Explanation of Significant Differences Operable Unit 2 Central lmpoundment Area

Groundwater Collection System

Bunker Hill Mining and Metallurgical Complex

Superfund Site, Northern Idaho

EPA Identification Number: 100048340921

February 2018

Issued by: Date:

·~~~ Sheryl Bilbrey, Director Office of Environmental Cleanup U.S. Environmental Protection Agency

NG0822170756SEA

NG0822170756SEA III

Contents Acronyms and Abbreviations .................................................................................................................... v 1 Introduction ............................................................................................................................. 1-1

1.1 Site Name and Location ......................................................................................................... 1-1 1.2 Lead and Support Agencies.................................................................................................... 1-1 1.3 Statement of Purpose ............................................................................................................ 1-1 1.4 Administrative Record ........................................................................................................... 1-3

2 Background .............................................................................................................................. 2-1 2.1 Summary of Site History ........................................................................................................ 2-1

2.1.1 Overview ................................................................................................................... 2-1 2.1.2 History of the Central Impoundment Area ............................................................... 2-1

2.2 Nature and Extent of Contamination within Operable Unit 2 ............................................... 2-2 2.2.1 Contaminants of Concern ......................................................................................... 2-2 2.2.2 Contaminant Sources ................................................................................................ 2-2 2.2.3 Contaminant Release Mechanisms........................................................................... 2-5 2.2.4 Groundwater and Surface Water Interaction and Contaminant Transport ............. 2-5

2.3 Description of the Central Impoundment Area Groundwater Collection System Remedial Action ..................................................................................................................................... 2-6

3 Basis of Explanation of Significant Differences ........................................................................... 3-1 3.1 Groundwater Collection Technology Optimization ............................................................... 3-1 3.2 Option 1—Drain Only (2012 IRODA Selected Remedy Technology Approach) ..................... 3-2 3.3 Option 2—Drain and Cutoff Wall ........................................................................................... 3-2 3.4 Option 3—Extraction Wells and Cutoff Wall (Preferred Technology Approach) .................. 3-5 3.5 Option 4—Fully Enclosed Cutoff Wall and Extraction Wells ................................................. 3-5 3.6 Preferred Technology Approach for Groundwater Collection .............................................. 3-5

4 Description of Significant Differences ........................................................................................ 4-1 5 Support Agency Acceptance ...................................................................................................... 5-1 6 Statutory Determinations .......................................................................................................... 6-1 7 Public Participation Compliance ................................................................................................ 7-1 8 References ................................................................................................................................ 8-1

Table

Table 4-1. Summary of Significant Differences Between Selected Remedy Approach and Explanation of Significant Difference-Modified Design ......................................................................................... 4-1

Figures

Figure 1-1. Vicinity Map ................................................................................................................................... 1-2 Figure 2-1. Central Impoundment Area, Central Treatment Plant, and Related Features .............................. 2-3 Figure 2-2. Gaining and Losing Reaches of the South Fork of the Coeur d'Alene River and Dissolved

Zinc Concentrations in Upper Aquifer Groundwater ..................................................................... 2-7 Figure 2-3. Maximum Zinc Ambient Water Quality Criteria Ratios in Bunker Hill Box Surface Water,

2002 to 2008 .................................................................................................................................. 2-8 Figure 2-4. Selected Remedy: Bunker Hill Box Remedial Actions .................................................................. 2-10 Figure 3-1. Drain-Only Configuration Simulated Upper Aquifer Capture, Baseflow Conditions ..................... 3-3 Figure 3-2. Extraction Well/Cutoff Wall Configuration Simulated Upper Aquifer Capture,

Baseflow Condition ........................................................................................................................ 3-7

NG0822170756SEA V

Acronyms and Abbreviations AMD Acid mine drainage AWQC ambient water quality criteria

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act of 1980 CERCLIS Comprehensive Environmental Response, Compensation, and Liability Information System CIA central impoundment area COC contaminant of concern CTP Central Treatment Plant

EPA U.S. Environmental Protection Agency ESD Explanation of Significant Differences

gpm gallons per minute GCS groundwater collection system

IDEQ Idaho Department of Environmental Quality IRODA Interim ROD Amendment

NCP National Contingency Plan

O&M operation and maintenance OU Operable Unit

RAO remedial action objective ROD Record of Decision

SFCDR South Fork of the Coeur d’Alene River Site Bunker Hill Mining and Metallurgical Complex Superfund Site

SECTION 1

NG0822170756SEA 1-1

Introduction 1.1 Site Name and Location The Bunker Hill Mining and Metallurgical Complex Superfund Site (Bunker Hill Superfund Site or Site) is located primarily in northern Idaho. The Site includes mining-contaminated areas in the Coeur d’Alene River corridor, adjacent floodplains, downstream water bodies,1 tributaries, and fill areas, as well as the 21-square-mile Bunker Hill “Box” where historical ore-processing and smelting operations occurred. The Site was listed on the National Priorities List in 1983 and, under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA), is assigned U.S. Environmental Protection Agency (EPA) identification number IDD048340921.

The EPA has divided the Bunker Hill Superfund Site into three Operable Units (OUs):

• OU 1 includes the populated areas of the Bunker Hill Box.

• OU 2 comprises the nonpopulated areas of the Bunker Hill Box where the former mining and metallurgical facilities were located.

• OU 3 includes all areas of the Coeur d’Alene Basin outside the Bunker Hill Box where mining-related contamination is located. OU 3 extends from the Idaho-Montana border into the state of Washington and contains floodplains, populated areas, lakes, rivers, and tributaries. OU 3 includes areas surrounding and including the South Fork of the Coeur d’Alene River (SFCDR) and its tributaries and areas surrounding and including the main stem of the Coeur d’Alene River down to the depositional areas of the Spokane River, which flows from Coeur d’Alene Lake into Washington State.2

The combined area of the 21-square mile Box and the upstream portion of OU 3 is often referred to as the “Upper Basin.” Figure 1-1 shows the location of the Bunker Hill Site and its OUs.

