United States Environmental Protection Agency 1200 Pennsylvania Avenue, N.W. Washington, D.C. 20460 March 2011 Office of Solid Waste and Emergency Response 

# Support Document for the Revised National Priorities List Final Rule – ACM Smelter & Refinery

# Support Document for the Revised National Priorities List Final Rule ACM Smelter and Refinery March 2011

Site Assessment and Reme dy Decisions Branch Office of Superfund Remediati on and Technology Innovation Office of Solid Waste and Emergency Response U.S. Environmental Protection Agency Washington, DC 20460

## Table of Contents

Executive Summary ....................................................................................................................................v

Introduction

............................................................................................................................... .................vi

Background of the NPL .........................................................................................................................vi

Development of the NPL .......................................................................................................................vii

Hazard Ranking System .......................................................................................................................vii

Other Mechanisms for Listing ............................................................................................................viii

Organization of this Document .............................................................................................................ix

Glossary ..................................................................................................................................................ix

1. List of Commenters and Correspondence ........................................................................................1

2. Site Description ...................................................................................................................................3

3. Summary of Comments ......................................................................................................................4

3.1 Need for Listing ...........................................................................................................................5

3.2 Purpose of NPL ...........................................................................................................................6

3.3 Impacts of Listing .......................................................................................................................7

3.4 Previous NPL Decision (Rationale for Listing) ........................................................................7

3.5 Liability ........................................................................................................................................8

3.6 Risk Associated with Site ............................................................................................................9

3.7 Permitted Releases and Regulatory Limits .............................................................................12

3.8 Adequacy of Site Description ...................................................................................................12

3.9 Extent of Site .............................................................................................................................13

3.10 Comments on HRS ....................................................................................................................14

3.11 Scoring Other Pathways ...........................................................................................................15

3.12 Consideration of Response Actions / Current Conditions .....................................................16

Sources Identified at Proposal .............................................................................................................17

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3.13 Additional Sources ....................................................................................................................17 Source 4 – Slag Waste ........................................................................................................................18

3.14 Source 1

............................................................................................................................... .......19 3.14.1 Hazardous Substances Associated with Source 1 ...............................................................19 3.14.2 Source 1 Delineation/Inference of Contamination ..............................................................21 3.14.3 Sampling Procedures ...........................................................................................................23 3.14.4 Other Sampling/Current Conditions ...................................................................................24 3.14.5 Source 1 Hazardous Waste Quantity ...................................................................................26 3.14.6 Source 1 Containment .........................................................................................................28

3.15 Source 2

............................................................................................................................... .......29 3.15.1 Source 2 Eligibility and Consideration of Response Activities ..........................................29 3.15.2 Association of Hazardous Substances with Source 2 .........................................................32 3.15.3 Source 2 Hazardous Waste Quantity ...................................................................................33 3.15.4 Source 2 Containment .........................................................................................................35

3.16 Other Possible On-site Sources ................................................................................................37

Surface Water Scoring at Proposal .....................................................................................................38

3.17 Evaluation of the Surface Water Pathway ..............................................................................38

3.18 Description of Pathway .............................................................................................................41

Likelihood of Release ............................................................................................................................44

3.19 Likelihood of Release - Observed Release by Direct Observation ........................................44 3.19.1 Documentation of Observed Release by Direct Observation ..............................................45 3.19.2 Quantity of Slag Dumped into the Missouri River .............................................................46

3.20 Surface Water Likelihood of Release - Observed Release by Chemical Analysis ...............48 3.20.1 Surface Water Observed Release - Significant Increase .....................................................51 3.20.1.1 Contamination Levels below Regulatory Limits ..............................................................52 3.20.1.2 Releases in Other Aquatic Media ......................................................................................53 3.20.1.3 Bias of Conclusions/Current Conditions ...........................................................................54 3.20.1.4 Downstream Contaminant Levels .....................................................................................56 3.20.2 Surface Water Observed Release - Attribution ...................................................................58 3.20.2.1 Attribution of Releases to On-Site Sources ......................................................................61 3.20.2.2 Other Possible Off-Site Sources ........................................................................................63 3.20.2.3 Permitted Storm Water Discharges ...................................................................................65 3.20.3 Surface Water Observed Release - Adequacy of Background ............................................65

3.21 Potential to Release ...................................................................................................................66

Surface Water – Drinking Water Threat ............................................................................................67

3.22 Surface Water - Drinking Water Threat ................................................................................67

Surface Water – Human Food Chain Threat .....................................................................................68

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3.23 Surface Water - Human Food Chain Threat Waste Characteristics ...................................69 3.23.1 Toxicity/Persistence/Bioaccumulation ................................................................................69 3.23.2 Human Food Chain Threat Waste Quantity ........................................................................71 3.23.3 Human Food Chain Threat Waste Characteristics ..............................................................72

3.24 Surface Water - Human Food Chain Threat Targets ............................................................73 3.24.1 Targets Subject to Actual Contamination ...........................................................................76 3.24.2 Consideration of Existing Fish Tissue Data ........................................................................77

3.25 Surface Water - Human Food Chain Threat Total Score .....................................................79

Surface Water – Environmental Threat .............................................................................................80

3.26 Surface Water - Environmental Threat Waste Characteristics ...........................................80 3.26.1 Environmental Threat Waste Quantity ................................................................................80 3.26.2 Environmental Threat Waste Characteristics ......................................................................81

3.27 Surface Water - Environmental Threat Targets ....................................................................82 3.27.1 Actual Contamination .........................................................................................................82 3.27.2 Aquatic Biota Targets .........................................................................................................84

3.28 Surface Water - Environmental Threat Total Score .............................................................85

3.29 Surface Water Overland/Flood Migration Component Score ..............................................86

Soil Exposure

............................................................................................................................... ..........87

3.30 Soil Exposure/Area of Observed Contamination ...................................................................87 3.30.1 AOC Sampling Method ......................................................................................................87 3.30.2 Significant Increase of Hazardous Substances ....................................................................90 3.30.3 AOC Delineation/Inference ................................................................................................94 3.30.4 Hazardous Substance Deposition/Attribution .....................................................................95 3.30.4.1 Aerial Deposition ..............................................................................................................95 3.30.4.2 Evaluation of Lead ............................................................................................................99 3.30.5 Contamination below Cleanup Criteria .............................................................................101

3.31 Soil Exposure Pathway Waste Characteristics .....................................................................101 3.31.1 Soil Exposure Hazardous Waste Quantity ........................................................................101 3.31.1 Calculation of Soil Exposure Pathway Waste Characteristics ..........................................103

3.32 Soil Exposure Targets .............................................................................................................104 3.32.1 Identification of Actually Contaminated Targets ..............................................................104 3.32.2 Count of Level II Targets ..................................................................................................106 3.32.3 Targets Not Scored ............................................................................................................107

3.33 Soil Exposure Pathway – Resident Population Threat Total Score ...................................108

3.34 Soil Exposure Pathway – Total Score ...................................................................................109

3.35 Alternate HRS Score ...............................................................................................................110

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iv 4.0 Conclusion

............................................................................................................................... ..........116ACM Smelter & Refinery NPL Listing Support Document March 2011



## Executive Summary

Section 105(a)(8)(B) of CERCLA, as amended by SARA, requires that the EPA prepare a list of national priorities among the known releases or threatened re leases of hazardous substances, pollutants, or contaminants throughout the United States. An orig inal National Priorities List (NPL) was promulgated on September 8, 1983 (48 FR 40658). CERCLA requir es that EPA update the list at least annually.

This document provides responses to public comment s received on the ACM Smelter and Refinery site, proposed on March 4, 2010 (75 FR 9843). This site is being added to the NPL based on an evaluation under EPA’s Hazard Ranking System (HRS) in a final rule published in the Federal Register in March 2011.

vACM Smelter & Refinery NPL Listing Support Document March 2011



## Introduction

This document explains the rationale for adding the ACM Smelter and Refinery site in Black Eagle, Montana, to the National Priorities List (NPL) of uncontrolled hazardous waste sites and provides responses to public comments received on this site listi ng proposal. EPA proposed this site to the NPL on March 4, 2010 (75 FR 9843). This site is being added to the N PL based on an evaluation under the Hazard Ranking System (HRS) in a final rule published in the Federal Register in March 2011. 

### Background of the NPL

In 1980, Congress enacted the Comprehensive Enviro nmental Response, Compensation, and Liability Act (CERCLA), 42 U.S.C. Sections 9601 et seq . in response to the dangers of uncontrolled hazardous waste sites. CERCLA was amended on October 17, 1986, by the Superfund Amendments and Reauthorization Act (SARA), Public Law No. 99-499, stat., 1613 et seq. To implement CERCLA, EPA promulgated the revised National Oil and Hazardous Substances Polluti on Contingency Plan (NCP), 40 CFR Part 300, on July 16, 1982 (47 FR 31180), pursuant to CERCLA Section 105 and Executive Order 12316 (46 FR 42237, August 20, 1981). The NCP, further revised by EPA on September 16, 1985 (50 FR 37624) and November 20, 1985 (50 FR 47912), sets forth guid elines and procedures needed to respond under CERCLA to releases and threatened releases of hazard ous substances, pollutants, or contaminants. On March 8, 1990 (55 FR 8666), EPA further revised the NCP in response to SARA.

Section 105(a)(8)(A) of CERCLA, as amended by SARA, requires that the NCP include

criteria for determining priorities among rel eases or threatened releases throughout the United States for the purpose of taking remedial action and, to the extent practicable, take into account the potential urgency of such action, for the purpose of taking removal action.

Removal action involves cleanup or other actions that are taken in response to emergency conditions or on a short-term or temporary basis (CERCLA Section 101). Remedial action is generally long-term in nature and involves response actions that are consistent with a permanent remedy for a release (CERCLA Section 101). Criteria for placing sites on the NPL, which makes them eligible for remedial actions financed by the Trust Fund established under CERCLA, were included in the HRS. EPA promulgated the HRS as Appendix A of the NCP (47 FR 31219, July 16, 1982). On December 14, 1990 (56 FR 51532), EPA promulgated revisions to the HRS in response to SARA, and established the effective date for the HRS revisions as March 15, 1991.

Section 105(a)(8)(B) of CERCLA, as amended, requires that the statutory criteria provided by the HRS be used to prepare a list of national priorities among th e known releases or threatened releases of hazardous substances, pollutants, or contaminants throughout th e United States. The list, which is Appendix B of the NCP, is the NPL.

An original NPL of 406 sites was promulgated on September 8, 1983 (48 FR 40658). At that time, an HRS score of 28.5 was established as the cutoff for lis ting because it yielded an initial NPL of at least 400 sites, as suggested by CERCLA. The NPL has been expanded several times since then, most recently on September 29, 2010 (75 FR 59975). The Agency al so has published a number of proposed rulemakings to add sites to the NPL. The most recent proposal was on October 21, 2010 (75 FR 64976).

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### Development of the NPL

The primary purpose of the NPL is stated in the le gislative history of CERCLA (Report of the Committee on Environment and Public Works, Senate Repor t No. 96-848, 96th Cong., 2d Sess. 60 [1980]).

The priority list serves primarily informa tional purposes, identifying for the States and the public those facilities and sites or other releases which appear to warrant remedial actions. Inclusion of a facility or site on the list does not in itself reflect a judgment of the activities of its owner or operator, it does not require those persons to undertake any action, nor does it assign liability to any person. Subsequent govern ment actions will be necessary in order to do so, and these acti ons will be attended by all appropriate procedural safeguards.

The NPL, therefore, is primarily an informational a nd management tool. The identification of a site for the NPL is intended primarily to guide EPA in determ ining which sites warrant further investigation to assess the nature and extent of the human health and environmental risks associated with the site and to determine what CERCLA-financed remedial action(s), if any, may be appropriate. The NPL also serves to notify the public of sites EPA believes warrant further investigation. Finally, listing a site may, to the extent potentially responsible parties are identifiable at the time of listing, serve as notice to such parties that the Agency may initiate CERCLA-financed remedial action.

CERCLA Section 105(a)(8)(B) directs EPA to list prio rity sites among the known releases or threatened release of hazardous substances, pollutants, or c ontaminants, and Section 105(a)(8)(A) directs EPA to consider certain enumerated and other appropriate factors in doing so. Thus, as a matter of policy, EPA has the discretion not to use CERCLA to respond to cer tain types of releases. Where other authorities exist, placing sites on the NPL for possible remedial action under CERCLA may not be appropriate. Therefore, EPA has chosen not to place certain types of sites on the NPL even though CERCLA does not exclude such action. If, however, the Agency later determines that sites not listed as a matter of policy are not being properly responded to, the Agency may consider placing them on the NPL. 

### Hazard Ranking System

The HRS is the principle mechanism EPA uses to place uncontrolled waste sites on the NPL. It is a numerically based screening system that uses inform ation from initial, limited investigations -- the preliminary assessment and site inspection -- to assess th e relative potential of sites to pose a threat to human health or the environment. HRS scores, however, do not determine the sequence in which EPA funds remedial response actions, because the inform ation collected to develop HRS scores is not sufficient in itself to determine either the extent of contamination or the appropriate response for a particular site. Moreover, the sites with the hi ghest scores do not necessarily come to the Agency's attention first, so that addressing sites strictly on the basis of ranking would in some cases require stopping work at sites where it was already underway. Thus, EPA relies on further, more detailed studies in the remedial investigation/feasibilit y study that typically follows listing.

The HRS uses a structured value analysis approach to scoring sites. This approach assigns numerical values to factors that relate to or indicate risk, b ased on conditions at the site. The factors are grouped into three categories. Each category has a maximum value. The categories are:

• likelihood that a site has released or has the potential to release hazardous substances into the environment;

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• characteristics of the waste (toxicity and waste quantity); and

• people or sensitive environments (targets) affected by the release.

Under the HRS, four pathways can be scored for one or more threats as identified below:

• Ground Water Migration (S gw ) - drinking water

• Surface Water Migration (S sw ) The following threats are evaluated for two separate migration components, overland/flood migration and ground water to surface water. - drinking water - human food chain - sensitive environments

• Soil Exposure (S s ) - resident population - nearby population - sensitive environments

• Air Migration (S a ) - population - sensitive environments

After scores are calculated for one or more pathways according to prescrib ed guidelines, they are combined using the following root-mean-square equati on to determine the overall site score (S), which ranges from 0 to 100 :



### 4 S + S + S + S = S 2

a 2

s 2

sw 2 gw

If all pathway scores are low, the HRS score is low. However, the HRS score can be relatively high even if only one pathway score is high. This is an important requirement for HRS scoring because some extremely dangerous sites pose threats through only one pathway. For example, buried leaking drums of hazardous substances can contaminate drinking water wells, but -- if the drums are buried deep enough and the substances not very volatile -- not surface water or air. 

### Other Mechanisms for Listing

There are two mechanisms other than the HRS by whic h sites can be placed on the NPL. The first of these mechanisms, authorized by the NCP at 40 CFR 300. 425(c)(2), allows each State and Territory to designate one site as its highest priority regardless of score. The last mechanism, authorized by the NCP at 40 CFR 300.425(c)(3), allows listing a site if it meets the following three requirements:

• Agency for Toxic Substances and Disease Registry (ATSDR) of the U.S. Public Health Service has issued a health advisory that recommends dissociation of individuals from the release;

• EPA determines the site poses a significant threat to public health; and

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• EPA anticipates it will be more cost-effective to use its remedial authority than to use its emergency removal authority to respond to the site. 

### Organization of this Document

The following section contains EPA responses to s ite-specific public comments received on the proposal of the ACM Smelter and Refinery s ite on March 4, 2010 (75 FR 9843). The site discussion begins with a list of commenters, followed by a site description, a summary of comments, and Agency responses to each comment. A concluding statement indicates the effect of the comments on the HRS score for the site.


### Glossary

The following acronyms and abbreviations are used throughout the text:

ACM ACM Smelter and Refinery Agency U.S. Environmental Protection Agency
AOC Area of observed contamination
ARCO Atlantic Richfield Company
ATSDR Agency for Toxic Substances and Disease Registry bgs Below ground surface
BNSF Burlington Northern Santa Fe
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act of 1980, 42 U.S.C. Sections 9601 et seq ., also known as Superfund
CFR Code of Federal Regulations
CLP Contract Laboratory Program
CRDL Contract required detection limit CRQL Contract required quantitation limit
EPA U.S. Environmental Protection Agency
ESI Expanded site investigation
FR Federal Register
GFR Great Falls Refinery
HRS Hazard Ranking System, Appendix A of the NCP
HRS score Overall site score calculated using the HRS; ranges from 0 to 100
MDEQ Montana Department of Environmental Quality
MDFWP Montana Department of Fish, Wildlife and Parks
MDL Method detection limit mg/kg Milligram per kilogram
NCP National Oil and Hazardous Substances Po llution Contingency Plan, 40 C.F.R. Part 300

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x NPL National Priorities List, Appendix B of the NCP
PPE Probable point of entry
PRP Potentially responsible party
RCRA Resource Conservation and Recovery Act
RI/FS Remedial investigation and feasibility study
SARA Superfund Amendments and Reauthorization Act
SCDM Superfund Chemical Data Matrix SQL Sample quantitation limit
TAL Target analyte list
TDL Target distance limitACM Smelter & Refinery NPL Listing Support Document March 2011



## 1. List of Commenters and Correspondence

EPA-HQ-SFUND-2010-0072-0004 Corres pondence, dated March 04, 2010, from Brian Schweitzer, Governor, State of Montana, Office of the Governor, letter requesting placement of the Newtown Creek site on the National Priorities List (NPL)
EPA-HQ-SFUND-2010-0072-0005 Correspondence, dated April 09, 2010, from Terry Jeng, memorandum authorizing extension of the commentperiod without an FR Notice for EPA-HQ-SFUND2010-0072
EPA-HQ-SFUND-2010-0072-0006 Comment , dated April 12, 2010, from William Duffy, Davis Graham & Stubbs LLP on behalf of Atlantic Richfield Company, re: Extension Request
EPA-HQ-SFUND-2010-0072-0007 Comment , dated April 12, 2010, from Douglas Ammon OSWER, USEPA, re: Response to Extension Request
EPA-HQ-SFUND-2010-0072-0008 Comment , dated April 23, 2010, from Gwenette R. Christiansen, NPL Coordinator, Site Assessment and Brownfields Program, Region 8, USEPA, re: ACM Smelter and Refinery Information Request
EPA-HQ-SFUND-2010-0072-0009 Comment, da ted May 11, 2010, from S. Peck, Public Commenter
EPA-HQ-SFUND-2010-0072-0010 Comment, dated May 11, 2010, from Graham and Maria Taylor, Public Commenter’s
EPA-HQ-SFUND-2010-0072-0011 Comment, dated May 14, 2010, from Andrew Finch, Senior Planner on behalf of Gregory T. Doyon, City Manager, City of Great Falls, Montana
EPA-HQ-SFUND-2010-0072-0011.1 Comment att achment, dated May 14, 2010, from Andrew Finch, Senior Planner on behalf of Gregory T. Doyon, City Manager, City of Great Falls, Montana
EPA-HQ-SFUND-2010-0072-0012 Comment, date d May 14, 2010, from Phil Partain, Public Commenter
EPA-HQ-SFUND-2010-0072-0013 Comment, dated May 14, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0013.1 Comment attachme nt, dated May 14, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0013.2 Comment attach ment, dated May 14, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.

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EPA-HQ-SFUND-2010-0072-0013.3 Comment attachme nt, dated May 14, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0014 Comment, dated May 14, 2010, from Gerry and Chuck Jennings, Public Commenter’s
EPA-HQ-SFUND-2010-0072-0015 Comment, dated May 17, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0015.1 Comment attachme nt, dated May 17, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0015.2 Comment attachme nt, dated May 17, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0015.3 Comment attach ment, dated May 17, 2010, from Stuart Lewin, Member of Board of Directors, Missouri River Citizens, Inc.
EPA-HQ-SFUND-2010-0072-0016 Comment, da ted May 18, 2010, from Robin Bullock, BP Remediation Management, A tlantic Richfield Company (Atlantic Richfield)
EPA-HQ-SFUND-2010-0072-0016.1 Comment attach ment, dated May 18, 2010, from Robin Bullock, BP Remediation Management, Atlantic Richfield Company (Atlantic Richfield)
EPA-HQ-SFUND-2010-0072-0016.2 Comment attach ment, dated May 18, 2010, from Robin Bullock, BP Remediation Management, Atlantic Richfield Company (Atlantic Richfield)
EPA-HQ-SFUND-2010-0072-0016.3 Comment attach ment, dated May 18, 2010, from Robin Bullock, BP Remediation Management, Atlantic Richfield Company (Atlantic Richfield)
EPA-HQ-SFUND-2010-0072-0016.4 Comment attach ment, dated May 18, 2010, from Robin Bullock, BP Remediation Management, Atlantic Richfield Company (Atlantic Richfield)
EPA-HQ-SFUND-2010-0072-0017 Comment , dated May 21, 2010, from Jack Gannon, Public Commenter
EPA-HQ-SFUND-2010-0072-0017.1 Comment att achment, dated May 21, 2010, from Jack Gannon, Public Commenter

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## 2. Site Description

The ACM Smelter and Refinery (ACM) site, situated in the vicinity of Black Eagle and Great Falls area of the State of Montana, consists of sources contaminated with metals, an observed release of metals to the Missouri River affecting fisheries and sensitive environments, and metals-contaminated soil located on residential property.

In 1893, the Boston and Montana Consolidated Co pper and Silver Mining Company built a smelter adjoining the Missouri River to treat and process ore mostly generated from mines in Butte, Montana. The Missouri River was used to generate electricity for an electrolytic refining process. By 1980, all process operations at the facility ceased and demolition of the facility structures took place from 1980 through 1986. Other closure activities continue d through 1999, including: basement substructure backfilling; salvaging; on-site burial of flue dust, granulated slag, asbestos-containing material, demolition debris, and other wastes ; and covering of wastes with soil. Other reclamation activities followed, and maintenance of recl aimed areas continues at present.
As a result of various operations at the facility, la rge quantities of granulated slag and tailings were documented as dumped into the Missouri River. Th e Montana Department of Environmental Quality estimated that millions of cubic yards of waste we re dumped into the Miss ouri River between 1893 and 1915, and that dumping of waste into the Missouri Ri ver continued at a reduced rate after 1915 until the facility closed.
An operating smoke stack for the plant, known as "the Big Stack," was built to a total length of 506 feet and operated from 1909 through 1972. The wind in the Great Falls area primarily blows from the southwest to the east-northeast, although it is documente d to blow from the northeast or east toward Black Eagle more than 20 percent of the time. The Big Stack was designed to eject a volume of 1,575,000 cubic feet of air per minute at a velocity of 450 feet per minute, and metals are documented to have been associated with the air emissions. An internal Anaconda Copper Mining Company memorandum from 1951 stated that approximately 15.04 pounds of arse nic passed out of the stack every 24 hours.
The EPA conducted an Expanded Site Inspection of the ACM Smelter and Refinery facility in April 2003 which found elevated concentrations of arsenic, cad mium, copper, lead, manganese, mercury, silver, and zinc in sediment or soil samples. A 2007 Site A ssessment and 2008 Expanded Site Inspection of the residential soils in the community of Black Eagle, ad jacent to the west of the site, identified a large area of elevated concentrations of arsenic, ca dmium, and lead in the residential soils.
The EPA also conducted an Expanded Site Inspection of the sediments in the Missouri River adjacent to and downstream of the ACM Smelter an d Refinery property in July 2009. This investigation identified a stream segment with elevated concentrations of cadmium, copper, lead, and zinc, as compared to background. This segment of the Missouri River includes widely used fishing areas, along with a number of wetland areas.
An observed release by chemical analysis and an observed release by direct observation to the surface water pathway were established in the HRS documentati on record at proposal. The observed release by chemical analysis included sedim ent samples adjacent to and downstream of the ACM property that contained levels of cadmium and/or lead at levels greater than three times over the respective background levels. For the observed release by direct observation, the direct disposal of slag and tailings from the facility into the Missouri River was established. A fishery subject to actual contamination from the site was established and is documented to be fished for human consumption. In addition, greater than 0.1 miles of wetland frontage was identified along the Mi ssouri River within the area encompassed by the observed release.

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For the soil exposure pathway, an area of obser ved contamination was scored, encompassing soil contaminated with metals at levels 3 times or gr eater above background. The HRS documentation record at proposal counted 8 samples associated with residen tial properties subject to Level I soil contamination and 178 residences as subject to Level II contamin ation within the area of observed contamination attributable to the ACM site. 

## 3. Summary of Comments

Several public commenters submitted comments regardi ng the proposed NPL listing of the ACM Smelter and Refinery site (hereinafter referred to as ACM site). Most of the commenters indicated unconditional support for the NPL designation. Atlantic Richfield Company (herein referred to as ARCO) submitted extensive technical and policy comments, summarized below.
ARCO asserted that NPL listing of the ACM site is not necessary for several reasons. ARCO asserted that further response actions at the site should be completed using the “proven and expeditious model of company-Agency cooperation,” rather than utilizi ng the Superfund process and that listing the ACM site would have unnecessary negative impacts. It asserted that listing the site would slow the pace of further cleanup efforts and would consume federal and State r esources. It contended that areas identified at the site have been reclaimed or are subject to ongoing operation and maintenance activities.
ARCO asserted that the site does not pose an unacceptabl e risk to human health and the environment, and that releases to the surface water migration pathwa y and the contaminants associated with the soil exposure pathway are within applicable regulatory or otherwise acceptable limits . ARCO asserted that conditions within both the community of Black Ea gle, and the Missouri River have been thoroughly investigated by EPA, the Montana Department of Environmental Quality (MDEQ), and ARCO and that the HRS does not reflect the presence of unacceptable risk.
ARCO submitted extensive comments challenging the scoring of the soil exposure pathway and the establishment of the area of observed contamination ( AOC) in the Black Eagle community. ARCO stated that the residential properties sampled by EPA incorrec tly included multiple residences that did not have concentrations of hazardous substances at concen trations greater than three times background concentrations. Thus, ARCO alleged, both the h azardous waste quantity and the number of affected targets in the soil exposure pathway scoring were exa ggerated, resulting in an inflated scoring of the pathway. Challenging the attribution of the hazar dous substances detected in the AOC, ARCO asserted that the substances found in Black Eagle residential soils are not necessarily the result of emissions from the smelter facility and that lead contamination of residential properties could be the result of deteriorating lead-based paint, other consumer products, or from other possible off-site sources.
Additionally, ARCO commented that the HRS sco ring of the surface water migration pathway was incorrect – challenging the establishment of the obser ved release by direct observation and the observed release by chemical analysis.
ARCO argued that the evaluation of the observed rel ease by direct observation does not identify a source material for the slag in the Missouri River and that the documentation of the disposal of slag into the Missouri River was inadequate. Regarding the observed release by chemical analysis, ARCO stated that the comparison of upstream background samples to downstream release samples was flawed due to the locations of probable points of entr y to the Missouri River from site sources, and a lack of consideration of other off-site contributors of contamination to the river. Further, ARCO asserted that other studies show that sediment, surface water, and fish tissues in the Missouri River were generally comparable to upstream background concentrations.

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ARCO disputed the scoring of actually contaminated targets in the human food chain threat and the environmental threats of the surface water migra tion pathway scoring on the basis that no observed release was established to the Missouri River. It stated that both human food chain threat targets and environmental threat targets could not be evaluate d as subject to Level II contamination and that only targets subject to potential contam ination should have been scored. 

### 3.1 Need for Listing

Comment : Atlantic Richfield Company (ARCO) challenged whether NPL listing of the ACM Smelter & Refinery site (hereinafter, ACM site) is necessar y to protect the Black Eagle community or the environment.
ARCO asserted that further response actions at th e site should be complete d using the “proven and expeditious model of company-Agency cooperation,” rather than utilizing the Superfund process. ARCO stated:
Listing the Site at this juncture will only ser ve to slow the pace of further cleanup efforts (if needed) and will consume federal and State resources that could better be directed to sites that pose greater risks to human health and the environment.
ARCO asserted that it has successfully worked with EPA and the State of Montana to address potential risks that may reasonably be traced to the former facility operations, and th at it is committed to completing appropriate further response actions. A RCO highlighted the previous cleanup work it had completed and contended that add itional cleanup can and should be co mpleted without NPL listing.

ARCO stated that it has committed to continue site -related investigation and reclamation activities and has offered to work with the appropriate agencies to determine what remedial actions, if any, are appropriate to reduce unacceptable risk. ARCO contin ued that agency cooperation with ARCO will more effectively utilize resources and result in a more efficient investigation and remediation process.
Furthermore, ARCO asserted that based on the available data, existing institutiona l controls that are in place, prior EPA findings, and ARCO’s commitment to perform future work at the former refinery operations, NPL designation is not warranted.
Response : The ACM Smelter & Refinery site qualifies for listing on the NPL. This site has received an HRS score of greater than 28.50 and has met all CE RCLA and NCP listing criteria. Furthermore, the State concurred with the listing, citing the long-term ri sk to the health of Mont anans in the area as a result of the site.
That the commenter asserted it is committed to comple ting response actions at the site outside of NPL listing does not prohibit or encumber listing the site on the NPL. The willingness of a potentially responsible party (PRP) to complete various response or investigation activities at a site is not a factor considered under the HRS. However, PRPs may affe ct remedy selection, as can any other member of the public, through the public comment process during rule making. PRPs may undertake the RI/FS and/or remedial design/remedial action stages under EPA supe rvision and pursuant to appropriate agreements with governmental authorities (under enforcement aut horities of CERCLA or those of other statutes). The listing process does not preclude PRPs from entering into these agreements.

This comment has no effect on the site score or the decision to place the site on the NPL.

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### 3.2 Purpose of NPL

Comment : ARCO stated that the intended use of the NPL is not properly applied to the ACM site. ARCO stated:
The NPL is reserved for the worst sites, where the available data supports the Agency's belief that there is a potential risk to human health or the environment. . . .NPL designation provides a mechanism to direct fe deral funding to site investigations and studies to investigate and assess site conditi ons and determine whether unacceptable risk is present under the procedures set forth in the National Contingency Plan ("NCP"), 40 CFR Part 300.
ARCO asserted that there is no risk to human health or the environment at this site to support NPL designation as conditions at the “Former GFR Site” [for mer Great Falls Refinery facility property], Black Eagle, and the Missouri River have been thor oughly investigated by EPA, MDEQ, and ARCO.
ARCO contended that NPL designation is most appr opriate at “orphan” sites, where, it stated:
...the parties that caused the contamination are bankrupt or otherwise unable to perform the work, or at those sites where the responsib le parties are solvent, but recalcitrant and unwilling to investigate or remediate the site. In those cases, EPA may utilize NPL listing to obtain federal funding to address c ontamination. The Former GFR Site is not an "orphan" site as Atlantic Richfield and other viable entities have voluntarily financed and performed investigations and reclamation to date.
ARCO asserted that EPA is not obligated to list all poten tial NPL sites. It stated that an HRS score above 28.5 only indicates that there is a potential risk to publ ic health or the environment that merits further investigation, and that EPA has the authority to require further investigations and cleanup without NPL designation.
Response : Listing the ACM Smelter & Refinery site on the NPL is consistent with the purpose of the NPL.
The primary purpose of the NPL is stated in the legislative history of CERCLA (Report of the Committee on Environment and Public Works, Senate Repor t No. 96-848, 96th Cong., 2d Sess. 60 [1980]), as follows:
The priority list serves primarily informa tional purposes, identifying for the States and the public those facilities and sites or other releases which appear to warrant remedial actions. Inclusion of a facility or site on the list does not in itself reflect a judgment of the activities of its owner or operator, it does not require those persons to undertake any action, nor does it assign liability to any person. Subsequent government actions will be necessary in order to do so, and these acti ons will be attended by all appropriate procedural safeguards.

Thus, the identification of a site for the NPL is in tended primarily to guide EPA in determining which sites warrant further investigation to assess the nature and extent of the human health and environmental risks associated with the site, and to determine what CERCLA-financed remedial action(s), if any, may be appropriate. The NPL also serves to notify the public of sites that warrant further investigation.
Listing a site may, to the extent PRPs are identifiabl e at the time of listing, serve as notice to such parties that the Agency may initiate CERCLA-financed re medial action. However, CERCLA contains no restriction that limits its application to orphan sites or insolvent or recalcitrant parties. Therefore, whether

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or not a site is associated with a PRP that is able to perform investigative or remedial work at the site has no bearing on the eligibility of a site for the NPL.

This comment has no effect on the site score or on the decision to place the site on the NPL. 

### 3.3 Impacts of Listing

Comment : ARCO contended that listing the ACM site would have unnecessary negative impacts. ARCO asserted that listing the site would slow the pace of further cleanup efforts and would consume federal and State resources, which it stated “could bett er be directed to sites that pose greater risks to human health and the environment.”
ARCO asserted that NPL listing would last for many years and that developers and prospective future purchasers of the many private properties encompassed by the site as proposed would “look elsewhere for economic development opportunities.”
Response : Potential negative impacts such as those raised by the commenter are not considered in the assessment of whether a site belongs on the NPL. In clusion of a site or facility on the list does not in itself reflect a judgment on the activities of the owner(s ) or operator(s), but rather reflects the Agency’s judgment that a significant release or threat of release has occurred and that the site is a priority for further investigation under CERCLA.
The Agency notes that there are both costs and benefits that can be associated with listing a site. Among the benefits associated with listing a site on the NPL are increased health and environmental protection as a result of increased public awareness of potential hazar ds. In addition to the potential for federally financed remedial actions, the addition of a site to the NPL could accelerate privately financed, voluntary cleanup efforts. Listing sites as national priority sites also may give States increased support for funding responses at particular sites. As a result of the additional CERCLA remedies, there will be lower human exposure to high-risk chemicals and higher quality surface water, ground water, soil, and air.
Therefore, it is possible that any perceived or actual negative fluctuations in property values or development opportunities that may result from contamin ation may be countered by positive fluctuations when a CERCLA investigation and any necessary cleanup are completed.
Further, NPL listing does not necessarily hinder further cleanup efforts at a site, as discussed in Section 3.1, Need for Listing, of this support document. PR Ps may undertake the RI/FS and/or remedial actions at a site, which is not precluded by placing a site on the NPL. Results of work undertaken will be taken into account during the evaluation of the need for remedial actions at the site.



### 3.4 Previous NPL Decision (Rationale for Listing)

Comment : In challenging the proposal to list the ACM site, ARCO commented that EPA had initially scored the “Former GFR Site” [former Great Falls Refinery facility property] in 1982 and did not calculate an HRS score greater than 28.50. ARCO asserted that:
the potential contribution of the multitude of sources that release contaminants to surface waters tributary to the Missouri River in the Great Falls area was a factor in EPA's decision not to propose NPL designation in 1983. See Action Memorandum dated September 2, 1983 (This site is not on the National Priorities List (NPL). It did not score above 28.5 on the NPL because its location is downstream of a major metropolitan area). (AR Reference #42)

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Response : A previous EPA decision regarding the “GFR” site based on the information available at the time and using the original HRS evaluation process does not mean that the present ACM site does not warrant listing based on more recent information and on the present HRS which was revised in 1990. That the site was evaluated by EPA as not qualifyi ng for the NPL based on the original HRS, is not relevant to the listing decision for three reasons. Firs t, the site previously evaluated is not necessarily the same site as evaluated at this proposal. The extent of the ACM Smelter & Refinery site at this proposal is not the same as the “site” evaluated under the 1982 eval uation described by the commenter. The site at this proposal includes sources and releases of hazardous substances to the Missouri River, and an area of observed contamination in the Black Eagle residen tial area, that was not included in the previous evaluation.
Second, a significant amount of information has been obtained at the site at this proposal since the previous NPL decision regarding the ACM site. Th is information has further contributed to the understanding of the nature and extent of the site. Information used in the scoring of the ACM site has been gathered after 1982 (including but not limite d to a 2003 Expanded Site Inspection, a 2007 Site Assessment and 2008 Expanded Site Inspection of resi dential soils in Black Eagle, and a 2009 Expanded Site Inspection of Missouri River sediments ad jacent to and downstream of the site).

Third, the 1982 HRS evaluation of the site was devel oped under a prior version of the HRS. The HRS as revised in December of 1990 (55 FR 51532), included several modifications. Notably in this case, the soil exposure pathway was added to the revised HR S, which was not accounted for under the original HRS, and has been scored in this HRS evaluation. Also, significant changes to the evaluation of releases or threatened releases to surface water were included in the revised HRS, such as the evaluation of threats to human health via the aquatic food chain (human food chain threat), and the evaluation of actually contaminated fisheries and the associated use of bi oaccumulation potential factor values. In establishing an observed release by chemical analysis, the revised HRS included wording allowing for partial attribution of the significant increase of an observed release, which was not considered in the original HRS.
Therefore, a previous decision on the eligibility of this site to be placed on the NPL does not discredit current HRS evaluation and scoring. This comment has no effect on the HRS site score or on the decision to place the site on the NPL. 

### 3.5 Liability

Comment : ARCO stated that other sources, facilities, a nd private parties within the Great Falls area are liable for contamination at the ACM site. ARCO commented that other sources are implicated and discussed in the HRS documentation record and that
a broad site description advanced by the EPA in the HRS Record generally attributes contamination in soils, sediment, and surface wa ter to the Former GFR Site and fails to describe other potential sources of contamina tion located both upstream and downstream of the site. Numerous industrial facilities ope rated in the greater Great Falls area over the past century, each a potential source of air emissions and water discharges that could impact soils, surface water and sediments in the Missouri River.
ARCO asserted that all of the private parties th at contributed contamination to the region share responsibility for cleanup. ARCO noted that Burli ngton Northern Santa Fe (BNSF), formerly Great Northern Railway, was involved in operations at the former refinery at the site starting in the late 1800s, and that BNSF owns track that served the refine ry. ARCO contended that PPL Montana owns and operates hydroelectric facilities on the Missouri River in the Great Falls area “which have influenced the distribution of potentially contaminated sediments,” and that Northwestern Energy and Pacific Steel and

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Recycling each owns land included in the site descrip tion of the HRS documentation record at proposal. Further, ARCO stated that “numerous other i ndustrial and commercial operators, such as Montana Refining Company, a documented source of metals a nd arsenic in air emissions,” are contributors of contaminants to soil, water, and sediment.
ARCO asserted that the City of Great Falls and Casca de County are also liable for contamination. ARCO commented that storm sewer systems that carry urba n runoff from Great Falls and Black Eagle empty into the Missouri River. ARCO stated that:
Cascade County, as the owner of the former Montana Silver Smelter property, owns lands that are known to be contaminated and a contributing source to the Missouri River. . . . This smelter could have impacted soils in Black Eagle, as well as river sediments, during its period of operation in the late 1800's and during the century that followed when the site was left unattended.

Further, ARCO stated that as an owner, the State of Montana shares responsibility for any contamination found in the Missouri River, and that
for more than a century, the State encourag ed and profited from businesses, including the historic smelter operators, that utilized the Mi ssouri River for waste disposal. As a result, the State shares responsibility for any inv estigation and cleanup of river sediments, including a substantial share of the orphan costs.
Response : Liability is not considered in evaluating a site under the HRS. EPA neither confirms nor denies the accuracy of the commenter’s statements as to who was liable for the contamination at and released from this site. The NPL serves primarily as an informational tool for use by the Agency in identifying those sites that appear to present a significant risk to public h ealth or the environment. It does not reflect a judgment on the activities of the owner(s) or operator(s) of a site. It does not require those persons to undertake any action, nor does it assign any liability to any person. This position, stated in the legislative history of CERCLA, has been explained more fully in the Federal Register (48 FR 40759, September 8, 1983 and 53 FR 23988, June 24, 1988) . See Kent County v. EPA, 963 F.2d 391 (D.C. Cir. 1992).



