DEPARTMENT OF THE ARMY BASE REALIGNMENT AND …

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DEPARTMENT OF THE ARMY BASE REALIGNMENT AND CLOSURE

ROCKY MOUNTAIN ARSENAL /~0f 5650 HAVANA STREET 2!.. ~~d

BUILDING 129 COMMERCE CITY, COLORADO 80022-1748

DAIM-ODB-RM 15 January 2009

MEMORANDUM FOR U.S. Environmental Protection Agency, (Mr. Greg Hargreavcs), Region VlIl, Mail Code 8EPR-F, 1595 Wynkoop Street, Denver, Colorado 80202-1129

SUBJECT: Explanation of Significant Differences for North Plants Soil Remediation Project Rocky Mountain Arsenal Federal Facility Site, Revision 0

I. Enclosed for your information is the signed Explanation of Significant Differences for orth Plants Soil Remediation Project Rocky Mountain Arsenal Federal Facility Site, Revision O. Thc final draft was provided for review and comment on 10 December 2008.

2. If you have any questions, please contact Mr. James L. Green at 303-289-0412.

6..«/- /7J f.I-~.L~./.. Enel BRUCE M. HUEM"EFELD

RMA Committee Coordinator

CF: Rocky Mountain Arsenal, (DAIM-BD-A-RM-CLIMr. M. Weslyn Erickson), ChiefCounscl, 5650 Havana Street, Building 129, Commerce City, Colorado 80022-1748 (wo/encl)

U.S. Environmental Protection Agency, (Mr. Ron Bertram), Region VIII, Mail Code 8EPR-F, 1595 Wynkoop Street, Room 5136, Denver, Colorado 80202-1129 (w/encl)

Pacific Western Technologies, Ltd, (Mr. Jim Bush/Mr. Levi Todd), 11049 West 44th Avenue, Suite 200, Wheat Ridge, Colorado 80033 (w/encl 2 copies)

Shell Oil Company, (Mr. Roger B. Shakely), P.O. Box 538, Commerce City, Colorado 80037 (wo/enel)

Washington Group, (Mr. Mark Thomson), P.O. Box 1717, Conm1erce City, Colorado 80022 (wo/encl)

Holme Roberts and Owens, (Mr. Daniel J. Dunn), 1700 Lincoln Street, Suite 4100, Denver, Colorado 80203 (wo/enel)

U.S. Fish and Wildlife Service, (Mr. Tom Jackson), Rocky Mountain Arsenal, 5650 Havana Street, Building 129, Commerce City, Colorado 80022-1748 (wo/encl)

Tri-County Health Department Environmental Health Division, (Ms. Melody H. Mascarenaz), 4201 East nnd Avenue, Commerce City, Colorado 80022-1488 (wo/enel)

Rocky Mountain Arsenal, (Document Tracking Center), 5650 Havana Street, Building 129. Commerce City, Colorado 80022-1748 (wo/enc!)

EXPLANATION OF SIGNIFICANT DIFFERENCES FOR NORTH PLANTS SOIL REMEDIATION PROJECT

ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE

Prepared by: Tetra Tech EC, Inc.

Prepared for: Rocky Mountain Arsenal Comminee

Department of the Army Shell Oil Company

U.S. Fish and Wildlife Service U.S. Environmental Protection Agency

Colorddo Department of Public Health and Environment

This document is the property of Rocky Mountain Arsenal Remediation Venture Office and was prepared by Tetra Tech EC, Inc. It is provided on the condition that it will neither be reproduced, copied, or issued to a third party; will be used solely for the intended purpose; and wil1 be used solely for the execution or review of the engineering, remediation, and/or construction of the subject project.

Revision Pre arcd By Reviewed Bv A proved By Dale Pages Affected

o S. Ache J. Schmuck B. DiDonato December 10. 2008 All

Rocky Mountain Arsl.:naJ Explanation of Signiticarlt Ditferences Revision 0North Plants Soil

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CONTENTS

1.0 INTRODUCTION I

2.0 SITE HISTORY, CONTAMINATION AND SELECTED REMEDy 2

2.1 RMA Operational History 2 2.2 North Plants History and Contamination Summary 3 2.3 Summary of the Selected On-Post Remedy 5 2.4 Summary of the Selected Remedy for North Plants Soi!.. 5 2.5 North Plants Remedy Implementation : 6

3.0 BASIS FOR THE ESD 6

3.1 Soil Remediation 7. 3.1.1 Soil Excavation 7. 3.1.2 Soil Sampling and Analysis 8 3.1.3 Residual Ecological Risk SoiL 9 3.1.4 Petroleum Contaminated Soil 10 3.1.5 Soil Remediation Summary 13

3.2 Sanitary Sewer Remediation 13 3.3 Underground Utility Removal 14 3.4 Structure Foundation Removal 14 3.5 Potential Agent Contamination 15 3.6 MEC Clearance Activities 16 3.7. Grading and Revegetation 16

4.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES 16

4.1 Changes to North Plants Soil Remediation 16 4.2 Summary of Cost Change 18

5.0 SUPPORT AGENCY COMMENTS 19

6.0 PUBLIC PARTICIPATION COMPLIANCE 19

7.0 STATUTORY DETERMINATIONS 20

8.0 REFERENCES 21.

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TABLES

4.1-1 Changes to North Plants Soil Remediation

FIGURES

1.0-1 Rocky Mountain Arsenal Regional Reference

2.2-1 North Plants Soil Remediation Areas

3.1.2-1 North Plants Verification and Confirmatory Soil Sample Locations

3.1.3-1 North Plants TRER Area 25CC-3 Soil Sample Locations

3.1.4-1 North Plants Petrolewn-Impacted Soil Sample Locations

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ACRONYMS AND ABBREVIATIONS

BAS

bey

BTEX

CCR

CDPIIE

CERCLA

CFR

DCN

DIMP

EPA

ESD

GB HH

HHE

HQ

HWL

JARDF

LNAPL

MEC

NCP

NPL

NPSA

OCPs

OPS

OU

PI

PAHs

PRER

RAB RBSL

RDIS

RER

RI RMA

ROD

Biological Advisory Subcommittee

bank cubic yard(s)

