Crystal Mine Operable Unit (OU) 5 Basin Mining Area Superfund Site Jefferson County, Montana Final Interim Record of Decision (ROD) Apr. 2015


Crystal Mine Operable Unit 05

of the Basin Mining Area Superfund Site

Jefferson County, Montana

Final Interim Record of Decision

#

U.S. Environmental Protection Agency Region 8

10 West 15th Street
Suite 3200
Helena, Montana 59626

April 2015

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Crystal Mine Operable Unit 05

of the Basin Mining Area Superfund Site

Final Interim Record of Decision

Part 1, Declaration

Part 2, Decision Summary

Part 3, Responsiveness Summary

Part 4, Acronyms and Abbreviations, and
References

Appendix A, ARARs Requirements and Waivers

April 2015

U.S. ENVIRONMENTAL PROTECTION AGENCY
REGION 8, MONTANA OFFICE

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Part 1
Declaration

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Contents

Section	Page

Part 1 Declaration

Site Name and Location	1

Statement of Basis and Purpose	1

Assessment of Site	1

Description of Selected Remedy	1

Statutory Determinations	3

ROD Data Certification Checklist	3

Authorizing Signatures	5

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Site Name and Location

Site Name and Location

Basin Mining Area Superfund Site
Jefferson County, Montana
CERCLIS ID: MTD982572562
Site ID No: 0801057
Crystal Mine Site Operable Unit 5

Statement of Basis and Purpose

This decision document presents the selected remedial action for the Crystal Mine Operable Unit (OU) 5
Superfund Site (Site) in Jefferson County, Montana. The remedy was selected in accordance with the
Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA), 42 USC §9601 et
seq., as amended, and the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), 40 CFR
Part 300, as amended. This decision was based on the administrative record established in accordance with
section 113(d) of CERCLA and is available for review at the Boulder Library in Boulder, Montana and at the
U.S. Environmental Protection Agency (EPA) Region 8 records center in Helena, Montana.

The Montana Department of Environmental Quality (MDEQ) and U.S. Forest Service, Region One (USFS),
both supporting agencies, concur with the selected interim remedy.

Assessment of Site

Hazardous substances in the form of metal contaminants are being released into the environment by the
Crystal Mine, and they pose a risk to human health and the environment. The response actions described in
this interim Record of Decision (ROD) are necessary to protect public health or welfare or the environment
from actual or threatened releases of hazardous substances.

Description of Selected Remedy

This interim ROD describes the selected remedy for the Crystal Mine OU5, located within the Basin Mining
Area Superfund Site, Jefferson County, Montana. This remedy complements previous removal actions by
remediating acid mine drainage (AMD) and soil contamination to finish Site cleanup. The Basin Watershed
OU2 ROD will make the final determination regarding the need and extent of any additional actions at OU5.

The AMD from the lower adit comprises a principal threat waste at the Site. Contaminated waste rock and
soil deposited by mining activities are considered a non-principal threat waste. A brief description of the
selected remedy (a combination of alternatives WR-3 and GW-6 from the proposed plan) is as follows:

Source Control:

• Implement source water control by constructing runoff conveyance features and by sealing latent mine
structures that allow water into underground workings (for example, exposed/caved shafts).

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DECLARATION

Treatment (Water and Soil):

• Design and construct an onsite repository to hold and encapsulate contaminated waste rock and soils
from the Site. Excavate and haul waste rock and contaminated soils to repository.

• Grade and stabilize excavated dump and waste rock areas, cover with clean soil and vegetate.

• Design and reconstruct Uncle Sam Gulch (USG) Creek adjacent to the mine boundary, stabilize banks and
vegetate.

• Open and stabilize lower adit portal. Portal opening will be secured to prevent unauthorized human
entry, if warranted, and accommodate appropriate wildlife access (for example, bats), if recommended
by the Montana Fish, Wildlife and Parks (MDFWP) or the U.S. Fish and Wildlife Service (USFWS).

• Design and construct passive treatment system.

• Collect mine adit flow using a diversion structure and piping. Convey the collected water to the semi-
passive treatment system.

The five stages of the semi-passive treatment system (SPTS) are as follows (see the Crystal Mine Feasibility
Study for more detail):

Stage 1 - Sulfate Reducing Biochemical Reactor (SRBR). The SRBR will adjust pH and convert sulfate and
trace metals in the water into metal sulfides that remain with the media.

Stage 2 - Aeration System. Two short series of cascades (riprapped channels) will run from the last SRBR
into the first aeration pond, and from the first pond into the second, to promote turbulence and aeration.

Stage 3 - Oxidation/Settling Ponds. The precipitation/settling ponds (two in series) will facilitate the
precipitation and settling of iron oxide sludges from the SRBR cells and aeration channels.

Stage 4 - Wetland. The wetland pond will allow for suspended solid polishing. It is assumed that discharge
from the adit will be naturally reduced during the winter months.

Stage 5 - Discharge Channel. An overflow and discharge channel (riprapped) will convey the treated mine
water from the distal end of the wetlands to USG Creek. This comprises the final stage of the SPTS.

Operation and Maintenance:

Periodic replacement of the SRBR media will be required. Sludge that settles in the deep end of the
oxidation ponds will also require removal, drying and disposal at the Luttrell Repository.

Institutional Controls:

• Institutional controls (ICs) to prohibit residential use, prevent installation of drinking water wells and to
protect the remedy will be required throughout the Site. ICs include administrative land management
methods necessary to maintain the effectiveness of the remedy and protect human health by
preventing exposure to contaminated soil and ground water that creates an unacceptable risk to human
health. ICs will be tailored to the size, location and complexity of the area.

• The EPA and MDEQ will work with adjacent landowner agencies (primarily USFS) on the specific
application of this remedy including protective ICs.

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OBPART 1
DECLARATION

Long-term Monitoring and Maintenance:

Long-term monitoring and maintenance activities will be needed to assure the remedy remains effective.
An Operation, Monitoring and Maintenance (OMM) Plan will be developed as part of the remedy and will
contain the following:

• Construction and post-construction monitoring of adit discharge and USG Creek water quality.

• Water quality monitoring and maintenance of the SPTS, including biochemical reactor media
replacement, sludge removal from the settling ponds and long-term monitoring.

• Periodic inspection/maintenance of the repository, soil and vegetative cover and erosion controls.

• Monitoring and maintenance of ICs.

Statutory Determinations

The selected remedy is protective of human health and the environment, complies with federal and state
requirements that are applicable or relevant and appropriate to the remedial action unless justified by a
waiver, is cost effective, and utilizes permanent solutions and alternative technologies to the maximum
extent practicable.

This remedy also satisfies the statutory preference for treatment as a principal element of the remedy (for
example, reduces the toxicity, mobility or volume of hazardous substances, pollutants or contaminants
through treatment).

Because this remedy will result in hazardous substances remaining onsite above levels that allow for
unlimited use and unrestricted exposure, a statutory review will be conducted within 5 years after initiation
of the remedial action, and at a minimum every 5 years thereafter, to ensure that the remedy is, or will be,
protective of human health and the environment.

ROD Data Certification Checklist

The following information is included in the decision summary section of this interim ROD. Additional
information can be found in the administrative record file for this Site.

1. Contaminants of concern and their respective concentrations (Section 5).

2. Baseline risks represented by the contaminants of concern (Section 7).

3. Cleanup levels established for contaminants of concern and the basis for these levels (Section 7).

4. Discussion of principal threat wastes (Section 11).

5. Current and reasonably anticipated future land use assumptions used in the baseline risk
assessment (Section 6).

6. Potential land use and ground water use that will be available as a result of the selected remedy
(Section 12).

7. Estimated capital, annual operation and maintenance (O&M), and total present worth costs,
discount rate, and the number of years over which the remedy cost estimates are projected
(Section 12).

8. Key factors that led to selecting the remedy (Sections 10, 11 and 12).

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Authorizing Signatures

This interim ROD documents the selected remedy for the Basin Mining Area Superfund Site, Crystal Mine
Operable Unit 5, Jefferson County, Montana. The following authorized officials from their respective
Agencies approve the selected remedy as described in this ROD.