### 3.6 Risk Associated with Site

Comment : ARCO asserted that the ACM site, as proposed, does not pose an unacceptable risk to human health and the environment.
ARCO stated that the completed mitigation measures at the “Former GFR Site,” such as the completion of demolition and removal actions, c ontainment of remaining site-rela ted debris, and improvement of storm water runoff structures have re duced the potential for transport of residual metals from the “Former GFR Site” to surface water and groundwater, and have resulted in no complete exposure pathways at the “Former GFR Site.” ARCO stated that EPA has not provided documentation that the community of Black Eagle, or fisheries and sensitive environments of the Missouri River have been negatively impacted.
ARCO stated that under the Superfund program, no furt her action is generally granted at sites where contaminant concentrations fall below “screening levels ,” and that these screening levels for contaminants in the Black Eagle residential soils are generally not exceeded. ARCO asserted that concentrations of arsenic, cadmium, and lead in the Black Eagle reside ntial soils are generally below established screening criteria or levels to be protective of human health. Specifically, ARCO stated that:

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None of the arsenic concentrations excee d a safe level established by EPA for Anaconda, Montana, a nearby similar site. Only one of the cadmium concentrations exceeds EPA's conservative soil screening level for cadmium. Eleven of the lead concentrations exceed EPA's conservative soil screening level for lead , but the pattern shown in the data is consistent with lead paint deterioration. These soil screening levels are considered by EPA to be protective of humans (including sensitive populations) over a lifetime. Further, many of these exceedances occur in subsurf ace soils where regular exposure to humans would not occur.
ARCO stated that the quality of the Missouri River adjacent to the site is not significantly impacted by releases of hazardous substances; that fisheries at the site do not pose a risk to human health; and, that there are no obvious increases of hazardous substances in fish tissues from the site. ARCO claimed that results from the Parametrix 1 investigation show concentrations of chemicals in sediment, surface water, and fish tissue are generally comparable to background , and that an EPA toxicologist determined that the risks from these media would be minimal. ARCO stated that EPA and the State reviewed the findings of the Parametrix report and concluded that existing cond itions in the river did not pose a threat, or require remediation to protect aquatic life or human health.

Further, ARCO asserted that the ri sks associated with soil exposure at residential properties of Black Eagle are not expected to be significant. ARCO stat ed that the soil contaminant concentrations of these residences are sporadic and non-uniform, and that many of the higher concentrations are found at depths below the surface, where ARCO asserted, “ongoing, chronic exposures by residents are likely to be limited.”
ARCO stated that the NPL is reserved for the worst sites, “where the available data supports the Agency's belief that there is a potential risk to human health or the environment.” ARCO stated that NPL designation:
provides a mechanism to direct federal fundi ng to site investigations and studies to investigate and assess site conditions and dete rmine whether unacceptable risk is present under the procedures set forth in the National Contingency Plan ("NCP"), 40 CFR Part 300.

ARCO asserted that the circumstances cited above ar e not present to support NPL designation of the site. It stated that conditions at the site have been thor oughly investigated and that risks from the “Former GFR Site” itself are expected to be negligible due to measures that have been undertaken to prevent contact with site-related contaminants with in the boundaries of the “Former GFR Site.” ARCO stated that EPA ignored relevant risk-assessment work from other rela ted NPL sites that could provide context for the evaluation of risk at the ACM Smelter & Refinery Site.
Response : The HRS is a numerically-based screening to ol that the Agency u ses to assess the relative degree of risk to human health and the environment po sed by a site compared to other sites subject to review. The HRS score is used to determine whethe r a site is eligible for placement on the NPL. The NPL is intended primarily to guide EPA in determining which sites warrant further investigation to assess the nature and extent of public health and environmental risks associ ated with a release of hazardous substances, pollutants or contaminants. See 75 FR 98 44 (Proposed Rule, ACM Smelter and Refinery Site, March 4, 2010); see also 55 FR 51532 (Final Rule, Hazard Ranking System, December 14, 1990).

1 The Parametrix investigation, as discussed throughout this support document, was a 2004 investigation performed by Parametrix for ARCO to assess the Missouri River in the proximity of the former ACM facility to determine whether sediment, surface water, and/or fish tissue may have been negatively impacted by the former smelter operations at the site.

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CERCLA § 105(a)(8)(a) requires EPA to determine NPL prio rities based on the “relative risk or danger to public health or welfare, or the environment.” The cr iteria EPA applies to determine this relative risk or danger is codified in the HRS, and is the Agency’s primary tool for deriving a site score based on the factors identified in CERCLA. In this case, the HRS evaluation and score above 28.50 represents EPA’s determination that the site may pose a relative risk or danger to human health and the environment and warrants further investigation under CERCLA.
In the preamble to the HRS, EPA stated:
The Agency stresses that the limited data generated at the SI stage are designed to support site screening, and are not intended to provide support for a quantitative risk assessment (55 FR 51541, December 14, 1990).
EPA performs a comprehensive risk assessment for the site as part of further investigations that typically follow listing. The results of ri sk assessment activities will be consid ered during the evaluation of the need for remedial actions at the site. At that time, EPA will consider relevant information provided by ARCO.
The Parametrix investigation of the surface water, sediment, and fish tissue of the Missouri River and subsequent review thereof by an EPA ecotoxicologist, does not establish that the contamination in the Missouri River adjacent to the site poses no risk. In reviewing the Parametrix investigation, the EPA ecotoxicologist concluded that metals results in surface water or sediment generally did not exceed applicable media-specific benchmark criteria 2 . (As explained in section 3.7, Permitted Releases and Regulatory Limits, of this support document, whether or not a releases is within applicable regulatory or otherwise acceptable limits does not eliminate those hazardous substances or their releases from consideration when evaluating a site using the HRS. ) In noting the locations of sediment water quality exceedances, the ecotoxicologist stat ed, however, that “[m]etals concentrations in the other two north shore sediment locations. . . were much higher th an any others perhaps indicating a pattern of contamination emanating from the site downstream.” Thus, in analyzing the sediment data from the Parametrix investigation, the EPA ecotoxicologist id entified that sediment samples taken from closer to the north side of the Missouri River (closer to the AC M facility) were contaminated at higher levels than other locations. This analysis by the EPA toxicol ogist calls into question the comprehensiveness of the Parametrix investigation, and conclu sions about site risk from the Parametrix investigation is therefore inherently limited.

Furthermore, ARCO has not shown that reclamation activ ities or other measures have mitigated all risk at the site. ARCO has not demonstrated that measur es taken at the “Former GFR Site,” such as the completion of demolition and removal actions, capping activities, and construction of storm water runoff structures have eliminated the risk to human hea lth or the environment. Refer to section 3.12, Consideration of Response Actions / Current Conditi ons, of this support document for further discussion on the topic of consideration of reclamation activities at the site.

2 In reviewing the Parametrix investigation, EPA Ecotoxicologist Dale J. Huff concluded that “[n]one of the concentrations of metals/metalloids in surface water samples taken from this sampling event exceed National Ambient Water Quality Criteria,” and that with the except ion of a few sample locations for bulk sediment metal concentrations “none of the metals ex ceed conservative ecotoxicolo gical benchmark screening values . . . . for Cd, Cu, Pb and Zn,” See Reference 20 of the HRS documentation record at proposal.

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### 3.7 Permitted Releases and Regulatory Limits

Comment : ARCO asserted that the releases to the surface water migration pathway, and the contaminants associated with the soil exposure pathway as presented in the HRS documentation record at proposal, are within applicable regulatory or otherwise acceptable limits.
ARCO stated that surface water discharge into th e Missouri River from the site is permitted at two discharge locations. It stated that EPA’s toxicolo gist concluded that “none of the water quality data exceeded applicable National Ambient Water Quality Criteria,” and that “metals concentrations in fish tissue are substantially less than concentrations an ticipated to impact survival, growth or fish reproduction.”
Response : That releases to the surface water migration pathway, or contaminant levels associated with the soil exposure pathway are within applicable re gulatory or otherwise acceptable limits does not eliminate those hazardous substances or their releases from consideration when evaluating a site using the HRS.
CERCLA Section 105(a)(8)(B) directs EPA to list on the NPL “releases” of hazardous substances, pollutants, and contaminants according to specific criteria set out in CERCLA Section 105(a)(8)(A). The definition of “release” in CERCLA Section 101(22) exempts certain releases from its scope, but it does
not exempt releases below regulatory limits. CERC LA exempts Federally-permitted releases only from the notification (Section 103(a)) and cost-recovery (107 (j)) sections of the statute); such releases remain subject to the listing requirements set out in CERCLA section 105.
As the Agency explained regarding observed releases specifically, on July 16, 1982, when responding to public comments on the proposed (original) HRS (47 FR 31188), and again on September 8, 1983 (48 FR 40665), the idea that releases within regulatory lim its should not be considered “observed releases” specifically, and not used in general in scoring a s ite, was rejected. As the Agency noted in 1982:
[e]mission or effluent limits do not necessarily represent levels which cause no harm to public health or the environment. These limitations are frequently established on the basis of economic impacts or achievability.

The relative hazard of the site is approximated by the total HRS sco re, which incorporates the observed release factors with other factors such as waste char acteristics (including waste quantity, toxicity, and persistence) and targets. This total HRS score reflects the hazard of the site relative only to the other sites that have been scored. The actual degree of cont amination and its effects are more fully determined during the Remedial Investigation. Refer to section 3.6, Risk Associated with the Site, for further discussion on the evaluation of risk. 

### 3.8 Adequacy of Site Description

Comment : ARCO challenged the description of the site as presented in the HRS documentation record at proposal. It asserted that EPA arbitrarily selected non-representative data to score and bias the HRS evaluation of the site.

ARCO asserted that “selective use of data” and the “arbitrary exclusion of relevant information” regarding site activities is evident throughout the HR S evaluation of the site. ARCO stated that EPA ignored reports of reclamation activities and other i nvestigations, and incorrectly represented the “former Great Falls Refinery operations” as the significant s ource of hazardous substances contributing to soil, water, and sediment impacts over a broad area.

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Response : The description of the site presented in th e HRS documentation record at proposal was sufficient to allow the public to review the HRS evalua tion of the site. The purpose of the site description is to inform the public of information that is relevant to the sources, re leases, and targets used in the site evaluation. Any information gaps or incorrect stat ements pointed out by the public during the comment period are addressed in this support document and the HRS documentation package has been revised accordingly and made available to the public at promulgation of the site.
The Site History and Description on pages 12 through 14 of the HRS documentation record at proposal presents information pertinent for the reader’s genera l comprehension of the site, including description of investigations that have taken place at the site. The commenter is incorrect in its assertion that EPA arbitrarily excluded relevant information in describ ing the site, as significant information regarding the site has been obtained from various studies and appropriately presented within the HRS documentation record at promulgation to support the HRS score for th e site. Sections 3.13 through 3.31 of this support document discuss in detail any and all scoring modifications of the HRS evaluation of this site, and section 3.35, Alternate HRS Score, of this support document presents the revised scoring scenario of the HRS documentation record at promulgation.



### 3.9 Extent of Site

Comment : ARCO challenged the extent of the ACM site as established in the HRS documentation record at proposal, questioning whether the soil contaminati on and surface water contamination was a result of releases from this site. ARCO referred to the site name as the “Former Great Falls Refinery” (GFR) site. ARCO argued that the soil contamination detected in the Black Eagle residential area is not part of the site and that the contamination in the residential pr operties was not part of the “Former GFR site,” nor was the contamination caused by the former operations at the site. Also, ARCO contended that the contaminated residential properties do not contribute to surface water releases associated with the site. Furthermore, ARCO argued that the extent of th e area of observed contamination (AOC) was much smaller in size than established in the HRS documentation record at proposal.
Response : The ACM Smelter & Refinery site, as establis hed at promulgation, consists of sources and areas that have become contaminated due to releases from the site sources.
The full extent of the ACM Smelter & Refinery site is not conclusively determined upon NPL listing. Placing a site on the NPL is based on an evaluati on, in accordance with the HRS, of a release or threatened release of hazardous substances, pollutants, or contaminants. However, the fact that EPA initially identifies and lists the releas e based on a review of contamination at a certain parcel of property does not necessarily mean that the site boundaries are limited to that parcel.
CERCLA Section 105(a)(8)(A) requires EPA to lis t national priorities among the known "releases or threatened releases" of hazardous substances; thus, the focus is on the release, not precisely delineated boundaries. Further, CERCLA Section 101(a) defi nes a "facility" as the "site" where a hazardous substance has been "deposited, stored, placed, or ot herwise come to be located." The "come to be located" language gives EPA broad authority to clean up contamination when it has spread from the original source. On March 31, 1989 (54 FR 13298), EPA stated:
HRS scoring and the subsequent listing of a release merely represent the initial determination that a certain area may need to be addressed under CERCLA. Accordingly, EPA contemplates that the prel iminary description of facility boundaries at the time of scoring will need to be refi ned and improved as more information is developed as to where the contamination has come to be located; this refining step generally comes during the RI/FS stage.

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The revised HRS (55 FR 51587, December 14, 1990) elaborates on the "come to be located" language, defining "site" as:
area(s) where a hazardous substance has been deposited, stored, disposed, or placed, or has otherwise come to be located. Such areas may include multiple sources, and may include the area between the sources.
Until the site investigation process has been complete d and a remedial action (if any) selected, EPA can neither estimate the extent of contamination at the site, nor describe the ultimate dimensions of the NPL site. Even during a remedial action, such as removing contaminated soils, EPA may find that the contamination has spread further than previous ly estimated, and the site definition may be correspondingly expanded. In addition, if anothe r, unrelated area of contamination is discovered elsewhere on the property, EPA may decide to evaluate that release for the NPL

. Regarding the commenter’s assertions that the “Former Great Falls Refinery (GFR) site” does not include the contaminated soil detected in the Black Eagle residential area, the commenter is highlighting the “former GFR” facility property – not the ACM Sme lter & Refinery site as established under the HRS. The HRS definition of “site,” as cited above is not bounded by facility or property boundaries – only where hazardous substances have come to be located. Properties within the Black Eagle residential area have been documented to contain hazardous substa nces and are considered a part of this site.
Upon consideration of the comments relating to the area of observed contamination in the Black Eagle Residential area, the area of observed contamination of the Black Eagle residential area has been revised in the HRS documentation record at promulgation (refer to section 3.29.1, Area of Observed Contamination, of this support document for a full discussion on this topic). However, at least some of the Black Eagle residential properties are still documen ted as contaminated and are included as part of the site.



### 3.10 Comments on HRS

Comment: ARCO submitted comments challenging the adequacy of the HRS to identify sites for the NPL on the basis that it does not identify unacceptable site-specific risk. Specifically, ARCO challenged whether the HRS procedures for establishing observed release, observed contamination, and determining the level of contamination, establish the presence of unacceptable risk. ARCO asserted that the evaluation of a significant increase of a hazardous substance in a sample medium compared to background concentrations, and the evaluation of SCDM benchmarks in the dete rmination of level of contamination, does not confirm the presence of an unacceptable risk posed by the presence of the hazardous substance in a sampled medium.
Regarding the HRS procedure for establishing an obs erved release to surface water, ARCO challenged the scoring of an observed release to surface water based on a single release sample, stating that:
[t]he only evidence EPA relies on to justify sco ring potential adverse effects to the River consists of one sediment sample with a weak ly elevated cadmium concentration collected above Rainbow Falls dam and sediment sa mples with weakly elevated metals concentrations that were collected from loca tions far downstream of the Site. Evidence of wide-spread or significant contamination is not documented by EPA for scoring or by any of the many studies that have been completed.
Response : The HRS was promulgated on December 14, 1990 (55 FR 51569), and comments on the HRS itself are not relevant to the proposal to place the ACM Smelter & Refinery site on the NPL and are outside the scope of this rulemaking.

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The HRS is intended to be a "rough list" of prioritized hazardous sites; a "first step in a process--nothing more, nothing less" Eagle Picher Indus. v. EPA, 759 F. 2d 922, 932 (D.C. Cir. 1985) (Eagle Picher II). EPA would like to investigate each possible site co mpletely and thoroughly prior to evaluating them for proposal for NPL, but it must reconcile the need for certainty before action with the need for inexpensive, expeditious procedures to identify potentially hazardous sites. The courts have found EPA's approach to solving this conundrum to be "reasonable and fully in accord with Congressional intent.” (Eagle Picher Industries, Inc. v. EPA, (759 F.2d 9 05 (D.C. Cir. 1985) Eagle Picher I)).

This comment has no effect on the HRS site score or on the decision to place the site on the NPL. 

### 3.11 Scoring Other Pathways

Comment : ARCO stated that it agrees with EPA's decision not to score the ground water and air migration pathways. It asserted that there are no active sources at the site that would contribute to releases by the air pathway, and that geologic fo rmations underlying the site limit the downward migration of contaminants to underlying aquifers.
Response : That the air and ground water migration path ways were not scored in the HRS evaluation of the ACM Smelter & Refinery site does not represent a decision that there is no risk associated with these pathways or that EPA does not intend to investigate these pathways during furthe r investigations at the site.
Regarding the scoring of the ground water migration pathway, the cover page of the HRS documentation record at proposal stated:
based on lack of analytical data supporting a release to the aquifers and on the fact that the ground water is not used as a drinking water source, the groun d water pathway will not be scored as part of this Hazard Ranking System (HRS) package.
However, this does not mean that the Agency is not concerned that there may be ground water contamination under the facility that may be reachin g targets such as by migration of ground water to surface water. In fact, future investigations at the site may document that ground water contamination at the site needs to be addressed.
Further, regarding the air migration pathway, th e cover page of the HRS documentation record at proposal stated:
Documentation of investigations from over 100 years ago conducted by the Boston and Montana Consolidated Copper and Silver Mining Company (Boston & Montana) and later by Anaconda Copper Mining Company (ACM) indicates that the operators of the smelter and refinery were aware that a si gnificant amount of contamination was being emitted by the stack. Topsoil samples were collected and analy zed by the Boston & Montana laboratory in 1904 and 1905. Comp any chemists concluded that over 40 pounds of arsenic trioxide (As203) had been deposite d per acre up to a distance of 6.24 miles to the northeast of the stack (Ref.32, p. 2). Samples collected from different locations in 1905 confirmed the aerial depositi on of contaminates [sic] from the stack (Ref. 47, p. 2). Internal ACM company correspondence from November 1951 indicated that the amount of arsenic leaving the stack per twenty-four hours in the early 1950s was approximately 15.04 pounds (Ref. 33).
The EPA chooses not to score the Air Migrati on Pathway at this time; however the EPA is interested in this potential pathway and may choose to pursue it further at a later date.

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The air migration pathway was not used in deve loping the HRS score, however emissions from the smelter facility at the site have been documented and are thought to be a source of contamination in soils surrounding the ACM facility and the risk associated with these emissions will be considered during further investigation of the site.
The HRS does not require scoring all four pathways if scoring those pathways does not change the listing decision. For some sites, data for scoring a path way are unavailable, and obt aining these data would be time-consuming or costly. In other cases, data for scoring some pathways are available, but will only have a minimal effect on the site score. In still ot her cases, data on other pathways could substantially add to a site score, but would not affect the listing decision. The HRS is a screening model that uses limited resources to determine whether a site should be placed on the NPL for possible Superfund response. A subsequent stage of the Superfund pro cess, the RI, characterizes conditions and hazards at the site more comprehensively.
In general, if reliable information shows that th e site scores 28.50 or above, EPA will not seek more information to increase the HRS score. In these cases, such as at this site, the HRS documentation record may include a brief qualitative discussion to present a more complete picture of the conditions and hazards at the site, and to identify areas that may requ ire further investigation at subsequent stages in the Superfund process. 

### 3.12 Consideration of Response Actions / Current Conditions

Comment : ARCO alleged that EPA did not present the current conditions of the site in the HRS evaluation nor consider the various mitigation/improve ment measures that have taken place at the site.
ARCO stated that the completed mitigation measures at the “Former GFR Site,” such as the completion of demolition and removal actions, c ontainment of remaining site-rela ted debris, revegetation actions, structure void filling, and improvement of storm wa ter runoff structures have removed potential exposure pathways at the “Former GFR Site.” In asserting that current conditions were not properly evaluated in the HRS evaluation, ARCO presented photographs of th e site from April 2010, and maintenance activity reports summarizing reclamation activities that have taken place at the site.
ARCO stated that the water quality exceedances reported in the 2003 Expanded Site Inspection, were not confirmed by the Parametrix investigation.
Response : ARCO has not shown that reclamation or other activities performed after the information used in the HRS evaluation resulted in changes to th e HRS evaluation or to the listing decision.
EPA has reviewed the reclamation activity reports and site photographs taken of the site, and agrees that vegetation activities, surface stabilization, and soil capping activities have taken place at the ACM Smelter & Refinery site. In fact, EPA agrees that the sampling information used to establish hazardous substances associated with Source 2 was from 2003 and that reclamation activities presented by the commenter have taken place since this information was gathered. However, ARCO has not presented information establishing that any of these activities have completely removed any sources from the site, or mitigated hazardous substances releases from thes e sources that are occurring or have occurred from the site. To lower the HRS site score, information re garding the current conditions at the site would need to demonstrate that hazardous substances associated with the site have been completely contained, or that any site source and any associated releases from the site have been addressed so as to no longer pose a risk. Such information was not supplied by ARCO. While the reclamation activities may have limited further migration from the site via runoff, none of the actions identified by ARCO have addressed past releases from the site. See Section 3.15.1, Source 2 Characterization and Consideration of Response

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Activities, of this support document for further di scussion on the consideration of response activities specifically associated with Source 2.
Information used in establishing observed release was collected from the 2009 Site Assessment Expanded Site Inspection of the Missouri River sediments, a nd the current conditions of the Missouri River are represented in the HRS evaluation. Refer to section 3.20.1.3, Bias of Conclusions/Current Conditions, of this support document for a discussion on ARCO’s assertion that the water quality exceedances reported in the 2003 ESI were not confirmed by the 2004 Parametrix investigation. 

### Sources Identified at Proposal

The HRS documentation record at proposal identified two sources at the site. Source 1 consisted of contaminated residential soil in the Black Eagle community located adjacent to the former smelter facility on the north bank of the Missouri River across from Great Falls, Montana. Arsenic, cadmium and lead have been detected in multiple soil samples througho ut the community area. Based on these samples, a contaminated area of approximately 1,926,569 square feet was delineated. Contaminants in Source 1are considered available to migrate to surface water. Th e contaminated soil area is also identified as an area of observed contamination for the soil exposure pathwa y. Pages 15-30 of the HRS documentation record at proposal present information used in establishing Source 1.
Source 2 was identified as the East Ditch Dump located on a ridge above the Missouri River approximately 400 feet north of the River and approxim ately 3,500 feet east of the former smelter facility stack, and was observed to contain building/construction debris, stained soils, blister copper, and slag. A waste sample from this area was found to contain arsenic, cadmium, lead, manganese and zinc. Contaminants in this source are considered availabl e to migrate to surface water. Pages 31-35 of the HRS documentation record at proposal present information used in establishing Source 2 (See Figure 1 of this support document).



### 3.13 Additional Sources

Comment : ARCO argued that the evaluation of the ACM site does not appropriately identify the source of hazardous substances found in Black Eagle residentia l soils or a source material for the slag in the Missouri River.
Response : After review of ARCO’s comments and th e HRS documentation record at proposal, two sources have been added to the HR S documentation record at promul gation. Source 3, facility air emissions (including stack emissions), has been added to the HRS documentation record at promulgation to more clearly show that hazardous substances documen ted in the Black Eagle residential soils came to be located due to migration from a site source. S ource 4, the slag waste, has been added to the HRS documentation record at promulgation to link the slag generated at the site with the slag found in the Missouri River. Refer to Figure 1 of th is support document for source locations.

HRS Section 1.1, Definitions , defines “source” as:
Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminat ed from migration of a hazardous substance. Sources do not include those volumes of air, ground water, surface water, or surface water sediments that have become contaminated by migration, except: in the case of either a ground water plume with no identif ied source or contaminated surface water sediments with no identified source, the plume or contaminated sediments may be considered a source.

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Sources 3 and 4, added to the HRS documentation record at promulgation, meet the above HRS definition of a source.


### Source 3 – Facility Air Emissions
Although the HRS documentation record at proposal did not designate the former stack at the ACM site as a source, facility air emissions, were discussed on page 12 of the HRS documentation record at proposal:
The Big Stack was constructed in 1908 to a le ngth of 506 feet and a total height of 751 feet above the smelter’s blast furnace (Ref. 8 p. 7A). The Big Stack went into service on June 12, 1909 and continued belching smoke until August 06, 1972 (Ref. 9, p. 15). The Big Stack was designed to eject a volume of 1,575,000 cubic feet of air per minute at a velocity of 450 feet per minute, a sufficien t volume and rate to entrain and remove from the smelter all the gases and dust generated dur ing the smelting process (Ref. 9, p. 20) . An internal Anaconda Copper Mining Company Memorandum from 1951 stated that approximately 15.04 pounds of arsenic passed out of the stack every 24 hours (Ref. 33, p. 1).
The HRS documentation record at proposal explaine d that “aerial deposition of contaminants emanating from the smelter smoke stacks betw een 1893 and 1972 is the most likely mechanism of contaminants transport to the residential soils in the community of Black Eagle” (Page 12 of the HRS documentation record at proposal). Thus, the addition of Source 3 at promulgation is consistent with the information presented in the HRS documentation record at proposal , and did not require the addition of information not already included in the HRS documentation package at proposal. Nor does the addition of this source change the site score. Addition of this source demons trates that the contamination in Source 1, the Black Eagle residential area, is at least partially due to a source at the site. That is, the hazardous substances associated with Source 3, facility air emissions, mi grated toward and deposited at the Black Eagle residential area.
Arsenic, antimony, cadmium, chromium, copper, lead, mercury, selenium and zinc are associated with Source 3 through flue dust samples taken from former sm elter stack operation. As discussed on page 40 of the HRS documentation record at promulgation, Source 3 is documented to have a containment value of greater than 0, as evidence of migration of hazardous substances from this source has been documented. Also, as hazardous substances are associated with the source yet the hazardous waste quantity is undetermined, Source 3 is assigned a source hazardous waste quantity value of greater than 0. Although emissions from this source may reasonably be e xpected to have also directly deposited into the Missouri River, this source is not included in the waste quantity evaluation for the surface water pathway because addition of this source would not affect the surface water pathway score which was already at the maximum score of 100.


### Source 4 – Slag Waste

Source 4, slag waste, consists of the slag prod uced from the operations at the ACM facility and the resultant dump areas associated with the slag producti on. In the HRS documentation record at proposal, slag was documented as generated from operations at the site (see Reference 14, p. 1 of the HRS documentation record at proposal). A slag dump was identified and sampled during a 1983 screening study and was documented to contain the hazardous substances arsenic, cadmium, chromium, copper, lead, nickel and zinc (see Reference 58 of the HRS do cumentation record at proposal, Table 4-5, results for slag dump “Fill Mtrl” samples).

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As discussed on page 44 of the HRS documentation reco rd at promulgation, this source (slag waste) is documented to have a containment value of greater than 0, as evidence of migration of hazardous substances from this source has been documented. Al so, as hazardous substances are associated with this source yet the hazardous waste quantity is undetermined, this source is assigned a hazardous waste quantity value of greater than 0. Overland drainage from this source enters the surface water of the Missouri River at PPE #7 (Figure 1 of this support document).
This comment has no effect on the HRS site score or the decision to place the site on the NPL.



### 3.14 Source 1

Comment : ARCO asserted that the HRS documentation r ecord at proposal incorrectly characterizes Source 1, contaminated soils in the Black Eagl e residential area, including the area of observed contamination associated with it, the evaluation of hazardous substances, and the calculation of source hazardous waste quantity.
Response : The following subsections of this support docu ment address specific assertions regarding the evaluation of Source 1:

• 3.14.1 Hazardous Substances Associated with Source 1

• 3.14.2 Source 1 Delineation/Inference of Contamination

• 3.14.3 Sampling Procedures

• 3.14.4 Other Sampling/Current Conditions

• 3.14.5 Source 1 Hazardous Waste Quantity

• 3.14.6 Source 1 Containment

3.14.1 Hazardous Substances Associated with Source 1

Comment : ARCO submitted comments that challenged the association of hazardous substances with Source 1, the contaminated soils in the Black Eagle residential area (and the area of observed contamination associated with the source). In cont esting the “attribution” of the arsenic, cadmium and lead in the area of contamination to the site based in part on their presence in this soil source, ARCO challenged the association of arsenic, ca dmium, and lead in this source.
Also, ARCO stated that none of the soil samples colle cted from residential areas in Black Eagle contained concentrations of antimony greater than three times background; therefore antimony cannot be regarded as elevated.
Response : Arsenic, cadmium, and lead were appropriately associated with Source 1 in the HRS documentation record because the soil in this source was shown to have come to be contaminated by these substances. Antimony has been dropped from the source discussion in the HRS documentation record at promulgation. For establishing sources, the HRS does not require “attribution” to be established to associate a substance with a source. (See secti on 3.30.4, Hazardous Substance Deposition/Attribution, of this support document regarding attributing co ntamination in the corresponding area of observed contamination to the site.)

HRS Section 1.1, Definitions , defines source

, in relevant part, as:
Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminat ed from migration of a hazardous substance.
Thus, contaminated soil is an eligible source if th e soil came to be contaminated due to migration.

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HRS Section 2.2.2, Identify hazardous substances associated with a source , explains what substances can be associated with a source. It states in relevant part:
For each of the three migration pathways, consider those hazardous substances documented in a source (for example, by sampling, labels, manifests, oral or written statements) to be associated with that source when evaluating each pathway. . . .
The HRS documentation record at proposal demonstrat ed that hazardous substances were associated with Source 1 through sampling. Page 15 of the HRS documentation record at proposal stated:
The 2007 EPA CERCLA Site Assessment sampled residences on both the north and south sides of the Missouri River adjacent to the Anaconda Minerals Co, Great Falls Refinery (Ref. 21, p. 11). Of the eleven residences sampled in Great Falls on the south side of the Missouri River in August of 2007 only one residence had an elevated concentration of arsenic (Ref. 21, p. 13). The results of the 2007 investigation of the residential soil in the community of Black Ea gle identified an area of arsenic, cadmium, and lead concentrations greater than three times background centered on eastern and central Black Eagle (Ref. 21, p. 13-14, 18).
The 2008 EPA CERCLA Expanded Site Investigation was focused in the unincorporated community of Black Eagle, Montana. The residential soil at fifty nine residential properties in Black Eagle was sampled in July 2008 to a depth of one foot (Ref. 5, p. 1). Concentrations of arsenic, cadmium, and lead elevated three times background were detected in the residential soil in Black Eagle centered on the central and southwestern area of Black Eagle (Ref. 5, p. 13; Ref. 22, pp. 1-2).
To document that the soils came to be contaminated by migration of arsenic, cadmium, and/or lead, as discussed above, it was documented that the levels of these hazardous substances are in the soil source at levels significantly greater than background using the HRS criteria necessary to establish a release has occurred to other media. Samples collected from S ource 1 were documented to contain arsenic, cadmium and lead at concentrations significantly greater than the highest background levels. The highest background level results of arsenic, cadmium and lead are presented on page 17 of the HRS documentation record at proposal. Section 2.2.1.2, Hazardous substances associated with the source, of the HRS documentation record at proposal, presents those Source 1 samples that were documented to contain arsenic, cadmium, and/or lead significantly ab ove background levels. Th ese results show that the soils at Source 1 came to be contaminated due to migration of hazardous substances, as the hazardous substances detected in Source 1 are documented as signif icantly greater than background concentrations.

Therefore, as the hazardous substances arsenic, cadmi um and lead were documented in Source 1 through sampling at levels significantly above background, each hazardous substance was properly associated with the source in the evaluation of the surface wate r migration pathway. Al though antimony and other hazardous substances were discussed as detected in Black Eagle residential soils, they are not included in the evaluation of hazardous substances associated wi th the source, and are not included in the HRS documentation record at promulgation. The removal of antimony from the Source 1 discussion of the HRS documentation record at promulgation has no impact on the scoring of the site, as it was never included in the HRS evaluation.
Regarding the commenter’s assertion that the dist ribution patterns of hazardous substances used in defining Source 1 were not adequa tely evaluated; the HRS does not require evaluation of hazardous substance distribution patterns as part of the HRS ev aluation. As cited above, the HRS requires that a hazardous substance be associated with a source, wh ich in this case was determined through sampling of soils at the site.

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This comment has no effect on the HRS site score or the decision to place the site on the NPL.

3.14.2 Source 1 Delineation/Inference of Contamination

Comment : In challenging the area associated with the c ontamination in the Black Eagle residential area, ARCO equated the extent of Source 1 with the ex tent of the area of observed contamination and challenged how this area was delineated. ARCO allege d that EPA’s inference of contamination between sample locations used to delineate the AOC is inco rrect according to the HRS, and that as a result EPA significantly overestimated the dimensions of Source 1.
ARCO stated, citing from the HRS Guidance Manual ( HRS Guidance Manual, Interim Final Draft) , that “for contaminated soil, consider the sampling locations that indicate observed contamination and the area lying between such locations to be an area of observed contamination, unless information indicates otherwise.”

ARCO stated:

EPA states that arsenic and cadmium results delineate an area of contaminated soil of approximately 1,926,569 ft 2 (AR Reference #53); however the area delineated is based on a few widely-distributed sample locations e xhibiting weakly elevated arsenic and/or cadmium concentrations with a broad area with no elevated arsenic or cadmium concentrations in between. In accordan ce with the EPA HRS Guidance Manual, a property can be inferred to have observed c ontamination if the property lies between points of observed contamination unless information indicates otherwise (AR Reference #46, p. 369). Atlantic Richfield has delineated two smaller areas of observed contamination in soil based on "elevated" c oncentrations of arsenic and cadmium that have a combined area of 153,000 ft 2 (see Figure A).
ARCO stated that the delineation established in the HRS documentation record at proposal “ignores dozens of included residences that do not have elevated arsenic or cadmium concentrations based on EPA's own data.” AR CO continued that:
Consideration of the 2009 data [collected by Atlantic Richfield] in combination with soil data from the 2007 and 2008 EPA investigations yields a large dataset inclusive of 790 xray fluorescence (XRF) soil samples for th e community and cove ring 122 residential yards plus a public ball field and playground.
ARCO discussed a soil investigation it completed in 2 009 and asserted that data gathered in this study indicates that the distribution of arsenic and cadmium in the residential soils is not uniform across Black Eagle residential properties. It stated:
Data from the 2009 residential soils investigati on indicate that concentrations of lead and arsenic are not uniformly elevated across all properties sampled despite the biased selection of these properties based on the highest arsenic and lead concentrations from the 2007 and 2008 EPA sampling investigations. Concentrations vary by property, depth, and sample type (e.g., drip line versus yard) for both arsenic and lead.
ARCO continued that: [a]lthough sporadically higher concentrations of arsenic and lead have been identified at various locations throughout Black Eagle, these occurrences are more indicative of random hot spots than a pervasively elevated soil exposure medium.

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ARCO stated that after review of all availabl e data, the HRS evaluation of this source and its corresponding AOC should be changed using its revised area of observed contamination in soil for the Black Eagle residential area.
Response : Upon a comprehensive review of all available sampling data taken from the Black Eagle residential area, the delineation of Source 1 and the associated AOC has been revised at promulgation. Specifically, after review of ARCO’s comments, the ex tent of source 1 at promulgation has been revised to only include those locations where samples show c ontaminated soils came to be located as being part of Source 1. The areas between those locations are no longer evaluated as part of the source (Figure 2 of this support document).
The HRS does not give explicit directions on how to delineate a contaminated soil source; however the Agency finds it reasonable to consider the Source 1 so ils that have come to be contaminated as the location of the source. That is, Source 1 is com posed of the locations from which samples showing hazardous substances significantly greater than backgr ound were taken. At proposal, the area of Source 1 was delineated in the same manner as the delineation of the AOC associated with it. Page 15 of the HRS documentation record at proposal states:
The 2007 EPA CERCLA Site Assessment sampled residences on both the north and south sides of the Missouri River adjacent to the Anaconda Minerals Co, Great Falls Refinery (Ref. 21, p. 11). Of the eleven residences sampled in Great Falls on the south side of the Missouri River in August of 2007 only one residence had an elevated concentration of arsenic (Ref. 21, p. 13). The results of the 2007 investigation of the residential soil in the community of Black Ea gle identified an area of arsenic, cadmium, and lead concentrations greater than three times background centered on eastern and central Black Eagle (Ref. 21, p. 13-14, 18).
The 2008 EPA CERCLA Expanded Site Investigation was focused in the unincorporated community of Black Eagle, Montana. The residential soil at fifty nine residential properties in Black Eagle was sampled in July 2008 to a depth of one foot (Ref. 5, p. 1). Concentrations of arsenic, cadmium, and lead elevated three times background were detected in the residential soil in Black Eagle centered on the central and southwestern area of Black Eagle (Ref. 5, p. 13; Ref. 22, pp. 1-2).
Those samples collected from the Blac k Eagle residential area, as cited above, were appropriately used in defining Source 1 and are the same samples used at promulgation as those at proposal to identify the presence of the source (see section 2.2.1.2 of the HRS documentation record at proposal and section 2.2.1.2 of the HRS documentation record at promul gation). However, at promulgation, the assumption that there is contamination between the 53 sample lo cations is no longer made. Information indicates that contaminant levels at certain sampling locations be tween the53sample locations may not be significantly above background levels. This indicates that soil betw een source sample points may not have come to be contaminated due to migration based on a signifi cant increase of hazardous substances above background concentrations (Figure 2 of this support document).
However, all 53 samples used at the time of propo sal to delineate this source are locations where the contaminant levels are significantly greater than background and thus are appropriately used to identify a soil source composed of the individual sample locatio ns, considering the proximity and type of the source samples, similarity of hazardous substances in each sa mple, similarity of containment considerations of each source sample, and increase in contaminant le vels. These changes are consistent with the recommendations in the HRS Guidance Manual and in th e HRS in that the inference of contamination between sample locations has been droppe d given information suggesting otherwise.

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These changes to the evaluation of the delineation of Source 1 and the associated AOC impact the calculation of Source 1 hazardous waste quantity and, as explained later in this support document, the evaluation of the soil exposure pathway. Refer to section, 3.14.5, Source Hazardous Waste Quantity; 3.29, Soil Exposure Pathway; 3.30, Soil Exposure Waste Characteristics; and 3.31, Soil Exposure Targets, of this support document for a discussi on on how these evaluations have changed at promulgation.
This comment has no effect on the decision to place the site on the NPL.
3.14.3 Sampling Procedures

Comment : ARCO asserted that the sampling procedures empl oyed to collect the soil samples used in the Source 1 evaluation were flawed. It contended that all samples collected by EPA for the 2007 and 2008 site investigations were “random gr ab samples rather than systematic composite samples” and were not representative of actual site conditions. ARCO asserte d that the sampling it completed in 2009 utilized a composite sampling method. It stated
Many of these residences were resampled in 2009 by Atlantic Richfield (AR Reference #14) using systematic composite sa mpling methods outlined in EPA's Superfund LeadContaminated Residential Sites Handbook (AR Reference #15). . . . In general, the composite sampling did not confirm the highest concentrations reported by EPA.
Response : The sampling methods used in establishing Source 1were appropriate for HRS purposes.