Benzene, Toluene, Ethylbenzene and Xylene

Construction Completion Report

Colorado Department of Public Health and Environment

Comprehensive Environmental Response, Compensation and Liability Act

Code of Federal Regulations

Design Change Notice

diisopropyl methylphosphonate

U. S. Environmental Protection Agency

Explanation of Significant Ditferences

isopropyl methylphosphonofluoridate (sarin)

Human Health

Human Health Exceedance

Hazard Quotient

Hazardous Waste Landfill

Joint Administrative Record Document Facility

Light Non-Aqueous Phase Liquid

Munitions and Explosives of Concern

National Contingency Plan

National Priorities List

North Plants Study Area

Organochlorine Pesticides

Division of Oil and Public Safety

Operable Unit

Priority I

polynuclear aromatic hydrocarbons

Petroleum Release Evaluation Report

Restoration Advisory Board

Risk-Based Screening Level

Remediation Design and Implementation Schedule

Residual Ecological Risk

Remedial Investigation

Rocky Mountain Arsenal

Record of Decision

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SAP

SEC

TCHD

TEPH

TRER

USFWS

·UXO

VOCs

Sampling and Analysis Plan

Site Evaluation Criteria

Tri-County Health Department

Total Extractable Petroleum Hydrocarbons

Terrestrial Residual Ecological Risk

U.S. Fish and Wildlife Service

Unexploded Ordnance

Volatile Organic Compounds


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1.0 INTRODUCTION This Explanation of Significant Differences (ESD) documents a significant change in a portion of the remedy for the North Plants Soil Remediation (North Plants Soil) Project of the Rocky Mountain Arsenal (R.J\1A) Federal Facility Site. The RMA On-Post Operable Unit (OU) is a federally owned facility located in southern Adams County, Colorado, approximately 10 miles northeast of do'Wntown Denver, directly north of the former Stapleton International Airport and west of Denver International Airport (Figure 1.0-1). The RMA On-Post OU site encompasses approximately 5.6 square miles and is currently on the U.S. Environmental Protection Agency (EPA) :-.Iational Priorities List (NPL) for environmental cleanup as a result of contamination released during previous RMA operations. The North Plants Soil Project consists of seven separate areas located in the north central part of the On-Post OU.

The Record of Decision (ROD), which describes the remedy for the entire On-Post OU of RMA, was signed by the U.S. Army (Army), the EPA, and the Colorado Department of Public Health and Environment (CDPHE) on June II, 1996 (FWENC 1996). The selected remedy includes 31 cleanup projects for soil, structures, and treatment of groundwater contamination (PMRMA 2006). As the site-wide remediation is completed, most of the On-Post OU ofRMA will become a National Wildlife Refuge, as provided for in Public Law #102-402.

The Army is the lead agency for RMA and is issuing this ESD as part of its responsibilities under Section 117 of the Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA), as amended by the Superfund Amendment and Reauthorization Act of 1986, and pursuant to the National Contingency Plan (NCP), 40 Code of Federal Regulations (CFR) Section 300.435(c)(2)(i). The NCP requires an ESD when the remedial action taken differs significantly from the remedy selected in the ROD with respect to scope, performance or cost. Regulatory Agency oversight is conducted by the EPA, CDPHE, and the Tri-County Health Department (TCHD). The TCHD oversees local public health and environmental issues in Adams, Arapahoe, and Douglas Counties.

This ESD summarizes modifications to the remedy for the North Plants Soil Project that result from new information developed by the Army since the ROD was signed. During design of the remedy for the North Plants Structure Demolition and Removal (North Plants Structures) Project, it was decided to incorporate North Plants soil excavation into the North Plants Structures Project due to the difficulty in segregating structure foundation removal and soil excavation. Although the ROD-identified remedy for the biota risk soil in North Plants did not include excavation, the biota risk soil was excavated to take advantage of implementation efficiency, prevent cross contamination between soil and structures debris during the structures demolition and foundation removal, and to support the possibility of eliminating the RODidentified soil cover. New information, based on soil sampling conducted following excavation of the contaminated soil, has resulted in a change to eliminate the soil cover requirement for the North Plants Soil Project. In addition to soil and chemical sewer excavation, remediation activities completed during the North Plants Structures Project, including remediation of sanitary sewers, removal of underground utilities, structure foundation removal, chemical agent screening, and munitions and explosives of concern (MEC) clearance activities were considered as factors for eliminating the soil cover. Elimination of the soil cover, while resulting in the need

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for an ESD, does not alter the overall hazardous waste management approach that was selected in the ROD.

In addition, deletion of the soil cover eliminates the remaining remedy requirement for North Plants. Although a Draft 30 Percent Design was issued for the North Plants Soil Project (TtFW 2005b), all requirements identified within the draft design have been completed or transferred to other projects or, in the case of the soil cover, eliminated. No field work, other than the design 'sampling described in Section 3.1.4, was conducted in support of the North Plants Soil Design. As a result, the Remediation Design and Implementation Schedule (RDIS) will be modified to reflect that no work is required under the North Plants Soil Project. No further design documents will be prepared and therefore no Construction Completion Report (CCR) is required.

This ESD will become part of the Administrative Record as required by the NCP, 40 CFR 300.825(a)(2) (EPA 1990). The Administrative Record is available to the public at the Joint Administrative Record Document Facility (JARDF) that is located on the RMA in Building 129, Room 1010. The JARDF is open Monday through Friday between oon and 4 pm or by appointment. The telephone number for the JARDF is 303-289-0983.

2.0 SITE HISTORY, CONTAMINATION AND SELECTED REMEDY

2.1 RMA Operational History The RMA was established in 1942 by the Army to manufacture chemical warfare agents and agent-filled munitions and to produce incendiary munitions for use in World War II. Following the war and through the early 1980s, the facilities continued to be used by the Army. Beginning in 1946, some facilities' were leased to private companies to manufacture industrial and agricultural chemicals. Shell Oil Company, the principal lessee, manufactured pesticides from 1952 to 1982 at the site. Common industrial and waste disposal practices during those years resulted in contamination of structures, soil, surface water, and groundwater.

The On-Post OU is one of two operable units at RMA. The Off-Post OU primarily addresses groundwater contamination north and northwest ofRMA. The On-Post OU addresses contamination within the approximately 26.6 square miles of RMA. As of August 2006, approximately 21 square miles of the On-Post OU have been determined to meet cleanup requirements and are no longer part of the NPL site. Implementation of the remedy for the remaining approximately 5.6 square miles is ongoing and is scheduled for completion in 2010.