Martin Hestmark

Assistant Regional Administrator

Office of Ecosystems Protection and Remediation

Date:

Tom Livers
Director

Montana Department of Environmental Quality

Date:

David E. Schmid

Regional Forester (Acting)

United States Forest Service, Region One

Date:

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5

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Authorizing Signatures

This interim ROD documents the selected remedy for the Basin Mining Area Superfund Site, Crystal Mine
Operable Unit 5, Jefferson County, Montana, The following authorized officials from their respective
Agencies approve the selected remedy as described in this ROD.

Martin Hestmark

Assistant Regional Administrator

Office of Ecosystems Protection and Remediation

Tom Livers
Director

Montana Department of Environmental Quality

Date:

David E. Schmid
Regional Forester (Acting)

¦ United States Forest Service, Region One

Date:

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Authorizing Signatures

This interim ROD documents the selected remedy for the Basin Mining Area Superfund Site, Crystal Mine
Operable Unit 5, Jefferson County, Montana. The following authorized officials from their respective

Agencies approve the selected remedy as described in this ROD.

Martin Hestmark

Assistant Regional Administrator

Office of Ecosystems Protection and Remediation

Tom Livers
Director

Montana Department of Environmental Quality

Date: i «- — / /

David E. Schmid

Regional Forester (Acting)

United States Forest Service, Region One

Date:

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Date:

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Authorizing Signatures

This interim ROD documents the selected remedy for the Basin Mining Area Superfund Site, Crystal Mine
Operable Unit 5, Jefferson County, Montana, The following authorized officials from their respective
Agencies approve the selected remedy as described in this ROD,

Martin Hestmark

Assistant Regional Administrator

Office of Ecosystems Protection and Remediation

Date:

Tom Livers
Director

Montana Department of Environmental Quality

Date:

Da\

Regional Forester (Acting)

United States Forest Service, Region One

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Part 2

Decision Summary

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Contents

Section	Page

Part 2 Decision Summary

Section 1. Site Name, Location, and Description	1-1

Section 2. Site History and Enforcement Activities	2-1

2.1	Site Background and History	2-1

2.2	Regulatory Activities	2-2

2.3	Enforcement History	2-3

Section 3. Community Participation	3-1

Section 4. Scope and Role of Operable Unit or Response Actions	4-1

Section 5. Summary of Site Characteristics	5-1

5.1	Conceptual Site Model	5-1

5.1.1	Potential Contamination Sources	5-11

5.1.2	Waste Rock and Acid Rock Drainage	5-11

5.1.3	Adit Discharge and Acid Mine Drainage	5-11

5.2	Movement and Behavior of Contaminants of Concern	5-12

5.2.1	Contaminants of Concern	5-12

5.2.2	Contamination Mobilization, Transport and Pathways, and the Exposure Model 5-12

5.3	Summary of Previous Site Response Actions	5-17

5.4	Site Description	5-18

5.4.1	Climate	5-18

5.4.2	Drainage and Hydrology	5-18

5.4.3	Soils and Geologic Setting	5-18

5.4.4	Disturbed Areas, Surface Features, Historical Features	5-18

5.5	Summary of Previous Site Characterization Water Studies	5-19

5.6	Summary of Previous Site Characterization Soil and Waste Rock Studies	5-21

5.7	2010 Field Activities	5-21

5.8	Surface Water Investigations	5-22

5.8.1	Synoptic Sampling of Uncle Sam Gulch Creek	5-22

5.8.2	Spring Inventory and Sampling Results	5-28

5.8.3	Comparison with Water Quality Standards	5-30

5.9	Soil and Waste Rock Investigations	5-31

5.9.1	Soil Concentrations and Ecological and Human Benchmark Values	5-37

5.9.2	Mine Waste Volumes and Locations	5-39

5.9.3	Sediment Concentration in Uncle Sam Gulch Creek	5-40

5.9.4	Aquatic Resource Investigation	5-50

5.10	Geology and Ground Water Investigations	5-52

5.10.1	2010 Investigation	5-52

5.10.2	2011-2012 Investigations	5-53

5.11	Hydrogeologic Findings	5-61

5.12	Crystal Mine Wetland Inventory	5-62

5.13	Riparian Wetland Health Assessment	5-62

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CONTENTS

Section 6. Current and Reasonably Anticipated Future Land and Resource Uses	6-1

6.1	Land Use	6-1

6.2	Human Land Uses	6-1

6.3	Ecological Land Uses	6-1

6.4	Surface Water Use	6-2

6.5	Ground Water Use	6-2

Section 7. Summary of Site Risks	7-1

7.1	Human Health Risk Assessment	7-1

7.1.1	Contaminants of Concern	7-1

7.1.2	Exposure Assessment	7-1

7.1.3	Toxicity Assessment	7-5

7.1.4	Risk Characterization	7-5

7.2	Ecological Risk Assessment	7-9

7.2.1 Baseline Ecological Risk Assessment (BERA) Problem Formulation	7-9

7.3	Basis for Action	7-13

Section 8. Remedial Action Objectives and Remedial Goals	8-1

8.1	Remedial Action Objectives and Remedial Goals	8-1

8.1.1	Surface Water RAOs	8-1

8.1.2	Ground Water RAOs	8-1

8.1.3	Soil RAOs	8-2

8.1.4	Stream Sediment RAOs	8-2

8.2	Remediation Goals	8-2

Section 9. Description of Alternatives	9-1

9.1	No Further Action Alternative	9-1

9.2	Waste Rock/Soil Alternatives	9-1

9.3	Ground Water Alternatives (GW)	9-1

9.3.1 Common Elements	9-6

Section 10. Comparative Analysis of Alternatives	10-1

10.1 Comparative Analysis of Alternatives	10-1

10.1.1	Summary of Comparative Analysis of Waste Rock Alternatives	10-2

10.1.2	Summary of Comparative Analysis of Ground Water Alternatives	10-6

Section 11. Principal Threat Waste	11-1

11.1 Principal Threat Determination	11-1

Section 12. Selected Interim Remedy	12-1

12.1	Short Description of the Selected Remedy	12-1

12.2	Rationale for the Selected Interim Remedy	12-2

12.3	Detailed Description of the Selected Interim Remedy	12-3

12.3.1	Site Access	12-3

12.3.2	Onsite Repository and Waste Rock Removal	12-3

12.3.3	Source Water Assessment and Control	12-3

12.3.4	Semi-Passive Water Treatment System	12-4

12.3.5	Institutional and Engineering Controls	12-11

12.3.6	Post-Remedy Construction Operation, Monitoring and Maintenance	12-11

12.4	Estimated Cost of the Selected Interim Remedy	12-12

12.5	Expected Outcomes of the Selected Remedy	12-16

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PART 2 DECISION SUMMARY

12.6	Performance Standards	12-16

12.6.1 Performance Evaluations for the Selected Interim Remedy	12-18

12.7	Safety Concerns	12-18

Section 13. Statutory Determinations	13-1

13.1 Statutory Determinations	13-1

13.1.1	Protection of Human Health and the Environment	13-1

13.1.2	Compliance with ARARs	13-1

13.1.3	Cost Effectiveness	13-1

13.1.4	Utilization of Permanent Solutions and Alternative Treatment (or Resource
Recovery) Technologies to the Maximum Extent Practicable	13-2

13.1.5	Preference for Treatment as a Principal Element	13-2

13.1.6	5-Year Reviews	13-2

Section 14. Documentation of Significant Changes	14-1

Tables

5-1	2002 Ecological and Human Health Benchmarks for Crystal Mine Contaminants of Concern

5-2	Surface Water Field Measurements

5-3	Total Elemental Levels (ng/L) in Surface Waters, S04 (mg/L)

5-4	Field Measurements in Waters from Springs

5-5	Total Elemental Levels (ng/L) in Waters Collected from Springs, Sulfate in mg/L

5-6	Surface Water and Ground Water Standards and Screening Benchmarks (mg/L)