HRS Section 1.1, Definitions , defines a source, in relevant part, as: “Any area where a hazardous substance has been deposited, stored, disposed, or plac ed, plus those soils that have become contaminated from migration of a hazardous substance.”

HRS Section 2.2.2, Identify hazardous substances associated with a source, states:
For each of the three migration pathways, consider those hazardous substances documented in a source (for example, by sampling, labels, manifests, oral or written statements) to be associated with that so urce when evaluating each pathway. In some instances, a hazardous substance can be doc umented as being present at a site (for example, by labels, manifests, oral or written statements), but the specific source(s) containing that hazardous substance cannot be documented. For the three migration pathways, in those instances when the specific source(s) cannot be documented for a hazardous substance, consider the hazardous subs tance to be present in each source at the site, except sources for which definitive information indicates that the hazardous substance was not or could not be present.
For an area of observed contamination in the soil exposure pathway, consider only those hazardous substances that meet the criteria fo r observed contamination for that area (see section 5.0.1) to be associated with th at area when evaluating the pathway.
Page 16 of the HRS documentation record at pro posal summarized how the samples establishing Source 1 were collected:
All samples were collected as grab sampl es following the guidelines of the UOS TSOPs using stainless steel spoons, bowls, and hand auge rs to collect the soil into 12X12 inch seal top poly bags. The 0-2 inch horizon was collected directly into the plastic zip lock bag using a stainless steel spoon, and the 6- 12 inch horizon was collected using a hand auger into a stainless steel bowl where the soil was lightly homogenized by stirring

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before a sample was collected into 12X12 inch seal top poly bags (Ref. 5, p. 11; Ref. 21, pp. 11-12). All of the background samples were collected from the area north of the Missouri River, west of the site, in soils de veloped on the Kootenai Formation which also underlies the community of Black Eagle (Ref. 5, p. 5; Ref. 10. 1 sheet; Ref. 21, p. 4). The soil was moist, fine-grained well sorted s ilty sand at the locations (Ref. 52, pp. 15-16 & 28). All soil samples collected were analy zed in the field using an x-ray fluorescence (XRF) unit (Ref. 5, p. 12; Ref. 21, p. 12). All samples for fixed laboratory analyses were analyzed through the EPA’s Contract Laboratory Program (CLP), Routine Analytical Services (RAS). The samples collected in 2007 were analyzed by A4 Scientific, Inc. of The Woodlands, Texas by EPA CLP method ILM05.4 (Ref. 60, p. 1; Ref. 61, p. 1; Ref. 62, p. 1). The samples collected in 2008 we re analyzed by Chemtech Consulting Group, Mountainside, New Jersey by EPA CLP method ILM05.4 (Ref. 63. p. 1; Ref. 64, p. 1).
These grab samples from the 2007 and 2008 sampling events were documented to contain hazardous substances, and thus meet the provisions of HRS Se ction 2.2.2 cited above. The HRS does not exclude grab samples in establishing a source or an area of observed contamination. As stated in the HRS, the scorer need only document that hazardous substances are associated with a source. Composite sampling methodology, as presented by the commenter, averages th e level of contamination over a collection area. Grab sampling methodology, which entails collection of one sample from a specific location, is used to determine the existence of hazardous substances at that given sample location, which is appropriate for HRS purposes. The HRS is a screening tool and is based on screening sampling. More comprehensive sampling and sampling methodologies may be employed as part of the RI/FS, which is separate from the listing process.
The handbook cited by the commenter, the Superfund Lead-Contaminated Residential Sites Handbook was written by the EPA’s Lead Sites Workgroup (20 03) to promote a nationally consistent decisionmaking process for assessing and managing risks associat ed with lead-contaminated residential sites across the country. As stated on page 1 of this handbook “[t]he primary audience for this risk management document is Superfund project managers worki ng on the characterization and cleanup of lead-contaminated residential site” [emphasis added]. This handbook is intended for response selection, not for HRS purposes. Sampling for the purpose of establishing a source or an area of observed contamination during an HRS evaluation is not intend ed for the determination of risk, but rather, to determine the existence of contamination at a sa mpling location. Establishing the existence of contamination at this site was conducted in accord ance with the HRS. Refer to section 3.6, Risk Associated with the Site, of this support document for further discussion on the evaluation of risk at this site.

This comment has no effect on the HRS site score or the decision to place the site on the NPL.
3.14.4 Other Sampling/Current Conditions

Comment : ARCO asserted that previous sampling of the Black Eagle residential soils by EPA and more current sampling performed by ARCO do not show the same extent or levels of contamination as established in the HRS documentation record at proposal . ARCO stated that its 2009 investigation of the Black Eagle residential soils “did not confirm the highest concentrati ons reported by EPA,” and submitted results from the Parametrix 2009 investigation performed for ARCO.
Response : ARCO has not shown that any of the sampling results used in establishing Source 1 were inappropriate in establishing Source 1 according to the HRS; sampling information presented by ARCO supports the conclusion that hazardous substances are appropriately associated with Source 1. More recent sampling performed by ARCO and the previ ous 2003 sampling completed by EPA do not indicate

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an error in the sampling results used in establishing th e extent of Source 1 nor document that there is not contamination present.

HRS Section 2.2, Characterize sources , states that source characterizati on includes: the identification of sources, hazardous substances associated with the so urces, and pathways potentially threatened by the hazardous substances. There is no reason to expect two nearby samples within the soil matrix at the Black Eagle residential area to yield exactly the same results, considering that residential yards may have been subject to modification or soil mixing by the resident.
The 2003 ESI performed by EPA involved the collec tion of multiple samples from the ACM site, only three of which were collected from residential proper ties in Black Eagle. The sampling data used in establishing Source 1 came from the more comprehensiv e investigations of the Black Eagle residential properties - the 2007 Site Assessment and 2008 Expande d Site Inspection, which involved the collection of multiple samples from this area.
Under the heading Description and Location of the sour ce, page 15 of the HRS documentation record at proposal states:
The 2007 EPA CERCLA Site Assessment sampled residences on both the north and south sides of the Missouri River adjacent to the Anaconda Minerals Co, Great Falls Refinery (Ref. 21, p. 11). Of the eleven residences sampled in Great Falls on the south side of the Missouri River in August of 2007 only one residence had an elevated concentration of arsenic (Ref. 21, p. 13). The results of the 2007 investigation of the residential soil in the community of Black Ea gle identified an area of arsenic, cadmium, and lead concentrations greater than three times background centered on eastern and central Black Eagle (Ref. 21, p. 13-14, 18).
The 2008 EPA CERCLA Expanded Site Investigation was focused in the unincorporated community of Black Eagle, Montana. The residential soil at fifty nine residential properties in Black Eagle was sampled in July 2008 to a depth of one foot (Ref. 5, p. 1). Concentrations of arsenic, cadmium, and lead elevated three times background were detected in the residential soil in Black Eagle centered on the central and southwestern area of Black Eagle (Ref. 5, p. 13; Ref. 22, pp. 1-2).
Thus, multiple residential properties were sampled and documented to contain hazardous substances that met observed contamination criteria during the 2007 and 2008 investigations. That three residential properties sampled during the 2003 study were not do cumented to contain hazardous substances that met observed contamination criteria does not discredit findings of contamination on the same properties based on data from the 2007 and 2008 investigations that were used in establishing Source 1. In addition, ARCO presented no rationale for rejecting the results from the 2007 and 2008 sampling events.
The analytical results from the 2009 sampling pe rformed by ARCO, as presented in its comments, support that elevated levels of metals have been det ected at several Black Eagle residential properties. Every property sampled by ARCO c ontained levels of lead significantly greater than background concentrations and 10 of these proper ties contained levels of arsenic si gnificantly greater than background concentrations. Thus, ARCO’s 2009 investigation of the Black Eagle residential properties presents data showing elevated levels of hazardous substances in parts of the Black Eagle residential area and does not preclude, or otherwise discredit, the data used to est ablish Source 1 in the HRS documentation record at promulgation.

This comment has no effect on the HRS site score or the decision to place the site on the NPL.

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3.14.5 Source 1 Hazardous Waste Quantity

Comment : ARCO stated that the estimated source hazardous waste quantity for Source 1 is overstated. ARCO asserted that the estimated source hazardous waste quantity value based on ARCO’s calculated area of contaminated soil would be 4.5 - instead of the 56.66 estimated in the HRS documentation record at proposal. In addition, ARCO asserted that the hazardous constituent quantity for Source 1 is adequately determined, not incomplete as the HRS doc umentation record at proposal indicates. [This estimate could then have been used to estimate the source waste quantity instead of the estimate based on area.]
Response : The source hazardous waste quantity has been revised at promulgation to reflect the redelineation of Source 1.
The source hazardous waste quantity for Source 1 ba sed on the area of the source is appropriately evaluated as “greater than 0.”

HRS Section 2.4.2, Hazardous waste quantity, states:
Evaluate the hazardous waste quantity factor by first assigning each source (or area of observed contamination) a source hazardous waste quantity value as specified below. Sum these values to obtain th e hazardous waste quantity factor value for the pathway being evaluated.
In evaluating the hazardous waste quantity f actor for the three migration pathways, allocate hazardous substances and hazardous wastestreams to specific sources in the manner specified in section 2.2.2. . . .

HRS Section 2.4.2.1, Source hazardous waste quantity , states:
For each of the three migration pathways, assign a source hazardous waste quantity value to each source (including the unallocated source) having a containment factor value greater than 0 for the pathway being evaluated. . . .
For all pathways, evaluate source hazardous waste quantity using the following four measures in the following hierarchy:
• Hazardous constituent quantity. • Hazardous wastestream quantity. • Volume. • Area.

HRS Section 2.4.2.1.1, Hazardous constituent quantity , states:
Evaluate hazardous constituent quantit y for the source (or area of observed contamination) based solely on the mass of CERCLA hazardous substances (as defined in CERCLA section 101(14), as amended) allocated to the source (or area of observed contamination)

. . . . If the hazardous constituent quantity for the source (or area of observed contamination) is adequately determined (that is, the total ma ss of all CERCLA hazardous substances in the source and releases from the source [or in the area of observed contamination] is known or is estimated with reasonable confidence), do not evaluate the other three measures

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discussed below. Instead assign these other thr ee measures a value of 0 for the source (or area of observed contamination) a nd proceed to section 2.4.2.1.5.
The hazardous constituent quantity was not adequately determined for Source 1 because the total mass of all CERCLA hazardous substances within the source was not known or able to be estimated with reasonable confidence. Only the Area tier of the source hazardous waste quantity evaluation was estimated for Source 1 because information was not av ailable to adequately score the other tiers.

HRS Section 2.4.2.1.4, Area , states:
Evaluate the area measure using the area of the source (or the area of the area of observed contamination). Based on this area, designated as A, assign a value to the area measure as follows:

• For the migration pathways, assign the source a value for area using the appropriate Tier D equation of Table 2-5.

Section 2.2.2.4.4, Area, of the HRS documentation re cord at promulgation presents the evaluation of the Tier D, Area measure of the source hazardous waste quantity evaluation. As hazardous substances were detected in all 53 source samples and the full extent of the area of contaminated soil source is not determined, the source hazardous waste quantity value of “greater than 0” was estimated (see page 32 of the HRS documentation record at promulgation).

As described in section 3.14.2, Source 1 Delineation/ Inference of Contamination, of this support document, contamination is no longer inferred between the 53 source sample locations at promulgation, and thus only the individual sample locations are cons idered part of this source (Figure 2 of this support document). As such, a specific total area for Source 1 in the Tier D, Area evaluation reflecting the area of the sample locations was not estimated, but is clearly greater than zero. Thus, a source area of “greater than 0” was assigned to the hazardous waste quantity evaluation of Source 1.
Further, even if the source hazardous waste quantit y value of 4.5 based on area as presented by ARCO was considered adequately determined and the va lue was used in evaluating Source 1 hazardous waste quantity, or if a hazardous constituent quantity was estim ated at an even lower value, it would not impact the evaluation of the respective pathway hazardous w aste quantity evaluations and would have no impact on the site score. That is, according to HRS Table 2-6, the source hazardous waste quantity value of 4.5 corresponds to a hazardous waste quantity factor value of 1. However, Footnote B of Table 2-6 of the HRS states that “if the hazardous constituent quan tity is not adequately determined, assign a value as specified in the text; do not assign the value of 1.” Given that the hazardous constituent quantity for this source was not adequately determined, as explained above, HRS Section 2.4.2.2, Calculation of hazardous waste quantity factor value , states in relevant part:
If the hazardous constituent quantity is not adequately determined for one or more sources. . . . assign a factor value as follows:
If any target for that migration pathway is subject to Level I or Level II concentrations (see section 2.5), assign either the value from Table 2-6 or a value of 100, whichever is greater , as the hazardous waste quantity factor value for that pathway. [emphasis added]
Thus, as targets in the scored pathways are subject to Level II contamination (or greater), the hazardous waste quantity factor value of 100 would be assigned to the scored pathway. Therefore, the source area of 4.5 presented by the commenter would not impact th e scoring of the respective pathway hazardous waste quantity evaluations and would ha ve no impact on the site score.

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Even though the Source 1 area estimate has been redu ced at promulgation, this change does not impact the pathway hazardous waste quantity, nor the scoring of the waste characteristics factor values for the surface water migration pathway or the soil exposure pathways.
This comment has no effect on the decision to place the site on the NPL.

3.14.6 Source 1 Containment

Comment : ARCO alleged that EPA ignored “the concep t of sediment mobilization and transport,” with respect to Source 1, i.e., lack of mobility of the Bl ack Eagle residential soil to be transported via surface water runoff, when it identified Source 1 as eligible for the surface water pathway. ARCO also claimed that since "subsurface soils in flat, vegetated areas" are not likely to be eroded and transported to the Missouri River, this would be an additional argumen t against the Black Eagle soils as a source for the surface water pathway.
Response : The Source 1 containment evaluation was c onducted in accordance with HRS requirements.

HRS Section 2.2.3, Identify hazardous substan ces available to a pathway , states that:
In evaluating each migration pathway, co nsider the following hazardous substances available to migrate from the sources at the site to the pathway:

• Surface water migration—overland/flood component.

- Hazardous substances that meet the cr iteria for an observed release to surface water in the watershed being evaluated.

- All hazardous substances associated with a source with a surface water containment factor value greater than 0 for the watershed (see sections 4.1.2.1.2.1.1 and 4.1.2.1.2.2.1).
Hazardous substances associated with Source 1 may be available to migrate to the surface water pathway if the HRS containment value assigned for that source is greater than 0.

HRS Section 4.1.2.1.2.1.1, Containment , directs the user to “assign a containment factor value from Table 4–2 to each source at the site that can potentially release hazardous substances to the hazardous substance migration path for this watershed.”
Table 4-2, Containment Factor Values for Surface Water Migration Pathway, of the HRS directs that when evidence of hazardous substance migration from a source area is not established, a containment value of 10 is assigned to the source when neither a maintained engineered cover is present, nor a functioning and maintained run-on control system/runoff management system is present.
Page 29 of the HRS documentation record at pro posal presents the rationale for why the Source 1 containment value was evaluated as 10:

There is no documentation of a maintained e ngineered cover or and [sic] functioning and maintained run-on control system and runoff management system. Building pads, streets, sidewalks, etc. cover less than half of the area, but are not engineered containment features. Surface water runoff flows directly to the Missouri River through a series of culverts.

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Thus, as no maintained engineered cover or functio ning and maintained run-on control system/runoff management system was documented to be in place at Source 1, the containment value of 10 was appropriately applied to Source 1.
ARCO’s undocumented assertion that Source 1 samples are from areas that are not likely to be eroded and transported to the Missouri River does not establish that Source 1 is contained. Whether or not the Source 1 samples were located on flat or vegetated areas or were otherwise perceived as not likely to be eroded and transported to the Missouri River are not factors considered in the HRS evaluation of containment.

This comment has no effect on the HRS site score or the decision to place the site on the NPL.



### 3.15 Source 2

Comment : ARCO asserted that the HRS documentation r ecord at proposal incorrectly characterizes Source 2, the East Ditch Dump. The following s ubsections of this support document address specific assertions regarding the evaluation of Source 2:

• 3.15.1 Source 2 Eligibility and Consideration of Response Activities

• 3.15.2 Association of Hazardous Substances with Source 2

• 3.15.3 Source 2 Hazardous Waste Quantity

• 3.15.4 Source 2 Containment
3.15.1 Source 2 Eligibility and Consideration of Response Activities

Comment : ARCO asserted that Source 2 (East Ditch Dump) was inaccurately depicted in the HRS documentation record at proposal. ARCO contended th at only a small portion of the East Ditch Dump is located in the “natural draw” near the Missouri Ri ver, and that EPA’s use of a single soil sample to characterize Source 2 does not accurately portray the full extent of the East Ditch Dump and reclamation activities that have taken place there.
ARCO asserted that significant work at Source 2 has minimized or eliminated the exposure of potential hazardous substances to surface water transport, citin g various response activities that have taken place (i.e., the moving equipment and de bris, construction of erosion c over and run-on control channels, revegetation actions, capping activities, etc.).
ARCO asserted that storm water monitoring reports indicate that spring/seep fl ows on-site “have been essentially eliminated since the establishment of a good grass cover following Site reclamation.” ARCO contended that “minor bare areas” that are present on the surface of the East Ditch Dump are scattered and consist of clean cover soil and that the area is managed by surface water control systems and a retention pond.
Response : Source 2 was appropriately evaluated as a source according to the HRS. ARCO has not shown that reclamation and surface water management activities in the vicinity of Source 2 are adequate to justify removing this source from co nsideration in the HRS evaluation.

HRS Section 1.1, Definitions , defines source

, in relevant part, as:
Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminat ed from migration of a hazardous substance.

HRS Section 2.2.1, Identify sources , states, “[f]or the three migration pathways, identify the sources at the site that contain hazardous substances. Identify the migration pathway(s) to which each source applies.”

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HRS Section 2.2.2, Identify hazardous substances associated with a source , states in relevant part:
For each of the three migration pathways, consider those hazardous substances documented in a source (for example, by sampling, labels, manifests, oral or written statements) to be associated with that source when evaluating each pathway.

HRS Section 2.2.3, Identify hazardous substan ces available to a pathway , states:
In evaluating each migration pathway, co nsider the following hazardous substances available to migrate from the sources at the site to the pathway:
• Surface water migrati on—overland/flood component.

-Hazardous substances that meet the criteria for an observed release to surface water in the watershed being evaluated.

-All hazardous substances associated with a source with a surface water containment factor value greater than 0 for the watershed (see sections 4.1.2.1.2.1.1 and 4.1.2.1.2.2.1).
Page 31 of the HRS documentation r ecord at proposal presented the in formation used in establishing Source 2, the East Ditch Dump. It stated that the:
East ditch dump is on a ridge above the Miss ouri River approximately 400 feet north of the River and approximately 3,500 feet east of the former stack (Ref. 18, Figure 2; Ref. 58, Appendix I). No continuous cap or soil c over is documented to be in place over the east ditch dump. Building/construction debris, stained soils, blister copper, and slag were observed on the surface and mixed in the soil throughout the landfill area during the 2003 EPA site inspection (Ref. 18, pp. 17 , 22, and 32; Ref. 53, 1 page).
The table displayed in section 2.2.2 of the HRS documentation record at proposal presents those hazardous substances detected in samples collected from the East Ditch Dump, and is reproduced below.

Evidence (total metals) Hazardous Substance Sample No. / CLP No. Concentration (mg/kg) Background References

Arsenic AM-SO-03 / MX0XH0 377 23.1

Cadmium AM-SO-03 / MX0XH0 89.8 1.2 J+

Lead AM-SO-03 / MX0XH0 4270 30.1

Manganese AM-SO-03 / MX0XH0 899 -

Zinc AM-SO-03 / MX0XH0 4060 - Ref. 18, p. 94; Ref. 21, p. 29-32; Ref. 51, pp. 2,

5, 9, 16, 17, 117, 120, 124, 138; Ref. 52. pp. 15-16, 28; Ref. 60, pp. 1, 3, 6, 8, 18, 78, 363-

365, 594, 599, 604; Ref. 61, pp. 1, 3, 5, 8,

9, 13-14, 219-224

Page 31 of the HRS documentation record at proposal , under the subheading Source Sample Information, presented information regarding the source samples:

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The source sample listed above was collected in April 2003 during the Expanded Site Inspection conducted by Tetra Tech EM Inc. for EPA. All samples were analyzed through the EPA CLP, RAS for total metals. Soil samples for inorganic analysis were delivered to Sentinal Inc, of Huntsville, Alab ama (Ref. 18, p. 3). The analytical data was validated by Tetra Tech, EM, Inc, of Chicago, IL on July 30, 2003 (Ref. 18, p. 56). Analytical results from the sample indicate the presence of arsenic, cadmium, copper, lead, manganese, silver, and zinc in the East Ditch Dump (Ref. 18, p. 94).
As materials were clearly dumped at the East Ditch Dump and it is evaluated as source type landfill, a comparison to background was not necessary to estab lish the existence of hazardous substances within Source 2. However, several hazardous substances in Source 2 were compared to background in the HRS documentation record at proposal. Page 31 of the HRS documentation record at proposal, under the subheading Background Sample Information, presented information regarding the background samples used in source samples:
The sample collected from the East Ditch Dump is compared to soil samples collected during the 2007 EPA Site Inspection. The E ast Ditch Dump sample was collected from the 0 to 24 inch horizon using a macrocore (Ref. 57, 1 page). The 0-2 inch horizon background soil sample was collected directly into a plastic seal top bag using a stainless steel spoon and the 6-12 inch horizon was collected with a hand auger into a stainless steel bowl where the soil was lightly homogenized by stirring before a sample was collected into 12X12 inch seal top poly bags (R ef. 5, p. 11; Ref. 21, pp. 11-12). The background samples were collected from the area north of the Missouri River, west of the site, in soils developed on the Kootenai Forma tion (Ref. 10. 1 sheet). The soil was moist, fine-grained well sorted silty sand at the locations (Ref. 52, pp. 15-16 & 28).
The highest background sample values ava ilable for the surface soil are 23.1 mg/kg arsenic, 1.2 J+ mg/kg for cadmium, and 30.1 mg/kg for lead.
Thus, a sample (AM-SO-03) collected from Source 2 was documented to contain hazardous substances.
Moreover, Source 2 was identified as eligible for evaluation in the surface water pathway. Section 2.2.3, Hazardous Substances Available to a Pathway, on page 34 of the HRS documentation record at proposal documents that the containment value for Source 2 was greater than 0. The table presented on page 34 of the HRS documentation record at proposal states:
No continuous maintained engineered cover, cap or soil cover is documented to be in place over the east ditch dump and waste was observed in the upper two feet of soil at the site in 2003. There is no documentation of a run-on control system and run-off is allowed to flow unchecked to concrete channels which direct run-off southward to the Missouri River.

Thus, hazardous substances were associated with Source 2 through sampling and the containment value for the source was documented as greater than 0.
Regarding the reclamation activities cited by ARCO , such activities do not impact the eligibility of this area to be identified as a source. ARCO’s assertions that the reclamation activities (such as revegetation efforts, construction of erosion cover and run-on control channels, and capping actions) have minimized or eliminated the potential migration of hazardous substances from Source 2 to the surface water migration pathway do not alter the identification of it as a source. Although the commenter presented and discussed re vegetation activities at the source and that bare soil areas at the East Ditch Dump are com posed of clean cover soils, the commenter has not

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supplied documentation establishing that a continuo us, maintained engineered cover, cap or soil cover is present at the East Ditch Dump. In fact , the commenter conceded that bare areas present on the surface of the East Ditch Dump are composed only “in most instances” of clean cover soil. Because ARCO has not demonstrated that the HR S containment value should be zero, Source 2 is an eligible source for the surface water migration pathway.
Additionally, despite ARCO asserting that surface water is managed at Source 2 with the use of run-on and runoff control systems, ARCO does not establish that hazardous substances have not been released from the source, or are not ava ilable to release to the surface water migration pathway. The HRS documentation record at propos al acknowledges that concrete channels in the vicinity of Source 2 direct surface water southward from the source area toward the Missouri River. The commenter has not shown that Source 2 does not contribute hazardous substances to these storm water conveyances. Thus, ARCO has not demonstrated that the HRS containment value should be zero, and Source 2 is therefore an eligible source for the surface water migration pathway. See also section 3.15.4, Source 2 Containment, of this support document.
Regarding ARCO’s assertion that only a portion of the East Ditch Dump is located within the “natural draw” of the Missouri River, only one sample was ta ken from this source and the entire East Ditch Dump was not fully evaluated. Whether or not a portion of th e East Ditch Dump is within the “natural draw” of the Missouri River does not establish that hazardous s ubstances are not available to migrate from Source 2 to surface water, and this assertion does not discred it the determination that overland flow from Source 2 enters the surface water pathway. Section 3.18, Description of Pathway, of this support document further discusses the description of the surface water pathway.
This comment has no effect on the HRS site score or the decision to place the site on the NPL.

3.15.2 Association of Hazardous Substances with Source 2

Comment : ARCO challenged the evaluation of hazar dous substances associated with Source 2. Specifically, ARCO asserted that manganese should not have been used to score the site as its concentration within the Source 2 soil sample was less than three times background. Also, ARCO contended that silver was not detected above the Cont ract Required Detection Limit (CRDL) in either the soil sample from the East Ditch Dump or in any of the background samples, yet is listed as associated with Source 2 in the HRS documentation record at proposal.
ARCO asserted that only five metals (arsenic, cad mium, copper, lead and zinc) were detected at concentrations “greater than three times background in the one grab sample collected from this possible source area.”
Response : Hazardous substances were appropriately associated with Source 2 as the hazardous substances were detected in Source 2 through sampling. HRS Section 2.2.2, Identify hazardous substances associated with a source , states in part:
For each of the three migration pathwa ys, consider those hazardous substances documented in a source (for example, by sampling, labels, manifests, oral or written statements) to be associated with that source when evaluating each pathway.
The HRS documentation record at proposal presents the hazardous substances associated with a source sample (AM-SO-03) taken from the East Ditch Dump. The table under the heading Hazardous Substances Associated with the Source on Page 31 of the HRS documentation record at proposal, presents that sample AM-SO-03 contained the CERCLA ha zardous substances: arsenic, cadmium, lead, manganese, and zinc.

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Although silver was discussed in the text of the HRS documentation record at proposal as detected in the Source 2 sample, it was not listed as a hazardous substance associated with Source 2 as no analytical data was presented to support this claim. The discussi on of the hazardous substances associated with Source 2 has been revised in HRS documentation record at promulgation and silver has been dropped from the discussion. Only those substances di splayed on the table on page 31 of the HRS documentation record at promulgation are associated with Source 2 and were included in the HRS evaluation.
Regarding ARCO’s assertion that manganese cannot be associated with the source because it did not exceed three times the background concentration, th ere is no HRS provision for a source (with source type landfill) to have substances present above background levels.

HRS Section 1.1, Definitions , defines source as:
Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminat ed from migration of a hazardous substance. Sources do not include those volumes of air, ground water, surface water, or surface water sediments that have become contamin ated by migration, except: in the case of either a ground water plume with no identif ied source or contaminated surface water sediments with no identified source, the plume or contaminated sediments may be considered a source.
This definition only restricts soils to be considered a source if the soil came to be contaminated due to migration, (which is often demonstrated by showing the contaminant levels are significantly greater than in background soils). It has no such restriction fo r other source types. However, several hazardous substances in Source 2 were compared to bac kground in the HRS documentation record at proposal simply to illustrate the high concentrations of hazardous substances found within Source 2.

This comment has no effect on the HRS site score or the decision to place the site on the NPL.
3.15.3 Source 2 Hazardous Waste Quantity

Comment : ARCO challenged the evaluation of the Source 2 hazardous waste quantity. ARCO stated that, using the area of the location where the source sample was collected, the estimated source hazardous waste quantity value should have been 0.44, instead of “> 0.”
Additionally, ARCO contended that Tier A, hazardous constituent quantity, is adequately determined, not incomplete as presented in the HRS documentation record at proposal.
Response : The source hazardous waste quantity was appr opriately evaluated for Source 2 as “greater than 0.” The exact boundaries of the East Ditch Dump have not been established with any certainty and the estimate provided by ARCO is itself only a portion of the East Ditch Dump area. Even if the source hazardous waste quantity value of 0.44 suggested by ARCO was used in evaluating Source 2 hazardous waste quantity, it would not impact the evaluation of the respective pathway hazardous waste quantity evaluations and would not change the site score.

HRS Section 2.4.2, Hazardous waste quantity, states:
Evaluate the hazardous waste quantity factor by first assigning each source (or area of observed contamination) a source hazardous waste quantity value as specified below. Sum these values to obtain th e hazardous waste quantity factor value for the pathway being evaluated.

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In evaluating the hazardous waste quantity f actor for the three migration pathways, allocate hazardous substances and hazardous wastestreams to specific sources in the manner specified in section 2.2.2. . . .

HRS Section 2.4.2.1, Source hazardous waste quantity , states:
For each of the three migration pathways, assign a source hazardous waste quantity value to each source (including the unallocated source) having a containment factor value greater than 0 for the pathway being evaluated. . . .
For all pathways, evaluate source hazardous waste quantity using the following four measures in the following hierarchy:
• Hazardous constituent quantity. • Hazardous wastestream quantity. • Volume. • Area.
For evaluating the hazardous constituen t quantity, HRS Section 2.4.2.1.1 Hazardous constituent quantity , states:
Evaluate hazardous constituent quantit y for the source (or area of observed contamination) based solely on the mass of CERCLA hazardous substances (as defined in CERCLA section 101(14), as amended) allocated to the source (or area of observed contamination). . . .
If the hazardous constituent quantity for the source (or area of observed contamination) is adequately determined (that is, the total ma ss of all CERCLA hazardous substances in the source and releases from the source [or in the area of observed contamination] is known or is estimated with reasonable confidence), do not evaluate the other three measures discussed below. Instead assign these other thr ee measures a value of 0 for the source (or area of observed contamination) a nd proceed to section 2.4.2.1.5.
If the hazardous constituent quantity is not ad equately determined, assign the source (or area of observed contamination) a value for hazardous constituent quantity based on the available data and proceed to section 2.4.2.1.2.
The HRS documentation record at proposal stated in section 2.4.2.1.1, Hazardous Constituent Quantity:
There are no Hazardous Constituent Quantity data available for the East Ditch Dump to calculate a Hazardous Constituent Quantity value.
As stated above, the hazardous constituent quantity (T ier A) is evaluated based on the mass of CERCLA hazardous substances in the source, and is properly established if this total mass is known or estimated with reasonable confidence. Since only one sample has been taken from Source 2, clearly the total mass of CERCLA hazardous substances could not have b een determined or estimated with reasonable confidence. Thus, hazardous constituent quantity (T ier A) could not be determined in the source hazardous waste quantity evaluation.
Page 30 of the HRS documentation re cord at proposal discussed that both Tier B (Hazardous wastestream quantity) and Tier C (Volume) were not adequately determined. As such, Tier D, Area, was evaluated according to the HRS in the source hazardous waste quantity evaluation.

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HRS Section 2.4.2.1.4, Area , states:
Evaluate the area measure using the area of the source (or the area of the area of observed contamination). Based on this area, designated as A, assign a value to the area measure as follows:

• For the migration pathways, assign the source a value for area using the appropriate Tier D equation of Table 2-5.
Section 2.2.2.4.4, Area, of the HRS documentation record at proposal, stated:
The area covered by the East Ditch Dump is currently not quantifiable; however as contamination has been documented at the source can be said to be unknown but greater than zero
Because the exact boundary of the East Ditch Dump remains unknown, assigning Source 2 a hazardous waste quantity value of “greater than 0” was consistent with the HRS.
Furthermore, even if the source hazardous waste quantity value of 0.44 based on an area of source estimate presented by ARCO was used in evalua ting Source 2 hazardous waste quantity, it would not impact the evaluation of the respective pathway hazar dous waste quantity evaluations and would have no impact on the site score. That is, according to HR S Table 2-6, the source hazardous waste quantity value of 0.44 corresponds to a hazardous waste quantity factor value of 1. Footnote B of Table 2-6 of the HRS states that “if the hazardous constituent quantity is not adequately determined, assign a value as specified in the text; do not assign the value of 1.” As the hazardous constituent quantity for this source was not adequately determined nor did ARCO s uggest it could be, HRS Section 2.4.2.2, Calculation of hazardous waste quantity factor value , states in relevant part:
If the hazardous constituent quantity is not adequately determined for one or more sources. . . . assign a factor value as follows:
If any target for that migration pathway is subject to Level I or Level II concentrations (see section 2.5), assign either the value from Table 2-6 or a value of 100, whichever is greater , as the hazardous waste quantity factor value for that pathway. [emphasis added]
Thus, as targets in the scored pathways are subject to Level II contamination (see section 3.27.1, Actual Contamination, of this support document), the h azardous waste quantity factor value of 100 would be assigned to the scored pathway. Therefore, the s ource hazardous waste quantity value of 0.44 presented by the commenter would not impact the scoring of the respective pathway hazardous waste quantity evaluations and would have no impact on the site score.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.

3.15.4 Source 2 Containment

Comment : ARCO alleged that the containment value of 10 assigned to Source 2 is incorrect as it does not account for the reclamation and stabilization work that has taken place. ARCO stated that the containment value of 9 should have been assigned to Source 2 as there is no evidence of hazardous substance migration from Source 2, and that a functiona l run-on control and runoff management system is in place at the site.

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Response : The Source 2 containment evaluation is correct and is consistent with the HRS criteria for assigning a containment value to a source.
In part, HRS Section 4.1.2.1.2.1.1, Containment , directs that for all source types:

Determine the containment factor value for the watershed as follows:

. . . . -If none of the sources is located in surface water in the watershed, assign a containment factor value from table 4–2 to each source at the site that can potentially release hazardous substances to the hazardous substance migration path for this watershed.
Table 4-2, Containment Factor Values for Surface Water Migration Pathway, of the HRS directs the scorer to assign a containment value of 10 to all sources (except surface impoundments, land treatment, containers, and tanks) when there is evidence of hazardous substance migration from a source area. When evidence of hazardous substance migration fro m a source area is not established, HRS Table 4-2 directs the scorer to assign a containment value of 10 to the source area for all source types (except surface impoundments, land treatment, containers, a nd tanks) when there is neither a maintained engineered cover nor a functioning and maintained run-on control system/runoff management system present.
Page 34 of the HRS documentation record at proposal stated:
No continuous maintained engineered cover, cap or soil cover is documented to be in place over the east ditch dump and waste was observed in the upper two feet of soil at the site in 2003. There is no documentation of a run-on control system and run-off is allowed to flow unchecked to concrete cha nnels which direct run-off southward to the Missouri River.
Thus, as no documented maintained engineered cove r or functioning and maintained run-on control system/runoff management system were documented to be in place at Source 2, the containment value of 10 was appropriately applied to Source 2. As noted in section 3.15.1, Source 2 Eligibility and Consideration of Response Activities, of this s upport document, although ARCO presented information on capping and revegetation activities in the vicini ty of Source 2, the commenter has not supplied documentation establishing that a continuous, maintained engineered cover, cap or soil cover is present at the source.
Moreover, the HRS documentation record at proposal identified that concrete channels in the vicinity of Source 2 direct surface water southward from the s ource area toward the Missouri River. ARCO’s assertions that surface water is managed at Source 2 with the use of run-on and runoff control systems, does not establish that hazardous substances have not re leased from the source, or are not available to release to the surface water migration pathway. Wa ter in the concrete channels reaches the river more rapidly than if the channels were not in place; the channels do not in any way rest rict the contamination in the runoff from reaching the river.
Further, even if a maintained engineered cove r and a functioning and maintained run-on control system/runoff management system as described in HRS Table 4-2 were present at Source 2, this condition would not result in a containment factor value of 0 fo r Source 2. That is, according to HRS Table 4-2, a containment value of 0 is assigned for all sources (except surface impoundments, land treatment, containers, and tanks) when the source is documented to be:

36ACM Smelter & Refinery NPL Listing Support Document March 2011

under maintained intact structure that provides protection from precipitation so that neither runoff nor leachate is generated, liqui ds or materials containing free liquids not deposited in source area, and functioning a nd maintained run-on control present.

ARCO has not demonstrated that the above considerations have been met and therefore a containment factor value of 0 does not apply to Source 2.

This comment has no effect on the HRS site score or the decision to place the site on the NPL.



### 3.16 Other Possible On-site Sources

Comment : ARCO asserted that the other possible source areas identified in the HRS documentation record at proposal are not correctly evaluated as possible sources. ARCO contended that the areas identified have been reclaimed or are subject to ongoing operation and maintenance activities.
Also, ARCO alleged that only those hazardous substa nces that are evaluated as "elevated" within each possible source area need be associated with each identified other possible source, and that hazardous substances detected below the CRDL 3 should be excluded. Response : The other possible sources identified in the HRS documentation record at proposal were appropriately described at proposal.

HRS Section 1.1, Definitions , defines source

, in part, as:
Any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminat ed from migration of a hazardous substance.

HRS Section 2.2.1, Identify sources , states, “[f]or the three migration pathways, identify the sources at the site that contain hazardous substances. Identify the migration pathway(s) to which each source applies.”

HRS Section 2.2.2, Identify hazardous substances associated with a source , states in part:
For each of the three migration pathways, consider those hazardous substances documented in a source (for example, by sampling, labels, manifests, oral or written statements) to be associated with that source when evaluating each pathway.
Pages 35 and 36 of the HRS documentation record at proposal present 14 other possible source areas that were sampled at the ACM site and were determined to contain hazardous substances. The discussion of these other possible sources in the HRS documentation reco rd identifies that EPA may further investigate these sources during the Remedial Investigation for the site and, if possible, determine whether releases from these other possible sources pose a sufficient risk and warrant remediation.

Also, hazardous substances need not be “elevated,” or significantly above bac kground to be considered associated with a potential source area. As quoted above , the HRS identifies that sampling can be used to associate hazardous substances with a source. The HR S does not require that the levels of a hazardous substance be significantly greater than background levels or that the sampling result be greater than a CRDL for consideration. The substance only needs to be present in the source. This can be documented

3 The Contract Required Detection Limit, as defined on page 51585 of the HRS, is a term equivalent to the Contract Required Quantitation Limit (CRQL). The HRS states that the CRQL is the “[s]ubstance-specific level that a CLP laboratory must be able to routinely and reliably detect in specific sample matrices. It is not the lowest detectable level achievable, but rather the level that a CLP laboratory should reasonably quantify. The CRQL may or may not be equal to the quantitation limit of a given substance in a given sample. For HRS purposes, the term CRQL refers to both the contract-required quantitation limit and the contract-required detection limit.”

37ACM Smelter & Refinery NPL Listing Support Document March 2011

by showing the substance is present above the detec tion limit of the analytical method employed. The CRDL is not the lowest detectable level achievable for a sample analysis; and detections below a CRDL can still exceed a detection limit specific to the analytical method, and thus still be documented in a source. Thus, hazardous substances were appropriate ly associated with other possible on-site source areas as these substances were detected in samples taken from the other possible on-site source areas.
This comment has no effect on the HRS site score or the decision to place the site on the NPL. 