The contaminated areas within the On-Post OU included approximately 3,000 acres of soil, 15 groundwater plumes, and 798 structures. The most highly contaminated areas were identified in South Plants (the central processing area, Hex Pit, Buried M-I Pits, and the chemical sewers), Basins A and F, the Lime Basins, and the Complex (Army) and Shell Trenches. The primary contaminants found in soil and groundwater in these areas are organochlorine pesticides (OCPs), solvents, metals, and chemical warfare agent byproducts.

The areas with the highest levels and/or the greatest variety of contaminants are located in the central manufacturing, transport, and waste disposal areas. The highest contaminant concentrations tend to occur in soil within five feet of the ground surface, although exceptions

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are noted, particularly where burial trenches, disposal basins, or manufacturing complexes were located.

The characteristics and locations of the groundwater plumes suggest that the greatest contaminant releases to the groundwater have occurred from Basin A and the Lime Basins, the South Plants chemical sewer, the South Plants tank fann and production area, the Complex (Army) and Shell Trenches in Section 36, and the former Basin F. The Motor Pool/Rail Yard and North Plants areas have been other sources of contaminant releases to the groundwater.

2.2 North Plants History and Contamination Summary

The North Plants was designed and constructed by the Army from 1950 to 1953 as a chemical nerve agent manufacturing facility to produce isopropyl methylphosphonofluoridate (GB), also known as sarin. The North Plants complex covered approximately 90 acres located within the central part ofSection 25 in T2S, R67W. Production of GB began in April 1953 and continued intermittently until 1957. The Army redistilled low purity GB intermittently from 1964 to 1970 (Ebasco 1988a). The facilities were also used by the Army to containerize, transfer, store, and demilitarize/neutralize chemicals and munitions.

The North Plants manufacturing facility included a network of chemical sewers consisting of three major parts: a cast iron collection network, the Building 1727 sump, and trunk lines from the sump to the disposal basins. The collection network was constructed as part of the original complex and connected various manufacturing facilities to the Building 1727 sump. Caustic solution and water were added to the liquid waste collected in the sump to neutralize potential chemical agent contamination. Two pressurized discharge lines transported waste from the Building 1727 sump to the disposal basins. Initially, a six-inch cast iron pipe conveyed the waste to Basin A for disposal. Sometime prior to 1961, a 12-inch steel line WiJ.S added to transport the ,vaste to the chemical sewer interceptor line running between South Plants and Basin F (Ebasco 1988b).

The Remedial Investigation (RI) of the North Plants area was conducted in 1988 and included soil sampling in the areas surrounding the buildings (Ebasco 1988a). Samples were located around the buildings to target investigation of potential spill areas and drainage ditches. Soil samples collected from the surface soil and drainage areas around Building 1611 (North Plants Study Area [NPSAJ-5) contained concentrations of arsenic in excess of the human health (HH) site evaluation criteria (SEC). Building 1611 was used in 1983 and 1984 for demilitarization of adamsite, an arsenic-containing chemical agent. Soil samples collected from the drainage areas around Building 1703 (NPSA-6) contained concentrations of arsenic and dieldrin in excess of the HH SEC. Building J703 was used as an ammunition demilitarization facility and was suspected to be contaminated with arsenic (Ebasco 1988a). Soil samples collected around Building 1501 (NPSA-3) did not exceed the I-IH SEC; however, concentrations of dieldrin in surface soils were sutlicient to result in delineation of a biota risk area. Dieldrin is a prevalent contaminant in surface soils at RMA, generally attributable to windblown contaminants from other RMA source areas.

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During the RI, sampling was not conducted along the cast iron pipe chemical sewer lines in the North Plants complex or the steel trunk lines downstream of the Building 1727 sump. However, soil samples collected in Section 36 and in South Plants along vitrified clay pipe sewer segments were used to provide a WOfSt case estimate of contaminated soil associated with the North Plants chemical sewers (Ebasco 1988b). Based on sampling results from the South Plants chemical .sewer RI, a contaminated soil corridor was defined for the chemical sewers in North Plants.

A summary of the information collected during the RemediallnvestigationlFeasibility Study process can be found in the North Plants Contamination Summary Reports (Ebasco 1988a, 1988b), the RI Swrunary Report (Ebasco 1992) and in the Detailed Analysis of Alternatives Report (FWENC 1995). Risk-based analysis of sample data collected during the RI resulted in designation of human health exceedance (HHE) soil and biota risk soil in the North Plants project area.

The North Plants soil remediation includes six areas from three ROD medium groups/subgroups and one Residual Ecological Risk (RER) area identified subsequent to the ROD. The contamination areas consist of chemical sewers, surface drainage areas and surface soils surrounding North Plants structures where contamination was present. These areas were grouped for implementation under the North Plants Soil Project since they are all present in the

orth Plants area. A site plan showing the North Plants soil remediation areas is included as Figure 2.2-1. The following ROD-identified areas are included in the North Plants soil remedy:

North Plants Medium SubQroup

NPSA-3, GB Manufacturing Area

NPSA-5, Special Weapons Plant

NPSA-6, Underground Spill Area

Chemical Scwers Medium SUbQrOUD

NPSA-I, North Plants Ch"mical Sewer System

DitcheslDrainage Areas Medium Group

NPSA-8c, Surface Drainage

NPSA-9f, Isolated Arsenic Detection

In addition, Terrestrial Residual Ecological Risk (TRER) area 25CC-3 was identified as a residual ecological risk area in accordance with the process described in the ROD for evaluating ecological risk. While not assigned to a medium group, residual risk areas are characterized by surface pesticide contamination. Area 25CC-3 includes the surface soil area encompassing the ROD-identified soil contamination areas and the North Plants manufacturing area.