5-7	Metal and Arsenic Levels (mg/kg) in Crystal Mine Site Soils and Waste Rock (Field XRF Data)

5-8	Soil and Sediment Screening Benchmarks

5-9	Contaminated Waste Rock and Soils Volume Estimates

5-10	Metal and Arsenic Concentrations in Uncle Sam Gulch Creek from Historic and 2005 RI/FS Reports

5-11	Summary of Sediment Results (Dry Weight)

5-12	Crystal Mine Ground Water Quality Laboratory Results—Validated

5-13	Jurisdictional and Functional Wetlands Delineated by Area

7-1	Summary of Human Health Risks Above Appropriate Risk Levels

7-2	Summary of Ecological Risk Hazard Quotients for Plants, Aquatic Organisms, and Benthic Infauna

7-3	Basis for Action

8-1	DEQ-7 Surface and Ground Water Standards the EPA Will Address
with the Basin Watershed OU2 ROD

8-2	Stream Sediment PRGs in mg/kga

9-1	Description of Primary Alternatives

9-2	Common Elements in Remedial Alternatives

10-1	Relative Ranking of Waste Rock Alternatives after Comparison Analysis
10-2	Listing of Site ARARs (Federal and State of Montana)

10-3	Relative Ranking of Acid Mine Drainage Alternatives after Comparison Analysis

12-1	Alternative GW-6 Design Parameters

12-2	Cost Breakdown of Selected Remedy (Waste Rock Alternative)

12-3	Breakdown of the Selected Remedy - GW Alternative 6

12-4	Surface Water Targets in mg/L

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CONTENTS

Exhibits

1-1	Location of Basin Mining District in West Central Montana

1-2	Basin Watershed OU2 Relative to the Boulder River Watershed

1-3	Uncle Sam Gulch and Crystal Mine Site

1-4	Crystal Mine Site

2-1	Location of Boulder River Watershed and Study Area, Montana.

5-1	Crystal Mine Prominent Site Features

5-2	Crystal Mine Conceptual Site Model (Plan View)

5-3	Crystal Mine Site Plan Trench and Dump Cross-Section

5-4	Acid Mine Drainage from Collapsed Adit at the Crystal Mine

5-5	Crystal Mine Conceptual Exposure Model for Potential Human Health and Ecological Receptors

5-6	Completed Removal Work on the Crystal Mine Trench

5-7	Creek and Spring Sampling Locations and Analytical Results

5-8	Total Arsenic (ng/L) in Uncle Sam Gulch Creek Waters

5-9	Total Copper (ng/L) in Uncle Sam Creek Waters

5-10	Total Zinc (ng/L) in Uncle Sam Gulch Waters

5-11	Soil Pits with Samples Analyzed

5-12	Mean Soil Arsenic (mg/kg) and Soil Depth (Inches)

5-13	Soil Lead (mg/kg) and Soil Depth (Inches)

5-14	Soil Copper (mg/kg) and Soil Depth (Inches)

5-15	Soil Zinc (mg/kg) and Soil Depth (Inches)

5-16	Crystal Mine Dump

5-17	Twin Ore Bins and Dump

5-18	Mammoth Road

5-19	Mammoth Dump Area

5-20	2012 RI/FS Sediment Monitoring

5-21	Arsenic Concentrations in Sediments, USG Creek

5-22	Copper Concentrations in Sediments, USG Creek

5-23	Lead in Sediments, USG Creek

5-24	Zinc Concentrations in Sediments, USG Creek

5-26	Mean BMI Community Density (Organism/m2)

5-25	Benthic Macro-Invertebrate Monitoring Locations

5-27	Total Taxa Richness and EPTTaxa Richness

5-28	Geologic Investigations: Test Pit, Borings, & Monitoring Well Locations

7-1	Crystal Mine Conceptual Exposure Model for Potential Human Health and Ecological Receptors

10-1	EPA's Evaluation Criteria

11-1	Metal Salt Crystals Formed Adjacent to Lower Mine Adit Discharge

12-1	GW-6 Semi-Passive Treatment Area (Plan View)

12-2	Process Flow Diagram

12-3	Bioreactors and Oxidation Ponds

12-4	Channel Cross Section and Wetlands

Appendixes

A	ARARs Requirements and Waiver

IV

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Section 1. Site Name, Location, and Description	

Basin Mining Area Superfund Site
Crystal Mine Site OU5
Jefferson County, Montana
Site ID Number:	0801057

CERCLIS ID:	MTD982572562

Lead Agency:	U.S. Environmental Protection Agency (EPA)

Support Agency:	Montana Department of Environmental Quality (MDEQ)

Cleanup Funding: The EPA Superfund Trust Fund

Site Type:	Abandoned Mine (Historic hard rock mine)

Mining-waste related contamination in the Basin watershed and in the Town of Basin resulted in the listing
of the Basin Mining Area on the National Priorities List (NPL) on October 22, 1999. The west-central
Montana mining area includes the watersheds of Basin and Cataract Creek and portions of the Boulder River
below the confluence with these heavily impacted streams (see Exhibits 1-1 and 1-2).

EXHIBIT 1-1

Location of Basin Mining District in West Central Montana

WYOMING

Kalispell

Great
Falls

Missoula

MONTANA

^Helena



Butte# Basin Mining Area

Billings
Bozeman	•

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46°30'

46»i5'f—j(

Deerlodge^

County / .

Jkj&py

45°45'

Jefferson
County

Madison
County

LOCATOR MAP

LEGEND

# City or Town
/V River or Creek
/V Interstate Highway
/V Highway
I I Study Area

Boulder River Watershed
^3 County Line

MONTANA

A

Exhibit 1-2

Basin Watershed OU2 Relative
to Boulder River Watershed

BASIN WATERSHED OPERABLE UNIT 2
JEFFERSON COUNTY, MONTANA

	Crystal Mine OU5 ROD	

Source: Draft RI for Basin Watershed 0U2 (CDM 2005b)
Crystal Mine OU5 Remedial Investigation

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PART 2 DECISION SUMMARY

The Basin Mining Area NPL site is divided into the following relevant Operable Units (OUs): the Town of
Basin OU1, Basin Watershed OU2, Luttrell Repository OU3, Buckeye/Enterprise Mine OU4, Crystal Mine
OU5, and Bullion Mine OU6.

Approximately 300 abandoned hard rock mines exist within the Basin Watershed OU2, according to a
remedial investigation (Rl) conducted by CDM Federal Programs Corporation (CDM) for the EPA (CDM,
2005b). Findings from the Basin Watershed OU2 Rl identified the Bullion OU6 and Crystal OU5 Mines, with
their associated AMD, as the largest contributors of mine-related contamination into the surface water
system (see Exhibit 1-3).

The Crystal Mine is located at the head of Uncle Sam Gulch (T7N, R5W, Sections 18,19, 20) within the
Cataract Creek Drainage, about 8 miles north of the Town of Basin. The Site is located adjacent to Uncle Sam
Gulch (USG) Creek, a small tributary to Cataract Creek. The watershed landforms consist of predominantly
steep slopes and narrow valleys. Access throughout the watershed is limited to existing, unpaved, secondary
roads maintained by the USFS. The roads are snow covered and typically impassible from late fall to early
summer (NRCS, 2009).

The mine resides on mining claims encompassing approximately 40 acres, 22 of which are disturbed from
mining activities. The Site is located between 7640 feet (ft) above mean sea level (amsl) and 8100 ft amsl,
and is surrounded by the Beaverhead-Deerlodge National Forest.

The surface expression of the Crystal Mine is superimposed on an east-west trending subbasin drainage
divide (at 8100 feet amsl). The east end of this subbasin intercepts an incised subdrainage (USG) which is
oriented north-south and drains to the south (see Exhibit 1-4). USG Creek originates northeast of the Site
from a series of alpine bogs and wet meadows. The creek flows north to south where it erodes and
undercuts waste rock piles while forming the eastern edge of the Site. The stream reach parallel to the Site
is slightly gaining in volume, relatively straight and approximately 1,100 feet long. The gradient along this
reach is approximately 22 percent, or a vertical foot of elevation for every 4.6 linear feet of stream channel.