### Surface Water Scoring at Proposal

The HRS documentation record at proposal evaluated observed releases of hazardous materials to the Missouri River adjacent to Source 1 (contaminated soil in the Black Eagle community) and Source 2 (East Ditch Dump, located east of the former stack on a ridge above the river). Five probable points of entry (PPEs) were identified where contaminant-carry ing runoff may drain from Source 1 to the Missouri River via outfalls. A sixth PPE was identified as a gully through which runoff from the Source 2 area may enter the Missouri River. Observed releases of hazardous substances were established both by direct observation of slag dumped into the river and chemical an alysis of metal levels in river sediment samples. The locations of the PPEs and the samples used in the characterization of the observed release are displayed on Figure 1 of this support document which follows. The targets scored for this pathway include a human food chain fishery and wetland sensitive environments subject to actual contamination. The scoring of the surface water migration pathway was presented on pages 37-49 of the HRS documentation record at proposal. 

### 3.17 Evaluation of the Surface Water Pathway

Comment : ARCO argued that it would be appropriate to not score the surface water pathway, stating that HRS guidance indicates this as the course of acti on when no observed release is established from the scored sources at a site. ARCO contended that th e entire pathway should be deleted, as it was based on faulty assumptions arising from the use of data not re flecting current conditions. ARCO stated that it had developed an alternative score for the surface wate r migration pathway in which it scored potential to release for Source 1 and Source 2 separately, with Source 2 yielding the slightly higher potential to release score.
Response : The surface water pathway was scored becau se it poses a threat to human health and the environment. That a threat exists is shown in the identification of observed releases to surface water within a human food chain fishery and the presence of wetlands subject to contamination, in accordance with the HRS (and consistent with the HRS Guidance Manual). The likelihood of release for the surface water pathway has been correctly scored based on the identification of observed releases to the Missouri River. The potential to release was not scored in eval uation of likelihood of rel ease, consistent with the HRS, as an observed release to surface water was established.

HRS Section 2.3, Likelihood of release, states:

Likelihood of release is a measure of the lik elihood that a waste has been or will be released to the environment. The likelihood of release factor category is assigned the maximum value of 550 for a migration path way whenever the criteria for an observed release are met for that pathway. If the crite ria for an observed release are met, do not evaluate potential to release for that pathway.
Establish an observed release either by dir ect observation of the release of a hazardous substance into the media being evaluated (fo r example, surface water) or by chemical analysis of samples appropriate to the pathway being evaluated (see sections 3, 4, and 6).

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39 The minimum standard to establish an observed release by chemical analysis is analytical evidence of a hazardous substance in the medi a significantly above the background level.
Further, some portion of the release must be attributable to the site. Use the criteria in table 2–3 as the standard for determining analytical significance.

HRS Section 4.1.2.1.1, Observed release , states in relevant part:
If an observed release can be established for a watershed, assign an observed release factor value of 550 to that watershed, enter th is value in Table 4-1, and proceed to section 4.1.2.1.3. If no observed release can be est ablished for the watershed, assign an observed release factor value of 0 to that watershed, enter this value in Table 4-1, and proceed to section 4.1.2.1.2.
The HRS documentation record at proposal, on page 39, stated:
According to the HRS, if an observed rel ease can be established, a value of 550 is assigned for Likelihood of Release (Ref . 1, Section 4.1.2.1.1, p. 51609).

As established in sections 3.19, Likelihood of Releas e - Observed Release by Direct Observation, and 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, an observed release by direct observation and an observed release by chemical analysis have been established. The evaluation of the entire surface water migration pathway should not be deleted in the HRS evaluation, as asserted by ARCO in its interp retation of the HRS guidance, because the observed releases have been properly established according to the HRS and there is no need to score potential to release.
Regarding the use of data not reflecting current c onditions, ARCO appears to be implying, though does not clearly state, that the site’s current threats are less than in the past. Ho wever the HRS evaluates the threat posed both by past and current releases. For example, the HRS specifies in Section 4.1.2.1.1 that the user is to “[e]stablish an observed release to su rface water for a watershed by demonstrating that the site has released a hazardous substance to the surface water in the watershed [emphasis added].” The HRS does not require that the release be ongoing, just that it has occurred, and the HRS evaluation shows that this release poses sufficient threat relative to othe r sites to warrant listing and further investigation. This topic is discussed in further detail in section 3.20.1.3, Bias of Conclusions/Current Conditions, of this support document. Furthermore, as discussed in sec tion 3.6, Risk Associated with the Site, of this support document, contrary to ARCO’s statements ARCO has not shown that no unacceptable risk is associated with releases from the site.

These comments have no effect on the site score or the decision to list the site on the NPL.ACM Smelter & Refinery NPL listing Support DocumentMarch 201140ACM Smelter & Refinery NPL Listing Support Document March 2011



### 3.18 Description of Pathway

Comment : ARCO challenged the description of the over land flow/flood component of the surface water pathway.

ARCO stated that the HRS documentation record at proposal documented that the outfalls draining the contaminated soil in the Black Eagle residential area enter the Missouri River at PPE #s 1 through 5. ARCO commented that the background sediment sample was collected in the vicinity of the most upstream outfall, PPE #1, and that, on this basis, the area drained by that outfall should be removed from consideration in the surface water pathway. ARCO further alleged that, if the sediment sample under consideration were not considered a background sam ple, then its results do not indicate contaminant transport from the Black Eagle residential area into the River.

ARCO noted that, while the PPE for Source 2 is i ndicated as being PPE #6, the two sediment samples with the highest contaminant concentrations in th e 2009 Expanded Site Inves tigation (ESI) (AGFSED-2 and AGFSED-3) were collected upstream from this PPE. On this basis, ARCO concludes that Source 2 cannot be the source of sediment contamination for this reach of the river.

R esponse : The surface water pathway for the overland/ flood migration component was appropriately characterized in accordance with the HRS require ments. These requirements are presented in HRS Section 4.1.1.1, Definition of hazardous substance migr ation path for overland/flood migration component , which specifies the following:

T he hazardous substance migration path includes both the overland segment and the inwater segment that hazardous substances would take as they migrate away from sources at the site:

• Begin the overland segment at a source and proceed downgradient to the probable point of entry to surface water.

• Begin the in-water segment at this probable point of entry.

- For rivers, continue the in-water segment in the direction of flow (including any tidal flows) for the distance estab lished by the target distance limit (see section 4.1.1.2).

- For lakes, oceans, coastal tidal waters, or Great Lakes, do not consider flow direction. Instead apply the target distance limit as an arc.

- If the in-water segment includes both ri vers and lakes (or oceans, coastal tidal waters, or Great Lakes), apply the target distance limit to their combined in-water segments.

F urther, HRS Section 4.1.1.2, Target distance limit , continues the description of the in-water segment of the migration pathway, as follows:

T he target distance limit defines the maximum distance over which targets are considered in evaluating the site. Determine a separat e target distance limit for each watershed as follows:

• If there is no observed release to surface water in the watershed or if there is an observed release only by direct observation (see section 4.1.2.1.1), begin measuring the target distance limit for the watershed at the probable point of

41ACM Smelter & Refinery NPL Listing Support Document March 2011

entry to surface water and extend it for 15 miles along the surface water from that point.

• If there is an observed release from the site to the surface water in the watershed that is based on sampling, begin measuring the target distance limit for the watershed at the probable point of entry; extend the target distance limit either for 15 miles along the surface water or to the most distant sample point that meets the criteria for an observed release to that watershed, whichever is greater.
In accordance with the requirements specified above, the HRS documentation record at proposal described the surface water migration pathway for Sour ces 1 and 2 in Section 4.1. 1.1, beginning on page 37, as follows:
Two sources have been evaluated at the ACM Smelter and Refinery site. The first source, Source 1, the contaminated soil in th e community of Black Eagle (Ref. 22, p. 2) would enter the surface water pathway of the Missouri River via a series of culverts draining surface water runoff from the community into the Missouri River. All outfall culvert locations are on the north bank of the Missouri River and are identified from information provided by the Cascade County Public Works Department (Ref. 55, 12 pages; Ref. 59, pp. 1-2). The five of the out fall locations are listed in the following table. A map showing the locations is also provided (Ref. 50, 1 page).

Outfalls draining from Contaminated Residential Soil to the Missouri River (Ref. 50, p. 2; Ref. 55, 11 pages; Ref. 59, pp. 1-2) Outlet Name Outfall Location Outfall Description Drainage Area Responsible Party PPE#1, North River

Road 9th to 15th

Street-No. 2 630 feet east of 10th Street

Bridge 24-inch diameter corrugated steel pipe 37 acres Western Black Eagle Cascade County

PPE#2, North River

Road 9th to 15th

Street-No. 3 1,400 feet east of 10th Street Bridge 24-inch diameter corrugated steel pipe 8 acres Western Black

Eagle along Missouri River Cascade County

PPE#3, 15th Street Bridge North 60 feet west of 15th Street

Bridge 30-inch diameter corrugated 132 acres Central Black Eagle MDT and Cascade County PPE#4, Black Eagle South East East of 15th Street Bridge under North

River Rd. 20-inch diameter corrugated steel pipe 22 acres South central Black Eagle Cascade County

PPE#5, Anaconda Hills West

(Coulee) 380 feet west of Black Eagle Dam Twin 36-inch diameter corrugated steel pipes 188 acres Cascade County Eastern Black Eagle, Moose Lodge, and golf

course
The most upstream outfall for surface water r unoff from the contaminated soil identified in the unincorporated community of Black Eagle would be outfall North River Road 9 th to 15 th Street No. 2 (PPE # 1) with a drainage area of approximately 37 acres (Ref 50, 1 page; Ref. 55, p. 7; Ref. 59, pp. 1-2). Th is outlet captures runoff from western edge of the community of Black Eagle and delivers th e runoff through the outfall to the Missouri River (Ref. 50; Ref. 59, pp. 1-2). PPE # 2 delivers runoff from approximately 8 acres in

42ACM Smelter & Refinery NPL Listing Support Document March 2011

western Black Eagle along the Missouri River to the river at the outfall named North River Road 9 th to 15 th Street. PPE # 3 drains approximately 132 acres in central Black Eagle to the Missouri River at the outfall named 15 th Street Bridge North. PPE # 4 drains approximately 22 acres in south central Black Eagle at the outfall named Black Eagle South East. The eastern area of Black Eagle, approximately 188 acres including the Moose Lodge and the northeast quadrant of th e golf course, drains through PPE # 5 into the Missouri River (Ref. 55, pp. 1-11; Ref. 50, 1 page; Ref. 59, pp.1-2). The surface water pathway for source 1-residential soil, would continue through Black Eagle Dam, past the sample collection locations of the 2003 and 2009 EPA investigations, through Rainbow Dam and Morony Dam to the 15-mile target distance limit (TDL) (Ref. 50, 1 page).
The probable point of entry (PPE #6) for wastes associated with source 2-the East Ditch Dump on the ACM Smelter and Refinery prope rty is shown on the surface water pathway map (Ref. 18, p. 38; Ref. 50; Ref. 58, Appendix I). The overland flow route from the East Ditch Dump to the Missouri River is south off the ridge to the river through a gully for approximately 400 feet to the Missouri River (Ref. 4, map 1; Ref. 50; Ref. 58, Appendix I). However, other sources at the f acility likely also have potential to release by overland flow due to lack of cover, the natural slope of the land toward the Missouri River, and culverts that drain surface water di rectly into the Missouri River (Ref. 18, p. 19, 20, 22, 32, & 38; Ref. 23, p. 31). The surface water pathway from PPE#6 would continue past the sample collection locati ons of the 2003 and 2009 EPA investigations, through Rainbow Dam and Morony Dam to the 15-mile target distance limit (TDL) (Ref. 50, 1 page).
As such, the overland segment of the hazardous subs tance migration path from each source has been appropriately described, as the migration path fro m each source and associated downgradient probable point of entry to surface water was detailed accordi ng to the HRS (see Figure 1 of this support document for site map presenting sources and PPEs). Also, the in -water segment and target distance limit (TDL) for the watershed were appropriately established as th e TDL was defined as 15 miles downstream of PPE#6 within the Missouri River.
ARCO is incorrect in its assertion that the drainage area associated with PPE #1 should be removed from the surface water pathway evaluation because it is ups tream of the most upstream background sample (AGFSED1). The HRS does not require that each in dividual PPE have an observed release associated specifically with it. PPE #1 was established follo wing the directions cited above from HRS Section 4.1.1.1, which contains no requirement that a PPE be ignored if it is upstream of a sample used to establish an observed release.
Sample AGFSED1 was selected to establish a background level and thus used to illustrate a significant increase in observed release samples. Even though this sample is located downstream of PPE #1, this does not mean that some contamination is not still migrating from Source 1 through PPE #1 to the Missouri River. In fact, as shown in Section 3. 20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support documen t, this sample does contain detectable levels of hazardous substances associated with Source 1.
Additionally, the possibility that this background sampl e may be affected by site contamination does not diminish its ability to fulfill the purpose of a background sample: to establish a background level in the medium of concern for the site to determine whethe r or not a significant increase in contaminant levels from the site has occurred. As established in section 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, the downstream observed release levels are clearly three times greater than the backgr ound levels. Therefore, there is no reason to remove

43ACM Smelter & Refinery NPL Listing Support Document March 2011

the area drained by PPE #1 from consideration in th e surface water pathway, and no reason to question the validity of the background leve ls established by sample AGFSED1.
ARCO is also incorrect in its assertion that higher le vels of contaminants found upstream of Source 2's probable point of entry (PPE #6) means Source 2 is not a source of contamination to the surface water pathway. Just because the two sedim ent samples with the highest contam inant concentrations in the 2009 ESI (AGFSED-2 and AGFSED-3) are upstream of Source 2, it does not mean that Source 2 is not contributing hazardous substances to surface water. Further, observed release samples AGFSED-5 and AGFSED-6 are downstream of Source 2, and while they could be affected by contamination migrating from Source 1, they also could be affected by contaminants migrating from Source 2. The information available is insufficient to demonstrate that Source 1 is the only contributor to the contamination found in samples AGFSED-5 and AGFSED-6.
Regarding ARCO’s assertion that a portion of Source 1 should not be included in the site simply because it has not been definitively shown to be releasing to surface water, the HRS has no such requirement. HRS Section 2.2.3, Identify hazardous substan ces available to a pathway, specifies that the surface water containment factor value need be greater than zero to consider hazardous substances identified in the source to be available to migrate to the surface water pathway. In addition, the HRS does not require that an observed release need be attributed to specific sources. HRS Section 4.1.2.1.1, Observed release
, specifies that the observed release concentration must be significantly above background and some portion of that significant increase must be attributable to the site (not specifically to an individual source) in order to establish an observed release in a wa tershed (see Section 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document).
These comments have no impact on the site score or the decision to list the site on the NPL.



### Likelihood of Release

The HRS documentation record at proposal established observed releases of hazardous substances to the Missouri River both by direct observation of slag dum ped into the river and by chemical analysis of metals levels in river sediment samples. Historical evidence was presented to document extensive dumping of slag from the former facility into the ri ver over the course of several decades, and showing that slag contained substantial concentrations of copper (see pages 38-39 of the HRS documentation record at proposal). Four contaminated sedimen t samples and one background sediment sample were described to show that concentrations of cadmium and lead significantly above background levels were present in river sediments; these samples establis hed observed releases of hazardous substances attributable to the site and meet the HRS criteria to establish observed release by chemical analysis (see pages 39-40 of the HRS documentation record at proposal).



### 3.19 Likelihood of Release - Observ ed Release by Direct Observation

C omment : ARCO made a number of comments in oppositi on to EPA's scoring of an observed release to surface water by direct observation, including objections to the use of alleged “anecdotal evidence,” objections to references, and objections to source association.

R esponses to specific comments are presented in th e following subsections of this support document:

• 3.19.1 Documentation of Observed Release by Direct Observation

• 3.19.2 Quantity of Slag Dumped into the Missouri River

• 3.19.3 Source of Slag Material

44ACM Smelter & Refinery NPL Listing Support Document March 2011

3.19.1 Documentation of Observed Release by Direct Observation

Comment : ARCO challenged the observed release by direct observation by alleging that EPA uses “anecdotal evidence from almost 100 years ago” of tailings and slag disposal in the Missouri River to establish the observed release by direct observation to the river. ARCO acknowledges that “the historical record reports some volume of tailings and slag were di sposed in the River nearly a century ago,” and that “this standard practice at the time was followed.”
ARCO also states that, in the Observed Release by Direct Observation section of the HRS documentation record at proposal, EPA states that cindery slag material was observed in the sediments during 2009 sampling; “however, three of the four references cited make no mention of slag in the Missouri River sediment.”
Response : The observed release by direct observation has been correctly documented and scored in accordance with the HRS. Furtherm ore, as quoted above, ARCO acknowledges the historical record of the disposal of slag and tailings into the river.

HRS Section 4.1.2.1.1, Observed Release , contains the requirements for establishing an observed release by direct observation:

E stablish an observed release to surfa ce water by demonstrating that the site has released a hazardous substance to the surface water in the watershed. (Emphasis added.)

H RS Section 4.1.2.1.1 provides that an observed rel ease by direct observation can be based on any one of three criteria:

- A material that contains one or more haza rdous substances has been seen entering surface water through migration or is known to ha ve entered surface water through direct deposition, or

- A source area has been flooded at a time that hazardous substances were present, and one or more hazardous substances were in contact with the flood waters, or

- When evidence supports the inference of a releas e of a material that contains one or more hazardous substances by the site to surf ace water, demonstrated adverse effects associated with that release may also be used to establish an observed release.

The first of these criteria has been satisfied in the HRS documentation record at proposal, which stated the following on pages 38 and 39:
The Boston and Montana smelter routinely dumped large quantities of granulated slag and tailings into the Missouri River (Ref. 6, p. 42; Ref. 15, p. 1). In 1907, the Boston and Montana Consolidated Copper and Silver Mining Company estimated that the facility could receive up to 4800 tons a day of ore and changing the di sposal of slag to railcar would significantly increase rail traffic (Ref. 14, p. 1). The Montana Department of Environmental Quality (Montana DEQ) estimates that between 27,500,000 and 31,000,000 cubic yards of waste were dumped into the Missouri River between 1893 and 1915 until onsite containment was instituted in 1915. Montana DEQ also reports that it is possible that dumping of waste into the Missouri River continued at a reduced rate after 1915 until the facility closed in the 1970s (Ref. 16, p. 1). So much smelter waste was dumped into the Missouri River between 1893 and 1915 that the management and legal department of the Boston and Montana Company were concerned that the Federal Government would intervene and enjoin the deposition of tailings in to the Missouri River

45ACM Smelter & Refinery NPL Listing Support Document March 2011

by virtue of the constitutional provisions re quiring the federal government to maintain navigable streams (Ref. 15, pp. 1 and 2). In 1884 Judge Lorenzo Sawyer of the United States Circuit Court in San Francisco had gran ted a perpetual injunction against hydraulic mining in California’s American River on these very grounds (Ref. 6, p. 38).

Numerous complaints were filed by proper ty owners and communities along the Missouri River downstream of the ACM Smelter and Refi nery of water quality issues and slag and smelter waste clogging the Missouri River (Ref. 6, p. 42; Ref. 29, p. 8A). These complaints increased after the 1908 flood of the Missouri River inundated the community of Fort Benton, 40 miles downstream of th e Boston and Montana Smelter with smelter waste (Ref. 6, p. 38; Ref. 29, p. 8A). A comp any analysis of slime (very fine grained slag and tailings, which tended to float in the Missouri River) taken above Rainbow Falls Dam in 1914 indicated that the slime containe d between .55 and 1.1 percent copper (Ref. 17, p. 1). The Boston and Montana Company sought to buy off disgruntled property owners and placate communities by providing alternate domestic water sources (Ref. 6, pp. 49-51).

Cindery slag material was also currently obser ved to be in the sediments of the Missouri River adjacent to the ACM Smelter and Refinery facility (Ref. 23, p. 57 and 202, Ref, 53 p. 1; Ref. 54, p. 1).

That a hazardous substance was associ ated with the slag is shown above by the documentation of the analysis of fine-grained slag and tailings (referred to as “slime”) that resulted from the slag deposition into the river, which contained betw een 0.55 and 1.1 percent copper.
In addition, the presence of other hazardous substa nces has been documented in the slag through sampling. As discussed in section 3.13, Additional Sources, of this support document, a slag source (Source 4) has been added to the HRS evaluation at promulgation. A sample of this slag is described in section 2.2.2, Source 4, of the HRS documentation record at promulgation, and reveals the presence of the hazardous substances arsenic, cadmium, copper, lead, manganese, and zinc. These substances are also added to the discussion in the Observed Release by Direct Observation section of the HRS documentation record at promulgation (see pages 49-50 of th e HRS documentation record at promulgation).
While ARCO objected to the timeframe in which th e documentation of slag disposal in the Missouri River originated, ARCO does not dispute the disposal of the slag, and in fact, actually acknowledges in its comments that the facility historically followed the st andard practice of disposing tailings and slag in the Missouri River (see the second paragraph on [bottom] page number 6 of ARCO’s Comment Section B: Atlantic Richfield's General Comments on the HRS Documentation Record Former Great Falls Refinery Site, docket ID EPA-HQ-SFUND-2010-0072-0016.2). Re gardless of when the slag dumping in the Missouri River occurred, the establishment of an obs erved release is based on whether or not a release “has occurred” – the HRS does not direct the scorer to determine a specific timeframe of a release.
Thus, the available documentation clearly shows that slag, demonstrated to contain hazardous substances, was documented in the Missouri River by visual observation of sediments and was documented through facility records and ARCO statements as be ing dumped into the Missouri River.

These comments have no effect on the site score or the decision to list the site on the NPL.
3.19.2 Quantity of Slag Dumped into the Missouri River

Comment : ARCO discounted the quantity of slag that was documented to have been dumped by the Anaconda Mining Company at the site, commenting th at there was no reference in the [MDEQ] site

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investigation report to document how MDEQ arrived at its estimate of 27,500,000-31,000,000 cubic yards of waste [slag] dumped in the Missouri River betw een 1893 and 1915. ARCO also dismissed as "pure speculation" MDEQ's statement that “it is possibl e that dumping of waste into the Missouri River continued at a reduced rate after 1915 until the fac ility closed in the 1970's.” In addition, ARCO commented on the ambiguity of a reference describi ng a possible increase in the amount of ore received by the facility.
Response : The observed release by direct observation was appropriately established, regardless of the quantity of slag disposed into the Missouri River. The quantity of slag that has entered the Missouri River as a result of facility operations is not considered in the establishment of an observed release by direct observation nor was the quantity used in the HRS scoring to assign any scoring factor.

As discussed in subsection 3.19.1, Documentation of Observed Release by Direct Observation, of this support document, in establishing an observed release by direct observation, HRS Section 4.1.2.1.1 requires that:

A material that contains one or more haza rdous substances has been seen entering surface water through migration or is known to ha ve entered surface water through direct deposition.
The HRS documentation record at proposal, on pa ges 38 and 39, fulfills these requirements by documenting, in relevant part:
The Boston and Montana smelter routinely dumped large quantities of granulated slag and tailings into the Missouri River (Ref. 6, p. 42; Ref. 15, p. 1). . . So much smelter waste was dumped into the Missouri River between 1893 and 1915 that the management and legal department of the Boston and M ontana Company were concerned that the Federal Government would intervene and enjoin the deposition of tailings into the Missouri River by virtue of the constitutional provisions requiring the federal government to maintain navigable streams (Ref 15, pp. 1 and 2).
Cindery slag material was also currently obser ved to be in the sediments of the Missouri River adjacent to the ACM Smelter and Refinery facility (Ref. 23, p. 57 and 202, Ref. 53, p. 1; Ref. 54, p. 1).
The HRS does not require that an exact amount of the material containing hazardous substances that entered the surface water be known to score an observed release by direct observation. Therefore, the amount of slag waste dumped into the river was not considered in establishing the observed release by direct observation, nor was the amount of waste slag considered in the hazardous waste quantity evaluation for the surface water pathway. Therefore, the MDEQ estimate of the quantity of slag dumped into the Missouri River, and whether slag waste was dumped into the Missouri River after 1915 have no bearing on the establishment of an observed release by direct observation in the HRS evaluation. Also, the amount of slag dumped into the river is not used in estimating the waste quantity of the new Source 4 (slag waste) added in the HRS doc umentation at promulgation.

These comments have no effect on the site score or the decision to list the site on the NPL.
3.19.3 Source of Slag Material

Comment : In its specific comments objecting to the observed release by direct observation, ARCO contended that “no observed release has been establishe d for Source 1 or Source 2,” and that slag is not related to these sources.

47ACM Smelter & Refinery NPL Listing Support Document March 2011

Response : An observed release by direct observation has been appropriately established from the site, in accordance with the provisions of the HRS, and an additional origin (s ource) of slag material has been associated with the site at promulgation. Source 4, slag waste, has been added to the HRS documentation record at promulgation as discussed in section 3.13, A dditional Sources, of this support document. Slag from this source was sampled and documented as cont aining hazardous substances as described in the HRS documentation record at proposal.

HRS Section 4.1.2.1.1, Observed release , directs the user to:

Establish an observed release to surface wate r for a watershed by demonstrating that the site [emphasis added] has released a hazardous substance to the surface water in the watershed.
The HRS specifies that the observed release has to have come from the site, not that it has to have come from individual sources at the site.
The HRS documentation record at proposal, while not associating the slag in the observed release by direct observation with Source 1, the Black Eagle soils, or Source 2, the East Ditch Dump, clearly associates it with the site, as specified by the HRS. This is shown on page 38 of the HRS documentation record at proposal, excerpted in relevant part as follows:
The Boston and Montana smelter routinely dumped large quantities of granulated slag and tailings into the Missouri River (Ref. 6, p. 42; Ref. 15, p. 1). . . So much smelter waste was dumped into the Missouri River between 1893 and 1915 that the management and legal department of the Boston and M ontana Company were concerned that the Federal Government would intervene and enjoin the deposition of tailings into the Missouri River by virtue of the constitutional provisions requiring the federal government to maintain navigable streams (Ref 15, pp. 1 and 2).

In addition, as discussed in section 3.13, Additional Sources, of this support document, two sources have been added to the HRS documentation record at promul gation, one of which (Source 4) represents onsite disposal of slag waste. This source is described in detail in section 3.13 of this support document. Its addition clarifies that the slag wastestream gene rated on site is part of the site as scored.

This comment has no effect on the site score or the decision to list the site on the NPL.



### 3.20 Surface Water Likelihood of Rele ase - Observed Release by Chemical Analysis

Comment : ARCO provided a number of comments contesting the observed release by chemical analysis to the Missouri River. ARCO referred to the sediment samples meeting observed release criteria as having "weakly elevated metals concentrations," and pointed out that they were also located downstream of urban and industrial areas. ARCO also argue d against the adequacy of the background sediment sample.

R esponse : The observed release by chemical analysis was appropriate, adequate, and established in accordance with HRS requirements. Four sediment sample locations were documented as meeting observed release criteria in the HRS documentation r ecord at proposal (see Figure 1 in section 3.17, Description of Pathway, of this support documen t for sample locations). These requirements are presented in HRS Section 4.1.2.1.1, Observed release , which directs the user to:

48ACM Smelter & Refinery NPL Listing Support Document March 2011

Establish an observed release to surface water for a watershed by demonstrating that the site has released a hazardous substance to the surface water in the watershed. Base this demonstration on either:

• Direct observation:

. . . . [or]

• Chemical analysis:

- Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substan ce(s) has increased significantly above the background concentration for the site for that type of sample (see section 2.3).

– Limit comparisons to similar types of samples and background concentrations - for example, compare surface water samples to surface water background concentrations.

– For benthic samples, limit comparisons to essentially sessile organisms.

- Some portion of the significant increase mu st be attributable to the site to establish the observed release, except: when the site itself consists of contaminated sediments with no iden tified source, no separate attribution is required.

- If an observed release can be established for a watershed, assign an observed release factor value of 550 to that watershed, enter this value in Table 4-1, and proceed to section 4.1.2.1.3.

I n addition, HRS Section 2.3, Likelihood of Release , specifies that the scorer “[u]se the criteria in Table 2- 3 as the standard for determining analytical signi ficance.” HRS Table 2-3 states that, if a sample measurement is greater than or equal to the SQL, an observed release is established:

• If the background concentration is not de tected (or is less than the detection limit), an observed release is established when the sample measurement equals or exceeds the sample quantitation limit.

• If the background concentration equals or exceeds the detection limit, an observed release is established when the sample measurement is 3 times or more above the background concentration.
In accordance with these requirements, the HRS documen tation record at proposal, beginning on page 39, presents the rationale and cites the supporting documentation for the observed release by chemical analysis by describing sediment sample collection and analysis from two investigations conducted in the Missouri River adjacent to and downstream of the site. The discussion of the April 2003 field investigation, in relevant part, stated:
Eight sediment samples from the Missouri River were collected by Tetra Tech EM, Inc for the EPA’s Expanded Site Inspection in April 2003. These samples were collected from the north bank of the Missouri River a nd consisted of medium to medium fine grained tan particles (Ref. 54, 1 page). All samples were analyzed through the EPA CLP,

49ACM Smelter & Refinery NPL Listing Support Document March 2011

RAS for TAL total metals. Soil samples for inor ganic analysis were delivered to Sentinal Inc, of Huntsville, Alabama (Ref. 18, p. 3). The analytical data was validated by Tetra Tech, EM, Inc, of Chicago, IL on July 30, 2003 (Ref. 18, p. 56) . . .

Analyses of sediment samples collected during the April 2003 sampling event indicate that concentrations of arsenic, cadmium , copper, lead, manganese, and zinc are significantly above background concentrations in the Missouri River for a distance of 2.39 miles downstream of the probable point of entry (PPE) at the Black Eagle Dam (Ref. 18, p. 39, Figure 3).
The discussion of the 2009 sampling investigation, the primary support for the observed release to surface water by chemical analysis, was presented at proposal in both text and table formats. The text portion of this description, on page 39 of the HRS documentation record at proposal, is as follows:
Seven release sediment samples were collected by START for the EPA Region 8 on July 14, 2009. All release samples were collected by boat from the north side of the Missouri River from beneath the water. A background sample (AGFSED1) was collected from the north bank of the Missouri River, downstream of the Montana Refining Company facility and upstream of the outfall North River Road 9 th to 15 th Street No. 3 (PPE#1) and the community of Black Eagle (Ref. 50, 1 page). The samples were analyzed by Compuchem Laboratory in Cary, North Carolina, for total Target Analyte List (TAL) metals analysis with a supporting Level 4 data package (Ref. 23, pp 6, 15, & 21). The data package was validated by a START chemist who determined that the background cadmium result was biased high as well as two downstream results (R ef. 23, pp. 77-198). The analytical data was revised in accordance with the EPA gui delines for using qualified data, which resulted in the high bias for the background sample being retained while the two downstream, analytical results were revi sed downward (Ref. 43, pp. 1-9, 18). The following table provides analytical data to document the observed release by chemical analysis.

Analyses of sediment samples collected during the July 2009 sampling event indicate that concentrations of lead are above background concentrations in the Missouri River for a distance of 2.39 miles downstream of the probable point of entry (PPE) at the Black Eagle Dam (Ref. 24, p. 28, Table 3; Ref. 50). According to the HRS, if an observed release can be established, a value of 550 is assigned for Likelihood of Release (Ref. 1, Section 4.1.2.1.1, p. 51609).
The observed release samples were presented on page 40 of the HRS documentation record at proposal, and are shown in the following table.

Sediment Samples Collected from the Missouri Rive r adjacent and downstream of the ACM Smelter and Refinery, July 2009

Sample ID Sample Location Sample Date Cadmium (Cd) mg/kg Lead (Pb) mg/kg CRQL HRS Documentation Record at Proposal Reference

AGFSED1 0907118-01 Background Sample at 11 th Street 07/14/09 0.36 J+ 12.3 Cd :0.06 Pb: 0.21- Ref. 2, p. BII-2, BII-8; Ref. 23, pp. 28, 30, 37, 40, 46, 69, 70, 85, 174-176, 177, 179, 182, 183, 190, 204.

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Sediment Samples Collected from the Missouri Rive r adjacent and downstream of the ACM Smelter and Refinery, July 2009
Cadmium Lead Sample Sample Sample ID CRQL Location Date (Cd) (Pb) mg/kg mg/kg HRS Documentation Record at Proposal Reference

AGFSED2 0907118-02 1.89 miles downstream of PPE No. 1 07/14/09 (11.7 J+) 8.3 168 Cd: 0.06 Pb: 0.21- Ref. 2, p. BII-2, BII-8; Ref. 23, pp. 28, 30, 37, 40, 47, 69, 70, 85, 174-176., 177, 179, 182, 183, 191, 202.

AGFSED3 0907118-03 1.95 miles downstream of PPE No. 1 07/14/09 11.8 78.4 Cd: 0.06 Pb: 0.21 Ref. 2, p. BII-2, BII-8; Ref. 23, pp. 28, 31, 37, 40, 48, 69, 70, 85, 174-176, 177, 182, 192, 202.

AGFSED4 0907118-04 2.06 miles downstream of PPE No. 1 07/14/09 3.9 18.9 Cd: 0.06 Pb: 0.21 Ref. 2, p. BII-2, BII-8; Ref. 23, pp. 28, 31, 37, 40, 49, 69, 70, 85, 174-176, 177, 182, 193, 202.

AGFSED5 0907118-05 2.39 miles downstream of PPE No. 1 07/14/09 (4.2 J+) 2.9 49.9 Cd: 0.06 Ref. 2, p. BII-2, BII-8; Ref. 23, pp. 28, 32, 37, 40, 50, 69, 70, 85, 174-176, 177, 179, 182, 183, 194, 202-203. Pb: 0.21

These samples (AGFSED2, AGFSED3, AGFSED4, AGFSED5) illustrate that concentrations of cadmium or lead in sediments are present in levels greater that three times the levels found in the background sample (AGFSED1), meeting the criteria in HRS Section 2.3, Likelihood of Release , cited above.

As further detailed in section 3.20.2 of this support document, Surface Water Observed Release – Attribution, the HRS documentation record at propos al documents that the released substances are associated with the ACM site as they are in on-site so urces and at least part of the contamination in the river came from the site based on direct observation of slag being dumped into the river. Therefore, the significant increase in contaminant levels has been appr opriately attributed at least in part to the ACM site. In addition ARCO only allege d contributions from othe r off site sources; it did not document actual contribution.

T hus, an observed release by chemical analysis has been appropriately established according to the HRS requirements, as the presence of hazardous substances attributable to the site were found in release samples adjacent to and downstream of the site at levels significantly above background concentrations.

S pecific issues related to ARCO's objections to the observed release to chemical analysis are addressed in the following subsections of this support document:

• 3.20.1 Surface Water Observed Release - Significant Increase

• 3.20.2 Surface Water Observed Release - Attribution

• 3.20.3 Surface Water Observed Release - Adequacy of Background

3 .20.1 Surface Water Observed Release - Significant Increase

C omment : ARCO made a number of comments arguing that the contamination used to score the significant increase for the surface water observed rel ease was not sufficient. These arguments were that:

• the contamination levels did not exceed regulatory criteria,

• recent data do not indicate contamination of the water column,

51ACM Smelter & Refinery NPL Listing Support Document March 2011

• EPA's conclusions regarding the contamination are biased, and

• the contamination levels are not indicative of release.

S pecific comments are addressed in the follo wing subsections of this support document:

• 3.20.1.1 Contamination Leve ls Below Regulatory Limits

• 3.20.1.2 Releases in Other Aquatic Media

• 3.20.1.3 Bias of Conclusions/Current Conditions

• 3.20.1.4 Downstream Contaminant Levels

3.20.1.1 Contamination Levels below Regulatory Limits

Comment : In challenging the sufficiency of the si gnificant increase in the observed release, ARCO stated that “[a] recent and multiple sampling station comparison of Missouri River water quality upstream and downstream of the Site indicates that, wh ile a few statistically significant differences in concentrations were noted, none of the reported tota l or dissolved concentra tions exceeded applicable aquatic life water quality standards at any sampling st ation.” ARCO also maintained that only a few concentrations from similar sampling “exceeded relevant sediment quality biological effect threshold concentrations.”

R esponse : Whether or not the contaminant levels docum ented that the Missouri River exceeded various regulatory limits does not discredit that an observed release by chemical analysis was appropriately evaluated. As identified in section 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, the an alytical data provided for the observed release by chemical analysis in the HRS documentation record at proposal meet the HRS criteria for an observed release by chemical analysis.

A s discussed in more detail in section 3.20, Surf ace Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, in establishing an observed release by chemical analysis, HRS Section 4.1.2.1.1, Observed release , states:

• Chemical analysis:

- Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substance(s) has increased significantly above the background concentration for the site for that type of sample (see section 2.3).

. . . . .

- Some portion of the significant increase mu st be attributable to the site to establish the observed release, except: when the site itself consists of contaminated sediments with no identif ied source, no separate attribution is required.

- If an observed release can be established for a watershed, assign an observed release factor value of 550 to that waters hed, enter this value in Table 4-1, and proceed to section 4.1.2.1.3.

H RS Section 2.3, Likelihood of Release , specifies that the user is to “[u] se the criteria in Table 2-3 as the standard for determining analytical significance.” HRS Table 2-3 states the following:

52ACM Smelter & Refinery NPL Listing Support Document March 2011

A n observed release is established as follows:

• If the background concentration is not de tected (or is less than the detection limit), an observed release is established when the sample measurement equals or exceeds the sample quantitation limit.

• If the background concentration equals or exceeds the detection limit, an observed release is established when the sample measurement is 3 times or more above the background concentration.

Because cadmium and lead were found at detectable levels in the background sample as presented in section 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, the HRS requires that, to meet the criteria for analytical significance, these contaminants be present in a release sample at levels three times or greater than the respective background concentrations. As shown in section 3.20 of this support document, the background level for cadmium was 0.36 mg/kg and the release sample results for cadmi um ranged from 2.9 to 11.8 mg/kg. Not only do the release sample concentrations exceed the background level by more than 3 times, all but one of the release sample concentrations ex ceed the background by more than 10 times. For lead, the background level was 12.3 mg/kg and the release samples ranged from 18.9 to 168 mg/kg; three of four release sample concentrations clearly exceed the background level by more than 3 times.
In addition, although not included in the HRS evalua tion, other metals were also detected in release samples at levels that meet the criteria for significant increase in downstream sediment samples. In downstream sediment sample AGFSED2, copper and zinc were detected at concentrations of 472 and 2370 mg/kg, respectively. In sample AGFSED3, copper and zinc were detected at 129 and 1,060 mg/kg, respectively. These concentrations ar e more than three times greater than their related concentrations in the background sample AGFSED1—41 mg /kg copper and 99.2 mg/kg zinc (see HRS Reference 23, pages 190-192).
Regarding exceedances of water quality standards or sediment quality threshold concentrations, the HRS does not require that any standards or benchmarks be exceeded to document an observed release, as shown by HRS Sections 4.1.2.1.1 and 2.3, above. Section 3.7, Permitted Releases and Regulatory Limits, of this support document, addresses this topic in furt her detail. On this basis, the cadmium and lead concentrations presented on pages 39 and 40 of the HR S documentation record at proposal meet the HRS criteria for significant increase and are therefore suffici ent to identify an observed release by chemical analysis.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.