1.2 Lead and Support Agencies The EPA is the lead agency for implementation of the cleanup. The Idaho Department of Environmental Quality (IDEQ) is the support agency.

1.3 Statement of Purpose Record of Decisions (RODs) were issued for OU 1 in 1991 (EPA, 1991), OU 2 in 1992 (EPA, 1992), and OU 3 in 2002 (EPA, 2002). An Interim ROD Amendment (IRODA) was issued in 2012 (EPA, 2012a) that amended portions of all three RODs. The 1992 ROD for OU 2 and 2012 IRODA include remedial actions for collecting and treating select metals-contaminated source waters within OU 2. The 2012 IRODA clarifies and modifies some of the OU 2 water collection and treatment actions that had previously been selected in the 1992 OU 2 ROD. The estimated costs of the remedial actions scoped within the 2012 IRODA for OU 2 and OU 3 is approximately $550 million.

As described in the 2012 IRODA, the OU 2 water collection and treatment actions focus on the following:

• Intercepting metals-contaminated groundwater (using gravel-filled trenches and piping systems (referred to as a “drain”) before the flows enter into surface water creek and river system

1 Downstream water bodies extend to portions of the Spokane River, located in eastern Washington. 2 Note that the river corridor portions of the SFCDR and Pine Creek located within the Bunker Hill Box are considered part of OU 3.

Washington Idaho Montana

Spokane

0 10 205 Miles

Figure 1-1Vicinity MapOU 2 Groundwater Collection System Explanation of Significant DifferencesBUNKER HILL SUPERFUND SITE

! Spokane River!

Coeurd'Alene OU 1 and OU 2

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St. Joe River

BUNKER HILL BOX

South Fork Coeur d’Alene RiverCoeu

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’Alene River

North Fork Coeur d’Alene River

Little North Fork Coeur d’Alene River

Upper Basin Portionof OU 3

Lower Basin Portionof OU 3

§̈Coeurd’AleneLake

LakePendOreille

Pinehurst

KingstonSmelterville Kellogg

Wardner

Osburn Silverton

WallaceMullan

KOOTENAI COUNTYBENEWAH COUNTY

SHOS

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Coeur d’Alene River

S. Fork Coeur d’Alene River

Pine

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Moon C

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Canyon Creek

Ninemile

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Placer Creek

Upper Basin

The Bunker Hill Box

OU = Operable Unit Note: The river corridor portions of the South Fork of the Coeur d’Alene River and Pine Creek located within the Bunker Hill Box are considered to be part of OU 3.

Vicinity Map of Coeur d’Alene Basin

Note: OU 3 includes all areas of the Coeur d’Alene Basin outside the Bunker Hill Box where mining-related contamination is located. OU 3 includes areas surrounding and including the South Fork of the Coeur d'Alene River and its tributaries and areas surrounding and including the main stem of the Coeur d'Alene River down to the depositional areas of the Spokane River, which flows from Coeur d'Alene Lake into Washington State.

ES070913163331SEA . GCS_Fig2-1_Vicinity_Map_09-10-13

Woodland Park

SECTION 1 – INTRODUCTION

NG0822170756SEA 1-3

• Pumping the collected waters to an existing treatment facility located within OU 2

• Treating the waters to achieve the surface water quality discharge requirements established by EPA for the Central Treatment Plant (CTP; EPA, 2015).

• Discharging the treated water back into the SFCDR

During pre-design activities for the OU 2 Central Impoundment Area (CIA) Groundwater Collection System (GCS) remedial action, the process option for groundwater collection for this action was re-evaluated to optimize collection efficiency and to minimize capital and long-term operation and maintenance (O&M) costs. The pre-design evaluations indicated that a groundwater collection technology consisting of a soil-bentonite cutoff wall and a multi-well pumping system would be a more efficient and cost effective method to collect the contaminated groundwater than the drain system assumed in the Selected Remedy.

In compliance with Section 117(c) of CERCLA, and Section 300.435(c)(2)(i) of the the National Contingency Plan (NCP), , this Explanation of Significant Differences (ESD) describes the process option change from a drain system to a cutoff wall/well system for the groundwater collection portion of the OU 2 CIA GCS remedial action. This process option change has been determined by EPA to be a significant, but not fundamental, change to the Selected Remedy of the 2012 IRODA.

1.4 Administrative Record This ESD and its supporting documents will become part of the Administrative Record file for this site, in accordance with the NCP, Section 300.825(a)(2). The ESD and its supporting documents are available for review at the following locations:

EPA Seattle Office (contains the entire Administrative Record) Superfund Records Center 1200 6th Avenue, Suite 900 Seattle, Washington 98101 206-553-4494 or 800-424-4372

Wallace Public Library 415 River Street Wallace, Idaho 83873 208-752-4571

Kellogg Public Library 16 West Market Avenue Kellogg, Idaho 83837 208-786-7231

Molstead Library, North Idaho College (contains the entire Administrative Record) 1000 Garden Avenue Coeur d’Alene, Idaho 83814 Tel. 208-769-3355

St. Maries Library 822 West College Avenue St. Maries, Idaho 83861208-245-3732

Spokane Public Library 906 West Main Avenue Spokane, Washington 99201-0976 509-444-5336

EPA Coeur d’Alene Field Office 1910 Northwest Boulevard, Suite 208 Coeur d’Alene, Idaho 83814 208-664-4588

SECTION 2

NG0822170756SEA 2-1

Background 2.1 Summary of Site History 2.1.1 Overview Mining within the Coeur d’Alene Basin began more than 100 years ago, and the region became one of the leading silver, lead, and zinc producing areas in the world. Mining activities were concentrated in the Upper Basin where more than 1,000 historical mining or milling-related features had been identified. The Bunker Hill Mine and the Basin’s primary mineral processing and smelting facilities were located within OU 2.

