mg/L.
3. Stabilize and contain the tailings impoundment. Stabilize the tailings impoundment embankment to ensure long-term geotechnical integrity and construct containment systems to prevent further seepage of contaminated water from the impoundment to surface water and groundwater.
4. Manage hazardous waste. Properly inventory, characterize, manage, treat, and/or dispose of all hazardous wastes associated with the SX/EW circuit in compliance with RCRA § 3004 and applicable RCRA closure and corrective action requirements.
5. Restore groundwater quality. Restore groundwater quality to applicable Montana groundwater quality standards and/or federal maximum contaminant levels ("MCLs") at designated compliance points within a reasonable timeframe, recognizing that groundwater restoration is a long-term objective that may extend through and beyond the 30-year remediation obligation period.
6. Long-term site stability. Achieve and maintain long-term site stability to protect human health and the environment, including ensuring the physical integrity of mine waste containment structures, maintaining institutional controls, and providing for long-term stewardship of the Site beyond the active remediation period.
3.2 Contaminant-Specific Cleanup Standards
The remedial action will be designed to achieve the following contaminant-specific cleanup standards, which are derived from applicable regulatory standards, risk-based calculations, and the performance requirements of the Consent Decree:
Surface Water Standards. The surface water cleanup standards for the Ridgecrest Site are based on Montana surface-water quality standards for B-1 classified waters, as codified in the Administrative Rules of Montana (ARM) Title 17, Chapter 30, Subchapter 6. The primary standards applicable to Ridgecrest COCs include:
• Copper: 0.0087 mg/L (chronic aquatic life standard, hardness-dependent, calculated at the site-specific hardness)
• Molybdenum: applicable standard as determined under Montana DEQ guidance
• Selenium: 0.005 mg/L (chronic aquatic life standard)
• Sulfate: 250 mg/L (secondary standard for drinking water / narrative standard for aquatic life)
• pH: 6.5–8.5
Groundwater Standards. Groundwater cleanup standards are based on Montana groundwater quality standards (ARM Title 17, Chapter 30, Subchapter 10) and, where applicable, federal MCLs established under the Safe Drinking Water Act. The specific cleanup standards for each COC in groundwater will be established in the Final Work Plan based on the applicable Montana standards and federal MCLs.
Soils and Tailings. Cleanup levels for soils and tailings will be derived from site-specific human health and ecological risk assessments conducted as part of the RI/FS. Risk-based cleanup levels will be developed using EPA risk assessment guidance (RAGS) and will reflect both current and reasonably anticipated future land uses at the Site.
Hazardous Waste. The SX/EW hazardous waste management units will be closed in accordance with RCRA closure and corrective action standards, as specified in 40 CFR Part 265, Subpart G, and applicable Montana hazardous waste management rules.
3.3 Points of Compliance
Surface Water Compliance Points. Surface water compliance monitoring will be conducted at designated locations on Flint Creek and its tributaries where applicable water quality standards must be met. The specific locations of compliance points will be established in the Final Work Plan, in consultation with EPA and MT DEQ. Anticipated compliance points include: (a) a point immediately downstream of the mine property boundary on Flint Creek; (b) points on each tributary that receives discharge from the Site, measured at or near the confluence with Flint Creek; and (c) a downstream integration point on Flint Creek located approximately 8 miles downstream of the Site, where mixing of all site-related inputs with ambient flows is expected to be complete.
Groundwater Compliance Points. Groundwater compliance monitoring will be conducted at designated monitoring wells within and downgradient of the tailings impoundment and the SX/EW area. The monitoring well network will be refined during Phase 1 based on additional hydrogeologic characterization. Compliance wells will be located at the downgradient edge of the source areas (near-field compliance) and at intermediate locations between the source areas and Flint Creek (far-field compliance).
The detailed compliance monitoring network, including specific well and station identifications, analyte lists, sampling frequencies, and statistical evaluation methods, will be set forth in the Sampling and Monitoring Plan to be developed as part of the Final Work Plan and cross-referenced in the companion Draft Sampling and Monitoring Plan (All Sites), dated February 24, 2025 (see Section 8 below).