Also, in 1993, light non-aqueous phase liquid (LNAPL) associated with groundwater was identified beneath the North Plants manufacturing area. The LNAPL was identified in the NPSA-6 area where the RI had noted the occurrence of several fuel oil spills. Delineation of an

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Rocky Mountain Arsenal Explanation of Signiticant Differences North Plants Soil . Revision 0 WBS 2.05.27.02 Dccl."fTlbcr 10, 2008

LNAPL plume was initially conducted in July 2001 as part of the North Plants Groundwater Monitoring (FWENC 2002c). The LNAPL was measured at six monitoring wells located within the impacted area. An oil/water interface probe was used to determine the presence or absence of LNAPL in each well. The results showed one well, Well 25055, to contain approximately 0.65 foot of LNAPL. Well 25055 was located hydraulically downgradient of Building 1712. The surrounding wells, which were approximately 200-feet to 750-feet away from Well 25055, were also measured for LNAPL during the July 2001 investigation and LNAPL was not present. This effort gave an estimated extent of the LNAPL plume (FWENC 2002b).

2.3 Summary of the Selected On-Post Remedy The overall remedy required by the 1996 ROD for the On-Post au includes the following:

• Interception and treatment of contaminated groundwater at the three existing on-site treatment plants .'

• Construction of a new Resource Conservation and Recovery Act- and Toxic Substances Control Act-compliant Hazardous Waste Landfill (HWL) on-post

• Demolition of structures with no designated future use and disposal of the debris in either the new, on-post HWL or the Basin A consolidation area, depending upon the degree of contamination

• The contaminated soil at RMA is addressed primarily through containment in the on-post HWL or under caps/covers, or through treatment depending upon the type and degree of contamination. Areas that have caps or covers require long-term maintenance and will be retained by the Anny. These areas will not become part of the wildlife refuge.

• The Basin A disposal area is used for consolidation of biota risk soil and structural debris from other RMA contamination areas and is covered with a soil cover including a biota barrier.

2.4 Summary of the Selected Remedy for North Plants Soil The ROD identifies the following major remedial actions for North Plants soil project:

• Excavate chemical sewers and associated HHE soil and dispose in the on-post HWL (NPSA-I).

• Excavate North Plants surface HHE soil and dispose in the on-post HWL (NPSA-5 and NPSA-6).

• Monitor for chemical agent during excavation and treat agent-contaminated soil by caustic solution washing.

• Backfill HHE soil excavations with clean soil.

• Construct a two-foot-thick soil cover over soil posing a potential risk to biota (NPSA-3, PSA-5 and NPSA-6) and the footprint of the North Plants processing area.

• Excavate potential biota risk soil that is located outside the 2-foot-thick soil cover and consolidate to Basin A for containment under soil cover (NPSA-8c and NPSA-9f).


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• Revegetate all disturbed areas.

• Institutional controls consistent with ROD requirements for soil covers.

• Long-term groundwater monitoring to assess the effectiveness of the soil cover.

In addition, the ROD includes a requirement for continued biomonitoring for areas that may pose potential risk to biota in order to refine design boundaries for surficial soil contamination areas. This evaluation includes surface soil areas surrounding the ROD-identified North Plants area.

During the North Plants S,ructures Project design, the soil cover configuration was modified to more accurately cover the chemical agent material processing area and to eliminate areas with no process-related structures or chemical sewers. Although this minor change to the ROD is documented in the North Plants Structures Project 100 Percent Design, both the ROD soil cover configuration and the final design configuration are shown on Figure 2.2-1 for information.

2.5 North Plants Remedy Implementation The ROD remedy for North Plants includes the above mentioned soil remediation as well as structures demolition. In the RDIS, elements of the North Plants remediation were divided into the North Plants Structures Project and the orth Plants Soil Project. During design of the remedy for the North Plants Structures Project, it was decided to incorporate North Plants soil excavation into the North Plants Structures Project due to the difficulty in segregating structure foundation removal and soil excavation. The combination of structures demolition and soil removal scope within the demolition project allowed an integrated design and implementation approach, resulting in ove~all program schedule and cost savings. The remaining scope of the North Plants soil remediation (i.e., soil cover if required) was left for completion under the onh Plants Soil Project.

Also, to address the LNAPL plume, a Design Change Notice (DCN) was approved for the North Plants Structures Project. Pursuant to DCN-NPD-002, a passive recovery system was installed in Well 25055 in September 2001 (FWENC 200Id). The LNAPL recovery was to continue until 90 percent of the LNAPL :hickness in the well was removed; however, since Well 25055 was in close proximity and interfered with the North Plants Structures Project, the well was abandoned and LNAPL recovery ceased in January 2002. At the time, LNAPL removal was estimated at only 50 percent reduction in thickness, therefore DCN-NPD-006 was issued to allow well abandonment and to require installation of a replacement well and continuation of recovery efforts upon completion of demolition activities (FWENC 2002c). Subsequent LNAPL recovery would continue until the LNAPL thickness is reduced by 90 percent. Although demolition activities are complete, installation of the replacement recovery well has been delayed pending completion of evaluations related to the LNAPL plume.

3.0 BASIS FOR THE ESD The ROD remedy for North Plants soil includes excavation of HHE soil, including chemical sewers, and construction of a two-foot-thick soil cover over soil posing a potential risk to biota and the footprint of the North Plants processing area. During the North Plants Structures Project design, evaluation of the project activities indicated that demolition of structures and foundations


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could not be accomplished without distltrbing the chemical sewers and'virtually all the biota risk soil. Therefore, to take advantage of implementation efficiency and prevent cross contamination between soil and structures debris during the structures demolition and foundation removal, excavation of HHE and biota risk soil was incorporated into the North Plants Structures Project (FWENC 2001 b). The decision to incorporate removal of biota risk soil to the North Plants Structures Project is documented in a separate ESD (TtFW 2004d).

With the soil excavation included in the North Plants Structures Project and the addition of biota soil removal, the possibility of modifying the remedy to eliminate the soil cover was considered. This modification would result in reducing the long-term operation and maintenance costs and, following deletion from the NPL, would make this 32 acre area available for transfer to the U.S. Fish and Wildlife Service (USFWS) and refuge use consistent with most of the rest ofRMA. In order to support this remedy modification, the North Plants Structures design included excavation of all HHE soil including chemical sewers, removal of all biota risk soil and both confirmatory and verification sampling. Composite sampling for potential residual ecological risk soil was also completed. There is no evidence of contaminated soil remaining in place that represents a contaminant source to groundwater. In addition to soil excavation and sampling, remediation acti vities completed during the North Plants Structures Project, including remediation of sanitary sewers, removal of underground utilities, structure foundation removal, chemical agent screening, and MEC clearance activities were considered as factors for eliminating the soil cover. The excavation of contaminated soil and other issues considered for elimination of the North Plants soil cover are discussed in the following sections.