As a result of past mining, milling, and smelting practices, high concentrations of metals (primarily lead, zinc, cadmium and arsenic) are pervasive in soils, sediments, surface water and groundwater in substantial portions of the Basin and these metals are potentially hazardous to human health and the environment. Within the Upper Basin, elevated concentrations of metals in the environment resulted primarily from the discharge or erosion of mill tailings and other mine-generated wastes into rivers and streams. In turn, these water bodies deposited millions of tons of mine tailings into stream beds, floodplains, and shorelines throughout the Site.

Tailings impoundments were frequently built in floodplains adjacent to river systems providing contaminant migration pathways from source materials to groundwater and surface water. Tailings were also frequently used as fill materials on which communities were built and for commercial and infrastructure construction projects. Particulates released to the air from smelting operations contained high concentrations of metals and were transported as airborne dust and deposited over a large area. Acid mine drainage (AMD), which is metals-affected drainage water from mine portals, also impacts surface water and groundwater.

2.1.2 History of the Central Impoundment Area In the early stages of historical mining and milling activity within OU 2, tailings and mine wastes resulting from mining, milling, and other processing activities were discharged either to the SFCDR, its tributaries, or the immediate areas surrounding mine workings, mills, or process facilities. In addition, the widespread availability of tailings, tailings mixtures, and mine waste rock resulted in these waste materials being used as fill in construction projects throughout OU 2 for decades. Most fill used to construct the towns of Kellogg and Smelterville and the area between the communities comprised mine-related material.

In 1928, construction of the unlined CIA tailings impoundment began, ending the direct discharge of tailings from OU 2 sources to the SFCDR. However, the existing tailings piles and waste materials placed throughout the valley floor were frequently reworked by the SFCDR during flood events, as well as miners’ efforts to reprocess mine tailings that were thought to have additional value. These activities resulted in the mine waste and tailings becoming intermixed with native soil throughout the SFCDR valley alluvium.

The current CIA configuration (Figure 2-1) covers approximately 260 acres with embankments ranging in height from about 30 to 70 feet above the valley floor. The CIA is bordered by Interstate 90 on the north and Bunker Creek on the south. The SFCDR is located to the north of Interstate 90; the CTP, an existing facility that currently primarily treats AMD flow from the Bunker Hill Mine, is located at the base of the southeast corner of the CIA. Prior to remediation, the CIA was used as an impoundment area for tailings and other process and mine wastes generated from Bunker Hill facilities. The Bunker Hill Superfund Site was placed on the National Priorities List (NPL) in 1983 and following initial investigation of OU 1 and OU 2, cleanup actions began in the late 1980s. The 1992 OU2 ROD (EPA, 1992) identified the CIA as a repository for the consolidation of tailings, gypsum, and other non-principal threat materials removed as part of site removal activities. From the middle to late 1990s, the CIA was used as an on-site waste consolidation area during the

SECTION 2 – BACKGROUND

2-2 NG0822170756SEA

initial phase of EPA’s OU 2 remedial actions and was closed with a geomembrane cover system in 2000 (EPA, 2010). Since the CIA was closed in 2000, discharge from an area of discrete seepage on the southern bank of the SFCDR (often referred to as the “CIA seeps”) has reduced by an order of magnitude but continues to the present. Groundwater elevations, in the area’s shallow aquifer, suggest that the current discharge associated with this discrete seepage location is associated with the shallow groundwater in the area and not direct seepage from the closed CIA (EPA, 2010).

2.2 Nature and Extent of Contamination within Operable Unit 2 2.2.1 Contaminants of Concern The contaminants of concern (COCs3) for the Upper Basin include arsenic, cadmium, lead, mercury, and zinc, with primarily cadmium, lead, and zinc affecting environmental media in the Upper Basin (soil, sediments, surface water, and groundwater). Contaminant releases within the Upper Basin are driven primarily by surface water and groundwater moving within the environmental system. Dissolved zinc in surface water and groundwater and total (or particulate) lead in surface water are used as indicators to identify potential contaminant sources having negative effects on water quality in the SFCDR and its tributaries; other COCs have been discussed in detail in previous EPA documents (EPA, 2001; EPA 2012).

Dissolved zinc is considered an appropriate indicator for dissolved metals in surface water and groundwater because it occurs at the highest concentrations; it is relatively mobile compared to other metals; and dissolved metals (particularly cadmium) appear well-correlated with dissolved zinc throughout the Upper Basin. Zinc is widely distributed in the environment, and SFCDR site-specific ambient water quality criteria (AWQC) for zinc are exceeded throughout the Upper Basin, often at levels acutely toxic to aquatic organisms. As one of the most mobile of the heavy metals, zinc is readily transported in most natural waters and can occur in both suspended and dissolved forms in surface water. The AWQC ratio is the concentration of a chemical in surface water divided by the AWQC for that chemical. An AWQC ratio of one or less indicates that the water quality criteria are met.

2.2.2 Contaminant Sources The long history of mining activities within the Upper Basin, combined with the dynamic and complex hydrologic system and anthropogenic modifications to that system, have resulted in widespread and commingled sources of contamination. Contaminant sources within the Box can be placed into several broad categories: tailings; impoundments and stockpiles; soils and sediments; and mine water. The sources specifically related to the OU2 GCS are:

• Tailings—Metal-rich tailings from mining activities are widespread throughout OU 2. Tailings have become mixed with natural alluvium on the main valley floor and in some tributary valleys over time. In general, the tailings/alluvium mixture is between 4 and 7 feet thick across the main valley floor of the Box. Tailings were also widely used as fill for construction projects within the Box, including residential areas, industrial facilities, railroad grades, and roadway fill.

• Impoundments and stockpiles—Impoundments were used to consolidate and contain tailings and other process wastes within OU 2. The two major tailings impoundments within OU 2 are Page Ponds and the CIA. In addition, mined ore, processed concentrates, finished materials, waste rock, and other mine wastes were stockpiled at several locations within OU 2.

3 COCs are those chemicals that are identified as threats to human health or the environment based on risk assessment results and need to be addressed by the remedial actions included in the Upper Basin Selected Remedy.