### 3.20.1.2 Releases in Other Aquatic Media

Comment : ARCO asserted that both surface water and fish tissue data collected from the Missouri River in the vicinity of the site do not support the establishment of an observed release. ARCO stated that the majority of the site-specific water quality data from sampling locations in the vicinity of the ACM facility do not indicate contamination of the water column. ARCO also stated that the Montana Department of Fish, Wildlife and Parks (MDFWP) Pollution Control Biologist, upon review of the 2004 Parametrix investigation, concluded that “the results indicate no obvious increase in metals in fish between the Black Eagle and Rainbow Dams.”

R esponse : Sediment samples were appropriately used in the documentation of an observed release by chemical analysis; other aquatic media, such as surface water and fish tissue samples, were not used in

53ACM Smelter & Refinery NPL Listing Support Document March 2011

establishing the observed release nor are they required. Furthermore, other sampling results not establishing observed releases do not negate th at other observed releases were found.

I n establishing an observed release by chem ical analysis, HRS Section 4.1.2.1.1, Observed release , states:

• Chemical analysis:

- Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substa nce(s) has increased significantly above the background concentration for the site for that type of sample (see section 2.3). [emphasis added]

. . . . .

- Some portion of the significant increase mu st be attributable to the site to establish the observed release, except: when the site itself consists of contaminated sediments with no iden tified source, no separate attribution is required.

- If an observed release can be established for a watershed, assign an observed release factor value of 550 to that watershed, enter this value in Table 4-1, and proceed to section 4.1.2.1.3.

As detailed further in section 3.20, Surface Water Like lihood of Release - Observ ed Release by Chemical Analysis, of this support document, pages 39-40 of th e HRS documentation record at proposal correctly established an observed release of ha zardous substances in Missouri Rive r sediments. As stated in HRS Section 4.1.2.1.1 , Observed Release , an observed release by chemical analysis is established in either surface water, benthic, or sediment samples; ther e is no requirement that an observed release be established in all three media. Thus, the HRS allows for an observed release to surface water to be documented by sediment samples alone.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.



### 3.20.1.3 Bias of Conclusions/Current Conditions

Comment : ARCO stated that EPA's conclusions regard ing metals concentrations in Missouri River sediment are biased to support the HRS score, and do not reflect data that describe current conditions. ARCO stated:
The water quality exceedances reported in Te tra Tech (AR Reference #10), from that limited distribution of sampling stations adj acent to or within the Site boundaries, were not confirmed by the more recent and far more widely distributed sampling transects and stations proximate to the Former GFR Site and reported in Parametrix (AR Reference #16).

R esponse : ARCO has not presented data that refute EPA’s identification of an observed release by chemical analysis. Further ARCO has not documented any improper bias in the data or in how the data were used by the Agency to perform the HRS evaluation.

54ACM Smelter & Refinery NPL Listing Support Document March 2011

I n establishing an observed release by chem ical analysis, HRS Section 4.1.2.1.1, Observed release , states:

• Chemical analysis:

- Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substan ce(s) has increased significantly above the background concentration for the site for that type of sample (see section 2.3).

. . . . .

- Some portion of the significant increase mu st be attributable to the site to establish the observed release, except: when the site itself consists of contaminated sediments with no iden tified source, no separate attribution is required.

- If an observed release can be established for a watershed, assign an observed release factor value of 550 to that watershed, enter this value in Table 4-1, and proceed to section 4.1.2.1.3.
The HRS documentation record at proposal stated on page 39:
Eight sediment samples from the Missouri River were collected by Tetra Tech EM, Inc for the EPA’s Expanded Site Inspection in Ap ril 2003. . . .Analyses of sediment samples collected during the April 2003 sampling event indicate that concentrations of arsenic, cadmium, copper, lead, manganese, and zinc are significantly above background concentrations in the Missouri River for a distance of 2.39 miles downstream of the probable point of entry (PPE) at the Black Eagle Dam (Ref. 18, p. 39, Figure 3). . . .
Seven release sediment samples were collected by START for the EPA Region 8 on July 14, 2009. All release samples were collected by boat from the north side of the Missouri River from beneath the water. A background sample (AGFSED1) was collected from the north bank of the Missouri River, downstream of the Montana Refining Company facility and upstream of the outfall North River Road 9th to 15th Street No. 3 (PPE#1) and the community of Black Eagle (Ref. 50, 1 page).
Data from the 2009 Site Assessment Expanded Site Inspection of the Missouri River sediments were used in the documentation of an observed release by chemical analysis. Thus, the more recent 2009 investigation (compared to the 200 4 Parametrix investigation cited by ARCO) of the Missouri River sediments was utilized in establishing the observed release by chemical analysis.
Although the 2003 Expanded Site Inspection performed by Tetra Tech found concentrations of various metals in Missouri River sediments, data from this study was not used in establishing the observed release by chemical analysis. Furthermore, HRS Section 4.1.2.1.1, Observed Release , states in part: “Establish an observed release to surface water for a watershed by demonstrating that the site has released a hazardous substance to the surface water in the waters hed” [emphasis added]. As discussed in Section 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, an observed release was established by de monstration that hazardous substances detected within release samples was significan tly greater than background levels. There is no requirement that the release is ongoing, nor is there specification of any tim eframe at all. ARCO’s assertion that the “water quality exceedances” reported in the 2003 Tetra Tech investigation were not confirmed by the more recent Parametrix investigation does not negate the sed iment concentrations detected in the more recent 2009 investigation used in establishing the observed release by chemical analysis.

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To the extent ARCO has asserted that EPA was bias ed in its conclusions regarding sediment metals concentrations, as shown above, EPA used the most recent data available in establishing observed releases at this site in a manner consistent with HRS requirements. If ARCO is alleging that the data used by EPA was not as comprehensive as that in other st udies, the HRS evaluation is a screening tool based on limited sampling and only indicates further investigation is necessary—further sampling is performed at the remedial investigation phase of the Superfund process (see section 3.6, Risk Associated with Site, of this support document).

This comment has no effect on the HRS site score or the decision to list the site on the NPL.



### 3.20.1.4 Downstream Contaminant Levels

Comment : ARCO stated that in 2004 it retained Parametr ix to conduct an investigation, which concluded that metals concentrations in sediment, surface water, and fish tissues at the ACM facility were generally comparable to upstream (background) concentrations.
ARCO stated that results of an MDEQ rive r sampling event conducted approximately 32 miles downstream from the ACM facility caused MDEQ to specu late that “both upriver and downriver soils and sediments may contain elevated metals levels.” ARCO stated that “the inves tigators specifically focused on....slag particles mixed with natura l river sands,” and that MDEQ's speculations were “[b]ased on these limited data.”

R esponse : The HRS evaluation of the observed release by chemical analysis appropriately compared an upgradient background sample to downgradient release samples. The Parametrix study, as presented in Reference 16 of ARCO’s comments, does not indicate any flaw in the evaluation of the analytical results used in the establishment of observed releases by chemical analysis.

I n establishing an observed release by chem ical analysis, HRS Section 4.1.2.1.1, Observed release , states:

• Chemical analysis:

- Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substance(s) has increased significantly above the background concentration for the site for that type of sample (see section 2.3).

. . . . .

- Some portion of the significant increase mu st be attributable to the site to establish the observed release, except: when the site itself consists of contaminated sediments with no identif ied source, no separate attribution is required.

- If an observed release can be established for a watershed, assign an observed release factor value of 550 to that waters hed, enter this value in Table 4-1, and proceed to section 4.1.2.1.3.

H RS Section 2.3, Likelihood of Release , specifies that the scorer "[u]se the criteria in Table 2-3 as the standard for determining analytical significance." HRS Table 2-3 states the following:

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A n observed release is established as follows:

• If the background concentration is not de tected (or is less than the detection limit), an observed release is established when the sample measurement equals or exceeds the sample quantitation limit.

• If the background concentration equals or exceeds the detection limit, an observed release is established when the sample measurement is 3 times or more above the background concentration.
As presented in section 3.20, Surface Water Likeli hood of Release - Observed Release by Chemical Analysis, of this support document, an observed re lease of hazardous substances was established in Missouri River sediments. The background sampl e (AGFSED1) was compared to the downstream release samples (AGFSED2, AGFSED3, AGFSED4, and AGFSED5). Concentrations of lead and/or cadmium were detected in the downstream release samples at levels greater than 3 times the detected levels of lead and cadmium in the upstream background sample.
ARCO’s assertion that the Parametrix investigation conclusion that metals conc entrations in sediment, surface water, and fish tissues at the ACM facility were generally comparable to upstream (background) concentrations, does not establish that the HRS evaluation improperly compared release samples to background samples. In fact, upon a comprehensive review of the Parametrix report, data gaps and inconsistencies have been identified in the Parametr ix report that may be the cause of the differences in results between that report and the 2009 ESI data used to establish an observed release by chemical analysis in this HRS evaluation.
For example, “background” sampling locations within the Parametrix evaluation were collected from areas that are not representative of background conditi ons for the site. As shown in ARCO Ref. 16, Figure 2-1, the two Parametrix “background” loca tions, identified as Area C and Area D, are located downstream of four of the five PPEs associated with Source 1 (see HRS documentation record Reference 50), indicating that these areas are influenced by cont aminated runoff from Source 1. If the background locations presented in the Parametrix investigation are actually subject to contamination from the site, this could explain why the adjacent and downstream sample locations are not significantly elevated above their background.
Further, with respect to Parametrix' sampling Area A, the long stretch of river designated as being adjacent to the facility property, 4 of the 7 sampl es collected from this area were from the opposite bank (south) of the river (ARCO Ref. 16, Figure 2-1). Of the remaining 3 samples, 2 were collected south of midstream, and only 1 sample of the 7 was collected from the north bank. This configuration of samples is not likely to be representative of the sources’ influence on the river, whereas samples collected nearer to or directly from the north bank would be more lik ely to be representative of the release from the ACM facility because of its proximity to the north shore. This conclusion is supported by the fact that the Parametrix investigation Area A north bank sediment sample (A41) has, in general, much higher concentrations of metals than the other Area A samples (ARCO Ref. 16, Table 3-2). This lack of sampling near the more highly contaminated north shor e was also highlighted by an EPA toxicologist in his assessment of the Parametrix report, as further discu ssed in section 3.6, Risk Associated with the Site, of this support document.
Based on these considerations, nothing in the Parame trix study results is found to cast doubt upon the analytical results used to support the HRS evaluation or the finding of an observed release by chemical analysis from the site.

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Regarding the MDEQ river sampling event cited by ARCO (ARCO Ref. 9), the event was conducted to obtain analytical data to determine the contaminant impact of slag waste transported downriver from the ACM site in a historic flood to an area including property known as the Witt ranch. A few years subsequent to the flood, the Witt ranch property was purchased by the then-owners of the ACM property. MDEQ's statement that it was possible that elevated me tals levels in sediments would be found upstream referred to the area on the Missouri River between the Witt ranch and the ACM site. In fact, the analytical data used in support of the HRS eval uation does document elevated levels of metals in sediments along a portion of that stretch of the ri ver, by meeting the significant increase requirements found in HRS Section 2.3, as demonstrated in sec tion 3.20 of this support document, Surface Water Likelihood of Release - Observed Release by Chemical Analysis.

These comments have no effect on the site score or the decision to list the site on the NPL.

3 .20.2 Surface Water Observed Release - Attribution

C omment : ARCO made several comments challenging the attribution of the releases in the Missouri River to the ACM site. ARCO’s comments on attribution dealt the following topics:

• Attribution of Releases to On-Site Sources

• Other Possible Off-Site Sources

• Permitted Storm Water Discharges

Response : An observed release has been appropriately attributed at least in part to the ACM site, as the observed releases are associated with on-site sources c ontaining hazardous substances at the site, and the significant increase of hazardous substances in the ob served releases is documented as not caused in total by off-site sources.

HRS Section 2.3, Likelihood of release , states:

Likelihood of release is a measure of the lik elihood that a waste has been or will be released to the environment. The likelihood of release factor category is assigned the maximum value of 550 for a migration path way whenever the criteria for an observed release are met for that pathway. If the crite ria for an observed release are met, do not evaluate potential to release for that pathway. When the criteria for an observed release are not met, evaluate potential to release for that pathway, with a maximum value of 500. The evaluation of potential to release varies by migration pathway (see sections 3, 4 and 6).
Establish an observed release either by dir ect observation of the release of a hazardous substance into the media being evaluated (fo r example, surface water) or by chemical analysis of samples appropriate to the pathway being evaluated (see sections 3, 4, and 6). The minimum standard to establish an observed release by chemical analysis is analytical evidence of a hazardous substance in the medi a significantly above the background level. Further, some portion of the release must be attributable to the site . [emphasis added]

HRS Section 4.1.2.1.1, Observed Release , specifies the following concerning attribution as it discusses documenting an observed release by chemical analysis:
Some portion of the significant increase must be attributable to the site to establish the observed release, except: when the site itself consists of contaminated sediments with no identified source, no separate attribution is required.

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T he HRS does not expressly contain specific inst ruction regarding the methodology for establishing attribution. However, consistent with the HRS Gu idance Manual, the Agency considers it is appropriate to establish attribution by first documenting that hazar dous substances are present at the site by showing that the contaminants are associated with the sources, and second, by documenting that not all of the increase in contaminant concentrations could have co me from off-site sources. This is established by demonstrating that the site at least partially contri butes to the significant increase of hazardous substances in the release.

T he Attribution section of the HRS documentation reco rd at proposal, beginning on page 41, discussed the hazardous substances detected in Source 1 and Source 2 areas during 2003, 2007, and 2008 investigations. It stated that in Black Eagle soils:

• The 2003 investigation found “elevated concentrations of antimony, arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, nickel, silver, sodium, and zinc” in one or more samples

• The 2007 investigation found “an area of arsenic, cadmium, and lead concentrations greater than three times background centered on eastern and central Black Eagle”

• The 2008 investigation found “[c]oncentrations of arsenic, cadmium, and lead elevated three times background . . . centered on the central and southwestern area of Black Eagle.”
Page 41 of the HRS documentation record at proposal (Attribution section) also indicated that the soil contamination is localized in noting that during the 2007 investigation residences on both the north and south sides of the Missouri River adjacent to the s ite were sampled and “[o]f the eleven residences sampled in Great Falls on the south side of the Misso uri River in August of 2007 only one residence had an elevated concentration of arsenic.”

The Attribution section continues to state that th e 2003 investigation sampled several sources on the facility property, finding contaminants including an timony, arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, nickel, selenium, silver, and zinc. This investigation also noted a lack of containment in the area, and identified culverts that drain local surface water into the Missouri River.
The HRS documentation record at proposal, on page 41, added that two of these source contaminants— cadmium and lead—were detected in downstream river sediments:
Sediment samples from the north bank of the Mi ssouri River were collected by boat on July 14, 2009 (Ref. 23, p. 16). . . . Cadmium and lead elevated three times background was detected for a total distance of 2.39 miles downstream of PPE #1 (Ref. 23, p. 23; Ref. 50).
The Attribution section on page 41 of the HRS documentation record at proposal proceeded by referring to the observed release by direct observation, as fu rther proof that the contaminants in the observed release by chemical analysis are attributable to the site.
The Boston and Montana smelter [former owner/ operator of the facility at the ACM site] routinely dumped large quantities of granulated slag and tailings into the Missouri River (Ref. 6, p. 42; Ref. 15, p. 1). In 1907, the Boston and Montana Consolidated Copper and Silver Mining Company estimated that the fac ility could receive up to 4800 tons a day of ore and changing the disposal of slag to railcar would significantly increase rail traffic (Ref. 14, p. 1). The Montana Department of Environmental Quality (Montana DEQ) estimates that between 27,500,000 and 31,000 ,000 cubic yards of waste were dumped into the Missouri River between 1893 and 1915 until onsite containment was instituted in 1915. Montana DEQ also reports that it is possible that dumping of waste into the

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Missouri River continued at a reduced rate after 1915 until the facility closed in the 1970s (Ref. 16, p. 1). So much smelter waste was dumped into the Missouri River between 1893 and 1915 that the management and le gal department of the Boston and Montana Company were concerned that the Federal Government would intervene and enjoin the deposition of tailings into the Missouri River by virtue of the constitutional provisions requiring the federal government to maintain na vigable streams (Ref. 15, pp. 1 and 2). In 1884 Judge Lorenzo Sawyer of the United States Circuit Court in San Francisco had granted a perpetual injunction against hydrau lic mining in California’s American River on these very grounds (Ref. 6, p. 38).

Numerous complaints were filed by proper ty owners and communities along the Missouri River downstream of the ACM Smelter and Refi nery of water quality issues and slag and smelter waste clogging the Missouri River (Ref. 6, p. 42; Ref. 29, p. 8A). These complaints increased after the 1908 flood of the Missouri River inundated the community of Fort Benton, 40 miles downstream of th e Boston and Montana Smelter with smelter waste (Ref. 6, p. 38; Ref. 29, p. 8A). A comp any analysis of slime (very fine grained slag and tailings, which tended to float in the Missouri River) taken above Rainbow Falls Dam in 1914 indicated that the slime containe d between .55 and 1.1 percent copper (Ref. 17, p. 1). The Boston and Montana Company sought to buy off disgruntled property owners and placate communities by providing alternate domestic water sources (Ref. 6, pp. 49-51).

Cindery slag material was also currently obser ved to be in the sediments of the Missouri River adjacent to the ACM Smelter and Refinery facility (Ref. 23, p. 57 and 202, Ref, 53 p. 1; Ref. 54, p. 1).
Finally, the HRS documentation record at proposal identified a nearby facility as another potential contributor to the contamination in the Missouri River, and concluded that the contamination is still at least partially attributable to the ACM site:

The former Montana Silver Smelter site is located on the south bank of the Missouri River in Giant Springs State Park, approxim ately 2.85 miles east-northeast of the 15 th Street Bridge (Ref. 4, 2 pages; Ref. 50, p. 1). The Montana Silver Smelter was constructed in 1888 by Great Falls city father , Paris Gibson (Ref. 56, p. 2). The smelter had two small stacks. Many of the structures at Giant Springs State Park and Giant Springs State Fish Hatchery were built on mineral processing wastes that contained extremely high levels of arsenic and lead. This property was cleaned up by the Montana Mine Waste Cleanup Bureau’s Abandoned Mine Lands Program in 2004 (Ref. 57, pp. 9- 11). The Montana Refining Company facility is located upstream of the unincorporated community of Black Eagle on the north bank of the Missouri River (Ref. 50, 1 map). The Montana Refining Company produces various grades of gasoline, diesel, jet fuel, distillates, and high grade asphalt products (Ref. 65, 1 page.). The facility has an operable capacity of 9,500 barrels per calendar day ranking the facility as number 132 of the 143 U.S. Refineries (Ref. 66, p. 4).

Both of the operations could potentially have contributed to the contamination documented in the Missouri River; however, the contamination found in the Missouri River sediments is at least partially attri butable to the sources found on the ACM Smelter and Refinery property because of the evidence presented above.

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T hus, the HRS documentation record at proposal docum ents that the released substances are associated with the ACM site as they are in and associated with on-site sources and at least part of the contamination in the river came from the site based on direct observati on of slag being dumped into the river. Thus, the significant increase in contaminant levels has been appr opriately attributed at least in part to the ACM site.

A RCO’s specific comments on this subject are addres sed in the following subsections of this support document:

• 3.20.2.1 Attribution of Releases to On-Site Sources

• 3.20.2.2 Other Possible Off-Site Sources

• 3.20.2.3 Permitted Storm Water Discharges



### 3.20.2.1 Attribution of Releases to On-Site Sources

Comment : ARCO challenged the attribution of the signi ficant increase in contaminant levels to the site sources because the observed release substances were not shown to be migrating from the scored sources. ARCO maintained that EPA provided no information, su ch as analysis of metals concentrations in runoff from Sources 1 and 2, to support attribution of metals concentrations in sediment to the sources. ARCO, quoting from the HRS Guidance Manual, stated:
An observed release is established for the . . . surface water pathway. . . either by chemical analysis or by direct observation. The minimum requirements for establishing an observed release by chemical analysis are analytical data demonstrating the presence of a hazardous substance in the medium significantly above background level, and information that some portion of that increase is attributable to the site . The minimum criterion for establishing an observed release by direct observation is evidence that the hazardous substance was placed into or has b een seen entering the medium. [emphasis in comment]

ARCO also stated that EPA had “no information whatsoever” to document that some portion of the significant increase was attributable to the sour ces at the facility (Source 1 or Source 2). ARCO concluded that no observed release by chemical anal ysis had been established, and that the pathway should therefore be scored on potential to release.
ARCO noted that storm water from Black Eagle is c onveyed to the Missouri River by approximately five culverts and stated that there are no known water quality sample results for any of these culverts.
ARCO also pointed to a sample identified as representa tive of background but downgradient of one of the five culverts associated with Source 1 (Black Eagle so ils) and argued that, if this sample (AGFSED-1) is background, then the PPE and associated drainage ar ea should be removed from consideration in the surface water pathway. ARCO also maintained that, if AGFSED-1 is not background, then the results of the sample “show that metals are not transporte d from soil underlying the Black Eagle residential area into the Missouri River.”
Further, ARCO asserted that samples AGFSED-2 and AGFSED-3, containing the highest relative concentrations of metals detected in the release sed iment samples, were collected from locations that are upstream of the East Ditch Dump (Source 2) and PPE #6, and concluded that “the East Ditch Dump cannot be the source of contamination for the surf ace water migration pathway EPA describes for this reach of the Missouri River.”

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Response : The HRS documentation record at proposal presented and documented adequate rationale for documenting that at least part of the significant incr ease in contaminant concentrations is due to release from the ACM site. Nothing in ARCO’s comments demonstrates this not to be the case.
As excerpted in section 3.20.2, Surface Water Observed Release - Attribution, of this support document, HRS Section 4.1.2.1.1 specifies that “Some portion of the significant increase must be attributable to the site . . .” [Emphasis added]. As such, attribution fo cuses on the site, as opposed to individual sources. Notwithstanding, to clarify the extent of the s ite, two new sources have been added to the HRS documentation record at promulgation, and the Attr ibution discussion has been revised to demonstrate how the hazardous substances in the observed releas e to surface water are associated with these new sources. Specifically, section 3.13, Additional Source s, of this support document discusses that sampling has revealed arsenic, cadmium, copper, lead, and zinc in flue dust at the former stack and also in Source 4, slag waste. As such, the contaminants meeti ng the significant increase criteria (as demonstrated in section 3.20.1.1, Contaminant Levels Below Regulator y Limits, of this support document) are associated with these two sources.

Further, as cited in section 3.20.2, Surface Water Ob served Release - Attribu tion, of this support document, the HRS documentation record at propos al documented an observed release by direct observation. This is discussed in further detail in section 3.19.1, Documentati on of Observed Release by Direct Observation, of this support document. Do cumenting the direct deposition of site-related waste into surface water demonstrates that the site contributed to the significant increase in the contamination in the river. This is sufficient to conclusively show attribution at least in part to the site.
It is significant to note that, as shown above by the citation of HRS Sections 2. 3 and 4.1.2.1.1 in section 3.20.2, Surface Water Observed Release – Attribut ion, of this support document, the HRS does not require documenting the observation of migration of contaminants from the sources overland and into surface water to establish an observed release by chemical analysis; therefore, it is not necessary to establish contaminant results in the runoff as suggested by ARCO's comments.
Regarding ARCO's assertion that the PPE (PPE#1) and associated drainage area upstream of the sediment background sample should be removed from consideration in the surface water pathway, the results of the background sample do not demonstrate that the most ups tream outfall is an invalid probable point of entry or that hazardous substances are not being conveye d through PPE#1. Because hazardous substances were lower in the background sample than those found in the release samples, this does not establish that hazardous substances are not conveyed through PPE #1 to surface water. The background sediment sample, while downstream of PPE#1, is still upstream of the other four outfalls from Source 1. Thus, sample AGFSED1 still acts as an appropriate sediment background in establishing an observed release. See section 3.20.3, Adequacy of Background, of th is response document, for further discussion on the adequacy of background used in establishing an observed release.
EPA concurs that sediment samples AGFSED-2 a nd AGFSED-3 are upstream of Source 2 and PPE #6; however, these samples are downstream of four PPEs from Source 1, as well as being downstream of the newly added sources (Source 3, facility air emissions, and Source 4, the slag waste). Therefore, these two sediment sample locations are expected to be impact ed by three sources (Sources 1, 3, and 4). And, as explained in section 3.18, Description of Pathway, of this support document, observed release sample AGFSED5 is downstream of PPE #6 an d, while it may be affected by contamination migrating from Source 1, it is also likely to be affected by contaminants migrating from Source 2.
Thus, as explained in section 3.20. 2, Surface Water Observed Release – Attribution, of this support document, the significant increases in contaminants in sediment in the Missouri River were reasonably attributed to the site in accordance with the HR S at proposal. And, two additional sources (Source 3, facility air emissions, and Source 4, slag waste) adde d to the site at promulgation further clarify the

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association between observed release sample hazardous substances and the site. This rationale is consistent with the suggestions in the HRS Guidance Manual cited by ARCO.

This comment has no effect on the site score or the decision to list the site on the NPL.



### 3.20.2.2 Other Possible Off-Site Sources

Comment : ARCO challenged the attribution of the obser ved release to the site on the basis that many other sources of contamination existed upstream and downstream of the site. ARCO argued that discharges reaching the river from Black Eagle and Great Falls may contain metals in concentrations typical of urban and industrial areas, and that th ere are numerous storm water and industrial discharge sources both upstream and downstream of the site. ARCO asserted that EPA omitted or did not adequately describe these sources and their respectiv e contributions of hazardous substances to the Missouri River.

ARCO named a number of potential contributors, via disch arge and/or runoff, to the contamination in the Missouri River: Northwestern Energy, Pacific St eel and Recycling, Montana Refining Company, the City of Great Falls, and Cascade County.
ARCO also noted that besides storm water and industrial discharge sources, other human uses of the river including recreational and agricultural use contributed to water quality problems in the Missouri River, such as “sedimentation/siltation and nutrient enri chment...and occasional metal/metalloid listings.” ARCO presented a table providing Missouri River reaches (downgradient or in the vicinity of the ACM facility) that were documented with exceedances of sp ecific Clean Water Act Section provisions (such as metals, nitrates, sedimentation, etc.) This table is shown below:

Missouri River Regulated Reach Description Listed Parameter(s)
Headwaters to Toston Arsenic, total nitrogen, sedimentation/siltation Toston Dam to Canyon Ferry Reservoir Cadmium, copper, lead, sedimentation/siltation Canyon Ferry Reservoir Dam to Hauser Lake Nitrogen (nitrate), dissolved oxygen, total phosphorus Total phosphorus, total Kjehldahl nitrogen, sedimentation/siltation Holter Dam to Little Prickly Pear Creek

Sun River to Rainbow Dam (includes Former GFR Site at Great Falls) Total chromium, mercury, pentachlorobenzene, selenium, sedimentation/siltation, suspended and bedload solids, turbidity Rainbow Dam to Morony Dam Arsenic, copper, pentachlorobenzene, sedimentation/siltation, water temperature, turbidity

Response : An observed release has been appropriately attri buted to the ACM site, and at least part of thesignificant increase of hazardous substances in the observed releases is documented as not caused by offsite sources.

As stated in HRS Section 4.1.2.1.1, Observed Release , “[s]ome portion of the significant increase must be attributable to the site to establish the observed release.”

A s presented on page 40 of the HRS documentation record at proposal, an observed release by chemical analysis was documented that included the compar ison of an upgradient background sample to downgradient release samples. Potential off-site ups tream sources of contamination have been screened out by this background sample because it would be affected by these sources in the same way that observed release samples would. A significant in crease of lead and cadmium was detected in the downstream release samples at levels greater than thr ee times the levels of lead and cadmium detected in

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the upstream background sample. Thus, samples taken in the Missouri River adjacent to and downgradient of the ACM site show contaminant levels significantly hi gher than the levels established upstream. Thus, the contributions of upstream o ff-site sources to the surface water migration pathway have been documented as not contributing to the significant increase of hazardous substances documented in the observed release.

R egarding the table of water quality exceedances pr esented by ARCO (see Reference 68 of the HRS documentation record at promulgation):

• The segment of the Missouri River that includes the ACM site (Sun River to Rainbow Dam), includes portions of the river upstream of the site and includes exceedances for some metals (chromium, mercury, and selenium). However, th ese metals are not associated with any source or observed release at the site, and would have no affect on the observed releases scored or on their attribution to the site. (See Section 2.2.1, Source Identification, and Section 4.1.2.1, Likelihood of Release, of the HRS documentation record at promulgation for a discussion of the hazardous substances associated with site sources and the observed releases.)

• The segment of the Missouri River downstream of the ACM site (Rainbow Dam to Morony Dam) includes exceedances of arsenic and copper—two site-related hazardous substances (arsenic was detected in Source 1 and Source 2; copper was associated with the observed release by direct observation). These may have resulted from site-related releases. Releases in this area were not scored in the HRS evaluation.

• In the segments of the Missouri River upstream of the ACM site there are detections of some metals which are also scored at this site (arsen ic, cadmium, copper, and lead). However, these exceedances are located in the two most upstream segments (Headwaters to Toston and Toston Dam to Canyon Ferry Reservoir). Two more river segments exist between those and the site in which no metals exceedances are noted (Canyon Ferry Reservoir Dam to Hauser Lake and Holter Dam to Little Prickly Pear Creek). There are well over 100 miles between Canyon Ferry Reservoir and the ACM site in which no metals exceedances are shown; it is therefore highly unlikely that the detected metals in the extreme upstream areas of the river have any effect on the sediments at this site or on the observed releases es tablished and attribution to the site. And, even if these distant upstream contaminants could affect the ACM site, as discussed above, the background sample used in establishing the ob served release (located downstream of these locations ) would screen out their effects, as any contamination coming from upstream would have impacted the background sample.

Therefore, there is no indication that off-site sources affect the attribution of observed releases to the site.
ARCO’s assertion that EPA did not adequately addr ess off-site sources is unfounded. As quoted above in section 3.20.2, Surface Water Observed Release – At tribution, of this support document, the HRS documentation record at proposal identifies and discusses other possible off-site contributors of hazardous substances that could have reasonably affected th e ACM site (Montana Silver Smelter and Montana Refining Company). At least part of the contaminati on in the river is documented to have come from the ACM site, and the significant increase in contaminant levels has been appropriately attributed at least in part to the site.

This comment has no effect on the site score or the decision to list the site on the NPL.

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3.20.2.3 Permitted Storm Water Discharges

Comment : ARCO argued against attribution of the obser ved release to any recent runoff conditions on the basis that ARCO’s current st orm water discharges are permitted. ARCO maintained that it has expended considerable effort to keep storm water fro m running across reclaimed areas. ARCO stated that its storm water control system was designed to direct storm water to the Missouri River at two existing, permitted discharge locations. ARCO also stated that, after the completion of its capping and revegetation activities, with the exception of copper and zinc, storm water sampling and reporting continued to comply with the gene ral storm water discharge permit.
Response : An observed release has been appropriately a ttributed to the ACM site, and whether or not hazardous substances in the observed releases are assoc iated with permitted discharges is not considered in the HRS evaluation. As discussed in section 3. 6, Permitted Releases and Regulatory Limits, of this support document, emission or effluent limits do not necessarily represent levels which cause no harm to public health or the environment, and are not considered in the HRS scoring process.
Further, over the multiple decades of operations at th e site, unpermitted discharges to surface water prior to regulatory oversight would not be addressed in later permits. ARCO’s efforts do not address the releases that have occurred during the long operational history at the site.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.
3.20.3 Surface Water Observed Release - Adequacy of Background

Comment : ARCO argued against the adequacy of the background sediment sample, challenging that its location was downstream of PPE #1 associated with th e Black Eagle community. ARCO alleged that, if the sample were not considered a background on this basis, then its results do not indicate contaminant transport from the Black Eagle residential area into the river.
Response : Background sample AGFSED1, although collect ed a short distance downstream of PPE#1, provides an adequate background level for compar ison to the downstream sediment samples collected from the Missouri River.

T he HRS does not identify requirements or define conditions for establishing background levels of contaminants. The HRS addresses background only in the context of identifying an observed release of a hazardous substance to the environment by chemical analysis. HRS Section 4.1.2.1.1, Observed release
, states in relevant part:

E stablish an observed release to surface water for a watershed by demonstrating that the site has release a hazardous subs tance to the surface water in the watershed. Base this demonstration on either:

• Direct observation: . . .

• Chemical analysis:

- Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substan ce(s) has increased significantly above the background concentration for the site for that type of sample (see section 2.3)

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- Limit comparisons to similar types of samples and background concentrations—for example compare surface water samples to surface water background concentrations.
The background sample (AGFSED1) is used as a reference point in the HRS evaluation to establish that a significant increase in contaminant levels in the release samples has occurred, and to screen out upgradient sources of contamination. Page 39 of the HRS documentation record at proposal documents that all samples collected during the 2003 Expanded S ite Inspection were collected from the north side of the Missouri River, which consisted of “medium to medium fine grained tan particles.” A significant increase of hazardous substances between the backgr ound and release samples was documented. The release samples collected during the 2009 sampling ev ent, upon which the observed release by chemical analysis was documented in this HRS evaluation, were collected from the same area at the north side of the Missouri River and showed significant incr ease of hazardous substances between background and release samples.
The background sample AGFSED1 was located not only up stream of the majority of Source 1's influence (the other four outfalls), but also upstream of the influence of Source 2 (East Ditch Dump) and Source 4 (slag waste), as shown in Reference 18, pages 38-39, and Reference 50 of the HRS documentation record at proposal.
While levels of some metals in the backgroun d sample may be a result of runoff from PPE #1, a significant increase of hazardous substances is still documented between the ups tream background sample and the downstream release samples. The concentra tions of source-related me tals in the downstream (observed release) samples still clearly meet the HR S significant increase criteria for observed release by chemical analysis (i.e., more than 3 times greater than the background levels), as discussed in section 3.20.1.1, Contamination Levels Below Regulat ory Limits, of this support document.
Therefore, sample AGFSED1 is a valid background sa mple, and its location acts as a reference point to establish significant increases in downstream observe d release samples and to screen out upgradient sources of contamination.

This comment has no effect on the site score or the decision to list the site on the NPL.



### 3.21 Potential to Release

Comment : ARCO maintained that, since (as it previously argued) no observed release had been established, the surface water pathway should be scored based on "likelihood of a release," or potential to release, as specified by the HRS. ARCO sugg ested potential to release scores from both sources identified in the HRS documentation record at proposal.
Response : The potential to release was not scored in eval uation of likelihood of release, as an observed release was appropriately evaluated from the site to the surface water pathway according to HRS requirements.

HRS Section 2.3, Likelihood of release, states:

Likelihood of release is a measure of the lik elihood that a waste has been or will be released to the environment. The likelihood of release factor category is assigned the maximum value of 550 for a migration path way whenever the criteria for an observed release are met for that pathway . If the criteria for an observed release are met, do not evaluate potential to release for that pathway. [emphasis added]

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The HRS documentation record at proposal section 4.1.2. 1.2, Potential to Release, stated that “Potential to Release by overland flow was not evaluated because an observed release was established.”
As established in sections 3.19, Observed Release by Direct Observation, and 3.20, Observed Release by Chemical Analysis, of this support document, an ob served release by direct observation and an observed release by chemical analysis have been established. As such, and according to the HRS directions stating to not evaluate potential to release for that pathway if an observed release is established, the potential to release was not evaluated for the surface water migration pathway.

This comment has no effect on the HRS score or the decision to list the site on the NPL.



### Surface Water – Drin king Water Threat

The surface water drinking water threat was not included in the scoring of the surface water migration pathway because, as stated on page 38 of the HRS doc umentation record at proposal, “[n]o information was found regarding existing surface water intakes on the 15-mile downstream segment.” 

### 3.22 Surface Water - Drinking Water Threat

Comment : ARCO commented that, while the HRS documen tation record at proposal stated that no information was found regarding existing surface wate r intakes on the 15-mile downstream segment, the 2009 ESI report states that there are two surface water intakes located upstream of the site supplying the city of Great Falls (and Black Eagle, which purchases its water from Great Falls).
Response : The surface water drinking water threat was not scored in the HRS evaluation of the site, and thus the consideration of intakes was not included in any scoring.

HRS Section 4.2.2, Drinking water threat , provides instruction on scoring the drinking water threat in the surface water migration pathway. HRS Section 4.1.2.3, Drinking water threat-targets , states to:
[e]valuate the targets factor category for each watershed based on three factors: nearest intake, population, and resources.
To evaluate the nearest intake and population factors, determine whether the target surface water intakes are subject to actual or potential contamination as specified in section 4.1.1.2. Use either an observed rel ease based on direct observation at the intake or the exposure concentrations from samples (or comparable samples) taken at or beyond the intake to make this determinati on (see section 4.1.2.1.1). The exposure concentrations for a sample (that is, surface water, benthic, or sediment sample) consist of the concentrations of those hazardous s ubstances present that are significantly above background levels and attributable at least in part to the site (that is, those hazardous substance concentrations that meet the criteria for an observed release).

HRS Section 4.1.1.2, Target distance limit , states that:

T he target distance limit defines the maximum distance over which targets are considered in evaluating the site. Determine a separate target distance limit for each watershed as follows:

. . .

• If there is an observed release from the site to the surface water in the watershed that is based on sampling, begin measuring the target distance limit for the

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watershed at the probable point of entry; extend the target distance limit either for 15 miles along the surface water or to the most distant sample point that meets the criteria for an observed release to that watershed, whichever is greater.
[i]n evaluating the site, include only su rface water targets (for example, intakes
, fisheries, sensitive environments) t hat are within or contiguous to the hazardous substance migration path and located, pa rtially or wholly, at or between the probable point of entry and the target di stance limit applicable to the watershed

. [emphasis added]

A lso, HRS Section 4.1.1.1, Definition of hazardous substance migration path for overland/flood migration component , states the following:

T he hazardous substance migration path includes both the overland segment and the inwater segment that hazardous substances would take as they migrate away from sources at the site:

• Begin the overland segment at a source and proceed downgradient to the probable point of entry to surface water.

• Begin the in-water segment at this probable point of entry.

- For rivers, continue the in-water segment in the direction of flow (including any tidal flows) for the distance estab lished by the target distance limit (see section 4.1.1.2). [emphasis added]
Therefore, in a river, only intakes downstream of the probable point of entry and within the target distance limit are evaluated in the drinking water threat of the surface water migration pathway.
The HRS documentation record at proposal stated on page 38 that “[n]o information was found regarding existing surface water intakes on the 15-mile downs tream segment.” The intakes discussed by the commenter are upstream of the site and not located within the target distance limit and would not be available for evaluation if the drinking water threat were scored.
If, during the course of subsequent investigations it is determined that hazardous substances released from the ACM site are impacting surface water drinking wate r supplies, that threat will be evaluated during a separate stage of the Superfund process. Furthe rmore, as discussed in section 3.11, Scoring Other Pathways, of this support document, EPA need not score all pathways in an HRS evaluation if the site score is above 28.50 based on tho se pathways already evaluated.

This comment has no effect on the HRS score or the decision to list the site on the NPL.