3.1 Soil Remediation The following sections describe remediation efforts associated with soil contamination as they relate to the decision to eliminate the North Plants soil cover.

3.1.1 Soil Excavation Both HHE soil and biota risk soil volumes were modified from the ROD estimates during design. The primary change for HH remediation volume was caused by changes to the excavation depth and width of the chemical sewer trenches. These changes were based on the difference in the actual depth of the pipe compared to what was expected and over excavation of depth and width requirements during implementation. In addition, two surface remediation areas were added to the North Plants soil during design data review. These changes increased the HHE volume from ] 1,442 bank cubic yards (bey) to 43,110 bey. This change was documented in an ESD for the North Plants Structures Project (TtFW 2004d). All HHE soil was disposed in the HWL.

The biota risk soil remediation volume was modified during design and implementation to eliminate structures footprints and other asphalt and concrete areas from the excavation boundaries. These areas were eliminated because these features provided a sufficient barrier to surficial soil contamination from North Plants operations as well as a barrier to potential windblown contamination from the enhanced evaporation efforts in 1973 at Basin F, located to the west of North Plants. Elimination of these surface areas decreased the total biota risk soil volume from the ROD-identified 17,140 bey to an actual excavation volume of7,234 bey, an


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approximate 58 percent decrease. This change was also documented in the North Plants Structures ESD (TtFW 2004d). All biota risk soil was disposed in Basin A.

The North Plants Structures Project included backfill of excavation areas, interim grading and interim vegetation to minimize ponding and promote runoff. Excavations were backfilled with soil from on-site borrow areas including Priority I (P I) soil and clean borrow soil. All PI soil was placed a minimum of2 feet below final grade. During project implementation, the project requirements were modified to eliminate the use of PI soil with Hazard Quotient (HQ) > 2 for backfill. This soil was subsequently disposed in Basin A. Additional discussion related to use of P I soil is included in Section 3.1.4.

3.1.2 Soil Sampling and Analysis

Following excavation of contaminated soil, confirmatory and verification sampling was conducted throughout the North Plants project area. These two sampling programs were conducted simultaneously with a common objective to verify that no HHE soil was remaining in the North Plants area following demolition and soil removal activities, and to provide additional data to assist in determining final soil cover requirements for North Plants (FWENC 2001 a). The verification sampling was focused in excavation areas including HHE and biota excavations, structure foundation excavations and chemical sewer excavations. A total of 65 verification samples were collected along with seven duplicates. Figure 3.1.2-1 shows the location of verification samples collected during the sampling program. Analyses were selected for each sample based on review of site histories and soil data from RI sample results and included metals, OCPs, volatile organic compounds (VOCs) and mercury depending on the location. For metals, eight samples contained arsenic at concentrations greater than background and two samples contained lead concentrations greater than background (TtFW 2004f). However, concentrations were below the ROD-identified HH SEC. All other metals detections were within the established RMA background ranges. There were no detections ofOCPs, VOCs, or mercury. Six additional verification samples were collected along the North Plants sanitary sewer to check for diisopropyl methylphosphonate (DIMP) contamination from potential sewer leakage (FWENC 2002a). No DIMP was detected in the sanitary sewer samples (TtFW 2004f).

Confirmatory samples were collected at 23 select locations to identify SEC exceedances and/or groundwater plume constituents of the North PlantslBedrock Ridge Plume. This sampling was designed to provide additional information to evaluate potential impacts to groundwater quality and specifically to provide data for the proposed cover deletion. Prior to finalizing sample locations, additional groundwater data was collected in July 200\. Water quality samples were collected from nine wells near Well 25055 and water level measurements were collected from 35 monitoring wells in the North Plants area (FWENC 200Ic). The Regulatory Agencies reviewed the groundwater level and water quality data to refine selection of confirmatory sample locations. Based on the groundwater analytical results, four confirmatory sample locations were added near Well 25051 to investigate possible sources related to elevated DIMP concentrations in groundwater (FWENC 2002a).

Confirmatory samples were collected at five-foot increments beginning at the excavation surface or 10 feet below the original ground surface, whichever was deeper, to the depth of thc soil

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groundwater interface. A total of 94 confirmatory samples were collected along with three duplicates. Figure 3.1.2-1 shows the confirmatory sample locations. Confirmatory samples were analyzed for VOCs, metals and DIMP based on review of site histories and presence of contaminants in the North Plants groundwater plume. Samples from selected locations were also analyzed for OCPs or mercury. Analytical results for all COCs were compared to the RODidentified HH SEC. There were no detections ofVOCs, D[MP, OCPs or mercury. There were four samples containing arsenic, lead and/or cadmium at concentrations greater than background; however, concentrations were well below the ROD-identified HH criteria (TtFW 2004g).

3.1.3 Residual Ecological Risk Soil

The ROD included a requirement for continued biomonitoring for areas that may pose potential risk to biota in order to refine design boundaries for surficial soil and aquatic contamination areas (FWENC 1996). The Biological Advisory Subcommittee (BAS) was tasked with determining a level of acceptable risk to biota remaining after implementation of the RODidentitied remedial activities. Based on the results of the BAS-directed evaluations, areas of RER soil were identified and selected for remediation. Initially, the BAS identified two classes of soil that exhibited generally higher and lower risk, or Hazard Quotient (HQ), and ranked them as P I and Priority 2 areas. Surface soil outside the ROD-identified HHE and biota risk soil areas in North Plants was identified as a PI soil area. However, the assessment and agreement completed in 1997 indicated that this area did not require remediation (RMA Committee 1997). The agreement for P I soil remediation also adopted the recommendation for use of PI soil when removed. Specitically, for P I soil used as common gradefill, the soil was restricted to backfill a minimum of I t(i) 2 feet below final grade.