WARDNER

KELLOGG

SMELTERVILLE

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South ForkCoeur d'Alene River

Bunker Creek

Central Treatment Plan Outfall to Bunker Creek (006 Outfall)

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Base Map Data:NHDPlus (Hydrography, 2005);ESRI (Roads, Jurisdictional Boundaries, 2006);IDWR (Aerial Imagery, 2006).

Figure 2-1Central Impoundment Area, Central Treatment Plant, and Related FeaturesOU 2 Groundwater Collection System Explanation of Significant DifferencesBUNKER HILL SUPERFUND SITE

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Upper Basin,Coeur d'Alene

River, North Fork

Lower Basin,Coeur d'Alene

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SECTION 2 – BACKGROUND

NG0822170756SEA 2-5

2.2.3 Contaminant Release Mechanisms Following are the primary contaminant release mechanisms from sources that are related to the OU 2 GCS:

• Inflow from upstream—Metals in both dissolved and particulate phases enter OU 2 in surface waterand groundwater at the eastern, upstream boundary from sources upstream of OU 2.

• Dissolution—The most widespread and highly concentrated contaminants found in sources within OU 2are cadmium, lead, and zinc. All three metals have the common attribute of occurring as insolublesulfide minerals and are classified as hazardous substances. The most common sulfide ores of cadmium,lead, and zinc do not form acid upon oxidation, but they do release soluble metals in the presence ofwater and atmospheric oxygen. Lead is released in soluble (dissolved) form from galena, but it is quicklysorbed to organic and inorganic materials such as sediment and suspended solids. Cadmium and zincreleased in soluble form have a much greater tendency to stay dissolved in OU 2 surface water andgroundwater.

Water moving through sources within OU 2 releases dissolved cadmium and zinc and, to a lesser extent,dissolved lead. Typically, water moving through source materials within OU 2 is the result of thefollowing:

– Infiltration of precipitation and snowmelt– Infiltration of surface water– Groundwater elevation fluctuations– Groundwater flux from areas of high to low head– Groundwater and surface water interaction at gaining and losing reaches of SFCDR

Infiltration of precipitation and surface water through contaminant sources is a predominant mechanism for release of dissolved metal to groundwater within OU 2.

2.2.4 Groundwater and Surface Water Interaction and Contaminant Transport Contaminants within OU 2 are transported primarily in the groundwater and surface water systems after release from their sources. In groundwater, contaminants are primarily in the dissolved phase, and in surface water, they are present in both the dissolved and particulate phases. Fate and transport of metals within OU 2 is complicated by the heterogeneous nature of the physical system, the varied and widespread nature of contaminant sources, and the long timeframe over which contaminants have been present in the OU 2 environmental system. Groundwater and surface water interaction plays a significant role in both contaminant release from sources and the transport and migration of contamination within OU 2.

Alluvial aquifers within the Upper Basin are often located in the valley fill sediments and are typically shallow, unconfined, and long and narrow in dimension. Alluvium and floodplain deposit sources are widespread contaminant sources in the Upper Basin, spreading across the floodplains and valleys of the SFCDR and other SFCDR tributaries. In most of the Upper Basin, a single aquifer is beneath the SFCDR and its tributaries, but in the Bunker Hill Box and downstream, both upper and lower alluvial aquifers are present. The upper aquifer is present in alluvial materials in the SFCDR valley, and a lower, confined aquifer is present downstream from the eastern end of the Box. The shallow upper aquifer in the Box has more contamination than the lower aquifer because of the groundwater interaction with contaminant sources; this shallow aquifer is the focus of the remedial action to collect contaminated groundwater from beneath the CIA for conveyance and treatment at the CTP.

A high degree of hydraulic interaction exists between the shallow groundwater aquifer and surface water. In general, the following characteristics are important to the interaction of groundwater and surface water in the Box:

SECTION 2 – BACKGROUND

2-6 NG0822170756SEA

• Groundwater quality in the shallow aquifer is impacted by tailings used as fill when the communities were developed, contaminated material impoundments (e.g., the CIA), and floodplain deposit sediment sources.

• The SFCDR tends to be losing (surface water discharging to groundwater) where the valley widens entering the Kellogg area from the east (upstream), and gaining (groundwater discharging to surface water) where the valley narrows adjacent to the western end of the CIA. This results in surface water leaving the river, entering the groundwater, contacting contaminant source areas below the town of Kellogg and the CIA, and then re-entering the river as contaminated groundwater seepage inflow.

• During low-flow conditions (late summer/early fall), surface water flow is dominated by groundwater inflow.

The groundwater-surface water interaction within the Box is significant in terms of the volume exchanged and its impact on SFCDR water quality. The eastern (upstream) gaining reach in the Box (see Figure 2-2) is located near the CIA, which results in a major negative effect on water quality due to highly contaminated groundwater entering the SFCDR. Furthermore, the CTP currently discharges treated water to Bunker Creek, and much of this treated water enters the groundwater system through losing reaches of Bunker Creek. This results in additional discharge of high-concentration groundwater to the SFCDR.

Figure 2-3 shows the maximum zinc AWQC ratios in surface water in the Bunker Hill Box using data from 2002 to 2008. It is important to note the correlation between the location of the gaining reach of the SFCDR (as shown on Figure 2-2) and the higher SFCDR zinc AWQC ratios on Figure 2-3 (the ratios increase along the eastern end of the CIA).

2.3 Description of the Central Impoundment Area Groundwater Collection System Remedial Action

The primary remedial action objectives (RAOs) applicable to the CIA GCS remedial action are related to ecological receptors’ exposure to COCs in surface water and groundwater. These RAOs, as stated in the 2012 IRODA, are as follows:

• Ecosystem physical structure and function—Reduce COCs in soil, sediments, and surface water to support a functional ecosystem for aquatic and terrestrial plant and animal populations (including waterfowl, riparian songbirds, and other species protected under the Endangered Species Act, the Fish and Wildlife Conservation Act, and the Migratory Bird Treaty Act) in the Upper Basin.

• Surface water—Reduce risks from COCs in surface water in the Upper Basin to acceptable exposure levels that are protective of ecological receptors.

• Groundwater—Reduce discharge to surface water of groundwater containing COCs at concentrations that cause surface water to exceed levels protective of ecological receptors.