Section 4: Remedy Selection Summary
4.1 Selected Remedy Overview
The selected remedy for the Ridgecrest Site, as agreed upon in the Consent Decree, addresses three principal components of environmental contamination:
1. Acid Mine Drainage Treatment System — Construction and long-term operation of a permanent active water treatment plant to intercept, collect, and treat AMD from the open pit, waste rock dumps, and other sources prior to discharge.
2. Tailings Impoundment Stabilization and Containment — Installation of a seepage interception and collection system, construction of an engineered cover system over the tailings surface, embankment stability improvements, and stormwater management infrastructure.
3. Flint Creek Water Quality Restoration — Implementation of in-stream and riparian measures to address contamination that has already reached Flint Creek, coordination with the SEP, natural attenuation evaluation for downstream areas, and a comprehensive biomonitoring program to track ecological recovery.
In addition, the remedy includes proper management and disposal of hazardous waste from the SX/EW circuit and RCRA closure of associated hazardous waste management units, as well as institutional controls to restrict land use and ensure long-term protection of human health and the environment.
The remedy represents a negotiated approach, with the total estimated cost of $198.6 million over 30 years reflecting a resolution between TMC's estimate of $187.5 million and EPA's estimate of $224.0 million. The negotiations addressed specific technical disagreements regarding treatment system design capacity, cover system specifications, and O&M requirements over the 30-year period.
4.2 Component 1: Acid Mine Drainage Treatment
Active Water Treatment Plant. The centerpiece of the AMD management component is the construction and operation of a permanent active water treatment plant ("WTP") designed to collect, neutralize, and treat AMD from the Ridgecrest Site prior to any discharge to Flint Creek. The WTP is anticipated to utilize a high-density sludge ("HDS") process, which is a proven technology for treatment of AMD at mining sites with similar water chemistry. The HDS process involves lime neutralization of acidic water, precipitation of dissolved metals as hydroxide sludge, and sedimentation in high-rate thickeners to produce a high-density, dewatered sludge suitable for on-site disposal.
The WTP will be designed to treat a peak flow of approximately 3,500 gallons per minute during spring runoff conditions, with a sustained treatment capacity of approximately 1,200 gallons per minute during baseflow periods. Effluent quality targets will be established to ensure that treated water discharged to Flint Creek meets all applicable Montana surface-water quality standards, including the copper standard of 0.0087 mg/L.
Alternative or supplemental treatment technologies, including passive treatment systems (constructed wetlands, anoxic limestone drains) and sulfate-reducing bioreactors, will be evaluated during the remedial design process and may be incorporated as components of the overall treatment system where they can enhance performance or reduce long-term O&M costs.
Source Control Measures. In addition to the WTP, the remedy includes source control measures designed to intercept AMD at or near its points of generation and convey it to the WTP for treatment. These measures include: (a) collection of AMD from the open pit via a pump-back system as the pit lake rises; (b) installation of toe drains and collection ditches at the base of the waste rock dumps to capture AMD before it reaches surface water; and (c) diversion channels to route clean surface water around contaminated areas, reducing the volume of water requiring treatment. The source control system will be designed to function effectively across the range of seasonal flow conditions experienced at the Site.
Sludge Management. The HDS process will generate a significant volume of treatment sludge containing precipitated metal hydroxides. Sludge production is estimated at approximately 50–80 dry tons per day during peak treatment periods. Sludge will be dewatered using belt filter presses or similar equipment and disposed of in a dedicated, engineered on-site sludge repository. The sludge repository will be designed with an appropriate liner system, leachate collection, and cover system to prevent release of metals to the environment. RCRA hazardous waste characterization testing will be performed on representative sludge samples to confirm that the sludge does not exhibit hazardous waste characteristics; if it does, disposal arrangements will be modified accordingly.
Interim Measures. Pending construction and commissioning of the permanent WTP, the existing temporary lime-dosing treatment system installed during EPA's removal action will continue to operate under the terms of the AOC and the Consent Decree. TMC will maintain the interim system in good working order and will operate it in a manner that minimizes AMD impacts to Flint Creek during the pre-design and construction phases.