Although the North Plants P I soil area did not require remediation under the 1997 agreement, a portion of the P I soil was excavated during structures demolition activities due to the proximity to the structures being demolished or its location overlying chemical sewer excavations. The PI soil was segregated for use as backtill in accordance with the 1997 agreement and the Site-Wide Remediation Waste Management Plan. Additional PI soil from Borrow Area 6 and Borrow Area 7a was also used for backfill of chemical sewer excavations. Although the PI soil was managed in accordance with the remediation requirements, concerns over the potential for future disturbance of the backfilled soil led to a design modification to limit use of PI soil for backfill to only soil with HQ::: 2. The PI soil with HQ > 2 was removed and disposed at Basin A (FWENC 2003). Because some PI soil with HQ > 2 had already been used as backfill for chemical sewer excavations, the North Plants Structures CCR indicated that areas having PI backfill with HQ > 2 required institutional controls to prevent future intrusive activities that could expose the soil. Subsequent to completion of the CCR, the backfill soil was sampled to verify that sufticient mixing had occurred during backfill placement to reduce the risk to an HQ of two or less. Sample results showed no detections of OCPs in the backfill soil (TtFW 2004a).

[n 2002, the BAS completed a detailed risk assessment to identify the magnitude and extent of residual ecological risks and provide recommendations for additional refinements to surficial soil remediation areas (BAS 2002). This assessment included all areas remaining after implementation of remedial actions including P I areas designated in the 1997 agreement as excepted from remediation. This assessment resulted in designation ofTRER areas for


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remediation. The PI soil area surrounding North Plants was redesignated as TRER area 25CC-3 as shown on Figure 2.2-1. In June 2003, the RMA Committee completed an agreement detailing the remediation requiremei1ts for all TRER areas. Remediation requirements under this agreement included soil removal or a specific three-step soil tilling process to reduce risk. Area 25CC-3 was identified for soil tilling as the remedial action (RMA Committee 2003).

Remediation of25CC-3 occurred in pieces. In 2003, the North Plants Structures Project removed RER soil from the drainage ditch along the western parcel of25CC-3. The balance of the western parcel was til1ed using the approved three-step soil tilling process. Biota risk verification samples collected following tilling showed residual biota risk below action levels (TtEC 2006c). The remaiaing 25CC-3 area was ripped following completion of demolition and soil excavation activities. In 2004, a small area of soil in the northwest comer of25CC-3 was removed to facilitate construction of a haul road for enhanced hazardous waste landfill construction access. Also in 2004, the southeast portion of25CC-3 was tilled and sampled as part of the BAS Tilling Demonstration Sampling and Analysis Plan (SAP). Sample results from this area following tilling were below action levels (BAS 2006). Sample locations for the western parcel and southeast portion of 25CC-3 are shown on Figure 3.1.3-1.

Although most of25CC-3 was ripped during the North Plants Structures Project, this activity did not satisfy the requiremems of the approved three-step tilling process for risk reduction. Therefore, sampling in accordance with the BAS SAP (USFWS 2000) was conducted in 2005 to verify that no concentrations of OCPs that presented an unacceptable risk were present. The sample area was divided into approximate three-acre parcels and a five-point composite sample was collected from the surface soil in each parcel. Sample locations for 25CC-3 are shoWTI on Figure 3.1.3-1. Sample results showed that there were no concentrations of OCPs exceeding biota risk action levels (TtEC 2006b). As a result, all RER soil remediation requirements were completed. Overall remediation of25CC-3 is documented in the RER CCR Part I (TtEC 2006a).

3.1.4 Petroleum Contaminated Soil

During excavation of chemical sewers under the North Plants Structures Project in 2002, a petroleum impacted subsurface soil area was encountered in the overburden and HHE zone of the chemical sewer corridor in the Building 1712 area. Petroleum-impacted soil was excavated in a 20-foot wide and IO-foot average depth corridor south of Buildings 1704, 1711 and 1712 with legs extending north to Building 1703 and south to Building 1727. Approximately 2,000 bcy of petroleum-impacted soil was excavated and disposed in the on-site HWL during chemical sewer removal (TtFW 2004c). Soil samples collected from the corridor south of Building 1712 showed positive detections for total extractable petroleum hydrocarbons (TEPH).

Following removal of the petroleum-impacted soil, sampling was initiated to determine the extent and concentration of contaminants associated with the petroleum-impacted soil. Two rounds of soil sampling were conducted with initial sampling conducted under an amendment to the North Plants Structures SAP (FWENC 2003b). When this initial effort did not fully delineate the extent of the petroleum-impacted soils, a second sampling effort was conducted to further defme the extent of the petroleum contamination. Because the remediation activities associated


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",ith the North Plants Structures Project were completed, the additional SAP was issued as part of the North Plants Soil Project (FWENC 2003a).

Sampling procedures were implemented consistent with information provided in the Colorado Department of Labor and Employment, Division of Oil and Public Safety (OPS) Corrective Action Program to delineate the horizontal and vertical extent of contamination (COLE 2004). Sample locations were chosen considering the historical information n;lated to fuel oil spills in North Plants and overlaying the expected area of petroleum-impacted soils with a 50-foot by 50foot grid system. The combined soil sample efforts resulted in 170 samples collected from 85 locations. These sample locations are shown on Figure 3.1.4-1. Samples were collected at two varying depths from each sample location. One sample was collected from immediately above the water table and a second sample was collected at a shallower depth, depending on the conditions observed in the borehole. The thickness of the petroleum-i1p.pacted soil was also measured based on visual observation of staining. All soil samples were analyzed for TEPH and benzene, toluene. ethylbenzene and xylene (BTEX) consistent with the OPS regulations (COLE 2004). In addition, soil samples with TEPH concentration greater than 500 mg/kg were analyzed for the 14 OPS-identified polynuclear aromatic hydrocarbons (PAHs).

The combined analytical results from the two soil sampling programs were sufficient to delineate an area of petroleum-impacted soil approximately 400 feet by 900 feet based on a 500 mg/kg TEPH threshold. Complete soil sample analytical results for the combined sample efforts are presented in the North Plants Soil Remediation Project Petroleum-Impacted/Stained Soils Data Summary Report (TtFW 2004e). With the exception of two isolated, shallow (four to six feet below ground surface) TEPH detections near the detection limit, the elevated TEPH contamination and stained soil observations are 20 to 30 feet deep and near the groundwater table.

Consistent with the OPS rules, exposure pathways were evaluated and soil sample results were compared to Tier I Risk Based Screening Levels (RBSLs) for BTEX for completed exposure pathways. In addition, where TEPH levels were greater than 500 mg/kg, PAH levels remaining on the site were compared to the RBSLs found in the Colorado Department of Labor and Emplo}ment Petroleum Storage Tank Owner/Operator Guidance Document (COLE 2005). Sample results from the combined soil sample efforts showed concentrations of BTEX and PAHs in all samples were below the Tier I RBSLs (TtFW 2004b).