The principal vein minerals at the Site are crystalline quartz and fine-grained pyrite, which contained gold,
silver, copper, lead and zinc. Pyrite (iron sulfide), sphalerite (zinc sulfide), and galena (lead sulfide) are the
most abundant ore minerals in the mine. Gold is associated with pyrite, arsenopyrite and copper minerals.
Silver is associated with tetrahedrite and galena.

The Site is now a significant source of AMD that is impacting water quality in USG Creek and Cataract
Creek. Elevated concentrations of arsenic and trace metals (particularly antimony, aluminum, cadmium,
copper, lead, selenium and zinc) are present in Site soils, mine discharge, and downstream surface water
and sediment. The principal source of AMD is discharge from the lower Crystal adit and several springs
within the mine area, which contribute to the total metal load in USG Creek downstream of Crystal Mine.
USG Creek flows into Cataract Creek approximately 2 miles downstream of its confluence with the
Crystal Mine discharge.

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1-3

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Luttrell Repository^

UPPER TENMILE
CREEK WATERSHED

Basin Creek Mine Area>

LITTLE BLACKFOOT
AND ONTARIO
WATERSHEDS

PRICKLY PEAR
WATERSHED



\

R7W R6V

RED ROCK CREEK
WATERSHED

R6W R!

Q

Interstate 15~|



Town of
Basin

I

IS
\

Upper
Boulder
River
Subarea

BOULDER RIVER

WATERSHED
(PORTION IN OU2
STUDY AREA)

Lower
Boulder

River
Subarea

LEGEND

A

Local Mountain Peak Over 8,000
Feet Above Mean Sea Level
River or Creek
Basin Creek Mine Area
Interstate

Luttrell Repository
Study Area
Watersheds
I I Subbasins

i i Township and Range Block

Note: Study area is within the Beaverhead-Deerlodge National Forest

A

2 Miles

Exhibit 1-3

UNCLE SAM GULCH AND
CRYSTAL MINE SITE

BASIN WATERSHED OPERABLE UNIT 2
JEFFERSON COUNTY, MONTANA

Crystal Mine OU5 ROD

Source: Draft Rl for Basin Watershed 0U2 (CDM 2005b)

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BO! \\OWL\PROJ\EPA\406950CRYSTALMINE\GIS\MAPFILES\2014\MINESITE.MXD JCARR3 4/28/2014 3:29:26 PM

VICINITY MAP

Helena

Butte

LEGEND

— Digitized\DEM Generated Streams

Notes:

1.	Area of interest subject to change.

2.	2011 Imagery -ArcGIS Streaming Map Service.

3.	30 meter USGS DEM used to generate streams.

ISI

0	250	500

	1	I	I

Feet

EXHIBIT 1-4
Crystal Mine Site

Crystal Mine OU5 ROD

CH2MHILL

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Section 2. Site History and Enforcement Activities

2.1 Site Background and History

The development of the Crystal Mine dates back to 1883. Mining activities were conducted by several
different companies from 1885 to 1901. The Crystal Mine was reportedly idle from 1926 until 1936 and then
operated by different companies throughout the 1940s and 1950s. The Bullock brothers operated the
Crystal Mine on a small scale between 1969 and 1984. In 1983, the Bullock Brothers Construction Company
sold and shipped about 350 tons of ore to the ASARCO Smelter at East Helena, Montana. This was the last
ore shipment for the Crystal Mine.

Relevant historic activities and Site investigations are explained in more detail in Section 1.5 of the 2013
Crystal Mine OU5 Rl (EPA).

(1897) Claim Surveyed. The Crystal claim was located in 1883 by unnamed parties.

(1891 to 1974) Mining Begin/Finish. The mine was initially worked intermittently through the 1890s. A small
tunnel was opened and ore shipped by Kennedy and Reed of Butte in 1900 (RTI, 2011). Mining on the Site
was terminated in the mid-1980s with excavation of the Crystal Mine surface trench.

(1994 to 2008) Site Investigations

March 1994. Abandoned Hardrock Mine Priority Sites - Summary Report (Red Book). Identified and
inventoried abandoned and inactive hard rock mine sites in Montana (state and federal lands) that exhibited
severe environmental degradation to surface water and ground water. The Site ranked 20 out of 263, or in
the top 10 percent.

April 1994. Abandoned-Inactive Mines Program Deerlodge National Forest, Cataract Creek Drainage. Volume
II. Prepared by Montana Bureau of Mines and Geology (MBMG). Contains a preliminary characterization of
abandoned and inactive mines on Deerlodge National Forest Lands. The results of the sampling and analysis
were used to estimate the nature and extent of contaminants as well as potential threat to human health
and environment.

1998. Final Report—Remote Mine Site Demonstration Project, Mine Waste Technology Program Activity III,
Project I. In 1994, MSE Technology Applications, Inc. (MSE) initiated a treatment study on the discharge from
the Crystal Mine lower adit.

2004. Integrated Investigations of Environmental Effects of Historical Mining in the Basin and Boulder Mining
Districts, Boulder River WatershedJefferson County, Montana. In 1996 the U.S. Geological Survey (USGS)
initiated a 5-year study of the impacts of mining and issues related to AMD on Upper Boulder River Basin.
This area included the Basin Mining District and the Cataract Creek Watershed (see Exhibit 2-1).

2005. Remedial Investigation Report Addendum, Basin Mining Area Superfund Site, Operable Unit 2,

Jefferson County, Montana. In 2001, the EPA authorized a remedial investigation/feasibility study (RI/FS) of
the Basin Watershed OU2 in which the Crystal Mine was included. The RI/FS was published in 2005 and
concluded that water quality degradation in Cataract Creek during low-flow months was predominantly
attributable to the tributaries—in particular, USG Creek. The results exceeded both ecological and human
health benchmarks for arsenic, cadmium, copper, lead and zinc (CDM, 2005b).

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SECTION 2. SITE HISTORY AND ENFORCEMENT ACTIVITIES

2.2 Regulatory Activities

Regulatory and government interest in the site began in the 1990s. The following is a list of relevant
regulatory activities that have occurred at the Site.

1998-99. Preliminary Assessment/Site Investigation. EPA conducted a preliminary assessment (PA) and site
investigation (SI) of the Basin Mining Area in 1998 and 1999. The Crystal Mine OU5 and Bullion Mine OU6
were included in the PA/SI. Elevated concentrations of arsenic, copper, lead and zinc were detected in soils,
mine wastes and surface water.

1999. National Priority Listing. The Crystal Mine was proposed for the Superfund NPL as part of the
Basin Mining Area in October 1999.

EXHIBIT 2-1

Location of Boulder River Watershed and Study Area, Montana.

2000. Action Memorandum. Formal
designation of the Site as OU5 occurred on
April 12, 2000.

2001 to 2002. Time Critical Removal Action

for the Crystal "trench area." The objective
of the time critical removal action (TCRA)
was to reduce the collection of snow melt
and precipitation in the "trenched" surface
feature caused by previous mining. The
collection of precipitation in this feature
was thought to contribute to the recharge
of the Crystal Mine underground workings
and production of AMD. The TCRA
consisted of back-filling the trench with
rock and capping it with an impervious liner
to prevent surface water runoff from
entering the underground workings
through the trench. It was completed in

2002 and appeared to help reduce the rate
of AMD discharge from the lower adit. This
work was also performed in anticipation of
future remedial work to capture and treat
the remaining AMD.

2010-2013. Remedial Investigation,
Feasibility Study, Human Health and
Ecological Risk Assessment. A focused
RI/FS and risk assessment of the Site was
initiated by the EPA in 2010. The RI/FS was
completed in November 2013.

2014. Proposed Plan. The proposed plan
for the Crystal Mine OU5 was distributed
for public review in March 2014. A public
meeting to explain the proposed remedial
action, answer questions and accept
comments was held on March 19, 2014.