4.3 Component 2: Tailings Impoundment Stabilization
Seepage Interception and Collection. A seepage interception and collection system will be installed along the downstream toe of the tailings impoundment embankment and at other identified seepage discharge points. The system may include some or all of the following elements: (a) toe drain trenches backfilled with drainage aggregate and perforated collection pipe; (b) a groundwater cutoff wall (e.g., soil-bentonite slurry wall or cement-bentonite barrier) installed in the foundation soils downgradient of the embankment to intercept deep seepage; and (c) sumps, pumps, and conveyance pipelines to route collected seepage to the AMD treatment plant. The specific design of the seepage collection system will be developed during the remedial design phase based on detailed geotechnical and hydrogeologic characterization.
Impoundment Cover System. An engineered cover system will be constructed over the approximately 425-acre tailings surface to reduce infiltration, limit oxygen contact with acid-generating tailings, and support revegetation. The cover system design will be developed during the remedial design phase and is anticipated to include: (a) a low-permeability barrier layer (compacted clay or geosynthetic clay liner) to minimize infiltration; (b) a drainage layer to manage lateral flow above the barrier; (c) a growth medium layer (soil cover) of sufficient depth to support native vegetation establishment and provide protection against erosion and frost damage; and (d) a surface layer of topsoil seeded with native grasses, shrubs, and, where feasible, native tree species appropriate to the elevation and climate. Cover system performance requirements, including maximum allowable infiltration rates and minimum vegetation establishment standards, will be established in the Final Work Plan.
Embankment Stability Improvements. Based on the findings of Pinnacle Creek's October 2024 stability assessment, additional embankment stability improvements may be required to ensure long-term geotechnical integrity under static, seismic, and hydrologic loading conditions. Potential stability improvements include: (a) construction of a downstream buttress fill; (b) installation of additional drainage features within the embankment to reduce pore water pressures; and (c) remedial grading of the downstream slope to reduce steepness. These improvements will be designed during the remedial design phase to meet applicable dam safety standards and MT DEQ requirements for the post-closure period.
Stormwater Management. Stormwater management infrastructure will be constructed around and across the tailings impoundment to divert clean surface water runoff away from the tailings surface, manage runoff from the cover system, and prevent erosion of the cover and embankment. Elements of the stormwater management system may include diversion ditches, drop structures, energy dissipators, and an emergency spillway for the impoundment.
Long-Term Monitoring. The tailings impoundment will be subject to ongoing monitoring throughout the remediation obligation period and beyond. Monitoring will include piezometric levels within the embankment and foundation, seepage rates and water quality, settlement and deformation measurements, vegetation establishment and cover system performance, and periodic geotechnical stability reassessments.
4.4 Component 3: Flint Creek Water Quality Restoration
In-Stream and Riparian Measures. The remedial action includes measures to address contamination that has already reached Flint Creek, including: (a) removal of contaminated streambed sediments in areas where metal concentrations pose ecological risk; (b) placement of clean gravel substrates to restore spawning habitat; (c) riparian buffer plantings and bank stabilization to reduce erosion and provide shading that moderates stream temperatures; and (d) installation of in-stream biogeochemical treatment features (e.g., reactive barriers or constructed wetlands at tributary confluences) where site conditions are favorable.
Coordination with the Flint Creek Watershed Restoration Project (SEP). The in-stream and riparian components of the remedial action will be closely coordinated with the $5.5 million SEP, which encompasses stream-bank stabilization, riparian replanting, and fish-passage improvements along 12 miles of Flint Creek downstream of the mine. While the SEP is contractually and operationally separate from the Work Plan, the two programs share overlapping geographic areas and ecological objectives. A coordination protocol will be developed to ensure complementary scheduling, avoid redundancy, and maximize the combined ecological benefit. The SEP compliance and reporting schedule is governed by separate provisions of the Consent Decree.