### Surface Water – Human Food Chain Threat

The HRS documentation record at proposal identifie d an active fishery within the zone of actual contamination in the Missouri river established by th e observed releases in sediment samples. Based on hazardous substances—arsenic, cadmium, lead, mang anese, and zinc—available to migrate to surface water from Sources 1 and 2, this fishery was determined to be subject to Level II concentrations, due to a release of a contaminant (cadmium) with a bioconcentra tion factor of greater than or equal to 500. The food chain individual, level II concentrations, and pot ential human food chain contamination targets were scored as described in pages 43-46 of the HRS documentation record at proposal.

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### 3.23 Surface Water - Human Food Ch ain Threat Waste Characteristics

C omment : ARCO commented on EPA's assigned values for both the toxicity/persistence/ bioaccumulation factor and the hazardous waste quan tity value, and as part of its comments assigned alternative, lower values than those in th e HRS documentation record at proposal.

S pecific comments are addressed in the follo wing subsections of this support document:

• 3.23.1 Toxicity/Persistence/Bioaccumulation

• 3.23.2 Human Food Chain Threat Waste Quantity

• 3.23.3 Human Food Chain Threat Waste Characteristics
3.23.1 Toxicity/Persistence/Bioaccumulation

Comment : ARCO challenged EPA's use of manganese to assign the toxicity/persistence/bioaccumulation values for the human food chain threat, citing to the HRS Guidance as its reason, ARCO stated the substance(s) used should be present at concentrations greater than three times background. ARCO instead assigned the value for cadmium, which has a lo wer toxicity/persistence/bioaccumulation value for the human food chain threat than assigned in the EPA HRS scoring.
Response : The toxicity/persistence/bioaccumulation va lues for the human food chain threat were correctly assigned in the HRS documentation record at proposal. The HRS does not require that the hazardous substance used to determine the surface water pathway toxicity/persistence/bioaccumulation factor value meet the criteria for an observed rel ease by chemical analysis. Rather, it specifies the evaluation of toxicity/persistence/bioaccumulation factor values for any hazardous substance meeting observed release criteria and any hazardous substance associated w ith a source whose containment value is greater than zero. Furthermore, as explained below and in section 3.15.2, Association of Hazardous Substances with Source 2, of this support document, the HRS does not require the substance be present at concentrations significantly above b ackground (e.g., three times) to be associated with a non-soil source, to be used in this calculation if it is otherwise associated with a source, nor does the HRS Guidance Manual suggest that this be demonstrated. In th is case the manganese is associated with Source 2, a landfill.

HRS Section 4.1.3.2, Human food chain threat-waste characteristics , states:

Evaluate the waste characteristics factor category for each watershed based on two factors: toxicity/persistence/bioaccumulation and hazardous waste quantity.

HRS Section 4.1.3.2.1, Toxicity/persistence/bioaccumulation , then directs to:
Evaluate all those hazardous substances eligible to be evaluated for toxicity/persistence in the drinking water threat for the watershed (see section 4.1.2.2).

T he HRS sets forth the procedures for each factor in Section 4.1.2.2, Drinking water threat-waste characteristics , which states:

E valuate the waste characteristics factor category for each watershed based on two factors: toxicity/persistence and hazardous waste quantity. Evaluate only those hazardous substances that are available to migr ate from the sources at the site to surface water in the watershed via the overland/flood hazardous substance migration path for the watershed (see section 4.1.1.1). Such hazardous substances include:

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• Hazardous substances that meet the criteria for an observed release to surface water in the watershed

• All hazardous substances associated with the source that has a surface water containment factor value greater than 0 for the watershed...

R egarding bioaccumulation, HRS Section 4.1.3.2.1.3, Bioaccumulation potential , states to:

U se the following data hierarchy to assign a bioaccumulation potential factor value to each hazardous substance:

• Bioconcentration factor (BCF) data.

• Logarithm of the n-octanol-water partition coefficient (log K ow ) data.

• Water solubility data.

A ssign a bioaccumulation potential factor value to each hazardous substance from table 4–15.

I f BCF data are available for any aquatic hum an food chain organism for the substance being evaluated, assign the bioaccumulation po tential factor value to the hazardous substance as follows:

• If BCF data are available for both fresh water and salt water for the hazardous substance, use the BCF data that correspond to the type of water body (that is, fresh water or salt water) in which the fisheries are located to assign the bioaccumulation potential factor value to the hazardous substance.

• If, however, some of the fisheries being ev aluated are in fresh water and some are in salt water, or if any are in brackish water, use the BCF data that yield the higher factor value to assign the bioaccumulation potential factor value to the hazardous substance.

• If BCF data are available for either fresh water or salt water, but not for both, use the available BCF data to assign the bioaccumulation potential factor value to the hazardous substance.
If BCF data are not available for the hazardous substance, use log K ow data to assign a bioaccumulation potential factor value to organic substances, but not to inorganic substances. If BCF data are not available, and if either log K ow data are not available, the log K ow is available but exceeds 6.0, or the substa nce is an inorganic substance, use water solubility data to assign a bioaccumulation potential factor value.
To determine the toxicity/persistence/bioaccumula tion factor value, HRS Section 4.1.3.2.1.4, Calculation of toxicity/persistence/bioaccumulation factor value , states:
Assign each hazardous substance a toxicity/persistence factor value from Table 4-12, based on the values assigned to the hazardous s ubstance for the toxicity and persistence factors. Then assign each hazardous substance a toxicity/persistence/bioaccumulation factor value from Table 4-16, based on th e values assigned for the toxicity/persistence and bioaccumulation potential factors. Use the hazardous substance with the highest toxicity/persistence/bioaccumulation factor value for the watershed to assign the value to this factor.

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Page 15 of the HRS documentation record at proposal states that according to a 2003 Expanded Site Inspection for Source 1, “elevated concentrations of antimony, arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, nickel, silv er, sodium, and zinc were found in one or more samples.” Also, Section 2.2.2, Hazardous substa nces associated with the source, of the HRS documentation record at proposal identifies arsenic, cadmium, lead, manganese, and zinc associated with Source 2. Sections 2.2.1.3 and 2.2.3, Hazardous substances available to a pathway, of the HRS documentation record at proposal, state that Sources 1 and 2 were assigned a containment factor value of 10 for the release via overland migration and/or flood to the surface water pathway. Therefore, manganese was appropriately evaluated as a hazardous substance for the human food chain threat in the surface water pathway.
Section 4.1.3.2.1, Toxicity/Persistence/Bioaccumula tion, of the HRS documentation record at proposal listed the toxicity/persistence/bioaccumulation va lues for manganese and other eligible hazardous substances for the human food chain threat in the su rface water pathway, and provides the citation to the source of the assigned values. In accordance with HRS Section 4.1.3.2.1.4, Calculation of toxicity/persistence/bioaccu mulation factor value , the highest toxicity/persistence/bioaccumulation factor value of 5 x 10 8 was selected - that for manganese.

This comment has no effect on the site score or the decision to list the site on the NPL.
3.23.2 Human Food Chain Threat Waste Quantity

Comment : ARCO disputed the Human Food Chain threat hazardous waste quantity value. Specifically, it included in its evaluation of this category value only the East Ditch Dump source (Source 2). ARCO stated that the source waste quantity value for the East Ditch Dump should be 0.44, yielding an assigned food chain threat waste quantity of 1.
Response : The human food chain threat pathway waste quantity value was calculated correctly. The source hazardous waste quantity values for Source 2 (East Ditch Dump), Source 1 (Black Eagle residential soil), and Source 4 (slag waste) are each assigned a value of “> 0”, as detailed in sections 3.15.3, Source 2, 3.14.5, Source 1, a nd 3.13, Addition of Sources, respectiv ely, of this support document. However, as explained below, the human food chain threat hazardous waste quantity factor value remains 100 as it was assigned in the HRS documentation record at proposal. Note that as discussed in section 3.13, Additional Sources, of this support document, Source 3 (facility air emissions) is not included in the waste quantity evaluation of the surface water pathway.

HRS Section 2.4.2.1, Source hazardous waste quantity , states:
For each of the three migration pathways, assign a source hazardous waste quantity value to each source (including the unallocated source) having a containment factor value greater than 0 for the pathway being evaluated.

As established in section 2.2.2 of the HRS documentati on record at promulgation, Sources 1, 2, and 4 all have surface water containment factor values “greater than 0”, and are therefore applicable to the waste quantity evaluation for the surface water migration pathway.

HRS Section 2.4.2.2, Calculation of hazardous waste quantity factor value , states:

S um the source hazardous waste quantity va lues assigned to all sources (including the unallocated source) or areas of observed cont amination for the pathway being evaluated and round this sum to the nearest integer, ex cept: if the sum is greater than 0, but less than 1, round it to 1. Based on this value, select a hazardous waste quantity factor value for the pathway from table 2–6.

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For a migration pathway, if the hazardous c onstituent quantity is adequately determined (see section 2.4.2.1.1) for all sources (or all portions of sources and releases remaining after a removal action), assign the value fro m table 2–6 as the hazardous waste quantity factor value for the pathway. If the hazar dous constituent quantity is not adequately determined for one or more sources (or one or more portions of sources or releases remaining after a removal action) assign a factor value as follows:

• If any target for that migration pathway is subject to Level I or Level II concentrations (see section 2.5), assign either the value from table 2–6 or a value of 100, whichever is greater, as the hazardous waste quantity factor value for that pathway.
Therefore, the source hazardous waste quantity values are added together to obtain the pathway waste quantity. Footnote B of Table 2-6 of the HRS stat es that “[f]or the pathway, if hazardous constituent quantity is not adequately determin ed, assign a value as specified in th e text; do not assign the value of 1.”

As discussed in sections 3.14.5, Source 1 Hazardous Waste Quantity, and 3.15.4, Source 2 Hazardous Waste Quantity, of this support document, Sources 1 and 2 were both assigned a hazardous waste quantity value of “greater than 0” at promulgation. Add itionally, the hazardous waste quantity value for Source 4 was appropriately evaluated as “greater than 0, ” as discussed on pages 44-45 of the HRS documentation record at promulgation and as explained in section 3.13, Additional Sources, of this support document.
As the hazardous constituent quantity was not adequate ly determined for any site source, a value was assigned to the pathway hazardous waste quantity as direct ed from the text of HRS Section 2.4.2.2, which directs the scorer to assign a value for the pathwa y hazardous waste quantity from Table 2–6 of the HRS, or a value of 100, whichever is greater, if any target for that migration pathway is subject to Level I or Level II concentrations. Because the presence of Level II targets has been appropriately identified (see section 3.24.1, Actual Contamination, of this s upport document), the assigned hazardous waste quantity value at promulgation remains 100—the same as that assigned at proposal.

This comment has no effect on the site score or the decision to list the site on the NPL.

3.23.3 Human Food Chain Threat Waste Characteristics

Comment : ARCO claimed that the HRS assigned “human food chain threat waste characteristics product” was incorrect, based on its allegation that th e toxicity/persistence/bioaccumulation values should have been derived from cadmium instead of manganese, and that a lesser human food chain threat hazardous waste quantity factor value should have been assigned.
Response : The waste characteristics category value [ i.e., the product] of the human food chain threat was correctly determined at proposal. The human f ood chain threat waste characteristics factor category value remains 320 in the HRS documentation record at promulgation, unchanged from that assigned at proposal, despite the change of the Source 1 hazardous waste quantity to > 0, and the addition of Source 4.
As described in section 3.23.1, Toxicity/Persisten ce/Bioaccumulation, of this support document, in accordance with HRS Section 4.1.3.2.1.4, the greatest toxicity/persistence/bioaccumulation factor value of 5 x 10 8 based on manganese was correctly assigned in the HRS documentation record at proposal.

Also, as detailed in section 3.23.2, Human Food Chai n Threat Waste Quantity, of this support document, the human food chain threat hazardous waste quantity factor value was correctly scored as 100 in the HRS documentation record at proposal and remains so at promulgation.

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H RS Section 2.4.3.2, Factor category value, cons idering bioaccumulation potential, states:

F or the surface water-human food chain th reat and the surface water environmental threat, multiply the toxicity or combined f actor value, as appropriate, from section 2.4.1.2 and the hazardous waste quantity factor value from section 2.4.2.2, subject to:

• A maximum product of 1×10 12 , and

• A maximum product exclusive of the bioaccumulation (or ecosystem bioaccumulation) potential factor of 1×10 8

. Based on the total waste characteristics product, assign a waste characteristics factor category value to these threats from table 2–7.
The greatest toxicity/persistence/bioaccumulation factor value of 5 x 10 8 (that for manganese, as shown on page 43 of the documentation record at proposal) was assigned. This value multiplied by the pathway hazardous waste quantity factor value of 100 produces a value of 5x10 10 . According to HRS Table 2-7, the value of 5x10 10 corresponds to a waste characteris tics factor category value of 320.

Thus, the human food chain threat waste characteristi cs factor category value at promulgation has not changed from that assigned at proposal.

This comment has no effect on the site score or the decision to list the site on the NPL.



### 3.24 Surface Water - Huma n Food Chain Threat Targets

Comment : In its comments on the human food chain threat targets scoring, ARCO presented arguments against scoring an actually contaminated fishery, conc luding that an observed release was not established, and that fish tissue data indicated no risk to fish or humans.
Response : The scoring of human food chain targets correctly identified actually contaminated targets within the zone of contamination, and potentia lly contaminated targets within the TDL.
In establishing the TDL, HRS Section 4.1.1.2, Target distance limit , states that:

[ t]he target distance limit defines the maximum distance over which targets are considered in evaluating the site. Determ ine a separate target distance limit for each watershed as follows:

. . .

• If there is an observed release from the site to the surface water in the watershed that is based on sampling, begin measuring the target distance limit for the watershed at the probable point of entry; extend the targ et distance limit either for 15 miles along the surface water or to the most distant sample point that meets the criteria for an observed release to that watershed, whichever is greater.

I n evaluating the site, include only surface wate r targets (for example, intakes, fisheries, sensitive environments) that are within or contiguous to the hazardous substance migration path and located, partially or wholly, at or between the probable point of entry and the target distance limit applicable to the watershed:

. . .

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• Determine whether targets within or con tiguous to the hazardous substance migration path are subject to actual or potential contamination as follows:

- If a target is located, partially or wholly , either at or between the probable point of entry and any sampling point that meets the criteria for an observed release to the watershed or at a point that meets the criteria for an observed release by direct observation, evaluate that target as subject to actual contamination, except as otherwise specified for fisheries in se ction 4.1.3.3 and for wetlands in section 4.1.4.3.1.1. If the actual contamination is based on direct observation, assign Level II to the actual contamination. Ho wever, if the actual contamination is based on samples, determine whether the actual contamination is at Level I or Level II concentrations as specified in sections 4.1.2.3, 4.1.3.3, and 4.1.4.3.1.

- If a target is located, partially or wholly , within the target distance limit for the watershed, but not at or between the probable point of entry and any sampling point that meets the criteria for an observed release to the watershed, nor at a point that meets the criteria for an observed release by direct observation, evaluate it as subject to potential contamination.

Thus, the HRS instructs the scorer to consider that ta rgets are subject to actual contamination if they are between the PPE and the furthest downstream sample th at meets observed release criteria. HRS Section 4.1.3.3, Human food chain threat-targets, identifies further requirements for determining a human food chain fishery within the zone of contamination:

E valuate two target factors for each waters hed: food chain indivi dual and population. For both factors, determine whether the target fisheries are subject to actual or potential human food chain contamination.

C onsider a fishery (or portion of a fishery) within the target distance limit of the watershed to be subject to actual human food chain contamination if any of the following apply:

• A hazardous substance having a bioaccumu lation potential factor value of 500 or greater is present either in an observed release by direct observation to the watershed or in a surface water or sediment sample from the watershed at a level that meets the criteria for an observed release to the watershed from the site, and at least a portion of the fishery is within the boundaries of the observed release (that is, it is located either at the point of direct observation or at or between the probable point of entry and the most distant sampling point establishing the observed release).

• The fishery is closed, and a hazardous s ubstance for which the fishery has been closed has been documented in an ob served release to the watershed from the site, and at least a portion of the fishery is within the boundaries of the observed release.

• A hazardous substance is present in a tissu e sample from an essentially sessile, benthic, human food chain organism from the watershed at a level that meets the criteria for an observed release to the watershed from the site, and at least a portion of the fishery is within the boundaries of the observed release.

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For a fishery that meets any of these three cr iteria, but that is not wholly within the boundaries of the observed release, consider only the portion of the fishery that is within the boundaries of the observed release to be subject to actual human food chain contamination. Consider the remainder of the fishery within the target distance limit to be subject to potential food chain contamination.

In addition, consider all other fisheries that are partially or wholly within the target distance limit for the watershed, including fi sheries partially or wholly within the boundaries of an observed release for the watershed that do not meet any of the three criteria listed above, to be subject to poten tial human food chain contamination. If only a portion of the fishery is within the target distance limit for the watershed, include only that portion in evaluating the targets factor category.
When a fishery (or portion of a fishery) is subject to actual food chain contamination, determine the part of the fishery subject to Le vel I concentrations and the part subject to Level II concentrations. If the actual food chain contamination is based on direct observation, evaluate it using Level II concentrations. However, if the actual food chain contamination is based on samples from the watershed, use these samples and, if available, additional tissue samples from aquatic human food chain organisms as specified below, to determine the part subj ect to Level I concentrations and the part subject to Level II concentrations. . . .

As established in sections 3.19, Likelihood of Releas e - Observed Release by Direct Observation, and 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, an observed release by direct observation and an observed release by chemical analysis have been established at this site in acco rdance with the HRS. Regarding the zone of contamination, page 44 of the HRS documentation record at proposal documents that the farthest downstream Level II observed release sediment sample (AGFSED5) is located 2.39 miles downstream of the most upstream PPE (PPE#1). Page 45 of the HRS documentation record at proposal stated:

The extent of the Level II contamination of the Missouri River fishery is 2.39 miles (distance from the most upstream PPE (PPE #1) in Missouri River at 11th Street in Black Eagle, Montana to Sample AGFSED5 (Ref. 23, p. 28; Ref. 50).
Targets subject to actual contamination within this zo ne include the Missouri River fishery. Page 45 of the HRS documentation record at proposal stated:

Actual contamination of the Missouri Ri ver fishery was documented by Level II concentrations in sediment sample AGFSED5 (0907118-05). The stretch of the Missouri River between Black Eagle Dam and Rainbow Dam is an active fishery.

T herefore, the establishment of the zone of contamina tion and consideration of targets within the zone of contamination to be subject to actual contamination was consistent with the HRS. The Missouri River fishery was considered for scoring as a human food ch ain fishery as it was identified as subject to actual human food chain contamination, as samples collected within this zone were documented to contain hazardous substances with a bioaccumulation concentra tion factor of greater than 500 (cadmium in sample AGFSED5).

R esponses to specific comments on the evaluation of human food chain targets are presented in the following subsections of this support document:

• 3.24.1 Targets Subject to Actual Contamination

• 3.24.2 Contaminants in Fish Tissue

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3.24.1 Targets Subject to Actual Contamination

Comment : ARCO commented that, since it asserted that an observed release to surface water was not established, the presence of an actually contaminated fishery was not documented, and that the Missouri River fishery should be scored based on potentia l human food chain contamination (not actual contamination). ARCO challenged the value of 45 for the Food Chain Individual factor in the Human Food Chain Threat, stating that since no observed release is established, the factor value should be based on the dilution weight of the Missouri River.
Response : An actually contaminated fishery was correctly established based on a zone of contamination in the Missouri river and correctly delineated usi ng observed release samples; the food chain individual factor value of 45 was appropriately assigned at proposal in accordance with the HRS.
As explained in section 3.24, Surface Water - Human Food Chain Threat Targets, of this support document, the portion of the Missouri River within the zone of contamination (from PPE#1 downstream to observed release sample AGFSED5) was documented as an actually contaminated fishery according to the HRS.

HRS Section 4.1.3.3.1, Food chain individual , directs the scorer how to est ablish the scoring of the food chain individual as follows:
Evaluate the food chain individual factor based on the fisheries (or portions of fisheries) within the target distance limit for the waters hed. Assign this factor a value as follows:
• If any fishery (or portion of a fishery) is subject to Level I concentrations, assign a value of 50.
• If not, but if any fishery (or portion of a fishery) is subject to Level II concentrations, assign a value of 45.
Page 45 of the HRS documentation record at proposal presented how food chain individual was scored. It stated:

Actual contamination of the Missouri Ri ver fishery was documented by Level II concentrations in sediment sample AGFSED 5 (0907118-05). The stretch of the Missouri River between Black Eagle Dam and Rainbow Dam is an active fishery. Fishing was observed and photo documented by the sampling team during the 2009 Expanded Site Inspection at the Rainbow Boat Ramp. Photo documentation of fishing and a statement attesting to the consumption of fish caught in that reach of the Missouri River at the Rainbow Boat Ramp is documented (Ref. 23, pp. 27, 28, 33; Ref. 44, 1 page; Ref. 45; Ref. 46, 1 page). The Montana Department of Fish, Wildlife and Parks estimates that a population of 1,292 rainbow trout and 302 brow n trout are found in the 1.5 mile stretch between Black Eagle Dam and Giant Springs Stat e Park (Ref. 34, p. 1). The fish that are caught along this stretch of the river are eaten by the anglers and their families (Ref. 44, 1 page; Ref. 46, 1 page). This fishery is subj ect to Level II concentrations, due to a release of a contaminant with a BCF of greater than or equal to 500; a value of 45 is assigned to the Human Food Chain Individual Factor Value (Ref. 1, Sec. 4.1.3.3.1 and Sec. 4.1.3.3).
ARCO is incorrect in its assertion that all of (or “t he entire”) Missouri River fishery within the TDL for the site is subject to potential contamination. As documented on page 45 of the HRS documentation record at proposal, the Missouri River downstream of sample location AGFSED05 to the 15-mile TDL was considered subject to potential contamination, a nd dilution values were appropriately considered in

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the assessment of this segment of the Missouri River fishery. However, a fishery subject to actual contamination was correctly established within the zo ne of contamination, and it was appropriate to assign the resulting food chain i ndividual score of 45.

This comment has no effect on the site score or the decision to list the site on the NPL.
3.24.2 Consideration of Existing Fish Tissue Data

Comment : ARCO asserted that fish tissue data were av ailable and should have been recognized in the HRS evaluation of the human food chain threat. ARCO also claimed that language on page 44, Section 4.1.3.3 Human Food Chain Threat Targets, Fish Tissue, of the HRS documentation record at proposal “arbitrarily portrays the sediment data as deleterious to fish in the Missouri River when the contrary conclusion has previously been documented and accepte d in the course of prior investigations.” In support of this statement, ARCO referred to a MD FWP biologist review of the Parametrix study (AR Reference #16 of the ARCO comments) data and stated th at the review concluded that there is “no health risk to people from eating fish from this area and there is no risk to the fish themselves.”
Response : Fish tissue contaminant data were not used in the HRS evaluation of the human food chain threat, and are not required to be used in any threat or factor value evaluation. While fish tissue data, if available, can be used to establish an observed release, to identify actually contaminated fisheries, and to assign the level of actual contamination of the fisheri es, in none of these cases is the use of fish tissue samples required.

HRS Section 4.1.2.1.1, Observed release , states in part that an observed release to surface water is established if:
Analysis of surface water, benthic, or sediment samples indicates that the concentration of hazardous substance(s) has increased significantly above the background concentration for the site for that type of sample. . . . [emphasis added]

As shown in section 3.20, Surface Water Likelihoo d of Release - Observed Release by Chemical Analysis, of this support document, an observed re lease by chemical analysis has been established because hazardous substances significantly greater than background concentrati ons were documented in sediments within the Missouri River. Since the anal ysis of either surface water, benthic, or sediment samples can be used to establish an observed release to surface water through chemical analysis, only the sediment sample contaminant data was necess ary to establish an observed release.

H RS Section 4.1.3.3, Human food chain threat – targets , provides for establishing actual contamination of a fishery using surface water, sediment or fish tissue samples from essentially sessile, benthic food chain organisms. It states:
Consider a fishery (or portion of a fishery) within the target distance limit of the watershed to be subject to actual human food chain contamination if any [emphasis added] of the following apply:

• A hazardous substance having a bioaccumula tion potential factor value of 500 or greater is present in an observed release by direct observation to the watershed or in a surface water or [emphasis added] sediment sample from the watershed at a level that meets the criteria for an observe d release to the watershed from the site,

. . .

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• A hazardous substance is present in a tissu e sample from an essentially sessile, benthic human food chain organism from th e watershed at a level that meets the criteria for an observed release from the site, . . .
The identification of an actually contaminated fishery was established by demonstrating that sediment samples from a human food chain fishery that meet the observed release criteria contain one or more hazardous substances that have a bioaccumulation potential factor value of 500 or greater. Page 45 of the HRS documentation record at proposal, under section 4.1.3.3.1, Fo od Chain Individual, stated:

Actual contamination of the Missouri Ri ver fishery was documented by Level II concentrations in sediment sample AGFSED5 (0907118-05). The stretch of the Missouri River between Black Eagle Dam and Rainbow Dam is an active fishery. Fishing was observed and photo documented by the sampling team during the 2009 Expanded Site Inspection at the Rainbow Boat Ramp. Photo documentation of fishing and a statement attesting to the consumption of fish caught in that reach of the Missouri River at the Rainbow Boat Ramp is documented . . . The Montana Department of Fish, Wildlife and Parks estimates that a population of 1,292 rai nbow trout and 302 brown trout are found in the 1.5 mile stretch between Black Eagle Dam and Giant Springs State Park . . . The fish that are caught along this stretch of the river are eaten by the anglers and their families . . . This fishery is subject to Level II concentra tions, due to a release of a contaminant with a BCF of greater than or equal to 500; a va lue of 45 is assigned to the Human Food Chain Individual Factor Value.

As cited in the HRS above, since either surface water, sediment, or fish tissue samples can be used for establishing actual contamination of a fishery, sediment samples were sufficient for use in this evaluation.
In general, fish tissue samples can al so be used to identify Level I fish ery targets. HRS Section 4.1.3.3, Human food chain threat—targets , states:
When a fishery (or portion of a fishery) is subject to actual food chain contamination, determine the part of the fishery subject to Le vel I concentrations and the part subject to Level II concentrations. If the actual food chain contamination is based on direct observation, evaluate it using Level II concentrations. However, if the actual food chain contamination is based on samples from the watershed, use these samples and if available , additional tissue samples from aquatic human food chain organisms as specified below, to determine the part subj ect to Level I concentrations and the part subject to Level II concentrations.

In this evaluation, Level II concentrations were determined within the fishery from sediment contamination data; therefore, by the HRS mechanism described above, if contaminant levels within fish tissue were great enough, the addition of fish tissue data to this analysis would only be used to raise the level of contamination from Level II to Level I.
Regarding the claim that the HRS documentation record at proposal incorrectly “portrays the sediment data as deleterious to fish in the Missouri River”, th e commenter is incorrect. The language on page 44 of the HRS documentation record at proposal, section 4. 1.3.3, Human Food Chain Threat Targets, Fish Tissue, stated:
Fish tissue samples were collected by ARCO’s contractor Paramatrix in 2004. Paramatrix concluded that the concentrations in tissue were largely comparable between upstream (background) and downstream (site) areas. A statistical analysis of the analytical data was not performed . . . A review of the report by the EPA’s Regional

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Ecological toxicologist noted that at three sample locations ecotoxicological benchmarks were exceeded in soil samples, and it is likely th at higher concentrations of metals in all samples could have been obtained by taking samples on the north side of the Missouri River, which is the same side as the facility rather than across from the facility on the south bank of the Missouri River (Ref. 20, p. 1, 2).
The text above simply repeats a fact stated by the EPA toxicologist—that the metals concentrations in 3 sediment samples were greater than the ecotoxicological benchmarks that the toxicologist used in his evaluation. This statement does not draw any final conclusions on the actual risk posed by the related contamination. Additionally, a biologist from the MDFWP also referred to by ARCO stated that his search for relevant risk-based levels was “cursory” and his conclusions were based on “crude risk calculations.”
Finally, as noted in section 3.6, Risk Associated with Site, of this support document, EPA performs a comprehensive risk assessment for the site as part of fu rther investigations that typically follow listing. As further discussed in section 3.6 of this s upport document, the Parametrix study upon which conclusions about site risk were ba sed is limited and is not considered sufficient to draw a comprehensive conclusion on risk at the site.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.



### 3.25 Surface Water - Human Food Chain Threat Total Score

Comment : ARCO attempted to rescore the entire human food chain threat of the surface water pathway, and in doing so claimed that it "refined" the component s of all of the major factor categories of likelihood of release, waste characteristics, and targets, as we ll as the resultant human food chain threat score. ARCO summarized the Human Food Chain Threat scoring by alleging that, since EPA had not established an observed release to surface water, there was no actual contamination in a fishery. ARCO asserted that the score for the surface water human food chain threat should be calculated as follows: (259 x 56 x 1.000003)/82,500 = 0.18.
Response : The human food chain threat remains the same at promulgation as that assigned in the HRS documentation record at proposal, and was performed co rrectly and appropriately per the HRS, thus the revised scoring provided by ARCO is not appropriate.

Sections 3.19.1, Documentation of Observed Releas e by Direct Observation, and 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, demonstrate that an observed release by direct observation and an observed release by chemical analysis were correctly documented and scored in accordance with the HRS.

Section 3.23.3, Food Chain Threat Waste Characteristi cs, of this support document, demonstrates that the human food chain threat waste characteristics factor category value of 320 assigned at proposal remains the same in the HRS documentation record at promulgation.
Finally, section 3.24, Surface Water - Human Food Chain Threat-Targets, of this support document, makes clear that a fishery subject to actual contamination was correctly established. Therefore, the human food chain threat score remains the same at promulgation as that assigned at proposal, and the revised scoring scenario provided by ARCO is not appropriate.

This comment has no effect on the HRS site score or on the decision to list the site on the NPL.

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### Surface Water – Environmental Threat

Pages 47-49 of the HRS documentation record at pro posal identified wetlands within the zone of actual contamination. These wetlands were determined to be subject to Level II contamination due to releases from the site. See Figure 1 of this support document fo r the locations of these wetlands in relation to the observed release locations.



### 3.26 Surface Water - Environmenta l Threat Waste Characteristics

C omment : ARCO commented on EPA's assigned values for the hazardous waste quantity value and the resulting waste characteristics factor category value, and as part of its comments assigned alternative, lower values than those in the HRS doc umentation record at proposal.

S pecific comments are addressed in the follo wing subsections of this support document:

• 3.26.1 Environmental Threat Waste Quantity

• 3.26.2 Environmental Threat Waste Characteristics
3.26.1 Environmental Threat Waste Quantity

Comment : ARCO stated that EPA's assigned value for the hazardous waste quantity value was incorrect, and assigned an alternative, lower value for this cat egory. ARCO disputed the hazardous waste quantity values EPA assigned for both sources at the site, and stated it had assigned alternative lower values in its own scoring scenario, which used only the waste quan tity for the East Ditch Dump (Source 2). ARCO stated that the source waste quantity value for the East Ditch Dump should be 0.44, yielding an assigned the hazardous waste quantity value for the site of 1, as opposed to the hazardous waste quantity value of 100 established in the HRS documentation record at proposal.
Response: After consideration of ARCO’s comments on th e site sources, while the source waste quantity for source 1 has been revised, the environmental th reat hazardous waste quantity value remains 100, the same as that assigned at proposal. This, as explaine d below, reflects the waste quantity for Sources 1, 2, and 4. Note that as discussed in section 3.13, A dditional Sources, of this support document, Source 3(facility air emissions) is not included in the w aste quantity evaluation of the surface water pathway.

HRS Section 4.1.4.2, Environmental threat-waste characteristics , states:

Evaluate the waste characteristics factor category for each watershed based on two factors: ecosystem toxicity/persistence/bioaccumulation and hazardous waste quantity.

HRS Section 4.1.4.2.2, Hazardous waste quantity , directs the scorer to “[a]ssign the same factor value for hazardous waste quantity for the watershed as would be assigned in section 4.1.2.2.2 for the drinking water threat.”

HRS Section 4.1.2.2.2, Hazardous waste quantity , directs the scorer to “[a]ssign a hazardous waste quantity factor value for the watershed as specified in section 2.4.2.”

HRS Section 2.4.2.1, Source hazardous waste quantity , states:
For each of the three migration pathways, assign a source hazardous waste quantity value to each source (including the unallocated source) having a containment factor value greater than 0 for the pathway being evaluated.

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As established in section 2.2.2, Hazardous Substances A ssociated with Source, of the HRS documentation record at promulgation, Sources 1, 2, and 4 all have surface water containment factor values of “greater than 0”, and are therefore included in the waste quantity evaluation for the surface water migration pathway.

HRS Section 2.4.2.2, Calculation of hazardous waste quantity factor value , states:
Sum the source hazardous waste quantity va lues assigned to all sources (including the unallocated source) or areas of observed cont amination for the pathway being evaluated and round this sum to the nearest integer, ex cept: if the sum is greater than 0, but less than 1, round it to 1. Based on this value, select a hazardous waste quantity factor value for the pathway from table 2–6.
For a migration pathway, if the hazardous c onstituent quantity is adequately determined (see section 2.4.2.1.1) for all sources (or all portions of sources and releases remaining after a removal action), assign the value fro m table 2–6 as the hazardous waste quantity factor value for the pathway. If the hazar dous constituent quantity is not adequately determined for one or more sources (or one or more portions of sources or releases remaining after a removal action) assign a factor value as follows:
• If any target for that migration path way is subject to Level I or Level II concentrations (see section 2.5), assign either the value from table 2–6 or a value of 100, whichever is greater, as the hazardous waste quantity factor value for that pathway.
Therefore, the source hazardous waste quantity values are added together to obtain the pathway waste quantity. Footnote B of HRS Table 2-6 states that “[ f]or the pathway, if hazardous constituent quantity is not adequately determined, assign a value as specified in the text; do not assign the value of 1.”

As discussed in sections 3.14.5, Source 1 Hazardous Waste Quantity, and 3.15.4, Source 2 Hazardous Waste Quantity, of this support document, Sources 1 and 2 were both assigned a hazardous waste quantity value of “greater than 0” at promulgation. Add itionally, the hazardous waste quantity value for Source 4 was appropriately evaluated as “greater than 0, ” as discussed on pages 44-45 of the HRS documentation record at promulgation, and as explained in section 3.13, Additional Sources, of this support document.
As the hazardous constituent quantity was not adequate ly determined for any site source, a value was assigned to the pathway hazardous waste quantity as dir ected by HRS Section 2.4.2.2, which instructs the scorer to assign a value for the pathway hazardous wast e quantity from Table 2–6 of the HRS, or a value of 100, whichever is greater, if any target for that migration pathway is subject to Level I or Level II concentrations. Because the presence of Level II targets has been appropriately identified (see section 3.27.1, Actual Contamination, of this support docum ent), the assigned hazardous waste quantity value at promulgation remains 100—the same as that assigned at proposal.

This comment has no effect on the site score or the decision to list the site on the NPL.

3.26.2 Environmental Threat Waste Characteristics

Comment : ARCO claimed that the environmental threat waste characteristics factor category was incorrect, based on its allegation that a lesser huma n food chain threat hazardous waste quantity factor value should have been assigned.

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Response : The human food chain threat waste characteri stics factor category value remains 320 in the HRS documentation record at promulgation, unchanged from that assigned at proposal, despite the change of the Source 1 hazardous waste quantity to ”g reater than 0,” and the addition of Source 4.

HRS Section 4.1.4.2.3, Calculation of environmental threat-wast e characteristics factor category value
, states:
For the hazardous substance selected for the watershed in section 4.1.4.2.1.4, use its ecosystem toxicity/persistence factor valu e and ecosystem bioaccumulation potential factor value as follows to assign a value to the waste characteristics factor category. First, multiply the ecosystem toxicity/persistence factor value and the hazardous waste quantity factor value for the watershed, subject to a maximum product of 1×10 8 . Then multiply this product by the ecosystem bioaccumulation potential factor value for this hazardous substance, subject to a maximum product of 1×10 12 . Based on this secondproduct, assign a value from Table 2–7 (section 2.4.3.1) to the environmental threatwaste characteristics factor category for the watershed.
Pages 47-48 of the HRS documentation record at pr oposal assigned an ecosystem toxicity/persistence/ bioaccumulation factor value of 5 x 10 8 , an ecosystem toxicity/persisten ce factor value of 10,000, and an ecosystem bioaccumulation potential factor value of 50,000 based on cadmium; these values were not challenged by ARCO. As described in section 3.26 .1, Environmental Threat Waste Quantity, of this support document, a hazardous waste quantity value of 100 was correctly assigned at proposal and remains at promulgation. Therefore, because the pr oduct of the ecosystem toxicity/persistence factor value and the waste quantity factor value remains 5 x 10 10 , the resulting environmental threat waste characteristics factor category value assigned from HRS Table 2-7 at promulgation is still 320, as assigned at proposal.

This comment has no effect on the site score or the decision to list the site on the NPL. 

### 3.27 Surface Water - Environmental Threat Targets

C omment : In its comments on the surface water pathway environmental threat, ARCO maintained that the targets scoring should be based on potential contamin ation rather than actual contamination, that there was no risk to the aquatic biota (i.e., fish in the Mi ssouri River) from the site, and that the target score should be substantially reduced.

R esponses to specific comments are presented in th e following subsections of this support document:

• 3.27.1 Actual Contamination

• 3.27.2 Aquatic Biota Targets

3.27.1 Actual Contamination

Comment : ARCO disputed the scoring of actual contamination in the environmental threat on the basis that no observed release was established. ARCO specifically commented that Level I and II concentrations were not applicable, and that the scoring should instead be determined solely from potential contamination to wetlands, incorporating th e dilution weight of the river. Based on this argument ARCO asserted that the environmental thre at-target factor category value should be 0.025 instead of 25 as assigned in the HRS documentation record at proposal.
Response: Environmental targets subject to actual contam ination were appropriate ly identified and were correctly assigned a Level II target value of 25 because wetlands subject to actual contamination were

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properly established in the HRS documentation record at proposal. ARCO is incorrect in its assertion that no observed releases were correctly identified in the HRS documentation record at proposal.

As established in sections 3.19, Likelihood of Releas e - Observed Release by Direct Observation, and 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, observed releases were properly establis hed for the surface water pathway by chemical analysis as well as by direct observation. See Figure 1 of this support document for the locations of the wetlands in relation to the observed release locations.

H RS Section 2.5, Targets , explains how to determine the level of contamination and the associated factor values associated with environmental targets:
The factor values that may be assigned to each type of target have the same range for each pathway for which that type of target is evaluated. The factor value for most types of targets depends on whether the target is s ubject to actual or potential contamination for the pathway and whether the actual cont amination is Level I or Level II:

• Actual contamination: Target is associat ed either with a sampling location that meets the criteria for an observed release (or observed contamination) for the pathway or with an observed release ba sed on direct observation for the pathway (additional criteria apply for establishing actual contamination for the human food chain threat in the surface water mi gration pathway, see sections 4.1.3.3 and 4.2.3.3). Sections 3 through 6 specify how to determine the targets associated with a sampling location or with an obs erved release based on direct observation.