Boulder River

Base from U.S. Geological Survey
1:100,000 Butte North, 1994

3 MILES

3 KILOMETERS

Adapted from USGS Professional Paper 1652 (2004)

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PART 2 DECISION SUMMARY

2014-2015. Time Critical Removal Action. Two sediment ponds containing contaminated mine water and
sludges are located below the lower adit. Accelerated erosion of support berms has resulted in the risk of
pond failure and release of sludges onto USFS land below the ponds. To address this risk, the EPA removal
program will drain water from the ponds to USG Creek. Consolidated sludge and liner material will be
transported to the Luttrell Repository for disposal.

2.3 Enforcement History

Between 1999 and 2000, a potentially responsible party (PRP) search was conducted for the Crystal Mine
that identified former operators, all now defunct mining companies, and past and current land owners. In
2000 and again in 2008 the EPA sent out information request letters to all owners and operators of mining
claims contributing to contamination of the Crystal Mine. Based upon this investigation, the EPA was unable
to identify any viable PRPs.

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Section 3. Community Participation

Community involvement in the cleanup of the Basin Mining Area began prior to the NPL listing of the site
and has continued through several EPA response actions taken within the NPL site. This involved four
different superfund activities, including the cleanup of the Town of Basin OU1 (2002-2004) with two 5-year
reviews (2007, 2012), a TCRA by the EPA at the Crystal Mine (2001-2002), the RI/FS of the Basin Watershed
OU2 (2001- 2005), and the current RI/FS and interim ROD process for the Crystal Mine OU5.

1)	Cleanup of the Town of Basin — Contaminated surface water from the Crystal Mine flows into Cataract
Creek, a tributary to the Boulder River located approximately one-quarter mile east of the Town of
Basin. The EPA prepared a detailed community involvement plan (CIP) for the Town of Basin in

March 2000 describing activities for which public participation would be solicited. Activities were posted
in local newspapers (Butte Standard, Boulder Monitor and Helena Independent Record) prior to their
occurrence, and public opinion and comments were captured in a responsiveness summary to a ROD for
the town in 2001 (CDM). From 2002 to 2004, cleanup of mining waste within the town commenced. This
activity triggered heightened community interaction as remedial activities progressed from property to
property. Public involvement continued as interviews of public officials and residents of Basin were
conducted to evaluate the success of the Town of Basin cleanup during subsequent 5-year reviews
(2007, 2012). A fact sheet discussing prudent use and contact with the surface water and soils from the
watershed was prepared and distributed to town residents by the EPA and MDEQ in December 2012.

2)	TCRA at the Crystal Mine — As described in Section 2.2 of this interim ROD, an EPA initiative to line and
backfill the Crystal Mine trench was implemented in 2002. Prior to this action, a notice was posted in
local newspapers (Butte Standard, Boulder Monitor and Helena Independent Record) to inform the local
community of increased traffic on Basin Creek Road and to solicit comments.

3)	RI/FS of the Basin Watershed OU2 — The EPA conducted the RI/FS for the Basin Watershed OU2
concurrent with cleanup of the Town of Basin. The Basin Watershed OU2 RI/FS included the Crystal
Mine OU5. Public participation was solicited through a public notice describing where the final
documents could be found for review. In June 2003, a final draft proposed plan was prepared by the EPA
describing the preferred remedy for the Basin Watershed OU2. The EPA did not publicly release the
draft proposed plan. Instead, EPA decided to conduct interim cleanups of the most detrimental sources
of surface water contamination within the Basin Watershed OU2 (Bullion Mine and Crystal Mine).

4)	Current RI/FS, Proposed Plan and ROD Process for the Crystal Mine OU5 — RI/FS reports for the Crystal
Mine were completed in November 2013 and distributed to local repositories in the towns of Boulder
and Basin. A proposed plan describing the preferred cleanup for the Site was prepared and distributed
to the local community on March 7, 2014. The official public comment period ran from March 19 to
April 21, 2014. Copies of the proposed plan were distributed to the State of Montana and the USFS,
property owners for the Crystal Mine, the Basin post office and community members who attended the
public meeting. The proposed plan was also posted on the EPA website for the Basin Mining Area
Superfund Site (under Crystal Mine OU5). A notice of availability of the proposed plan and a letter to the
editor were published in the local newspapers (Butte Standard, Boulder Monitor and Helena
Independent Record) at the beginning of the public comment period in an effort to further publicize the
availability of the proposed plan. EPA held a public meeting on March 19, 2014 at the Basin School to
explain the preferred remedy and the ROD process to the community and solicit their comments.
Comments verbalized at this meeting were generally supportive of the proposed clean-up plan. A
transcript of the meeting was placed in the Administrative Record for the Site. No written comments
were received during the public comment period.

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Section 4. Scope and Role of Operable Unit or
Response Actions

As with many Superfund sites, the problems at the Basin Mining Area NPL Site (77 square miles) are
complex. As a result, EPA has organized the work into the following 6 operable units, of which the Crystal
Mine is OU5:

• Operable Unit 1 - The Town of Basin. The Town of Basin is located at the mouth of the Basin Watershed
OU2. Mine wastes within town represented the most immediate threat to human health. The ROD was
completed on March 30, 2001, and remedial action was completed December 16, 2004.

• Operable Unit 2 - Basin Watershed. The Basin watershed is the largest operable unit (77 square miles)
and encompasses OUs 3, 4, 5 and 6. The RI/FS and a draft proposed plan were completed between 2002
and 2005. A final proposed plan and cleanup of the watershed will follow interim actions at OU5 and
OU6. EPA has decided to conduct interim actions at OU5 and OU6 first because the acidic adit
discharges from these OUs significantly degrades water quality within the Basin watershed. Upon
completion of the interim remedies at these two mine sites, a ROD for the remainder of the watershed
will be written.

• Operable Unit 3 - Luttrell Repository. Luttrell is the regional repository located on the divide between
Ten Mile Creek and the Basin watershed. Construction of this repository was initiated in 2000. The site
currently accepts mining wastes associated with response actions performed by the USFS, the State of
Montana and the EPA, Region 8.

• Operable Unit 4 - Buckeye/Enterprise Mines. Contaminated soils and mining waste removal were
completed at these sites in 2006.

• Operable Unit 5 - Crystal Mine. A removal action to line and cover a surface mine trench was
performed between 2001 and 2002. The purpose of the action was to prevent snow melt and
precipitation from infiltrating and migrating into underground mine workings. Contaminated mine
wastes and AMD from the lower adit remain unremediated. Another removal action will be performed
in 2014 and 2015 to remove two sediment ponds containing contaminated mine water and sludge.
Because of erosion of support berms, the ponds are in jeopardy of failing. Water will be discharged to
USG Creek. Consolidated sludge and liner material will be transported to the Luttrell Repository for
disposal.

• Operable Unit 6 - Bullion Mine. In a joint removal action by the USFS and the EPA, contaminated mine
and mill wastes were removed to the local repository at Luttrell. Another removal action will be
performed in 2014 and 2015 to treat pooled mine water, discharge the treated water to Jill Creek and
remove the contaminated adit debris plug materials to the Luttrell Repository.

As noted above, the EPA decided to prioritize remedial action at the two mine sites (Crystal OU5 and Bullion
OU6) in the Basin Watershed OU2 that contribute the most to water quality degradation. Upon completion
of the interim remedies at these two mine sites, a ROD for the Basin Watershed OU2 will be written. The
interim action at the Crystal Mine will focus on reducing surface water infiltration into the mine workings
and treating the mine-contaminated water discharging from the lower adit. In addition, waste rock will be
excavated and deposited in an onsite repository, the Site will be graded for slope stabilization and
vegetated, and USG Creek will be remediated to a stable configuration.