Natural Attenuation Evaluation. For downstream areas where active in-stream remediation is not warranted or feasible, the Work Plan will evaluate the potential for monitored natural attenuation ("MNA") to achieve water quality improvement over time, driven by source control measures (WTP operation and tailings impoundment stabilization) that reduce contaminant loading. The MNA evaluation will include fate-and-transport modeling, assessment of natural attenuation mechanisms (sorption, dilution, precipitation), and development of trigger criteria for implementation of contingency measures if MNA proves insufficient.
Biomonitoring Program. A comprehensive biomonitoring program will be established to track ecological recovery in Flint Creek and its tributaries. The program will include: (a) annual macroinvertebrate community surveys using standardized protocols; (b) biannual fish population assessments, including species composition, density, age structure, and condition factor; (c) riparian vegetation monitoring; and (d) assessment of periphyton (algae) communities as indicators of water quality improvement. Biomonitoring results will be reported annually and will be used to evaluate remedy effectiveness and inform adaptive management decisions.
4.5 Hazardous Waste Management (SX/EW Circuit)
Inventory and Characterization. A complete inventory of all hazardous waste and hazardous materials remaining at the SX/EW facility will be conducted as part of Phase 1 activities. The inventory will include the approximately 145,000 gallons of spent organic solvent, 80,000 gallons of spent electrolyte, and 3,200 cubic yards of filter cake and sludge, as well as any additional hazardous materials identified during decommissioning activities. Waste characterization will be performed in accordance with RCRA requirements to confirm hazardous waste determinations and identify appropriate treatment, storage, and disposal methods.
Proper Disposal. All inventoried hazardous wastes will be managed, treated, and disposed of in accordance with applicable RCRA requirements. Spent organic solvent will be transported to a licensed hazardous waste incineration facility for thermal destruction. Spent electrolyte will be treated on site (if feasible) or at an off-site licensed treatment facility to recover metals and neutralize acidity. Filter cake and sludge will be characterized, stabilized if necessary, and disposed of at a licensed RCRA Subtitle C hazardous waste landfill.
RCRA Closure. All unpermitted hazardous waste management units at the SX/EW facility, including the unlined surface impoundment, storage tank areas, and associated contaminated soil areas, will be closed in accordance with RCRA closure standards. RCRA closure plans will be developed during Phase 1 and submitted to EPA and MT DEQ for review and approval. Closure activities will include removal and decontamination of tanks and equipment, excavation and off-site disposal of contaminated soils, and confirmation sampling to verify that closure performance standards have been met.
Decontamination and Decommissioning. The SX/EW facility, including buildings, process equipment, tanks, piping, and concrete foundations, will be decontaminated and decommissioned. Decontamination will include removal of residual chemicals, cleaning of contaminated surfaces, and verification sampling. Decommissioned structures will be demolished and removed from the Site, with materials disposed of at appropriate facilities based on waste characterization results.
4.6 Institutional Controls
Land Use Restrictions. Institutional controls will be implemented to restrict future land uses at the Ridgecrest Site in a manner that is consistent with the remedial action and protective of human health and the environment. Deed restrictions or environmental covenants will be recorded against the real property held by Ridgecrest Mine Holdings, LLC, prohibiting residential use, prohibiting the extraction of groundwater for drinking water or irrigation purposes within defined restriction areas, and requiring maintenance of the cover system, treatment plant, and other remedy components.
Access Controls and Security. Physical access controls, including fencing, gates, and signage, will be maintained around the mine workings, tailings impoundment, treatment plant, and other restricted areas. Security measures will be maintained throughout the remediation period and the long-term stewardship period.
Notification Requirements. The institutional controls will include provisions requiring Ridgecrest Mine Holdings, LLC (or any successor property owner) to provide notice to EPA, MT DEQ, and the county government of any proposed change in land use, any proposed transfer of ownership, or any proposed activity that could affect the integrity of the remedy. These notification requirements will be binding on successors and assigns through recorded covenants.
Section 5: Remedial Action Phasing and Schedule
5.1 Phasing Overview
The remedial action at the Ridgecrest Site is organized into four sequential phases, each with defined objectives, activities, and mile