Bunker Creek

LittlePineCreek

HumboldtCreek

Grouse Creek

MagnetGulch

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Deadwood Gulch

RailroadGulch

Milo Creek

Italian Gulch

Jackass Creek

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Smelterville

Kellogg

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CIA

0 2,000 4,000 Feet

Dissolved zinc concentrations (mg/L)

Nondetect148

15

25

SFCDR Primarily Gaining ReachSFCDR Primarily Losing Reach

CIA = Central Impoundment AreaCTP = Central Treatment PlantOU = Operable UnitSFCDR = South Fork Coeur d’Alene River WWTP = Wastewater Treatment Plantmg/L = milligrams per liter

Source: (EPA, 2012a)

Notes:Figure 2-2Gaining and Losing Reaches of the SFCDR and Dissolved Zinc Concentrations in Upper Aquifer GroundwaterOU 2 Groundwater Collection SystemExplanation of Significant DifferencesBUNKER HILL SUPERFUND SITE

382081.TA.07.01.01.04_BunkerHill_ES031812185638SEA . Fig 5-10 Dissolved Zinc Conc Upper-Aquifer Groundwater Bunker Hill Box v4 16mar12.ai . gr

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Smelterville

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0 2,000 4,000 Feet

LittlePineCreek

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Italian Gulch

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AWQC Ratios<1 2 8

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25 Rep

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ntat

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esNotes:1. Dissolved zinc AWQC ratios are the maximum results based

on data collected from October 2002 to the present. Data sourcesinclude the OU 3 BEMP, the OU 2 EMP, and various studiesincluding the 2008 High-Flow and Low-Flow Surface Water Study, the Coeur d’Alene Basin Remedial Action Monitoring Program, and the 2008 Data Report for Fish Population Monitoring andEnvironmental Sampling in the SFCDR.

2. Source sites shown here are discrete, while most waste massis distributed more broadly, such as along streams and theSFCDR, and below communities and infrastructure.

AWQC = Ambient Water Quality CriteriaBEMP = Basin Environmental Monitoring Program EMP = Environmental Monitoring ProgramEPA = U.S. Environmental Protection AgencyOU = Operable UnitSFCDR = South Fork Coeur d’Alene River

Source: (EPA, 2012a)

Figure 2-3Maximum Zinc AWQC Ratios in Bunker Hill Box Surface Water, 2002 to 2008OU 2 Groundwater Collection System Explanation of Significant Differences BUNKER HILL SUPERFUND SITE

382081.TA.07.01.01.04_BunkerHill_ES031812185638SEA . Fig 5-5 Max Zinc AWQC Ratios Bunker Hill Box Surface Water Post-2002 v4 1aug12.ai . gr

SECTION 2 – BACKGROUND

NG0822170756SEA 2-9

To achieve these RAOs, the 2012 IRODA assumed that the approach to collect groundwater from beneath the CIA would be a 4,225-foot-long drain4 constructed along the northern edge of the CIA with a single pump station and conveyance piping back to the CTP. The groundwater collector drain would be used to collect contaminated groundwater near the CIA before it enters the SFCDR. Figure 2-4 from the 2012 IRODA shows the various remedial actions included for the Box in the Selected Remedy, with the GCS drain and location highlighted for emphasis. The drain was assumed to extend from the ground surface to the bottom of the upper alluvial aquifer at an average depth of 25 feet.

4 A drain is a trench with a pipe located in the trench bottom and backfilled with gravel to promote groundwater to flow into the pipe system.

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Smelterville

CTPExpansion

Cherry Raise andReed and RussellAdits

Extraction Wellsfor GroundwaterCollection

ConveyancePiping fromUpper Basin(See Figure 12-9)

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382081.TA.07.01.01.04_BunkerHill_ES031812185638SEA . 12-17_Selected_Remedy_Bunker_Hill_Box_v6_23apr12.ai . gr

Figure 2-4Selected Remedy: Bunker Hill Box Remedial ActionsOU 2 Groundwater Collection System Explanation of Significant Differences BUNKER HILL SUPERFUND SITE

Notes:Selected Remedy design element addressed in Explanation of Significant Differences

CIA = Central Impoundment AreaCTP = Central Treatment PlantOU = Operable UnitSFCDR = South Fork Coeur d’Alene RiverWWTP = Wastewater Treatment Plant

1CTP effluent discharge pieline may be conveyed to the SFCDR on the east side of the CIA (as pictured above) or along Bunker Creek.

SECTION 3

NG0822170756SEA 3-1

Basis of Explanation of Significant Differences 3.1 Groundwater Collection Technology Optimization The design objectives of the GCS are:

• Reducing contaminant loading to surface water (SFCDR; Bunker Creek) • Maximizing hydraulic isolation of the CIA and the extent of hydraulic capture • Minimizing infiltration of surface water into the GCS • Preventing contaminated groundwater surface ponding • Minimizing the flow to be treated at the CTP • Minimizing system capital and O&M costs

In consideration of these design objectives and as part of the GCS design definition phase, the groundwater collection approach was evaluated in greater detail from the perspective of groundwater and surface-water interaction, geochemical and geotechnical subsurface conditions, constructability, and screening-level costs. This technology optimization phase is summarized below and fully documented in the Central Impoundment Area Groundwater Collection System Design Definition Report (CH2M, 2013).

The groundwater collection approach optimization included the following activities:

• Groundwater model development—A local-scale, refined version of the basinwide groundwater model was developed to evaluate various potential groundwater collection configurations and approaches.

• Field investigations and laboratory testing—Geotechnical and hydrogeologic field investigations were conducted that included drilling soil borings, installing wells, excavating test pits, conducting four aquifer pump tests, and conducting geotechnical laboratory testing.

• Groundwater model refinement, calibration, and simulations—The groundwater model was refined and calibrated based on data collected during field investigations; groundwater simulations were conducted to evaluate groundwater collection technologies.