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SECTION 4. SCOPE AND ROLE OF OPERABLE UNIT OR RESPONSE ACTIONS

The anticipated sequence of cleanup activities for the Crystal Mine starts with design and construction of an
onsite repository. Waste rock from dumps and contaminated soils will be excavated and deposited in the
onsite repository and USG Creek will be remediated and stabilized. Contaminated wood, metal, and plastic
debris will be transported to the Luttrell Repository. Next, the Site will be graded, stabilized and revegetated
to discourage erosion. Source water control measures will be implemented by constructing surface runoff
conveyance features and by sealing latent mine structures (such as open trenches or mine shafts) that allow
water into underground workings. AMD from the mine will then be captured and treated through a semi-
passive biochemical treatment process to mitigate the existing impact of AMD on USG Creek. Land and
water use controls will be established. Prescribed monitoring of the reclamation and maintenance of the
treatment system will begin, and the implementation of ICs will conclude the sequence of remedial actions.

Remediation of the mine discharge will improve water quality, reduce risks to human and ecological
receptors, and contribute to meeting downstream total maximum daily load (TMDL) goals for Cataract
Creek. This interim ROD will be consistent with the previous removal actions as well as the final remedy
selected for the Basin Watershed OU2.

4-2

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Section 5. Summary of Site Characteristics

5.1 Conceptual Site Model

A conceptual site model (CSM) for the Site was prepared to help with identification of 1) potential sources of
metals and arsenic; 2) probable pathways of movement of these contaminants from source material into
soils, ground water and surface water; and 3) the potential assimilation into aquatic and terrestrial
receptors. An accurate conceptual site model facilitates evaluation of potential risks to human health and
the environment (EPA, 1989).

The Basin watershed is largely underlain by the Boulder batholith, a relatively small batholith, exposed at
the surface as granite (more specifically quartz monzonite) and serving as the host rock for rich mineralized
deposits. Regional uplift brought the deep-seated granite to the surface, where erosion exposed the granite
and the extremely rich mineral veins. Hundreds of millions of dollars' worth of copper, silver, gold, zinc, lead
and other metals have been mined from the batholith, using both underground and pit mining methods.

Snowmelt and precipitation infiltrates the shallow, unconsolidated glacial till and alluvial surface soils at the
Site. Ground water flow generally follows surface topography and infiltrates downward through the shallow
soils to the uppermost fractured and weathered zone of the Boulder batholith bedrock. This ground water
then migrates primarily through fractures or faults in the bedrock, some of which are mineralized and host
the ore deposits exploited by mining. The ground water at the Crystal Site flows into the underground mine
workings. This water moves through the workings and discharges from the lower adit at an average rate of
approximately 26 gallons per minute (gpm).

The model for the Site was developed from existing data (previous sampling and Basin Watershed OU2 Rl)
and information obtained from Rl field activities performed from 2010 to 2012. Prominent Site features are
presented in Exhibit 5-1.

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BO I \\OWL\PROJ\EPA\406950CRYSTALMiNE\GIS\MAPFILES\2014\SITEFEATURES.MXD JCARR3 4/28/2014 12:03:19 PM

VICINITY MAP

LEGEND

P Mine Adit

Surface Runoff Drainage Direction
j—-) MineTrench

Topographic Divide
— Digitized\DEM Generated Streams

Wetlands
Waste Dump

Mammoth Road
Mammoth Dump
Twin Ore Bin Dump
Crystal Dump

Notes:

1.	Area of interest subject to change.

2.	2011 Imagery -ArcGIS Streaming Map Service.

3.	30 meter USGS DEM used to generate streams.

IM

0	250	500

	1	I	I

Feet

Exhibit 5-1

Crystal Mine Prominent Site Features

Crystal Mine OU5 ROD

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PART 2 DECISION SUMMARY

The surface of the Site includes an east-west trending linear, previously mined, trench feature located on a
subbasin drainage divide. The trench was an 800- to 1,000-foot-long excavation that the EPA lined and
backfilled during the 2002 TCRA. Other features at the Site include numerous waste rock piles, twin ore bins
that held material mined from the upper adit, five historic out-buildings, a visible portal leading to the lower
underground workings, remnants of an old trestle, several ore chutes and two lined ponds built over a waste
rock dump. The slope below the east end of the trench is steep (greater than 25 percent) and covered with
waste rock. Waters from USG erode and undercut waste rock piles while forming the eastern edge of the
active mine site. Drainage from the lower adit is presently directed into the two lined settling ponds.
Overflow from the ponds runs approximately 300 yards downslope across USFS land, and then discharges
into USG Creek. During high flow, the adit discharge splits and flows to the ponds and directly to USG Creek
from the portal (Exhibit 5-2). The ponds will be removed during a 2014-2015 TCRA in an effort to avoid
failure of the pond liner and a release of the sludges they contain.

The majority of the Site is barren of top soil and vegetation due to mining impacts. Mineralized waste rock
and decomposed granite constitute the soil surface. This material is easily eroded and highly mobile during
rainfall and surface runoff events, as evidenced by numerous erosion rills. In the high elevations associated
with the Site, ground water is present in small, unconsolidated glacial/alluvial deposits as well as in the
fractured bedrock. Shallow ground water flow generally follows surface topography. Ground water is not
developed for drinking at or near the Site. Surface water associated with the drainage basin and wetlands
north of the trench area infiltrates the shallow soils, migrates through fractured bedrock and intercepts the
lower underground mine workings. This water moves laterally through the workings, discharges from the
lower adit at an average rate of approximately 26 gpm and intercepts USG Creek approximately 300 yards
downstream from the onsite settling ponds. Ground water discharge from the adit is highest in the spring,
responding to snowmelt and runoff. An illustration of the CSM is presented in Exhibit 5-3.

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Dying Trees from Acid Mine Old Settling Ponds on	Ore Loading Trestle on

Drainage	Mammoth Dump	Mammoth Dump

Legend

Mine Adit Discharge

Creek Water Impacted by Mine

^Unimpacted Creek Water

Access Road

Cross Section Locations
See Figure 5-3)

Crystal MineTrench
(Remediated)

Twin Ore Bins and Dump

Uncle Sam Gulch Creek
(Note: Proximity to Waste Rock).
Upstream is non-impacted

Crystal Dump
(Note: Erosion Rills and
lack of vegetation)

Lower Adit Portal with
culvert installed for safety
(Note: Acid Mine Drainage)

Waste Rock Dump Erosion
into Uncle Sam Creek

Mine Support Structures on
Mammoth Dump

EXHIBIT 5-2

CRYSTAL MINE CONCEPTUAL
SITE MODEL (PLAN VIEW)

Crystal Mine 0U5 ROD

	 CH2MHILL

ES032009002BQI Wowl\Proj\EPA\4O6950CrystalMine\ROD\DraftROD\ROD_FIGURES\Graphics_ROD_Draft

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Uncle Sam
Creek A

Precipitation and Snowmelt Runoff
Infiltrate into Fractured/Rock Aquifer
and Eventually into Mine Tunnels
and Shafts —

AirborneTransport
of Contaminated
Waste Rock and Soil

Wetlands

— Trench Backfilled
with Waste Rock

— Trench
Liner



%

(Covered)

UpperWorkings

Lower

Workings

Drainag

ThrougfrGeochemical
Processes, Groundwater is
Converted into Acid Mine
Drainage High in Dissolved
Metals



Waste Rock from Crystal Trench

— Waste Rock from
Trench - Surface
Erosion and Transport

— Precipitation and Snowmelt
Infiltrate into Waste Rock to
form Acid Rock Drainage

— Upper Adit

Acid Rock
Drainage

Granitic Host Rock

Groundwater'
Inflow to USG
Creek

Cross-cut
Lower Adit
to Portal

Lower Adit Portal —
(Before Culvert
Installation)

Surface Erosion
ofWaste Rock
and Sediments
into USG Creek

EXHIBIT 5-3

CRYSTAL MINE SITE PLAN
TRENCH AND DUMP
CROSS-SECTION

Crystal Mine 0U5 ROD

	 CH2MHILL

ES032009002BOI \\ow&ProflEPA\406950CrystalMine\ROD\DraftROD\ROD_FIGURES\Graphics_ROD_Draft

Road

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PART 2 DECISION SUMMARY

5.1.1 Potential Contamination Sources

Potential sources of contamination at the Site include waste rock, mineralized host rock, and mineralized
soils that generate acid rock drainage (ARD). Water pooled and flowing within the Crystal Mine workings is
converted into acid mine drainage (AMD), which discharges from the lower adit and into receiving streams.