HRS Section 4.1.4.3.1, Sensitive environments , states:
Evaluate sensitive environments along the h azardous substance migration path for the watershed based on three factors: Level I c oncentrations, Level II concentrations, and potential contamination.
Determine which factor applies to each sen sitive environment as specified in section 4.1.2.3, except: use ecological-based benchmar ks (Table 4–22) rather than health-based benchmarks (Table 3–10) in determining the level of contamination from samples. In determining the level of actual contamination, use a point of direct observation anywhere within the sensitive environment or samples (that is, surface water, benthic, or sediment samples) taken anywhere within or be yond the sensitive environment (or anywhere adjacent to or beyond the sensitive environment if it is contiguous to the migration path).

HRS Section 4.1.4.3.1.2, Level II concentrations , states:
Assign value(s) from table 4–23 to each sensitive environment subject to Level II concentrations. Do not include sensitive e nvironments already counted for table 4–23 under the Level I concentrations factor for this watershed.

For those sensitive environments that are wetlands, assign an additional value from table 4–24. In assigning a value from table 4–24, include only those portions of wetlands located along the hazardous substance mi gration path in the area of Level II concentrations, as specified in section 4.1.4.3.1.1.
Estimate the total length of wetlands along th e hazardous substance migration path (that is, wetland frontage) in the area of Level II concentrations and assign a value from table 4–24 based on this total length. Estimate this length as specified in section 4.1.4.3.1.1,

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except: for an isolated wetland or for a we tland where the probable point of entry to surface water is in the wetland, use the perimeter of that portion of the wetland subject to Level II (not Level I) concentrations as the length.
Section 4.1.4.3, Environmental Threat – Targets, of the HRS documentation record at proposal explained the assignment of the environmental targets fact or values specifically at this site as follows:
Wetlands along the banks of the Missouri River and on Black Eagle Island and Steamboat Island in the Missouri River are identified by the U.S. Fish and Wildlife Service as part of the National Wetlands Inventory . . . Most of the wetlands immediately below Black Eagle Dam are classified as Ri verine Perennial wetlands, and Palustrine Emergent wetlands are identified further dow nstream of the Black Eagle Dam within the zone of documented contamination . . . Most of the wetlands identified on the U.S. Fish and Wildlife Service National Wetlands Inve ntory Map for Northeast and Northwest Great Falls Montana within the zone of actu al contamination qualify as eligible wetlands defined in 40 CFR Sec 230.2 because most of these wetland types are likely to contain a prevalence of wetland plant species based on th eir classification such as the palustrine emergent wetlands mentioned above.

Section 4.1.4.3.1.2, Level II Concentrations, of the HRS documentation record at proposal stated that “[a]t least 0.1 mile of wetland frontage occurs within the zone of actual contamination and is subject to Level II contamination using sediment samples alone (R ef. 1, Table 4-24).” Citing to HRS Table 4–24, Wetlands Rating Values for Surface Water Migration Pathway , the HRS documentation record at proposal assigned the appropriate Leve l II concentrations factor value of 25 for a wetland frontage within the 0.1 to 1 mile category. See Figure 1 of this support document for the locations of the wetlands in relation to the observed release locations.
Thus, wetlands subject to actual cont amination at level II contaminations were appropriately established. Therefore, the commenter’s assertion that the enviro nmental threat-target factor category value should have been based only on potential contamination is incorrect, and the score at proposal was correctly calculated in accordance with the HRS instructions.
Regarding dilution weight, this para meter is only used in the environmental threat of the surface water pathway in calculating the potential contaminati on factor value (HRS Section 4.1.4.3.1.3, Potential contamination , describes this calculation). Because, as descr ibed above, wetlands were correctly scored as subject to actual Level II contamination and not pote ntial contamination, this parameter does not apply.

This comment has no effect on the site score or the decision to list the site on the NPL.
3.27.2 Aquatic Biota Targets

Comment : ARCO asserted that no environmental impact to fish species was shown.
Response: The HRS documentation record at proposal did not evaluate environmental impacts to a specific species as part of the environmental threat evaluation. The environmental threat-targets evaluated at this site only included HRS e ligible wetlands as sensitive environments.

HRS Section 4.1.4.3.1, Sensitive environments, states:
Evaluate sensitive environments along the h azardous substance migration path for the watershed based on three factors: Level I c oncentrations, Level II concentrations, and potential contamination.

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Determine which factor applies to each sen sitive environment as specified in section 4.1.2.3, except: use ecological-based benchmar ks (Table 4–22) rather than health-based benchmarks (Table 3–10) in determining the level of contamination from samples. In determining the level of actual contamination, use a point of direct observation anywhere within the sensitive environment or samples (that is, surface water, benthic, or sediment samples) taken anywhere within or be yond the sensitive environment (or anywhere adjacent to or beyond the sensitive environment if it is contiguous to the migration path).

HRS Section 4.1.4.3.1.2, Level II concentrations , directs the assignment of values from HRS Table 4-23 for each sensitive environment subject to Level II contamination or from HRS Table 4-24 for wetlands:
Assign value(s) from table 4–23 to each sensitive environment subject to Level II concentrations. . . . For those sensitive environments that are wetlands, assign an additional value from table 4–24. In assigni ng a value from table 4–24, include only those portions of wetlands located along the hazardous substance migration path in the area of Level II concentrations, as specified in section 4.1.4.3.1.1.

HRS Table 4-23, Sensitive Environments Rating Values , provides a list of sensitive environments and related values eligible for scoring in the HRS surface water migration pathway.
Section 4.1.4.3, Environmental Threat – Targets, of the HRS documentation record at proposal, on page 48 stated:
Wetlands along the banks of the Missouri River and on Black Eagle Island and Steamboat Island in the Missouri River are identified by the U.S. Fish and Wildlife Service as part of the National Wetlands Inventory (Ref. 35, pp 1-3). Most of the wetlands immediately below Black Eagle Dam are classified as Riverine Perennial wetlands, and Palustrine Emerge nt wetlands are identified further downstream of the Black Eagle Dam within the zone of documented contamination (Ref. 35, pp 1-3). Most of the wetlands identified on the U.S. Fish and Wildlife Service National Wetlands Inventory Map for Northeast and Northwest Great Falls Montana within the zone of actual contamination qualify as eligible we tlands defined in 40 CFR Sec 230.2 because most of these wetland types are likely to c ontain a prevalence of wetland plant species based on their classification such as the palu strine emergent wetlands mentioned above.
As explained above, HRS Table 4-23 identifies the sensitive environments which are eligible for evaluation within the surface water pathway; while this includes contamination of certain state and federal threatened, endangered, or otherwi se protected aquatic biota, none of these aquatic biota were identified as present within the TDL for this site. See section 3.6, Risk Associated with Site, of this support document for other information regarding risk to aquatic biota at the site.

This comment has no effect on the site score or the decision to list the site on the NPL.


### 3.28 Surface Water - Enviro nmental Threat Total Score

Comment : ARCO stated that it had rescored the entir e environmental threat of the surface water pathway, and in doing so had “refined” the components of all of the major factor categories of likelihood of release, waste characteristics, and targets, as well as the resultant environmental threat score. ARCO asserted that the score for the environmental th reat should be 0.0078, instead of 53.33.
Response: The environmental threat score was correc tly assigned as 53.33 at proposal and remains the same at promulgation.

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Sections 3.19.1, Documentation of Observed Releas e by Direct Observation, and 3.20, Surface Water Likelihood of Release - Observed Release by Chemical Analysis, of this support document, demonstrate that an observed release by direct observation and an observed release by chemical analysis were correctly documented and scored in accordance with the HRS.

Section 3.26, Surface Water - Environmental Threat Waste Characteristics, of this support document, shows that the environmental threat waste character istics factor category value of 320 remains unchanged from proposal.
Finally, section 3.27.1, Actual Contamination, of this support document, makes clear that sensitive environments subject to actual contamination were co rrectly established. Therefore, the environmental threat score remains the same at promulgation as that assigned at proposal, and the revised scoring scenario provided by ARCO is not appropriate.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.



### 3.29 Surface Water Overland/Flo od Migration Component Score

Comment : ARCO stated that it had rescored the entire surface water migration pathway. ARCO asserted that the score for the surface water overland/flood mi gration pathway should be 0.18, instead of 100 as assigned in the HRS documentation record at proposal.
Response : Although the assignment of certain HRS factor values were revised at promulgation, the surface water overland/flood migration pathway com ponent score remains the same at promulgation as that assigned in the HRS documentation record at proposal.

HRS Section 4.1.5, Calculation of overland/flood migration component score for a watershed , states to:
[s]um the scores for the three threats for the watershed (that is, drinking water, human food chain, and environmental threats). Assign the resulting score, subject to a maximum value of 100, as the surface water overland/ flood migration component score for the watershed.

HRS Section 4.1.6, Calculation of overland/flood migration component score , states to:
[s]elect the highest surface water overland/flood migration component score from the watersheds evaluated. Assign this score as the surface water overland/flood migration component score for the site, subject to a maximum score of 100.
As explained in section 3.22, Surface Water - Drinki ng Water Threat, of this support document, the drinking water threat was not scored in the HRS evaluation of the site.

Section 3.25, Surface Water - Human Food Chain Thr eat Total Score, of this support document, establishes that the human food chain threat score of 96.06 assigned at proposal remains the same at promulgation.

Section 3.28, Surface Water - Environmental Threat To tal Score, of this supp ort document, establishes that the environmental threat score of 53.33 assigne d at proposal remains the same at promulgation.
Therefore, the component score, generated by the su m of these threats and subject to a maximum of 100, remains 100 at promulgation.

This comment has no effect on the site score or the decision to list the site on the NPL.

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87 

### Soil Exposure

The HRS documentation record at proposal identif ied an area of observed contamination based on contaminated residential soil samples in Black Eagle. Background and contaminated soil samples utilized in the record at proposal were taken from a 2007 investigation and a 2008 investigation, and were collected from a 0-2 inch horizon and a 6-12 inch horiz on. Levels of metals meeting HRS criteria for establishing observed contamination (including arsenic , cadmium, and lead) were detected in 53 samples taken from 53 different residences (see Figure 2 of this support document which follows). Of the 53 sampled residences, 8 exhibited Level I concentrations. The remainder of the residences and their associated population within the area of contamina tion were scored as targets subject to Level II contamination.



### 3 .30 Soil Exposure/Area of Observed Contamination

C omment : ARCO alleged that the establishment of the AOC associated with Source 1 was flawed, and the soil exposure pathway score as presented in the HR S documentation record at proposal is incorrect. The following subsections of this support documen t address specific comments on this topic raised by ARCO.

• 3.30.1 AOC Sampling Method

• 3.30.2 Significant Increase of Hazardous Substances

• 3.30.3 AOC Delineation/Inference

• 3.30.4 Hazardous Substan ce Deposition/Attribution

• 3.30.5 Contamination Below Cleanup Criteria
3.30.1 AOC Sampling Method

Comment : ARCO asserted that the sampling procedures empl oyed to collect the soil samples used in the AOC evaluation were flawed. ARCO contended that all samples collected by EPA for the 2007 and 2008 site investigations were “random gr ab samples rather than systematic composite samples” and were not representative of actual site conditions. In addition, ARCO asserted that the sampling it completed in 2009 utilized a composite sampling method. It stated:
Many of these residences were resampled in 2009 by Atlantic Richfield (AR Reference #14) using systematic composite sa mpling methods outlined in EPA's Superfund LeadContaminated Residential Sites Handbook (AR Reference #15). . . . In general, the composite sampling did not confirm the highest concentrations reported by EPA.
Response : The sampling methods used to collect the n ecessary data in the evaluation of the area of observed contamination at the ACM site were appropriate. Grab samples are sufficient to collect the data necessary to identify the pr esence of contamination in establishing an area of contamination. Additional sampling is conducted in a later stage of the Superfu nd process when a site-specific risk assessment is performed.

HRS Section 5.0.1, General considerations , states:
Evaluate the soil exposure pathway b ased on areas of observed contamination:
• Consider observed contamination to be present at sampling locations where analytic evidence indicates that:ACM Smelter & Refinery NPL Listing Support DocumentMarch 201188ACM Smelter & Refinery NPL Listing Support Document March 2011

–A hazardous substance attributable to the site is present at a concentration significantly above background levels for th e site (see table 2–3 in section 2.3 for the criteria for determining analytical significance), and
–This hazardous substance, if not present at the surface, is covered by 2 feet or less of cover material (for example, soil).
• Establish areas of observed contamination based on sampling locations at which there is observed contamination as follows:
–For all sources except contaminated soil, if observed contamination from the site is present at any sampling location w ithin the source, consider that entire source to be an area of observed contamination.
–For contaminated soil, consider both the sampling location(s) with observed contamination from the site and the area lying between such locations to be an area of observed contamination, unless available information indicates otherwise.
• If an area of observed contamination (or portion of such an area) is covered by a permanent, or otherwise maintained, essentia lly impenetrable material (for example, asphalt) that is not more than 2 feet thick, exclude that area (or portion of the area) in evaluating the soil exposure pathway.

• For an area of observed contamination, consider only those hazardous substances that meet the criteria for observed contamination fo r that area to be associated with that area in evaluating the soil exposure pathway (see section 2.2.2).
The HRS does not give explicit direction on what sampling methodology to use in establishing an area of contamination. However, the HRS directs the scorer, as cited above, to consid er observed contamination to be present at sampling locations where analy tic evidence indicates that a hazardous substance attributable to the site is present at a concentration significantly above background levels for the site.
Page 50 of the HRS documentation record at pro posal documented the collection of samples used in establishing the area of contamination:
The 2007 EPA CERCLA Site Assessment sampled residences on both the north and south sides of the Missouri River adjacent to the Anaconda Minerals Co, Great Falls Refinery (Ref. 21, p. 11). Of the eleven residences sampled in Great Falls on the south side of the Missouri River in August of 2007 only one residence had a elevated concentration of arsenic (Ref. 21, p. 13). Thirty-three residences were sampled for CLP analysis north of the Missouri River in Black Eagle in August of 2007, and arsenic and lead concentrations greater than three tim es background were identified in central and eastern Black Eagle from the results of the 2007 investigation (Ref. 21, pp. 13-14, 18). All soil samples collected were analyzed in the field using a portable XRF device (Ref. 5, p. 12; Ref. 21., p. 12). All samples for fi xed laboratory analyses were analyzed through the EPA’s CLP, RAS. The samples collected in 2007 were analyzed by A4 Scientific, Inc. of The Woodlands, Texas by EPA CLP method ILM05.4 (Ref. 60, p. 1; Ref. 61., p. 1; Ref. 62, p. 1).
The 2008 EPA CERCLA Expanded Site Investigation was focused in the community of Black Eagle, Montana. The residential soil at fifty-nine residential properties in Black Eagle was sampled in July 2008 to a depth of one foot (Ref. 5, p. 1). Concentrations of arsenic, cadmium, lead and zinc elevated th ree times background were detected in the

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residential soil in Black Eagle (Ref. 5, pp. 13-14; Ref. 18, pp. 28-30; Ref. 21, pp. 13-14). Elevated arsenic and cadmium results delineated an area of contamination centered on southwestern and central Black Eagle (Ref. 5, pp. 13-14, 23-24).
All 2008 soil samples collected were analyzed in the field using a portable XRF device (Ref. 5, p. 12; Ref. 21., p. 12). All sampl es for fixed laboratory analyses were analyzed through the EPA’s Contract Laboratory Program (CLP), Routine Analytical Services. The samples collected in 2008 were anal yzed by Chemtech Consulting Group, Mountainside, New Jersey by EPA CLP method ILM05.4 (Ref. 63. p. 1; Ref. 64, p. 1).
The collection of the 2007 and 2008 samples described above, resulted in the collection of data meeting the HRS provisions for establishing an AOC. This me thod resulted in the determination that the soil at the sampling locations was contaminated at levels meeting observed release criteria.
While the HRS does not give explicit direction on the sampling methodology necessary to establish the concentrations of a hazardous substance at a particul ar location, page 51532 of the HRS, under Section I, Background , states:
In 1986, Congress enacted the Superfund Am endments and Reauthorization Act of 1986 (SARA). . . .requiring EPA to amend the HRS to assure ‘to the maximum extent feasible, that the hazard ranking system accurately assesses the relative degree of risk to human health and the environment posed by sites and facilities subject to review.’ Congress, in its Conference Report on SARA, stated that substantive standard against which HRS revisions could be assessed:
This standard is to be applied within the context of the purpose for the National Priorities List; i.e., identifying for the States and the public those facilities and sites which appear to warrant remedial actions....This stan dard does not, however, require the Hazard Ranking System to be equivale nt to detailed risk assessments , quantitative or qualitative, such as might be performed as part of remedial actions. The standard requires the Hazard Ranking System to rank sites as accurately as the Agency believes is feasible using information from preliminary assessments and site inspections.
The sample results used in establishing the area of contamination at the ACM site were collected from site inspections of the Black Eagle residential area which were specifically performed to collect data necessary to evaluate the site. As described in section 3.14.3, Sampling Pro cedures, of this support document, the collection of grab samples documented the level of hazardous substances at each sampling location to determine that observed contamination was present each sampling location. That hazardous substances within the area of contamination were detected significantly above background levels is discussed in section 3.29.2, Significant Increase of Hazardous Substances, of this support document.
Regarding the Superfund Lead-Contaminated Residential Sites Handbook, nothing in this guidance document precludes the use of grab samples for an HRS evaluation. This document is directed toward a detailed site characterization and th e remedy section for the site; this occurs during a later stage of the Superfund process, not during a screening HRS investigation.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.
3.30.2 Significant Increase of Hazardous Substances

Comment : In its comments challenging the establishment of the AOC, ARCO challenged the significant increase of various hazardous substances in samples used to determine the AOC. ARCO specifically stated that the residential properties sampled by EPA incorrectly included 114 residences that did not

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have concentrations of arsenic and cadmium greater than three times background concentrations and their respective SCDM 4 benchmarks .
In addition, ARCO stated that none of the soil samp les collected from residential areas in Black Eagle contained concentrations of antimony greater than three times background, therefore antimony cannot be regarded as elevated.
Response : A significant increase (i.e, three times background) of arsenic, cadmium, and lead in samples from the AOC were correctly identif ied in accordance with the HRS. The number of residences included the AOC evaluation has been reduced at promulgation (see section 3.32, Soil Exposure Targets, of this support document). Antimony has been removed from the HRS evaluation at promulgation.

HRS Section 5.0.1, General considerations , states:
Evaluate the soil exposure pathway based on areas of observed contamination:
• Consider observed contamination to be present at sampling locations where analytic evidence indicates that:
–A hazardous substance attributable to the site is present at a concentration significantly above background levels for th e site (see table 2–3 in section 2.3 for the criteria for determining analytical significance), and
–This hazardous substance, if not present at the surface, is covered by 2 feet or less of cover material (for example, soil).

Establish areas of observed contamination b ased on sampling locations at which there is observed contamination as follows:
–For all sources except contaminated soil, if observed contamination from the site is present at any sampling location w ithin the source, consider that entire source to be an area of observed contamination.
–For contaminated soil, consider both the sampling location(s) with observed contamination from the site and the area lying between such locations to be an area of observed contamination, unless available information indicates otherwise.
• If an area of observed contamination (or portion of such an area) is covered by a permanent, or otherwise maintained, essentia lly impenetrable material (for example, asphalt) that is not more than 2 feet thick, exclude that area (or portion of the area) in evaluating the soil exposure pathway.

• For an area of observed contamination, consider only those hazardous substances that meet the criteria for observed contamination fo r that area to be associated with that area in evaluating the soil exposure pathway (see section 2.2.2).

T he portions of HRS Table 2-3 used to establish observed contamination and the AOC are cited as follows:

4 Superfund Chemical Data Matr ix. EPA assumes that ARCO’s reference to the SCDM benchmarks is referring to the benchmarks cited in the HRS (HRS Section 5.1.3.2) as to be used for establishing Level I contamination. SCDM repeats the list of those HRS benchmarks and presen ts the contaminant levels currently associated with the benchmarks.

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An observed release is established as follows:

• If the background concentration is not de tected (or is less than the detection limit), an observed release is established when the sample measurement equals or exceeds the sample quantitation limit.

• If the background concentration equals or exceeds the detection limit, an observed release is established when the sample measurement is 3 times or more above the background concentration.
Pages 50-51 of the HRS documentation record at pr oposal document that samples taken from the Black Eagle residential area contained hazardous substances elevated significantly above background levels:
The 2007 EPA CERCLA Site Assessment sampled residences on both the north and south sides of the Missouri River adjacent to the Anaconda Minerals Co, Great Falls Refinery (Ref. 21, p. 11). Of the eleven residences sampled in Great Falls on the south side of the Missouri River in August of 2007 only one residence had an elevated concentration of arsenic (Ref. 21, p. 13). Thirty-three residences were sampled for CLP analysis north of the Missouri River in Black Eagle in August of 2007, and arsenic and lead concentrations greater than three tim es background were identified in central and eastern Black Eagle from the results of the 2007 investigation (Ref. 21, pp. 13-14, 18). . .
The 2008 EPA CERCLA Expanded Site Investigation was focused in the community of Black Eagle, Montana. The residential soil at fifty-nine residential properties in Black Eagle was sampled in July 2008 to a depth of one foot (Ref. 5, p. 1). Concentrations of arsenic, cadmium, lead and zinc elevated th ree times background were detected in the residential soil in Black Eagle (Ref. 5, pp. 13-14; Ref. 18, pp. 28-30; Ref. 21, pp. 13-14). Elevated arsenic and cadmium results delineated an area of contamination centered on southwestern and central Black Eagle (Ref. 5, pp. 13-14, 23-24).
All 2008 soil samples collected were analyzed in the field using a portable XRF device (Ref. 5, p. 12; Ref. 21., p. 12). All samp les for fixed laboratory analyses were analyzed through the EPA’s Contract Laboratory Program (CLP), Routine Analytical Services. The samples collected in 2008 were anal yzed by Chemtech Consulting Group, Mountainside, New Jersey by EPA CLP method ILM05.4 (Ref. 63. p. 1; Ref. 64, p. 1).
Page 52 of the HRS documentation re cord at proposal presented the background soil samples to which the samples taken from the area of observed contamination were compared. A background level was developed for arsenic, cadmium, and lead from two horiz ons of soil – the 0-2 and the 6-12 horizons. The following tables present the highest background levels established from each horizon in the evaluation of the area of observed contamination.

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ACM Smelter and Refinery – Background Samples 0-2 inch horizon collected in 2007 and 2008* ( mg/kg)

Analyte CRDL/ MDL (2007/2008) SCDMBenchmark RDSC Jaycee Park R04ST2400F102 CLP - MHIH26 Sampled 8/10/2007 North Middle School R08ST2601F102 CLP - MH1H34 Sampled 8/8/2007 724 25 th Ave. R25AV0724FY02 CLP - MH26P6 Sampled 7/11/2008 737 25 th Ave. R25AV0737SY02 CLP - MH26P7 Sampled 7/11/2008 Arsenic 1/(0.49/ 0.37) 0.43 13.3 (10.4 J) 18.1 6.3 16.1

Cadmium 0.5/(0.20/ 0.040) 39 1.2 J+ 0.82 J+ 0.53 (0.34 J) 0.48

Lead 1/(0.49/ 0.25) - 30.1 (17.5 J) 25.2 25.3 20.6

Note: * Sample IDs ending in 02 represent samples collected from 0-2 inches (Ref. 5, p. 10; Ref. 21, p. 11). Bold Indicates background value selected for this document / mg/kg CRSC Cancer Risk Screening Concentration RDSC Reference Dose Screening Concentration J The associated numerical value is an estimated quantity an d is the approximate concentration of the analyte in the sample. J+ The associated numerical value is an estimated quantity, but the result may be biased high. ( ) Validated value prior to adjustment as per EPA Fact Sheet “Using Qualified Data to Document an Observed Release and Observed Contamination” (Ref. 43, 18 pages). mg/kg milligrams per kilogram

ACM Smelter and Refinery – Background Sample s 6-12 inch horizon collected in 2007** ( mg/kg)

Analyte CRDL/ MDL(2007) SCDM Benchmark RDSC Jaycee Park R04ST2400F112 CLP - MHIH27 Sampled 8/10/2007 North Middle School R08ST2601F112 CLP - MH1H35 Sampled 8/8/2007 724 25 th Ave. 6-12 inch sample not collected in 2008 737 25 th Ave. 6-12 inch sample not collected in 2008 Arsenic 1/0.49 0.43 17.2 (13.3 J) 23.1 - - Cadmium 0.5/0.20 39 0.43 J+ 0.67 J+ - - Lead 1/0.49 - 10.6 (12.9 J) 18.6 - - Note: ** Sample IDs ending in 12 represent sa mples collected from 6-12 inches Bold Indicates background value selected for this document / mg/kg Milligrams per kilogram CRSC Cancer Risk Screening Concentration RDSC Reference Dose Screening Concentration
As identified above , pages 54-63 of the HRS doc umentation record at proposal presented multiple samples taken from the Black Eagle residential area that were documented to contain arsenic, cadmium, or lead at a concentration greater than 3 times the highest background level of each respective horizon. As arsenic, cadmium, and lead were all detected in the background samples, according to HRS Table 2-3, a significant increase is shown when the hazardous substance detected in the release sample (observed contamination sample) is greater than 3 times the hi ghest detected background. Each of the 53 sampling locations presented on pages 54-63 of the HRS documenta tion record at proposal contained either lead, cadmium, or arsenic at levels greater than 3 times detected background concentrations, thus satisfying HRS Table 2-3 requirements. These hazardous substances were associated with the AOC, as contamination associated with Source 3 was documented as present at sampling locations within the AOC within 2 feet of the surface.

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The commenter is incorrect in its assertion that a hazardous substance need be greater than 3 times background and above an HRS benchmark [referred to by ARCO as “SCDM benchmarks”] (in this case, an HRS health-based benchmark for hazardous substan ces in soils) to identify residences as within the AOC. As stated above, there is no requirement that a substance be above a benchmark to use it in identifying an AOC or identifying residences with th e AOC (EPA notes that it does impact whether the residences are considered exposed to Level I or Level II exposure. See section 3.32, Soil Exposure Targets, of this support document.)
Regarding antimony, it was not included in the evaluation of the AOC (or the corresponding source 1) at proposal. The discussion of antimony has been removed in the HRS documentation record at promulgation, and because this substance was not sco red in the HRS documentation record at proposal, its removal has no impact on the scoring of the site. The removal of this substance has no effect on the identification of residences exposed to arsenic, cadmium or lead.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.

3.30.3 AOC Delineation/Inference

Comment : ARCO alleged that the inference of contamination between sample locations in the Black Eagle residential soil AOC delineation is incorrect according to the HRS, and as a result EPA has significantly overestimated the AOC. ARCO asserte d that AOC evaluation at proposal “ignores dozens of included residences that do not have elevated ar senic or cadmium concentrations based on EPA's own data.”
Response : The evaluation of the AOC delineation at promul gation has been revised. As stated in section 3.14.2, Source 1 Delineation/Inference of Contamination, contamination is no longer inferred between sample locations in the evaluation of the AOC in the HRS evaluation at promulgation (see Figure 2 of this support document for a presentation of the sample locations used in establishing the AOC).

HRS Section 5.0.1, General considerations , states:

Establish areas of observed contamination b ased on sampling locations at which there is observed contamination as follows:

. . . . –For contaminated soil, consider both the sampling location(s) with observed contamination from the site and the area lying between such locations to be an area of observed contamination, unless available information indicates otherwise.
The samples used in defining the AOC remain the same at promulgation as defined at proposal (see pages 54-63 of the HRS documentation record at proposal); however, the area between the sample locations is no longer considered part of the AOC. As discuss ed in section 3.30.2, Significant Increase of Hazardous Substances, of this support document, 53 samples within the Black Eagle residential area were documented as meeting the HRS criteri a for observed contamination.
However, as noted by ARCO, certain information co llected from various investigations of the Black Eagle residential area indicates that sampling locatio ns between the 53 AOC samples exhibit levels of arsenic, cadmium, or lead not significantly greater than background levels. For example, during the 2008 ESI of the Black Eagle residential area, field sa mple location RMTAV1721F12, collected between AOC sample locations, did not show levels of arsenic, lead , and/or cadmium at levels significantly greater than background. Therefore, no contamination was in ferred between the 53 AOC sample locations at promulgation (See Figure 2 of this support document).

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Changes to the evaluation of the soil exposure pathwa y waste characteristics and targets have been made at promulgation in association with the change to no longer infer contamination in areas lying between the AOC sampling locations. Specifically, the soil exposure pathway hazardous waste quantity area evaluation at promulgation has been reduced from the de fined area at proposal to “greater than 0”. Also, the count of soil exposure targets subject to Level II concentrations has been reduced at promulgation. Refer to section 3.31, Soil Exposure Pathway Waste Ch aracteristics, and section 3.31.2, Count of Level II Targets, of this support document for a discussion on the scoring of the soil exposure pathway waste characteristics and targets at promulgation. How this change impacts the overall site score is discussed in the corresponding sections described above.

3 .30.4 Hazardous Substance Deposition/Attribution

C omment : ARCO submitted comments challenging the a ttribution of hazardous substances in the observed contamination samples at the site. ARCO asserted that the concentrations of arsenic, cadmium, and lead in the samples show a sporadic and variab le distribution that is not consistent with aerial deposition from a single dominant source and that the pr evailing wind direction at the site is not toward the Black Eagle residential properties associated with the AOC. Additionally, ARCO asserted that concentrations of lead in residential soils are cons istent with degradation of lead-based paint and other consumer products. ARCO subm itted sampling data to support its assertion (see pages 88-112 of ARCO’s comments at Docket ID: EPA-HQ-SFUND-2010-0072-0016.1).
The following subsections of this support documen t address specific assertions regarding the evaluation of the hazardous substance attribution with the AOC:

• 3.30.4.1 Aerial Deposition

• 3.30.4.2 Evaluation of Lead


### 3.30.4.1 Aerial Deposition

Comment : ARCO challenged the attribution of hazardous substances in the AOC to an aerial deposition source, stating that “the data collected by EPA demons trates that concentrations of arsenic, cadmium, and lead in the residential soil throughout Black Eagle do not exhibit a spatial distribution that is consistent with aerial deposition.”
ARCO asserted that the HRS criteria for determining impacts from an aerial source are “quite simple,” and that:
a more relevant consideration of potential impacts related to an aerial deposition source would consider not only the presence of potentia lly source-related contaminants, but also the spatial distribution of contaminant c oncentrations relative to the source.

ARCO stated that a more relevant consideration of an aerial deposition source would consider more factors than just the presence of site-related hazar dous substances. ARCO argued that other factors should have been evaluated, such as the distribution of contaminant concentrations in the soils in relation to the source (both horizontally and vertically), whether the distribution corresponds with prevailing wind direction and distance from the source, and the consideration of other potential sources, such as commercial operators, such as Montana Refining Company, “a documented source of metals and arsenic in air emissions.” ARCO cited to a Montana Depart ment of Environmental Quality (MDEQ) database in support of this assertion. Further, ARCO asserted:
The data collected by EPA demonstrate that concentrations of arsenic, cadmium, and lead in the residential soil throughout Black Eagle do not exhibit a spatial distribution that is

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consistent with aerial deposition from the Form er GFR Site. If aerial emissions were the primary source of arsenic, cadmium, and lead in Black Eagle soil, the concentrations of these metals should be higher at locations that are closer to the former stack location, and decrease with distance from the former st ack location. In fact, the few higher concentration locations for arsenic and cadmium are located in the southwestern portion of Black Eagle, distant from the former smoke stack location and the higher concentrations often occurred in the deeper (6-12 inch bgs) soil horizon. The spatial distribution of arsenic, cadmium, and lead, as shown on Figures 5-3, 5-4, and 5-5, do not exhibit any sort of gradient that could be linked to emissions from the former smoke stack. Instead, and as detailed above, the concen trations of these metals in the residential yards show a sporadic and variable distribu tion throughout Black Eagle and also within individual properties that is not indicative of a single dominant source. “

ARCO stated that its 2009 residential soil inves tigation showed a variability of arsenic and lead concentrations by sample location and depth, as well as a lack of discernible pattern in the distribution of arsenic relative to lead. ARCO stated that arsenic concentrations were generally higher in deeper depth horizons (6-18 inches bgs) than in upper depth horizo ns (0-6 inches bgs), whereas lead concentrations were generally higher in the more sha llow (0-6 inches bgs) depth intervals.

Also, ARCO noted that the distribution of lead a nd arsenic concentrations in their sampling across different properties is relatively consistent between front and back yard samples, whereas the distribution of concentrations in garden, driveway, and dripline samples was less consistent between properties, which it asserted may be the result of differences in structur es between individual properties that could influence dripline lead concentrations or soil mixing in gardens and during driveway construction.
Response : The HRS documentation record has been re vised at promulgation to better explain the attribution of the hazardous substances in the AOC to em issions to the site and specifically to Source 3, the facility air emissions.

HRS Section 5.0.1, General considerations , states:
Evaluate the soil exposure pathway based on areas of observed contamination:
• Consider observed contamination to be present at sampling locations where analytic evidence indicates that:

–A hazardous substance attributable to the site [emphasis added] is present at a concentration significantly above background levels for the site (see table 2–3 in section 2.3 for the criteria for determining analytical significance), and
–This hazardous substance, if not present at the surface, is covered by 2 feet or less of cover material (for example, soil).

Establish areas of observed contamination b ased on sampling locations at which there is observed contamination as follows:

. . . . –For contaminated soil, consider both the sampling location(s) with observed contamination from the site and the area lying between such locations to be an area of observed contamination, unless available information indicates otherwise.

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• If an area of observed contamination (or portion of such an area) is covered by a permanent, or otherwise maintained, essentia lly impenetrable material (for example, asphalt) that is not more than 2 feet thick, exclude that area (or portion of the area) in evaluating the soil exposure pathway.

• For an area of observed contamination, consider only those hazardous substances that meet the criteria for observed contamination fo r that area to be associated with that area in evaluating the soil exposure pathway (see section 2.2.2).

As described in section 3.13, Additional Sour ces, of this support document, although the HRS documentation record at proposal did not designate the former stack at the ACM site as a source, emissions from the stack were discussed on page 12 of the HRS documentation record at proposal:
The Big Stack was constructed in 1908 to a le ngth of 506 feet and a total height of 751 feet above the smelter’s blast furnace (Ref. 8 p. 7A). The Big Stack went into service on June 12, 1909 and continued belching smoke until August 06, 1972 (Ref. 9, p. 15). The Big Stack was designed to eject a volume of 1,575,000 cubic feet of air per minute at a velocity of 450 feet per minute, a sufficien t volume and rate to entrain and remove from the smelter all the gases and dust generated dur ing the smelting process (Ref. 9, p. 20). . . . An internal Anaconda Copper Mining Company Memorandum from 1951 stated that approximately 15.04 pounds of arsenic passed out of the stack every 24 hours (Ref. 33, p. 1).
The HRS documentation record at proposal on page 12 explained that “aerial deposition of contaminants emanating from the smelter smoke stacks between 1893 and 1972 is the most likely mechanism of contaminants transport to the residential soils in th e community of Black Eagle.” The addition of Source 3 at promulgation provides for a clear link between th e hazardous substances associated with the former stack emissions, and the hazardous substances detected in the AOC – as lead, cadmium, and arsenic were associated with both the AOC and with the former stack emissions.
As presented on pages 38-39 of the HRS documentati on record at promulgation and references cited therein, arsenic was detected in emissions from S ource 3 and samples taken from flue dust associated with smelter emissions. Large quantities of flue dust, a by-product of smelting operation emissions, were accumulated at the former stack. This flue dus t was documented as containing arsenic, lead, and cadmium. These hazardous substances were also detected in the AOC located in the Black Eagle residential area as described in section 3.30.2, Sign ificant Increase of Hazardous Substances, of this support document.
Regarding ARCO’s assertion that other sources of metals in air emissions, such as the nearby Montana Refining Company facility, could have caused the significant increase of hazardous substances within the AOC, no information from this facility or any other nearby facilities documents that air emissions of arsenic, cadmium, and lead have occurred that coul d have caused the significant increase of hazardous substances found within the AOC. Although ARCO a lluded that the Montana Refining Company emitted metals to the air, the information provided by ARCO does not support such an assertion. The MDEQ database cited by the ARCO to support this asserti on only notes the cleanup and maintenance of solid wastes at that facility, and no informa tion regarding air emission of metals.

Thus, it is reasonable to conclude that the emissi ons from Source 3, facility air emissions, which are documented to contain arsenic, cadmium, and lead resu lting from operations at the site, have caused the significant increase of hazardous substances in the AOC.

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Evaluation of Wind Direction
Regarding ARCO’s questioning of whether the hazar dous substance distribution in the AOC corresponds with established wind direction data at the site, evidence of the wind blowing from the former stack, southwest toward the Black Eagle residential area has been established (see Figure 1 of this support document for locational information regarding the former stack in relation to the Black Eagle residential area). Page 64 of the HRS documentation record at proposal, under the subheading Attribution, states:

The wind direction in the Great Falls, Mont ana area is usually from the southwest, particularly in the months of October thr ough March. Wind data collected at the Great Falls International Airport, approximately 5 miles southwest of the community of Black Eagle shows that during March, April, and May the winds can blow from the northwest or northeast more than twenty percent of the time and that in the summer months of June, July, and August wind direction over Black Eagle is from the northeast or east more than 20 percent of the time (Ref. 30, pp. 2-13). A south facing aerial photo published by the Great Falls Tribune in 2002 shows smoke from the 506 foot tall stack wafting westward across the community of Black Eagle (Ref. 29, p. 8A).
Thus, the wind is documented to blow at least part of the time from the former stack location toward the Black Eagle residential area and the AOC, allowing for the transport of hazardous substances from the former stack to the AOC.
Regarding ARCO’s assertion that concentrations of hazardous substances detected in the Black Eagle residential area did not display a specific pattern, such as a gradient dissipating to lesser concentrations farther from the former stack location, this informat ion does not negate the fact that hazardous substances came to be located in the Black Eagle residential area from the smelter emissions. The HRS documentation record at promulgation does not concl ude that a predictable pattern by which hazardous substances were deposited in the Black Eagle reside ntial area was determined, nor does the HRS require this analysis be performed. As explained above, the HRS only requires that at least part of the significant increase be due to releases from the ACM site, and this requirement was met.
Furthermore, the deposition of airborne hazardous s ubstances into a predictable pattern, as asserted by ARCO, could not have been adequate ly determined without the use of extensive modeling, and does not consider the multiple dynamic factors at play (i.e., stack height, discharge temperatures, variability in wind direction). Also, as documented on page 12 of the HRS documentation record at proposal, operations at the facility changed ove r the long history from the late 180 0’s and into the 1970’s, such as the 1916 addition of electrolytic zinc and copper operati ons, and the closure of zinc refining operations in 1972. It is reasonable to conclude that stack emission composition and rates varied during these operational time periods and that th e potential influence on the deposition patterns of emitted substances would vary.
Further, residential soils can be disturbed by a variet y of factors, such as gardening, landscaping, paving, and work on public utilities. Even if reasonabl y precise air modeling from the former stack were completed, deposition patterns of h azardous substances within the soil of the Black Eagle residential area would still be influenced by these factors.
Therefore, because a specific discernible gradient of hazardous substances geographically across the Black Eagle residential area in relationship with the smelter emissions was not identified, does not discredit that the hazardous substances detected within the AOC are attributable to the site.