The second soil sampling effort included installation of eight piezometers, later followed by the installation of four additional piezometers. Data collected from these piezometers were used to delineate the extent of the LNAPL plume to the north. The piezometers were placed where TEPH soil concentrations were greatest and near abandoned Well 25055. LNAPL and static water levels were measured with an oil/water interface probe after the piezometers were installed and developed and the water table equilibrated. Measurements were collected from late November 2003 through May 2004. Based on the piezometer information, the extent of the LNAPL plume existing on the water table was delineated. The location of the LNAPL plume is shown on Figure 3.1.4-1.

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Available groundwater data were also reviewed to assess potential impacts to groundwater. A water quality sample collected in 1994 after LNAPL was observed in Well 25055 showed detections of BTEX with a benzene concentration slightly above the groundwater standard. As discussed in Section 2.2, :i:Toundwater samples were collected from nine wells near or downgradient of Well 25C55 in July 2001. Analytical results showed concentrations for BTEX were all nondetect. These results were consistent with the historic data, which show that there are no apparent downgradient BTEX impacts (TtFW 2004b). Because BTEX are poor indicator compounds for fuel oil, two wells were sampled for PAHs. No PAHs were detected; however, these wells are not located directly downgradient of the LNAPL plume. As a result, additional characterization was perf0lmed to evaluate impacts of the LNAPL plume on groundwater quality.

The investigation and evaluation of the petroleum contaminated soil and LNAPL plume was documented in the North Plants Soil Remediation Petroleum Release Evaluation Report (PRER) (TtFW 2004b). Also in March of2004, the RVO prepared a draft North Plants Frec Product Remediation Plan, which presented a concept for LNAPL recovery. In the fall of2004, thc LNAPL recovery concept ',vas modified and presented in the North Plants Soil Design Scope of Work. A 30 Percent Design Analysis was then issued in 2005 that included the conceptual design for an LNAPL recovery system (TtFW 2005b). The PRER was also included as an attachment to that design. As a result the North Plants Soil 30 Percent Design provided a compilation of the no action evaluation for soil, the LNAPL recovery concept and the responses to Regulatory Agency comments received through late 2004. With approval of this ESD, the Regulatory Agencies concur that no further action is required for soil based on the current available information.

Subsequent to North Plants Soil 30 Percent Design, a SAP entitled the North Plams Soil Remediation Interim Free Product and Groundwater Characterization SAP (TtFW 2005a) was prepared. Under this effort, groundwater and LNAPL plume samples were collected to supplement data collected under the two soil sampling efforts. The LNAPL plume characterization included analysis for diesel range organics, gasoline range organics, volatile organics including BTEX, PAHs, specific gravity and viscosity. Groundwater characterization of two piezometers and three monitoring wells in and downgradient from the LNAPL included analysis for diesel range organics, volatile organics, DIMP, semivolatile organics (including lowlevel PAHs) and OCPs. In November 2007, additional groundwater quality samples were collected from two piezometers to further characterize groundwater immediately downgradiem from the LNAPL plume.

The results from that sampling are presented in the North Plant Soil Remediation Project Interim Free Product and Groundwater Characterization Data Summary Report (TtEC 2007). The LNAPL characterization was consistent with a historical release of fuel oil. 0 BTEX constituents were detected and non-carcinogenic PAHs were detected at low ppb levels in piezometers 25132 and 25133. DIMP and chlorinated VOCs were present at levels typical for groundwater in this vicinity. In April 2008, a second evaluation report, the North Plants Petroleum Release Evaluation and Action Plan/or LNAPL Associated with Groundwater, was completed to update the groundwater and LNAPL characterization to include data collected since


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2004 (TtEe 2008). This report included an action plan summary of a pilot system for LNAPL recovery.

Since issuance of the 'orth Plants Soil 30 Percent Design, experience at RMA with the groundwater mass removal system located in South Plants revealed the importance of both sitespecitic data and site-specific recovery experience when implementing a contaminant recovery system. For that reason, and because there is not a direct soil exposure issue, LNAPL plume evaluation and recovery efforts have been removed from the North Plants Soil Project and will continue under the North Plants LNAPL Remediation Project.

Consistent with OPS rules, the presence of LNAPL in several monitoring locations requires evaluation of actions to satisfy requirements for LNAPL removal. Specifically, the LNAPL must be removed to the ma-ximum extent practicable and in a manner that minimizes the spread of contamination. To satisfy these requirements, a pilot L -APL remo\;a1 system will be implemented to perform LNAPL removal and to gather operating data to evaluate the feasibility and cost effectiveness of continuing LNAPL removal. In evaluating options for LNAPL removal, construction of a soil cover over the North Plants area was discarded as unsuitable since it did not promote the requirement for removal of LNAPL. In addition, a localized soil cover would not affect the regional groundwater elevations and would not limit movement of groundwater in contact with the LNAPL.

3.1.5 Soil Remediation Summary

Extensive soil sampling. including contirmatory sampling, veritication sampling, residual biota risk sampling and petroleum-impacted soil sampling, has been conducted throughout the North Plants project area in conjunction with soil remediation activities. Based on the sample results. there is no evidence of contaminated soil remaining in place that exceeds the HH SEC or biota risk criteria or that represents a contaminant source for groundwater. This information, coupled with the excavation of all identified HHE soil and biota soil from the North Plants area indicates that there is no contaminated soil remaining to warrant retention of the soil cover requirement. Long-term maintenance requirements for the two-foot cover area may be eliminated and the protectiveness of the remedy is not compromised. The elimination of the two-foot cover will also result in a long-term cost savings, and will allow transfer of this area to Department of the Interior for inclusion as part of the wildlife refuge.

3.2 Sanitary Sewer Remediation During implementation of the North Plants Structures Project, DIMP was detected in the HWL leak detection system. As a result, an investigation was conducted to determine the source of the DIMP and confirm that the HWL primary liner had not been compromised. The results of that investigation indicated that the sanitary sewer in Borrow Area 5 was a potential source of DIMP since soil from Borrow Area 5 had been used in HWL liner construction. Since the sanitary sewer in North Plants is upstream of Borrow Area 5 and DIMP exists in the North Plants groundwater plume. the North Plants sanitary sewer was identified as a potential source ofDIMP contamination.