• Geochemical modeling—Geochemical modeling was conducted to evaluate the potential biogeochemical reactions and impact of mixing oxygen-rich surface water (such as that from the SFCDR) and anoxic, ferrous iron-rich groundwater entering a drain system.

• Conceptual-level screening cost comparisons—Capital costs and net present value (NPV) of future O&M costs were estimated. The cost comparisons did not include all components of the entire GCS system and were intended as an approximate comparison only to be used in combination with consideration of risk, uncertainty, and adaptability associated with each of the options.

Process options considered for groundwater collection included the approach assumed in the Selected Remedy (a drain) and an extraction-well pumping system, in combination with a soil-bentonite cutoff wall downgradient of either a drain or pumping well system (to minimize inflow of oxygen-rich river water into the collection system).

The following options were evaluated:

• Option 1: Drain Only— This is the technology assumed in the 2102 IRODA Selected Remedy.

• Option 2: Drain/Cutoff Wall—The drain would function to collect contaminated groundwater, and a cutoff wall immediately downgradient of the drain would minimize inflow of oxygen-rich river water into the drain.

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• Option 3: Extraction Wells/Cutoff Wall—Contaminated groundwater would be collected using a multiwell pumping system, and a cutoff wall immediately downgradient of the well system would minimize the inflow of oxygen-rich river water into the wells.

• Option 4: Fully Enclosed Cutoff Wall/Extraction Wells—This option assumed a cutoff wall would be installed that completely encircled the CIA (complete hydraulic isolation) with pumping wells operating from within the enclosed cutoff wall area.

The following subsections discuss the technology options evaluation.

3.2 Option 1—Drain Only (2012 IRODA Selected Remedy Technology Approach)

Groundwater model simulations of the drain only concept conducted as part of the Final Focused Feasibility Study (EPA, 2012b) focused on confirming a proof of concept. These simulations confirmed that a drain could achieve the objective of reducing the discharge of metals-laden groundwater to the SFCDR. The drain characteristics assumed in these simulations were designed to maximize drain inflow and reduction of metals-loading to the river. Post-IRODA and as part of the GCS predesign, additional groundwater model simulations indicated that the drain-only approach would require an average extraction rate of about 3,000 gpm to achieve hydraulic capture which would also result in a significant amount of surface-water leakage from the SFCDR into the aquifer system (that would subsequently be captured by the drain and conveyed to the CTP for treatment thus increasing treatment costs). Additionally, the drain only configuration resulted in fairly significant drawdown in groundwater levels in the areas around the drain. This drawdown could introduce atmospheric oxygen into the aquifer system and drain. Figure 3-1 shows the area of hydraulic capture of the Option 1 drain-only approach.

The technical concern associated with introducing oxygen-rich surface water and anoxic, ferrous iron-rich groundwater into the drain is that biogeochemical reactions would occur that would increase the potential for ferric iron hydroxide to precipitate and clog the drain piping and permeable drain backfill (that is, iron-fouling). Another, lesser, concern was that groundwater level fluctuations around the drain during operation could result in an influx of atmospheric oxygen into the drain system and could also result in similar potential for iron-fouling that could affect drain function.

Geochemical modeling was conducted that assumed varying ratios of SFCDR surface water mixed with groundwater. These analyses indicated that introducing even small amounts of dissolved oxygen into the drain system could result in iron precipitates fouling the drain, causing potential degradation of its performance over time and probably a shortened drain life. For this conceptual-level screening evaluation, a 15-year life was assumed for the drain, and complete replacement of the drain would be needed at that time. This significant O&M cost for drain replacement would be incurred to maintain the operational effectiveness of the drain system for a 30-year life. As a result of the geochemical analyses and the high risk of iron-fouling, isolating the drain system from oxygenated surface water became a high priority.

3.3 Option 2—Drain and Cutoff Wall This groundwater collection option assumed the same drain technology as the Selected Remedy (Option 1), but also added a soil-bentonite cutoff wall immediately downgradient of the drain to minimize the inflow of SFCDR river water to the drain.

Groundwater modeling of Option 2 indicated a significant reduction of river-water inflow into the collection system compared with the drain-only option. With this reduction of river-water inflow into the drain, the groundwater analyses indicated that the drain could pump at an average extraction rate of about 1,850 gpm and still maintain the desired area of contaminated groundwater capture.

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¯ 0 1,000 2,000500 Feet

Simulated Hydraulic CaptureCaptured by drain Not captured

Fall 2011 dissolvedzinc concentration (mg/L)!( <= 1.0!( 1.1 − 2.0!( 2.1 − 5.0!( 5.1 − 10.0!( 10.1 − 15.0!( 15.1 − 20.0!( 20.1 − 40.0!( 40.1 − 105.0!( Not sampled

RDD \\ODIN\PROJ\USEPA\BUNKERHILL_382081\BUNKERHILL\FIGURES\MXD\2013_05_GW_DDR\FIGURE_32_DRAINONLY_CAPTURE_BASEFLOW.MXD HPERRY 8/26/2013 3:02:58 PM

Figure 3-1Drain-Only Configuration Simulated Upper Aquifer Capture, Baseflow ConditionsOU 2 Groundwater Collection System Explanation of Significant Differences BUNKER HILL SUPERFUND SITE

Notes:CIA = Central Impoundment Area HGL = hydraulic grade line ft/ft = foot per footmg/L = milligrams per literOU = Operable Unit

Source: (CH2M, 2013)

LEGENDCIA drain segment (HGL=0.004 ft/ft)CIA drain segment (HGL=0.0068 ft/ft)Transverse drain segment (HGL=0.004 ft/ft)

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3.4 Option 3—Extraction Wells and Cutoff Wall (Preferred Technology Approach)

The impact of a lower overall extraction rate resulting from including a cutoff wall (Option 2) allowed for considering using individual extraction wells rather than a continuous horizontal drain system to achieve hydraulic capture of the contaminated groundwater. Previously, without including a cutoff wall, extraction wells were not considered optimal because of the high flow rates required would necessitate a large number of individual wells, and the limited saturated thickness of the upper alluvial aquifer would limit the quantity of water that could be extracted from a single well.