5.1.2 Waste Rock and Acid Rock Drainage

Waste rock, host rock and mineralized overburden not removed from the Site for processing are distributed
across the Site and represent one of the primary sources of arsenic and metals. The geologic zone of interest
for mining consisted of vein minerals of crystalline quartz and fine-grained pyrite. Lenses of sulfide minerals
occur within portions of the vein and contribute to the acid-generating potential of the waste rock. Pyrite
(iron sulfide), sphalerite (zinc sulfide) and galena (lead sulfide) are the most abundant ore minerals in the
mine. Gold, silver, copper, lead and zinc appear in the higher-grade veins. The depth of waste rock varies
across the Site from less than 1 foot in the vicinity of the trench to 18-plus feet near the twin ore bins and
the old settling ponds. Contaminant concentrations, depth and volumes of waste rock onsite are presented
in later sections of this document.

5.1.3 Adit Discharge and Acid Mine Drainage

The lower adit portal leads to an 800-foot-long cross-cut tunnel through granite that leads to the
mineralized zone where the underground mining occurred. While the mine was being worked, waste rock
and ore were transported out of the lower workings of the mine through this adit. Since mine closure,
several sections of timber in the adit have collapsed. Ground water has pooled behind these natural earth
and rock plugs. Exposure of the mineralized rock to infiltrating ground water and bacteria in the mine
workings has resulted in a constant discharge of acidic water from the adit portal (see Exhibit 5-4) and
ultimately into USG Creek. This represents a substantial and continuing source of arsenic and metals at the
Site.

EXHIBIT 5-4

Acid Mine Drainage from Collapsed Adit at the Crystal Mine

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SECTION 5. SUMMARY OF SITE CHARACTERISTICS

5.2 Movement and Behavior of Contaminants of Concern

Oxidation of metal sulfides produces acidity (hydrogen ions), free metal ions and sulfate. Acidic conditions
increase the mobility of most metals, whereas alkaline environments inhibit metal mobility. Arsenic, a
metalloid, behaves differently and may become more mobile in high pH environments. As free metal ions
move into the ground water or surface water, geochemical reactions can occur to enhance or inhibit
mobility. Oxidation reduction potential (ORP) also influences metal mobility in water. Because the reactions
influencing form and mobility of metals and arsenic in ground water and surface water are primarily
dissolution, precipitation and adsorption, the chemical and physical factors that dominate these reactions
will have a strong influence on the form and mobility of metals and arsenic as well. Therefore, the acidity,
alkalinity, oxidation-reduction conditions, hardness and the presence of organic material in ground water
and surface water are important factors influencing the movement and behavior of contaminants.

5.2.1 Contaminants of Concern

The contaminants of concern (COCs) at the Site are aluminum, antimony, arsenic, cadmium, copper, lead,
manganese, selenium, silver and zinc. In soils, antimony, arsenic, cadmium, copper, lead, manganese,
selenium and zinc are the focus for terrestrial life because significant concentrations of these contaminants
still remain throughout the Site. In surface water, and ground water discharging to surface water, elevated
concentrations of aluminum, cadmium, copper, lead and zinc are of particular concern because of their
toxicity to aquatic life and potential toxicity to plants in riparian areas. Stream sediment data show that
antimony, arsenic, cadmium, copper, lead, manganese, silver and zinc exist at concentrations high enough to
cause adverse effects on stream macroinvertebrates (aquatic life).

5.2.2 Contamination Mobilization, Transport and Pathways, and the Exposure
Model

Metals-laden materials can be mobilized from Site sources in a number of ways. These processes include
erosion, runoff, infiltration and wind-borne transport. The most likely transport pathways for contaminants
are through surface water, ground water, air, vegetation and soil pore water (vadose zone). Along these
pathways, exchange of COCs may occur between:

> Soil and ground water

> Stream sediment and surface water

> Soil and vegetation

> Surface water and ground water

> Vegetation and surface water

Specific pathways between abiotic and biological elements of the Site will be discussed in more detail by the
screening risk assessment. A source, pathway, receptor exposure diagram (conceptual exposure model
[CEM]) specific to the Site is presented in Exhibit 5-5.

The major mobilization mechanisms for the contaminants at the Site are summarized below. Detailed
descriptions of these mechanisms and contaminant transport phenomena are presented in Section 2.3 of
the RI/FS Report.

• Erosion and runoff. Stream bank erosion, especially during high flows, may cause stream bank materials
containing arsenic and metals to erode directly into the stream. The degree to which materials may be
transported is influenced by climatic conditions, infiltration, slope, soil conditions, animal-human
activity, the proximity of waste rock and metals-impacted soil and the presence of vegetation.

• Infiltration and vadose zone transport. Soluble metals of concern in source material may be leached by
infiltrating water and carried into underlying soil and shallow ground water.

5-12

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PART 2 DECISION SUMMARY

• Ground water inflow into the fluvial system. Ground water discharging to the surface through a mine
adit is a particularly important transport mechanism at the Site. Adit discharge water originates from the
infiltration of precipitation and snowmelt into the soil profile and its migration into underlying bedrock
fractures. Movement of water down through fractures intercepts the mineralized zone and
underground workings created by mining. Once the ground water discharges from the portal, exposure
to the atmosphere may result in precipitation, co-precipitation and absorptive processes that change
free metal ions to less-mobile forms. Arsenic and metals that remain in solution are transported as a
point discharge until they infiltrate into the soil or intercept runoff or other surface water such as USG
Creek.

• Physical transport of sediments. Transient sediment deposits may form along the creek as point bar
deposits or within the streambed itself, where metals may reside for a long time until they are
remobilized by a change in flow regime.

• Surface water flow to ground water. Surface water may transport contaminants into ground water
along stream reaches that lose water into shallow alluvial aquifers.

• Ground water flow into surface water. Ground water contributing to base flow for streams during low
flow periods may transport contaminants from floodplain areas. The floodplain for the first order
USG Creek is very small and poorly developed in the steep upper channel reach adjacent to the mine.

• Airborne transport. Contaminants could potentially be carried on dust particles entrained by the wind.
Variables influencing the degree to which this transport mechanism might occur include climatic
conditions, surface area, or exposed and sparsely vegetated source materials.

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Notes:

C = Potentially complete pathway; quantitatively evaluated in the risk assessment
— = Incomplete pathway

I = Potentially complete pathway considered insignificat and not quatitatively evaluated in the risk assessment

ES032009002BOI \tal\Proj\EPA\406950CiyitalMine\ROD\Draft ROD\ROD FIGURE5\Graphici ROD Draft

Potential
Exposure
Pathways



Incidential Ingestion

Dermal/Direct Contact



Uptake into Plants



Incidential Ingestion

Dermal/Direct Contact



Uptake into Plants

Uptake into Food Items

Dust Inhalation

Ingestion
Dermal Contact

Incidental Ingestion
Dermal Contact

Ingestion
Dermal/Direct Contact
Uptake into Food Items

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PART 2 DECISION SUMMARY

5.3 Summary of Previous Site Response Actions

The Crystal Mine has been subjected to several previous remedial activities including an adit discharge

treatment demonstration project, a removal action and remedial planning. These are summarized below;

additional details are found in Section 3.3 of the Rl report.

• A 1994-1996 remote mine site demonstration project consisted of a semi-active process focused directly
on the treatment of AMD discharging from the lower adit (MSE, 1998). Effluent draining from the lower
adit of the Crystal Mine was injected with quicklime (calcium oxide) where it was allowed to mix prior to
being discharged into one of two primary settling ponds. Effluent from the secondary settling pond was
discharged directly into USG Creek. Sludge buildup in the settling ponds was pumped periodically into
the Crystal Mine airshaft for disposal.

• The 2002 removal action was directed toward mitigating surface recharge to the underground workings
of the mine in an attempt to reduce the volume of AMD discharging from the lower adit. The objective
of the removal action was to reduce the collection of snowmelt and rain runoff in the trenched surface
feature caused by previous mining. The collection of surface water runoff in this feature was thought to
contribute to the recharge of the Crystal Mine underground workings and production of AMD, This work
(see Exhibit 5-6) appeared to help reduce the rate of AMD discharge from the lower adit by about

25 percent.