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Vertical Distribution of Hazardous Substances within Soil

ARCO’s assertion that arsenic was more often detect ed at concentrations of greater depth in the 2009 study it completed, and that lead was more often detected at more shallow depths, does not necessarily indicate and certainly does not demonstrate or prove that these hazardous substances were not deposited from the same aerial emission source.
Variability in the vertical contaminant distribution wi thin the Black Eagle residential soils was noted in ARCO’s 2009 study, as both arsenic and lead were de tected in both deep and shallow samples in the Black Eagle residential area.
Again, variations in former facility operations at the site would have influenced which hazardous substance and how much of that hazardous subs tance was emitted at va rious time periods, and disturbances in residential soils may have altered the relative location of hazardous substances within Black Eagle residential properties.
Regardless, HRS Section 5.0.1, General considerations , directs that in evaluating which hazardous substances are to be included in the establishment of observed contamination, the hazardous substance “ if not present at the surface, is covered by 2 feet or less of cover material (for example, soil).” Pages 53 of the HRS documentation record at proposal documents th at the samples used in identifying the AOC were collected less than 2 feet below ground surface, a nd that all AOC samples were documented to contain either arsenic, cadmium, or lead at levels greater th an 3 times background levels. It states: “[a]ll of the residential soil samples listed in the table [table beginning on page 54 of the HRS documentation record at proposal] were collected as grab samples from either the 0 to 2 or the 6 to 12 horizon...” Thus, it is reasonable to conclude that the emissions that ar e documented to contain arsenic, cadmium, and lead resulting from operations at the site have caused th e significant increase of hazardous substances in the Black Eagle residential area.

This comment has no effect on the HRS site score or the decision to list the site on the NPL.



### 3.30.4.2 Evaluation of Lead

Comment : ARCO asserted that detections of lead on residential properties within the AOC are indicative of deteriorating lead-based paint, varnishes, plumbing, or other consumer products. ARCO asserted that, of those concentrations that exceeded soil screening level for lead established by EPA, the pattern shown in the data is consistent with lead paint deterioration. ARCO stated that, among the shallower soil samples, higher lead concentrations were often associated with dripline sample locations, yet arsenic was not. ARCO stated that:
Given the age of housing stock in Black Eagle (most homes constructed prior to 1920), the presence of lead at these locations is not unexpected and generally higher concentrations associated with dripline samples is likely due to deteriorating lead-based paint. As shown in Table 5-5, paint chips were frequently identified in soil samples from the 2009 investigation where lead concentrations exceeded 400 mg/kg.”

Further, ARCO asserted that the HRS documentation reco rd at proposal attempted to justify the area of observed contamination by noting a "pervasive distribu tion of lead" in the residential soils, yet elevated lead concentrations were not used to justify the clai m that the area was contaminated. ARCO stated that “EPA's decision to use elev ated lead concentrations to support the breadth of its ‘area of contamination,’ but not the substance of its ‘area of contamination’ is transparent,” and that “EPA's CERCLA authorities do not generally support soils remediation to abate risks posed by deteriorating lead based paint.”

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Response : The significant increase in lead concentrations in samples collected from properties within the Black Eagle residential area was appropriately attr ibuted to releases of hazardous substances from the site. It is well documented that lead was emitte d from the facility and migrated via air toward the residential area of Black Eagle. Furthermore, the AOC samples were specifically collected from locations unlikely to be contaminated by th e lead sources identified by ARCO.
As discussed in section 3.30.4.1, Aerial Deposition, of this support document, HRS Section 5.0.1, General considerations , states:
Evaluate the soil exposure pathway based on areas of observed contamination:
• Consider observed contamination to be present at sampling locations where analytic evidence indicates that:

–A hazardous substance attributable to the site [emphasis added] is present at a concentration significantly above background levels for the site (see table 2–3 in section 2.3 for the criteria for determining analytical significance), and
–This hazardous substance, if not present at the surface, is covered by 2 feet or less of cover material (for example, soil).
As detailed on pages 38-39 of the HRS documentation r ecord at promulgation, flue dust samples collected from Source 3 (facility air emissions) document the presence of lead and several other hazardous substances as associated with facility emissions. Anal ytical data documentation shows that lead was one of the most common hazardous substances detected in the stack flue dust (Reference 58, p. 103 of the HRS documentation record at proposal). As emissions from the stack are documented to have migrated via air toward the Black Eagle residential area, it is reasonable to conclude that the hazardous substances migrating from the stack emissions and settling on the Black Eagle residential area would have included high amounts of lead.
Also, the sampling data used in establishing the AOC (the 2007 Site Assessment and 2008 Expanded Site Inspection) was collected using a sampling methodology cognizant of the potential influence of residential sources of lead. Sample sketches were developed for each AOC sample location detailing the spatial relationship between the sample locations and property features of note such as houses, driveways, and shed, etc. (Reference 5, pp. 149-208; Reference 21, pp. 98-161 of the HRS documentation record at proposal). These sketches demonstrate that the major ity of the sample locations were located away from buildings and often in the middle of an open area of the residential yards. For example, the sketches demonstrate that at least 32 of the AOC samples were collected at a distance of 5 feet or greater from any nearby structure on the residential properties - minimizing the potential influence from degrading paint or other lead containing materials.

Further, if lead were not included in the evalua tion of the AOC and overall soil exposure pathway, the hazardous substances arsenic and cadmium would still est ablish an AOC and targets would still be subject to Level I and Level II contamination for those substances within the AOC.
EPA also disagrees with ARCO’s unsupported stat ement that CERCLA authorities “do not generally support soils remediation to abate risks posed by de teriorating lead based paint.” CERCLA contains no provision precluding releases of lead from lead-based paint from coverage under CERCLA.

This comment has no effect on the site score or the decision to list the site on the NPL.

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3.30.5 Contamination below Cleanup Criteria

Comment : ARCO asserted that hazardous substances detect ed in samples associat ed with the AOC were below various screening levels and cleanup criteria. ARCO stated that “[n]one of the arsenic concentrations exceed a safe level established by EPA for Anaconda, Montana, a nearby similar site,” and that only one of the cadmium concentrations exceeds EPA's conservative soil screening level for cadmium.
Response : Hazardous substance levels identified as mee ting observed contamination criteria in the AOC evaluation were established according to the HRS; th ere is no HRS requirement that hazardous substance levels exceed cleanup or screening levels in orde r to establish an area of observed contamination.
As explained in section 3.7, Perm itted Releases and Regulatory Limits, of this support document, whether or not the level of a hazardous substance in a samp le is within applicable regulatory or otherwise acceptable limits does not eliminate that hazardous s ubstance from HRS scoring consideration or affect the establishment of an AOC. Section 3.30.2, Sign ificant Increase of Hazardous Substances, of this support document shows that a significant increase of hazardous substances was identified in the AOC according to the HRS.

Thus, that the levels of hazardous substances within specific soil samples at this site do not exceed cleanup or screening levels established at another Supe rfund site does not preclude the inclusion of those hazardous substances in the establishment of an AOC at the site in the HRS evaluation.

T his comment has no effect on the HRS site score or on the decision to list the site on the NPL.



### 3 .31 Soil Exposure Pathway Waste Characteristics

Comment : ARCO alleged that the waste characteristics factor category for the soil exposure pathway was incorrectly evaluated as the pathway hazardous waste quantity was incorrectly calculated. The following subsections of this support document address specifi c comments on this topic raised by ARCO.

• 3.31.1 Soil Exposure Pathway Hazardous Waste Quantity

• 3.31.2 Significant Increase of Hazardous Substances
3.31.1 Soil Exposure Hazardous Waste Quantity

Comment : ARCO claimed that its assessment of the AOC resulted in a smaller area from which the hazardous waste quantity would be evaluated. ARCO stated that it had:
calculated a revised area assigned value of 153,000 - 26,010 (17% pavement) = 142,290 x 69% (open grass/soil) = 98,180 ft. 98,180/ 34,000 = 2.8876, which is significantly less than the 32.5633 used by EPA.
Response : The evaluation of the soil exposure pathway hazardous waste quantity has been revised at promulgation to reflect that areas between soil samples are no longer inferred as part of the AOC. However, this revision results in a pathway waste ch aracteristics factor value of 18 at promulgation, the same as at proposal.
As discussed in section 3.30.3, AOC Delineation, of this support document, the delineation of the AOC in the Black Eagle residential area has been revised at promulgation.
In determining the hazardous waste quantity factor value for the AOC in the soil exposure pathway, HRS Section 5.1.2.2, Hazardous waste quantity , states:

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Assign a hazardous waste quantity factor value as specified in section 2.4.2. In estimating the hazardous waste quantity, use table 5–2 and:
• Consider only the first 2 feet of dept h of an area of observed contamination, except as specified for the volume measure.
• Use the volume measure (see section 2.4.2.1.3) only for those types of areas of observed contamination listed in Tier C of Table 5-2. . . .
• Use the area measure (see section 2.4.2. 1.4), not the volume measure, for all other types of areas of observed contamination, even if their volume is known.

HRS Section 2.4.2.2, Calculation of hazardous waste quantity factor value , states to:
[s]um the source hazardous waste quantity values assigned to all sources (including the unallocated source) or areas of observed cont amination for the pathway being evaluated and round this sum to the nearest integer, ex cept: if the sum is greater than 0, but less than 1, round it to 1. Based on this value, select a hazardous waste quantity factor value for the pathway from table 2–6.
Table 2-6 of the HRS, presented below, displays the criteria used in establishing the hazardous waste quantity factor value.
Hazardous Waste Quantity Value Assigned Value

0 0 1 a to 100 1 b Greater than 100 to 10,000 100 Greater than 10,000 to 1,000,000 10,000 Greater than 1,000,000 1,000,000 a If the hazardous waste quantity value is greater than 0, but less than 1, round it

to 1 as specified in text b For the pathway, if hazardous constituen t quantity is not adequately determined, assign a value as specified in the te xt; do not assign the value of 1.

HRS Section 2.4.2.2, Calculation of hazardous waste quantity factor value , continues: For the soil exposure pathway, if the h azardous constituent quantity is adequately determined for all areas of observed contamin ation, assign the value from table 2–6 as the hazardous waste quantity factor value. If the hazardous constituent quantity is not adequately determined for one or more areas of observed contamination, assign either the value from table 2–6 or a value of 10, whichever is greater , as the hazardous waste quantity factor value. [emphasis added]
Page 65 of the HRS documentation record at propos al documented that data were not available to evaluate Tier A, hazardous constituent quantity, Tier B, hazardous wastestream quantity, or Tier C, volume, in the evaluation of the soil exposur e pathway hazardous waste quantity. The HRS documentation record at proposal explained that Tier D, area, was evaluated, and that an area assigned value was calculated. It stated:
Arsenic and cadmium results from soil samples collected in the summers of 2007 and 2008 that are documented to be at Leve l I concentrations delineate an area of approximately 1,926,569 square feet (ft2) (Ref. 1, Sections 2.5.1, 2.5.2, 5.1.3.1; Ref. 22,

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p. 2). The area covered by streets within th e area was calculated to be 322,000 square feet (Ref. 25, p. 1). It was calculated that an average of 31 percent of each lot in Black Eagle, Montana is covered by a structure or pavement or 69 percent of the average property is open soil (Ref. 26, p. 3).
(1,926,569 ft 2 of contaminated soil) – (322,000 ft 2 of pavement) = 1,604,569 ft 2

. (1,604,569 ft 2 )(0.69 open soil) = 1,107,152.61ft 2

Thus, according to HRS Table 5-2, the value of 1,107,152.6 ft 2 calculated above for contaminated soil was divided by 34,000 (the divisor used for contam inated soil according to HRS Table 5-2) and an area assigned value of 32.5633 was calculat ed at proposal (1,107,152.61(ft 2 ) / 34,000 = 32.5633). This value equated to an assigned value of 1 from HRS Table 2-6. Footnote B of HRS Table 2-6, directs the scorer to assign a value as specified in th e text, instead of from HRS Table 2-6, if the hazardous constituent quantity was not adequately determined. As the hazardous constituent quantity was not adequately determined, a value of 10 was assigned as the hazard ous waste quantity factor value at proposal, in accordance with HRS Section 2.4.2.2 cited above.

However, as discussed in section 3.30.3, AOC Delineati on, of this support document, the delineation of the AOC in the Black Eagle residential area has been re vised at promulgation to reflect that contamination is no longer inferred between the AOC sample locations . At promulgation, as at proposal, no information was available to determine the hazardous constituen t quantity (Tier A), hazardous wastestream quantity (Tier B), and volume (Tier C) measures in eval uating the soil exposure hazardous waste quantity. Therefore, because the presence of hazardous substances meeting observe d contamination criteria within the AOC samples has been established and a defined contiguous area comprising the AOC has not been evaluated; the area of the AOC is evaluated as “greater than 0” at promulgation.
According to HRS Table 2-6, a value of “greater th an 0” corresponds to a hazardous waste quantity factor value of 1. Footnote B of HRS Table 2-6, states that “if the hazardous constituent quantity is not adequately determined, assign a value as specified in the text; do not assign the value of 1.” As the hazardous constituent quantity for this AOC was not ade quately determined at promulgation, a value of 10 was assigned as the hazardous waste quantity factor value at promulgation.

Furthermore, even if ARCO’s revised area value of 2.8876 for the AOC were used, it would not change the hazardous waste quantity factor value assigned at proposal. That is, ARCO’s area assigned value for the AOC [2.8876] would be subject to HRS Section 2. 4.2.2 cited above, which states that in situations when the hazardous constituent quantity is not ade quately determined—as is the case here—a hazardous waste quantity factor less than 10 determined by HRS Table 2-6 is replaced with a value of 10.

This comment has no effect on the HRS site score or the decision to place the site on the NPL.
3.31.1 Calculation of Soil Exposure Pathway Waste Characteristics

Comment : ARCO asserted that a value of 10 should have been assigned to the waste characteristics factor category value, instead of the value of 18 assigned at proposal
Response : The soil exposure pathway waste characteristics factor value of 18 assigned at proposal was correct according to the HRS; revisions to the hazar dous waste quantity evaluation have not changed this value at promulgation.
The soil exposure pathway waste characteristics value reflects the toxicity and quantity of the hazardous substances associated with the AOC. HRS Section 5.1.2 Waste characteristics , states:

103ACM Smelter & Refinery NPL Listing Support Document March 2011

Evaluate waste characteristics based on two factors: toxicity and hazardous waste quantity. Evaluate only those hazardous substances that meet the criteria for observed contamination at the site (see section 5.0.1).

HRS Section 5.1.2.3, Calculation of waste characteristics factor category value , directs the scoring of the waste characteristics factor category value. It states:
Multiply the toxicity and hazardous waste quantity factor values, subject to a maximum product of 1×10 8 . Based on this product, assign a valu e from table 2–7 (section 2.4.3.1) to the waste characteristics factor category.

Because the hazardous substances scored in the soil exposure pathway in the HRS documentation record at proposal are the same as those at promulgation (ar senic, cadmium, and lead), the toxicity factor value remains 10,000 as presented in section 5.1.2.1, Toxicity, of the HRS documentation record at promulgation. ARCO did not challenge this toxicity factor value in its comments.

As discussed in section 3.31.1, Soil Exposure Pathway Hazardous W aste Quantity, of this support document, the hazardous waste quantity factor value at promulgation is 10. Therefore, in accordance with HRS Section 5.1.2.3, Calculation of waste characteristics factor category value, the toxicity value of 10,000, multiplied by the hazardous waste quantity factor value of 10, results in a product of 100,000, which according to HRS Table 2-7 corresponds to a waste characteristics factor category value of 18. This waste characteristics factor category valu e assigned at promulgation remains unchanged from proposal.

T his comment has no effect on the HRS site score or the decision to list the site on the NPL.



### 3 .32 Soil Exposure Targets

C omment : ARCO submitted comments challenging the estab lishment of soil exposure targets in the HRS documentation record at proposal. The following sub sections of this support document address specific comments raised by the commenter on this topic.

• 3.32.1 Identification of Actually Contaminated Targets

• 3.32.2 Count of Level II Concentrations

• 3.32.3 Targets Not Scored
3.32.1 Identification of Actually Contaminated Targets

Comment : ARCO questioned the establishment of actually contaminated targets in the evaluation of the soil exposure pathway. ARCO stated that EPA delineated the AOC using soil samples from two different depth intervals (0-2 inches and 6-12 inches below gr ound surface [bgs]) and that residents would have the greatest exposure to contaminants in surface soil (0-2 inches bgs); ARCO noted that many of the higher concentrations in residential soils are present at depths greater than 6 inches bgs.
Response : The number of residences considered actua lly contaminated based on the presence of contamination on the property and within 200 feet of the residence was correctly identified at the time of proposal. The HRS establishes AOCs based on contam ination documented within 2 feet of the soil surface. The exact amount of exposure that results from this contamination is determined at a later stage of the Superfund process when a site-sp ecific risk assessment is performed.

HRS Section 5.0.1, General considerations , states:
Evaluate the soil exposure pathway based on areas of observed contamination:

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• Consider observed contamination to be present at sampling locations where analytic evidence indicates that:
–A hazardous substance attributable to the site is present at a concentration significantly above background levels for th e site (see table 2–3 in section 2.3 for the criteria for determining analytical significance), and
–This hazardous substance, if not present at the surface, is covered by 2 feet or less of cover material (for example, soil).

HRS Section 5.1.3, Targets , states:
Evaluate the targets factor category for the resident population threat based on five factors: resident individual, resident population, workers, resources, and terrestrial sensitive environments.
In evaluating the targets factor category for the resident population threat, count only the following as targets:
• Resident individual—a person living or attending school or day care on a property with an area of observed contam ination and whose residence, school, or day care center, respectively, is on or within 200 feet of the area of observed contamination.
• Worker—a person working on a property with an area of observed contamination and whose workplace area is on or within 200 feet of the area of observed contamination.
• Resources located on an area of observed contamination, as specified in section 5.1.

• Terrestrial sensitive environments located on an area of observed contamination, as specified in section 5.1.
The HRS documentation record at proposal presente d that samples meeting observed contamination criteria were taken on or within 200 feet of the resi dential properties. Section 5.1, Resident Population Threat, of the HRS documentation record at proposal stated:
Observed contamination samples were selected from the Analytical Results Reports for the 2007 CERCLA Site Assessment and from the 2008 Expanded Site Investigation to establish laboratory verified concentrations of arsenic lead, and cadmium, as well as sample ID, street address, property owner, and the date each sample was collected (Ref. 21, pp. 29-32; Ref. 5, pp. 28-31). All samples listed in Section 5.0.1 of this HRS documentation record were collected within 200 feet of the residence (Ref. 21, p. 1; Ref. 5, pp. 4 and 5). Analysis of sample R12ST0121FY12 (MH26N5) collected on July 11, 2008, with an arsenic concentration of 215 mg/kg, a cadmium concentration of 13 mg/kg, and a lead concentration of 706 mg/kg in surface soils from a collection depth < 2 feet bgs documents the resident population threat (Ref. 5, Appendix C, p. 49. Appendix D, p.8; Ref. 24, p. 1; Ref. 36, book 3, p. 17; Ref. 63 p. 15)

Since observed contamination was found to be within 2 feet or less of the surface, the samples at both 0 to 2 inch and 6 to 12 inch soil horizons were properly used to identify the AOC. Correspondingly, the

105ACM Smelter & Refinery NPL Listing Support Document March 2011

residences on the property and within 200 feet of th e sample locations were appropriately considered actually contaminated targets.

Furthermore, as the HRS allows for the evaluation of observed contamination within 2 feet or less of the ground surface; ARCO’s assertions regarding the lack of risk of exposure of deeper contamination are a challenge to the HRS itself in the establishment of observed contamination within an AOC. As explained in section 3.10, Comments on HRS, of this support do cument, comments on the HRS rule are not relevant to the proposal to place the ACM site on the NPL and are outside the scope of this rulemaking.

This comment has no effect on the site score or the decision to list the site on the NPL.
3.32.2 Count of Level II Targets

Comment : ARCO asserted that the evaluation of the number of residences exposed to Level II concentrations as presented in the HRS documentation record at proposal is incorrect. ARCO alleged that the AOC delineation as proposed incorrectly included soil sample locations that did not have arsenic and cadmium concentrations greater than three times background and exceeding their respective SCDM benchmarks.
ARCO presented its own AOC evaluation that excl uded these soil samples, and asserted that only 42 residences are associated with Level II concentrations , as opposed to the 170 resi dences associated with Level II concentrations in the HRS documentation record at proposal.
Response : The number of targets subject to Level II concen trations has been reduced in the scoring of the resident population soil exposure targets in the HRS documentation record at promulgation to reflect the decision to no longer infer contamination between AOC samples. However, as discussed in section 3.30.3, AOC Delineation/Inference, of this suppor t document, the 53 AOC samples are appropriately documented as containing hazardous substances meeti ng observed contamination criteria, and targets associated with these individual sampling points are correctly considered exposed to Level II concentrations.

HRS Section 5.1.3, Targets , states:
Evaluate the targets factor category for the resident population threat based on five factors: resident individual, resident population, workers, resources, and terrestrial sensitive environments.
In evaluating the targets factor category for the resident population threat, count only the following as targets:
• Resident individual—a person living or attending school or day care on a property with an area of observed contam ination and whose residence, school, or day care center, respectively, is on or within 200 feet of the area of observed contamination.

HRS Section 5.1.3.2, Resident population , states:
Evaluate resident population based on two f actors: Level I concentrations and Level II concentrations. Determine which factor appli es as specified in sections 2.5.1 and 2.5.2, using the health-based benchmarks from table 5–3. Evaluate populations subject to Level I concentrations as specified in section 5.1.3.2.1 and populations subject to Level II concentrations as specified in section 5.1.3.2.2.

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HRS Section 5.1.3.2.2, Level II concentrations , states:
Sum the number of resident individuals subject to Level II concentrations. Do not include those people already counted under the Level I concentrations factor. Assign this sum as the value for this factor. Enter this value in table 5–1.
Pages 68 of the HRS documentation record at proposal presented the evaluation of how targets subject to Level II contamination were scored:
Properties that are wholly or partially incl uded within the zone defined by arsenic and cadmium contamination at Level I concentra tions include approximately 178 residences that are within 200 feet of the zone of contam ination (Ref. 22, p. 2; Ref. 37, p. 1). Eight of these residences have documented Level I contamination within 200 feet of the residence leaving approximately 170 residen ces exposed to Level II contamination (Ref. 22. page 2; Ref. 37, p. 1). 170 residences with a county average of 2.41 individuals per residence results in 409.7 people being exposed to Level II Concentrations (Ref. 28, p. 1)
At promulgation, all 53 samples taken from the AOC we re determined to be on r esidential properties, and residences are documented within 200 feet of each AO C sample – the same as at proposal (see Figure 2 of the HRS documentation record at promulgation). Of the 53 residences within 200 feet of the AOC samples, 8 of the residences were already included in the evaluation of Level I concentrations. Because no contamination is inferred between the AOC sample s, at promulgation 45 residential properties were evaluated for Level II concentrations.
Thus, at promulgation, 108.45 individuals are subject to Level II concentrati ons, considering the county average of 2.41 persons per residence (as directed by HRS Section 5.1.3.2, Resident population ) (45 residences multiplied by 2.41 persons per residence = 108.45 persons).

This comment has no effect on the site score or the decision to list the site on the NPL.
3.32.3 Targets Not Scored

Comment : In its reassessment of the soil exposure pa thway score, ARCO made assertions regarding the lack of scoring potential of other targets. ARCO st ated that workers were not scored in the targets evaluation “because the area of contaminated soil was scored for residents.” Also, ARCO asserted that the nearby population threat did not warrant scori ng because “the data does not support the presence of such threat.”
Response : ARCO incorrectly asserted that both workers and the nearby population threat could not have been scored as part of the HRS documentation record at proposal.

H RS Section 5.1.3, Targets , states:

E valuate the targets factor category for the resident population threat based on five factors: resident individual, resident population, workers, resources, and terrestrial sensitive environments.

I n evaluating the targets factor category for the resident population threat, count only the following as targets:

• Resident individual...

107ACM Smelter & Refinery NPL Listing Support Document March 2011

• Worker – a person working on a property with an area of observed contamination and whose workplace area is on or within 200 feet of the area of observed contamination.

• Resources...

HRS Section 5.1.3.3, Workers , states:

Evaluate this factor based on the number of wo rkers that meet the section 5.1.3 criteria. Assign a value for these workers using table 5-4. Enter this value in table 5-1.

HRS Section 5.2, Nearby Population Threat , states:
Include in the nearby population only those i ndividuals who live or attend school within a 1-mile travel distance of an area of observed contamination at the site and who do not meet the criteria for resident individual as specified in section 5.1.3.
The HRS documentation record at proposal did not document the presence of workers, or potentially affected nearby population targets, that could have been scored in the soil exposure targets evaluation. The scoring of workers or nearby population targets would not have significantly added to the HRS site score, as a site score of greater than 28.50 was established.
Note, however, that although page 68 of the HRS docum entation record at proposal stated that workers were not scored in the targets evaluation of the so il exposure pathway, this statement does not establish that workers could not have been scored in the targets evaluation of the soil exposure pathway. For instance, a church located within 200 feet of the AOC (located at 2200 Smelter Avenue, Black Eagle; Ref. 18, pp. 27-30 of the HRS documentation record at proposal) could have been potentially evaluated for workers (e.g., the minister) concurrently with reside nt individuals under the resident population threat scoring.
In addition, further analysis of the nearby population threat could have resulted in the scoring of nearby populations, as the AOC is located within a residential area near schools (Ref. 4, Map 1 of the HRS documentation record at proposal).

This comment has not effect on the site score or the decision to list the site on the NPL. 

### 3.33 Soil Exposure Pathway – Reside nt Population Threat Total Score

Comment : In determining an alternate score for the soil exposure hazardous waste quantity and targets, ARCO asserted that the resulting score for the soil ex posure pathway resident population threat should be 1,892,110, instead of 6,459,750 assigned in the HRS documentation record at proposal.
Response : Incorporating the changes made to the waste quantity yields a soil exposure pathway resident population threat score of 3,477,375 in the HRS documentation record at promulgation.

HRS Section 5.1.3.6, Calculation of resident populati on targets factor category value , states to:
[s]um the values for the resident individual, resident population, workers, resources, and terrestrial sensitive environments factors. Do not round to the nearest integer. Assign this sum as the targets factor category value for the resident population threat.

HRS Section 5.1.3.2.3, Calculation of resident population factor value , states to:

108ACM Smelter & Refinery NPL Listing Support Document March 2011

[s]um the factor values for Level I concentra tions and Level II concentrations. Assign this sum as the resident population factor value.
ARCO’s comments did not challenge the reside nt individual value of 50 assigned in the HRS documentation record at proposal. ARCO also did no t challenge the Level I concentrations value of 192.8 assigned in the HRS documentation record at proposal. These values remain unchanged at promulgation.
Section 3.32.2 Count of Level II Targets, of this support document explains that 108.45 individuals are subject to Level II concentrations.
Therefore, per HRS section 5.1.3.2.3 quoted above, the resident population factor value at promulgation is 301.25 (192.8 + 108.45).
Per HRS Section 5.1.3.6 quoted above, the resident popula tion targets factor category value is thus 351.25 (50 + 301.25). Note that workers, resources, and terrestrial sensitive environments were not scored.

HRS Section 5.1.4, Calculation of resident population threat score , states to:
[m]ultiply the values for likelihood of exposure, waste characteristics, and targets for the resident population threat, and round the produc t to the nearest integer. Assign this product as the resident population threat score
The likelihood of exposure factor category value of 550 assigned in the HRS documentation record at proposal remains unchanged at promulgation; this value was not challenged by ARCO.
As explained in section 3.31.1, Calculation of So il Exposure Pathway Waste Characteristics, of this support document, the value of 18 assigned as the so il exposure pathway waste characteristics factor value assigned at proposal was corre ct according to the HRS, and revisions to the hazardous waste quantity evaluation have not change d this value at promulgation.
Per HRS Section 5.1.4 quoted above, the reside nt population threat score assigned in the HRS documentation record at promulgation is 3,477,375 (550 x 18 x 351.25). The effects of the changes to the resident population threat score on the soil exposure pathway are detailed in section 3.34, Soil Exposure Pathway – Total Score, of this support document; the effects on the total site score are taken into account in section 3.35, Alternate HRS Score, of this support document.

This comment has no effect on the decision to list the site on the NPL. 

### 3.34 Soil Exposure Pathway – Total Score

Comment : ARCO stated that it had rescored the soil exposure pathway. ARCO asserted that the score for the surface water overland/flood migration pathway should be 22.93, instead of 78.3 as assigned in the HRS documentation record at proposal.
Response : The soil exposure component score assigned in the HRS documentation record at promulgation is 42.15.

HRS section 5.3, Calculation of soil exposure pathway score , states to:
[s]um the resident population threat score and the nearby population threat score, and divide the sum by 82,500. Assign the resulting value, subject to a maximum of 100, as the soil exposure pathway score (S s ).

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As described in section 3.33, Soil Exposure Pathway – Resident Population Threat Total Score, of this support document, the resident population threat score assigned in the HRS documentation record at promulgation is 3,477,375.
Section 3.32.3, Targets Not Scored, of this support document explains that the nearby population threat was not scored.
Therefore, the soil exposure pathway score assigned at promulgation is 42.15 (3 ,477,375 / 82,500). The effects of the changes to the soil exposure pathway sco re on the total site score are taken into account in section 3.35, Alternate HRS Score, of this support document.

This comment has no effect on the decision to list the site on the NPL.



### 3.35 Alternate HRS Score

Comment : ARCO asserted that the Surface Water Migr ation Pathway Score of 100.00 and the Soil Exposure Pathway Score of 78.30 in the HRS documentation record at proposal are “grossly inflated.” ARCO presented an alternative HRS site score it considers appropriate for the ACM site.
ARCO argued that the surface water migration pathwa y in the HRS documentation record at proposal is incorrect. ARCO stated that a more realistic alternative HRS score for the surface water migration pathway would only assess the potential to release of hazardous substances from soil in the East Ditch Dump (Source 1) and Black Eagle residential soils (Source 2), and would result in a pathway score of0.18. ARCO included an attachment with its comments (Attachment 1, DOCKET # EPA-HQ-SFUND2010-0072, Document # 0016) to suppor t its contention that the surface water migration pathway should be rescored.
For the soil exposure pathway, ARCO asserted that “EPA selectively used the available soils data to drive the HRS score beyond the threshold required for listing.” ARCO asserted that two smaller areas of contamination should have been assessed, rather th an the AOC used in scoring. Based on using two smaller areas of contamination, ARCO assigned an alternate soil exposure pathway score of 22.93 (presented in Attachment 1 of ARCO’s comments at Docket # EPA-HQ-SFUND-2010-0072, Document # 0016).
ARCO submitted revised HRS scoresheets to illustrate their rescoring of the site, which present ARCO’s interpretation of each scoring factor value and score component.
Thus, ARCO argued that the overall site score shoul d be 11.47, which would be lower than the 28.50 score threshold for inclusion of a site on the NPL.
Response: After considering all comments received on the proposed NPL listing of the ACM site, and making revisions where appropriate as described thr oughout this support document, the final ACM site score is 54.26. The following HRS worksheet docum ents the final HRS scoring; this worksheet and detailed scoresheets for the surface water overland/flo od migration and soil exposu re pathways appear on pages 2-6 of the HRS documentation record at promulgation. Following the worksheet, are HRS scoresheets for the surface water migration and soil exposure pathways that identify the components of the site score at promulgation shown alongside the scores at proposal and the scores presented by ARCO in its comments. The far right column on the sco resheets cites the section(s) within this support document where each scoring factor or component is addressed. The comparison scoresheets are provided below to facilitate understanding of the changes made to the score at promulgation.

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WORKSHEET FOR COMPUTING HRS SITE SCORE

S S 2

1. Groundwater Migration Pathway Score (S gw ) NS NS (from Table 3-1, line 13)

2a. Surface Water Overland/Flood Migration Component 100 (from Table 4-1, line 30)

2b. Groundwater to Surface Water Migration Component NS (from Table 4-25, line 28)

2c. Surface Water Migration Pathway Score (S sw ) 100 10,000.00 Enter the larger of lines 2a and 2b as the pathway score.

3. Soil Exposure Pathway Score (S s ) 42.15 1,776.62 (from Table 5-1, line 22)

4. Air Migration Pathway Score (S a ) (from Table 6-1, line 12) NS NS

5. Total of S gw 2 + S sw 2 + S s 2 + S a 2 11,776.62

6. HRS Site Score Divide the value on line 5 by 4 and take the square root 54.26

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SURFACE WATER OVERLAND/FLOOD MIGRATION COMPONENT SCORE SHEET

Factor Categories and Factors Value Assigned at

Proposal ARCO Provided Explanation in Section # of this Support Document* Value Assigned at Promulgation Value

Drinking Water Threat

Likelihood of Release : 1. Observed Release 2. Potential to Release by Overland Flow: 2a. Containment 2b. Runoff 2c. Distance to Surface Water 2d. Potential to Release by Overland Flow (lines 2a[2b+2c]) 3. Potential to Release by Flood: 3a. Containment (Flood) 3b. Flood Frequency 3c. Potential to Release by Flood (lines 3a × b) 4. Potential to Release (lines 2d+3c, subject to a maximum of 500) 5. Likelihood of Release (higher of lines 1 and 4) 550

NS NS NS NS

NS NS NS NS

550 0

9 1 20 189

10 7 70 259

259 550

NS NS NS NS

NS NS NS NS

550 3.19, 3.20

3.19, 3.20

Waste Characteristics : 6. Toxicity/Persistence 7. Hazardous Waste Quantity 8. Waste Characteristics NS NS NS NS NS NS NS NS NS Targets : 9. Nearest Intake 10. Population 10a. Level I Concentrations 10b. Level II Concentrations 10c. Potential Contamination 10d. Population (lines 10a+10b+10c) 11. Resources 12. Targets (lines 9+10d+11) NS

NS NS NS NS NS NS NS

NS NS NS NS NS NS NS

NS NS NS NS NS NS Drinking Water Threat Score : 13. Drinking Water Threat Score ([lines 5 × 8 × 12]/82,500, subject to a maximum of 100) NS NS NS

Human Food Chain Threat

Likelihood of Release : 14. Likelihood of Release (same value as line 5) 550 259 550 3.19, 3.20
Waste Characteristics : 15. Toxicity/Persist ence/Bioaccumulation 16. Hazardous Waste Quantity 17. Waste Characteristics 500,000,000 100 320 50,000,000 1 56 500,000,000 100 320 3.23.1 3.23.2 3.23.3 Targets : 18. Food Chain Individual 19. Population 19a. Level I Concentrations 19b. Level II Concentrations 19c. Potential Human Food Chain Contamination 19d. Population (lines 19a+19b+19c) 20. Targets (lines 18+19d) 45

0 0.03 0.000003 0.030003 45.030003 1

0 0 0.000003 0.000003 1.000003 45

0 0.03 0.000003 0.030003 45.030003 3.19, 3.20 3.24.1

3.24.1, 3.24.2

3.24.1, 3.24.2 3.24.1, 3.24.2

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SURFACE WATER OVERLAND/FLOOD MIGRATION COMPONENT SCORE SHEET

Explanation in Section # of this Support Document* Value ARCO Value Assigned at Promulgation Factor Categories and Factors Assigned at Provided Proposal Value

Human Food Chain Threat Score : 21. Human Food Chain Threat Score ([lines 14 × 17 × 20]/82,500, subject to a maximum of 100) 96.06 0.18 96.06 3.25

Environmental Threat

Likelihood of Release : 22. Likelihood of Release (same value as line 5) 550 259 550 3.19, 3.20
Waste Characteristics : 23. Ecosystem Toxicity/Persistence/Bioaccumulation 24. Hazardous Waste Quantity 25. Waste Characteristics 500,000,000 100 320 500,000,000 1 100 500,000,000 100 320 3.26 3.26 Targets : 26. Sensitive Environments 26a. Level I Concentrations 26b. Level II Concentrations 26c. Potential Contamination 26d. Sensitive Environmen ts (lines 26a+26b+26c) 27. Targets (value from line 26d) 0 25 0 25 25 0 0 0.025 0.025 0.025 0 25 0 25 25 3.27.1, 3.27.2

Environmental Threat Score : 28. Environmental Threat Score ( [lines 22 × 25 × 27]/82,500, subject to a maximum of 60) 53.33 0.0078 53.33 3.28
Surface Water Overland/Flood Migration Component Score for a Watershed
29. Watershed Score (lines 13+21+28, subject to a maximum of 100) 100 0.18 100 3.29

Surface Water Overland/Flood Migration Component Score
30. Component Score (S of ) c (highest score from line 29 for all watersheds evaluated, subject to a maximum of 100) 100 0.18 100 3.29

NS = Not Scored. * = See Table of Conten ts of this support document.

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SOIL EXPOSURE PATHWAY SCORE SHEET

Factor Categories and Factors Value Assigned at Proposal Explanation in Section # of this Support Document* ARCO Provided

Value Value Assigned at Promulgation

RESIDENT POPULATION THREAT Likelihood of Exposure 1. Likelihood of Exposure 550 550 550 3.30

Waste Characteristics 2. Toxicity 10,000 10,000 10,000 3. Hazardous Waste Quantity 10 2.8876 10 3.31.1
4. Waste Characteristics 18 10 18 3.31 Targets
5. Resident Individual 50 50 50 6. Resident Population
6a. Level I Concentrations 192.8 192.8 192.8
6b. Level II Concentra tions 409.7 101.22 108.45 3.32.2 6c. Resident Population (lines 6a + 6b) 602.5 294.02 301.25 3.32.2, 3.32.3

7. Workers 0 0 0
8. Resources 0 0 0
9. Terrestrial Sensitiv e Environments 0 0 0 10. Targets (lines 5 + 6c + 7 + 8 + 9) 652.5 344.02 351.25 3.32, 3.33

Resident Population Threat Score 11. Resident Population Threat (lines 1 x 4 x 10) 6,459,750 1,892,110 3,477,375 3.33

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SOIL EXPOSURE PATHWAY SCORE SHEET

Factor Categories and Factors Value Assigned at

Proposal ARCO Provided

Value Explanation in Section # of this Support Document* Value Assigned at Promulgation

NEARBY POPULATION THREAT Likelihood of Exposure 12. Attractiveness/Accessibility NS NS NS 13. Area of Contamination NS NS NS 14. Likelihood of Exposure NS NS NS Waste Characteristics 15. Toxicity NS NS NS 16. Hazardous Waste Quantity NS NS NS 17. Waste Characteristics NS NS NS Targets 18. Nearby Individual NS NS NS 19. Population Within 1 Mile NS NS NS 20. Targets (lines 18 + 19) Nearby Population Threat Score 21. Nearby Population Threat (lines 14 x 17 x 20) NS NS NS

SOIL EXPOSURE PATHWAY SCORE Soil Exposure Pathway Score d 22. (S s ), (lines [11 + 21] ÷ 82,500 subject to a maximum of 100) 78.3 22.93 42.15 3.31

* = See Table of Conten ts of this support document.

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116 

## 4.0 Conclusion

The original HRS score for this site was 63.50. Based on the above responses to comments and the modifications detailed in section 3.35, Alternate HRS Score, of this support document, the HRS score at promulgation is 54.26. The final scores for the ACM Smelter and Refinery site are:

Ground Water Not Scored Surface Water 100 Soil Exposure 42.15 Air Not Scored
HRS Site Score 54.26