By including a low-permeability cutoff wall in the design, required extraction rates would be reduced significantly, and groundwater would “back-up” upgradient of the cutoff wall allowing for effective capture with fewer wells at greater spacing with essentially no or negative drawdown. Additionally, under this scenario, using extraction wells (rather than a drain system) was considered beneficial for the following reasons:

• Extraction wells are easier to install.

• More contractors with extraction well installation experience are available.

• Extraction wells are cheaper to install and replace.

• More well-established protocols for extraction well O&M and cleaning are available.

• Extraction wells provide for more flexibility in operating and refining pumping distribution.

• Extraction wells provide for better lateral distribution of drawdown to minimize the potential for surface ponding of water.

Groundwater modelling was conducted to optimize the configuration of the combined extraction well/cutoff wall GCS in achieving the design objectives listed above. The optimal configuration consisted of an 8,800-foot-long cutoff wall and a series of 10 extraction wells. The wells were positioned along an east-west transect south of the cutoff wall, spaced approximately 600 feet apart, and would have a nominal flow rate of about 200 gpm (for a total flow of 2,000 gpm). The layout of this cutoff wall/extraction well system is shown in Figure 3-2 along with the accompanying expected hydraulic capture area.

3.5 Option 4—Fully Enclosed Cutoff Wall and Extraction Wells A final set of groundwater model simulations were performed to investigate a design scenario that would provide complete hydraulic isolation of the CIA from the surrounding aquifer system and minimize groundwater collection required for hydraulic capture and treatment. For this scenario, an 18,250-foot-long cutoff wall was assumed to completely encircle the CIA with a series of five extraction wells inside the cutoff wall. The wells were modelled to have a nominal flow rate of 200 gpm, for a total flow of 1,000 gpm.

3.6 Preferred Technology Approach for Groundwater Collection During the design optimization phase, the drain characteristics that maximized drain inflow and, thus, reduced metals discharge to the river, were found to also induce a significant amount of surface-water leakage from the SFCDR into the aquifer system that would subsequently be captured by the drain and conveyed to the CTP for treatment. The higher extraction rate of the drain only option , coupled with the induced leakage of river water to the drain, resulted in higher flows that would require treatment and thereby increased treatment costs. Additionally, the drain configuration would result in a significant drawdown in groundwater levels in the area around the drain, which would introduce atmospheric oxygen into the system. Additional geochemical modeling demonstrated that introducing surface water into the

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drain, when mixed with groundwater, generated favorable conditions for iron (that is, ferric iron hydroxide) precipitation, potentially affecting drain function and efficiency. The results of the optimization and geochemical modeling evaluations resulted in a change in the remedial design strategy, which led to the changes described in this ESD.

Based on the optimization evaluations and screening-level cost estimates, Option 3 (extraction wells with a cutoff wall) was selected as the preferred technology for collecting groundwater and providing the best balance of achieving the design objectives. Although Option 4 (fully enclosed cutoff wall around the CIA) would provide complete hydraulic capture of groundwater beneath the CIA at a lower flow rate, it was not preferred because of its significantly higher capital cost for the cutoff wall; further, there was great uncertainty with respect to constructability in consideration of the extensive utilities in the CIA area. Option 1 (drain only) was not preferred because of the high risk of iron-fouling and associated high O&M costs. Option 2 (drain with a cutoff wall) was judged to be less preferred than Option 3 in consideration of the many advantages and flexibility provided by using extraction wells versus a collection drain.

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¯ 0 1,000 2,000500 Feet

LEGEND

!< Extraction wellCutoff wall

Simulated Hydraulic CaptureCaptured by extraction well Not captured

Fall 2011 dissolved zinc concentration (mg/L)!( <= 1.0!( 1.1 − 2.0!( 2.1 − 5.0!( 5.1 − 10.0!( 10.1 − 15.0!( 15.1 − 20.0!( 20.1 − 40.0!( 40.1 − 105.0!( Not sampled

RDD \\ODIN\PROJ\USEPA\BUNKERHILL_382081\BUNKERHILL\FIGURES\MXD\2013_05_GW_DDR\FIGURE_50_WALL_WELL_CAPTURE_BASEFLOW.MXD HPERRY 8/27/2013 4:33:59 PM

Notes:mg/L = milligrams per liter OU = Operable Unit

Source: (CH2M, 2013)

Figure 3-2Extraction Well/Cutoff Wall ConfigurationSimulated Upper Aquifer Capture,Baseflow ConditionsOU 2 Groundwater Collection System Explanation of Significant Differences BUNKER HILL SUPERFUND SITE

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Description of Significant Differences The RAOs, remedial technology, scope, and design objectives of the OU 2 CIA GCS remedial action remain the same as that described in the 2012 IRODA. However, with this ESD, EPA is changing a secondary technology process option of the remedial action for how contaminated groundwater is collected.

As described in Section 3 of this ESD, the preferred process option to collect the contaminated groundwater from beneath the CIA before it enters the SFCDR is an extraction well/cutoff wall system versus a drain that was part of the Selected Remedy. Table 4-1 summarizes key aspects for comparison between the 2012 Selected Remedy technology approach and the change described in this ESD. As shown, the ESD-modified design has the same general response action and remedial technology as the action described in the IRODA and achieves the same hydraulic capture performance, but at a lower extracted flow rate. From a cost perspective, the ESD-modified design has lower capital and lower annual O&M costs, resulting in a 30-year NPV of $33 million. In comparison, the Selected Remedy action has higher capital and higher annual O&M costs and includes a one-time cost to replace the drain, resulting in a 30-year NPV of $51 million.