• Initial remedial planning included a draft engineering evaluation/cost assessment (EE/CA) prepared to
evaluate potential remedial options for the Site. An evaluation of alternatives for effectiveness,
implementability and cost was completed. No one alternative consistently outperformed the others
with respect to meeting all the evaluation criteria. The EPA later decided to implement an interim ROD
rather than an EE/CA. Refer to Section 3.3 of the Rl and the draft EE/CA document (EPA, 2009a) for
detailed information.

• A sampling and analysis plan (SAP) and quality assurance project plan (QAPP) were prepared to guide
the 2010 field activities of the Rl. These documents (EPA, 2010) described the purpose and scope
designed to characterize the Site and the surrounding area. The field investigation focused on the
acquisition of data to define human and ecological risk associated with arsenic and metals
concentrations in soils, surface water and shallow ground water associated with the Site; develop an
accurate model depicting exposure pathways; and obtain the information necessary to complete the
RI/FS process.

EXHIBIT 5-6

Completed Removal Work on the Crystal Mine Trench

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SECTION 5. SUMMARY OF SITE CHARACTERISTICS

5.4 Site Description

5.4.1 Climate

The Site receives an average annual precipitation of approximately 29 inches. The highest precipitation for
the area generally occurs in May, June and July. Temperature extremes for the Site range from highs near
85 degrees Fahrenheit (°F) in late summer to lows near -40°F in December and January. Snowfall
accumulation typically occurs between October and March.

5.4.2 Drainage and Hydrology

USG Creek, a tributary to Cataract Creek, drains the eastern side of the Site. USG Creek flows south-
southeast approximately 2.5 miles to its confluence with Cataract Creek (see Exhibits 1-2 and 1-3). Surface
water from the west side of the Site and a wetland area northwest of the trench flows into an unnamed
tributary that joins USG approximately 0.8 miles downstream of the Site. Runoff from the lower adit and adit
discharge is intercepted by two sediment retention ponds that overflow into USG Creek.

The beneficial use classification for the entire Missouri River drainage (including Cataract Creek), unless
otherwise identified, is B-l. The B-l classification states that the water quality of the stream must be
sufficient to support recreational activities such as bathing, swimming and recreation; growth and
propagation of salmonid fishes and associated aquatic life and other wildlife; agricultural and industrial
water supply; and drinking, culinary and food processing purposes after conventional treatment. The Site
has three wetland areas that total 21 acres in size. The northernmost wetland is located directly northwest
of the Crystal trench. The second wetland area is the riparian zone associated with USG Creek near the
eastern portion of the mine site. The third wetland area is formed by a small seep at the southern-most
corner of the mine site (see Exhibit 5-1).

5.4.3 Soils and Geologic Setting

Bedrock geologic units in the vicinity include volcanic rocks of the Elkhorn Mountains Volcanics and intrusive
rocks of the Butte Pluton. The Elkhorn Mountain Volcanics are described as welded tuff and minor volcanic
sandstone and conglomerate. The Butte Pluton consists of granite or granodiorite.

Surficial geologic units in the vicinity include glacial till in high areas, alluvial deposits along USG, bog/swamp
deposits in the wetland area, and talus and colluvial deposits on steep slopes.

Geologic structures in the vicinity of the Site consist of faults/shear zones, joints, fractures and lineaments. The
geologic structure influences the orientation and location of the ore bodies, which in turn controls the
configuration and location of the mining activity. The Crystal vein occupies an east-trending shear zone that is
more than 3.5 miles in length. Faults shown on geologic mapping also indicate a north-trending cross fault in
the Crystal vein, and beneath USG, where the ore vein is offset to the north.

The majority of the soils consist of stoney loam to 15 inches in depth overlying bedrock. Along the lower
portion of the where glacial till has been deposited, soils consists of bouldery loamy sand to 60 inches in
depth.

5.4.4 Disturbed Areas, Surface Features, Historical Features

The majority of the land within the Basin Creek Watershed is managed by the USFS or U.S. Bureau of Land
Management (BLM). The historic land use for claim properties in the watershed includes mining, logging,
grazing, recreation and limited residential. A few residences are located along Cataract Creek, with the Town
of Basin at the mouth of the watershed. No known potable water supply wells are located within a 1-mile
radius of the Site, but water from Cataract Creek is used for irrigation, supports impaired fisheries and
discharges to the Boulder River, which is a drinking water source for the Towns of Basin and Boulder.

5-18

ES042314162 509BOI

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PART 2 DECISION SUMMARY

Disturbed, barren, erosion-prone surfaces cover the Site and include the filled-in surface trench, waste rock
dumps, ore load-out areas, and an access road across the Site. Most of the Site is not vegetated, as waste
rock piles and contaminated soil combined with harsh Site conditions have inhibited revegetation. A twin
ore bin is located below the Crystal dump in the vicinity of the upper adit. Other historic wooden mining
structures include a couple of miner's cabins, a covered ore rail load out, an assay building, and an
outhouse. The two sediment retention ponds and a concrete platform for the quick-lime injection
demonstration project are located in the lower portion of the Site.

Cultural and historic resources within the Site were characterized during the Rl. When the Site was first
examined in 1998, the surface workings of the mine covered portions of three mining claims. These claims
(along with the Crystal, St. Lawrence, Jack and Commerce) comprise the Crystal group. The investigation
noted that the remains at the Crystal Mine reflect basically two different periods of operations—the initial
production period at the turn of the twentieth century, and another period of operations during the late
1920s and 1930s when the Bullock family leased and mined the property for a number of years. Twenty-six
features were observed at the Site including residences and a variety of mining-related buildings and
structures in two distinct clusters (Rossillion and Haynes, 1999). In 2011, Renewable Technologies, Inc. (RTI)
conducted an updated inventory of the Site to note any changes that might have occurred since 1998 and
reconsider Site eligibility in light of the 2003 historic evaluation guidance document. RTI concluded that the
1998 National Register evaluation for the Crystal Mine (24JF1567) continues to stand as a thorough and
defensible evaluation (RTI, 2011). A complete copy of the inventory is presented as a reference to the
Rl report (EPA, 2013).

5.5 Summary of Previous Site Characterization Water
Studies

Water quality at, or in the vicinity of the Site was evaluated by several investigations in the past, all with the
intent of assessing impacts of mining on USG Creek, a tributary of Cataract Creek. This historic data, when
coupled with data gathered during the Rl, indicates the Crystal Mine has been a substantial and long-term
degrading influence on USG Creek that is sustained through its confluence with Cataract Creek.

At least five major investigations were carried out over a period from 1989 through 2010. They are
listed below.

• Abandoned-Inactive Mines Program Deerlodge National Forest, Basin Creek Drainage. Volume I,

Basin Creek Drainage and Volume II, Cataract Creek Drainage (MBMG, 1994 and 1995). Results showed:

Adit discharge from the Crystal Mine degraded water quality all along its flow path including
USG Creek.

Surface waters from above Crystal Mine, in USG Creek, and above and below the confluence with
Cataract Creek exceeded primary and secondary maximum contaminant levels (MCLs) in effect in
1992 and 1993. Primary and secondary MCLs for aluminum, cadmium, copper, iron, lead,
manganese, zinc and pH were all exceeded directly below the Crystal Mine (dissolved metals
analyses).

• Montana Department of State Lands sampled the Crystal Mine adit discharge (Pioneer Technical
Services and Thomas, Dean and Hoskins, Inc, 1994), finding:

Acidic water (pH 3.41).

Adit discharge exceeded Federal Safe Drinking Water Act (SDWA) MCLs for arsenic, cadmium and
copper.

Adit discharge exceeded the chronic and acute aquatic life criteria for arsenic, cadmium, copper,
lead and zinc presented in the Montana Numeric Water Quality Standards, Circular DEQ-7 (MDEQ,
2010).