Table 4-1. Summary of Significant Differences Between Selected Remedy Approach and Explanation of Significant Difference-Modified Design

Comparison Items Selected Remedy Approach Explanation of Significant Difference-Modified Design

General response action Hydraulic isolation Hydraulic isolation

Remedial technology Groundwater collection Groundwater collection

Process option Drain with pump stations Extraction wells and cutoff wall

Treatment volume 3,000 to 4,000 gpm 2,000 gpm

Hydraulic containment Achieved Achieved

Resistance to Iron Fouling and Ease of Cleanout Low Medium to High

Capital cost $18 million $16 million

NPV of annual O&M 1 $19 million $17 million

NPV of Year 15 O&M 2 $14 million $0 (zero; not applicable)

30-year NPV 3 $51 million $33 million

Note; The costs developed are feasibility study-level estimates with a nominal accuracy of -30 percent to +50 percent and have been rounded to two significant digits. The cost opinion is in 2013 dollars and was prepared for guidance in project evaluation from the information available at the time of preparation (CH2M, 2013). 1 Annual O&M costs include the conceptual-level cost for treatment at the CTP for the assumed flow for each alternative. 2 The Selected Remedy (drain with pump station) was assumed to require a single-year O&M expenditure to address drain clogging from iron fouling. 3 This includes capital cost, NPV of 15-Year expenditures, and 30-Year NPV of annual O&M.

EPA has determined that the change in approach to collect the contaminated groundwater (from a drain to a cutoff wall/well system) constitutes a “significant change” to the Selected Remedy, as defined by CERCLA. A significant change generally involves changing a component of a remedial action but not fundamentally altering the overall cleanup approach. For significant changes to the Selected Remedy, EPA is required to publish an ESD describing the reasons such changes were made. Consistent with EPA’s 1999 Guide to Preparing Superfund Proposed Plans, Records of Decision, and Other Remedy Selection Decision Documents

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and the 2012 Upper Basin IRODA (EPA, 2012a), EPA is proceeding with designing and constructing the project while the ESD is being prepared and made available to the public.

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Support Agency Acceptance IDEQ supported the water collection and treatment component of the 2012 IRODA that included the OU 2 CIA GCS that will collect metals-contaminated groundwater near the CIA to significantly reduce metals loading to the SFCDR. IDEQ and EPA worked collaboratively to evaluate and optimize the GCS remedial action that resulted in this ESD. During the remedial design process, as a support agency partner, IDEQ provided written review comments on all design documentation. In addition, IDEQ signed a Memorandum of Agreement with EPA (EPA, 2014) to fund response and operations and maintenance costs for water collection and treatment within OU 1 and OU 2 of the Bunker Hill Superfund site.

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Statutory Determinations The Selected Remedy for OU 2, as modified by this ESD, continues to satisfy the requirements of §121 of CERCLA to accomplish the following objectives:

• Protect human health and the environment, through a combination of treatment, engineering controls, and institutional controls.

• Comply with applicable or relevant and appropriate requirements.

• Be cost-effective.

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Public Participation Compliance Public participation requirements for an ESD, as set out in the NCP §300.435(c)(2)(i), will be met as follows:

• The ESD and supporting information will be added to the 2012 IRODA’s Administrative Record established under §300.815. The ESD and its supporting documents will be made available to the public at the locations listed in Section 1.4 of this ESD.

• The ESD will be made available on EPA’s web page (https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=1000195)

• When this ESD is issued, a public notice of its availability will be published in the local Shoshone News Press.

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References CH2M HILL Engineers, Inc. (CH2M). September 13, 2013. Central Impoundment Area Groundwater Collection

System Design Definition Report. Prepared for the U.S. Environmental Protection Agency, Region 10, Seattle, Washington.

U.S. Environmental Protection Agency (EPA). August 30, 1991. EPA Superfund Record of Decision, Bunker Hill Mining and Metallurgical Complex Residential Soils Operable Unit 1, Shoshone County, Idaho.

U.S. Environmental Protection Agency (EPA). September 22, 1992. EPA Superfund Record of Decision: Bunker Hill Mining & Metallurgical Complex, EPA ID: IDD048340921, OU 02, Smelterville, ID.

U.S. Environmental Protection Agency (EPA). December 2001. Second Record of Decision Amendment for Operable Unit 2 . Bunker Hill Mining & Metallurgical Complex, EPA ID: IDD048340921, OU 02, Smelterville, ID

U.S. Environmental Protection Agency (EPA). September 2002. Record of Decision, Bunker Hill Mining and Metallurgical Complex Operable Unit 3.Available at https://yosemite.epa.gov/r10/cleanup.nsf/ fb6a4e3291f5d28388256d140051048b/cbc45a44fa1ede3988256ce9005623b1!OpenDocument.

U.S. Environmental Protection Agency (EPA). November, 2010. 2010 Five-Year Review for the Bunker Hill Mining and Metallurgical Complex Superfund Site, Operable Units 1, 2, and 3, Idaho and Washington.

U.S. Environmental Protection Agency (EPA). August 27, 2012a. Interim Record of Decision Amendment, Upper Basin of the Coeur d’Alene River, Bunker Hill Mining and Metallurgical Complex Superfund Site.

U.S. Environmental Protection Agency (EPA). August 2012b. Final Focused Feasibility Study Report, Upper Basin of the Coeur d’Alene River, Bunker Hill Mining and Metallurgical Complex Superfund Site. Available at https://www3.epa.gov/region10/pdf/sites/bunker_hill/cda_basin/final_ffs_report_volume_1.pdf. Prepared by CH2M HILL Engineerse, Inc. for the U.S. Environmental Protection Agency, Region 10, Seattle, Washington.

U.S. Environmental Proection Agency (EPA). 2014. Appendix L—Memorandum of Agreement Between the United State Environmental Protection Agency and the Idaho Department of Environmental Quality Regarding the Release of Court Registry Funds. Signed by the USEPA and IDEQ on June 19, 2014.Case 3:96-cv-00122-EJL, Document 1632. United States District Court for the District of Idaho, United States of America, Plaintiff v. HECLA LIMITED, a Delaware Corporation, Defendant, and Consolidated Cases. Filed July 9, 2014.

U.S. Environmental Protection Agency (EPA). February 12, 2015. Central Treatment Plant Dischaarge Requirements Technical Memorandum, Bunker Hill Superfund Site. Prepared by the U.S. Environmental Protection Agency in conjunction with CH2M HILL.