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Therefore, the North Plants Structures Project was modified to include removal of most of the North Plants sanitary sewer (FWENC 2002a). The portion of the sewer originating from Buildings 1614 and 1615 was not removed since the building use history indicated that these structures were used as warehouses and not production facilities. Following sewer removal, 12 verification samples were collected along the sanitary sewer alignment to characterize the soil. Six of these samples were included as part of the initial verification sampling program and 6 were added due to the investigation of potential DIMP sources for Borrow Area 5. There were no detections of COCs orDlMP in any of the samples, eXGept for metals present at background concentrations (TtFW 200·4f). Although no contamination was detected, the inclusion of sanitary sewer removal in the North Plants project provides additional assurance that there arc no remaining potential contamination sources in the North Plants area.

3.3 Underground UtIlity Removal A variety of underground utilities were located within the North Plants project area including sanitary and storm sewers, natural gas, water lines, condensate lines and process chemical lines including GB transfer piping. In some cases, particularly for the GB lines, the utilities were located within underground concrete chases. During North Plants Structures design, the history and configuration of underground utilities was reviewed to determine utility removal requirements that would support eliminating the soil cover. Because of the potential hazards associated with the GB piping, all GB piping and associated concrete chases were excavated and disposed in the HWL. Chemical agent screening was conducted during GB concrete chase removal as described in Section 3.5.

In addition, there was concern over potential contamination with underground utilities identified as process chemicals, process auxiliary chemicals or drains. Although most of these underground utilities would be removed based on their proximity to foundations being removed or their locations v;lthin the chemical sewer excavation corridors, the design was modified to require removal. Pipes constructed of or v;Tapped in asbestos-containing material were also identified for removal. In addition, all shallow utilities, located within 2 feet of the ground surface, were removed. Deeper utilities were cut and plugged at the excavation face. This approach to underground utility removal provided an overall remedy that supported elimination of the soil cover by eliminating any potential contamination source associated with the utilities.

3.4 Structure Foundation Removal

The ROD requires demolition of all structures present in North Plants including demolition and removal of foundations. Because all foundations were being removed, the soil cover was not selected as a remedy for foundations. However, the soil cover was required to cover the North Plants processing area footprint following structure/foundation demolition. During design, the configuration of the soil cover was modified to more accurately cover the chemical agent material processing area and to eliminate areas with no process-related structures or chemical sewers. This minor change to the ROD is documented in the North Plants Structures Project 100 Percent Design and is shown on Figure 2.2-1.

To address the potential for residual contamination associated with structures, verification and confirmatory samples, discussed in Section 3.1.2, were located within structures footprints.


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Sample locations are shown on Figure 3.1.2-1. Samples were collected following foundation removal and were analyzed for ROD COCs. Confirmatory samples were also analyzed for DIMP. Analytical results from this sampling efrort showed no detections of ROD COCs above ROD-identified HH criteria and no detections of DIMP (TtFW 2004g).

Also during foundation removal, chemical agent screening was performed for structures identified with chemical agent history. The North Plants Structures design provides a detailed discussion and rationale for determining which structures required chemical agent screening (FWENC 200Ib). There were no confirmed detections ofehemical agent during structures demolition (TtFW 2004c). Removal of underground utilities during foundation demolition was also required. These utilities were removed when encountered during foundation removal and were either capped or plugged at the excavation boundary, unless additional removal was required as described in Seetion 3.3. . .

Removal of all foundations and associated utilities along with chemical agent screening and soil sample results provides additional assurance that there are no remaining potential contamination sourccs in the North Plants area and supports elimination or the soil cover.

3.5 Potential Agent Contamination

Portions of the North Plants project area were identified in the ROD as potential chemical agent contamination areas. The ROD requires certifieation of 3X decontamination for all soil or demolition debris identified as potentially agent contaminated unless the area is contained beneath a soil cover. During the North Plants Structures design, all structures and soil areas were reviewed to detennine chemical agent screening requirements for 3X certification. Structures, or portions of structures, that did not have exposure to liquid chemical agent were certified during the design review based on review of building use and spill history. Structures with historic chemical agent use and decontamination history were certified 3X for the aboveground ponion of the structure; however, chemical agent screening was required during foundation removal. Chemical agent screening was also required for structures lacking decontamination history, underground GB chases, and chemical sewer excavations upstream from the Building 1727 neutralization sump (FWENC 2001 b). Chemical agent screening was performed during remediation activities as part of the North Plants Structures Project. There were no confirmed detections of chemical agent during structures demolition or soil excavation and all RODidentified structures and soil areas achieved ROD 3X'certification (TtFW 2004c).

During the North Plants Structures Project, a liquid-filled vial was discovered during backfill operations. Emergency personnel responded and a field screen for chemical agent was conducted. The screen was negative and the vial was then transported to the RMA Analytical Laboratory where the liquid ,vas analyzed and neutralized. The results of the analysis estimated the liquid to be 5 percent GB with the remainder DIMP (TtFW 2004c). Although this liquid showed a positive result for GB, this singular discovery does not indicate an unidentified hazard for the North Plants area. Therefore, discovery of the vial does not affect the basis for elimination of the soil cover.

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3.6 MEC Clearance 'Activities

Although the North Plants"area was never identified as a potential MEC area, unexploded ordnance (UXO) personnel provided support for potential MEC clearance as a conservative safety measure. Prior to structure demolition and soil excavation, UXO technicians performed a sweep through the North Plants area. The sweep included the main structures, areas surrounding the structures and the equipment storage area and was intended to locate and clear any potential MEC. No MEC were rec<:>vered during the North Plants sweep.

During excavation and demolition activities, a UXO technician was assigned to the project to respond to anomalies discovered by field personnel. All anomalies discovered during the project were identified as munitions debris or cultural debris. Munitions debris responses were typically related to investigation and clearance of 4.2-inch mortar test slugs. These slugs were considered munitions debris as they contained no explosive charge. There were no UXO or MEC recovered during the North Plants Structures Project (TtFW 2004c). Therefore, these activities did not affect the basis for elimination of the soil cover.

3.7 Grading and Revegetation

The North Plants Soil 30 Percent Design included site grading to remove manmade features across the former North Plants area. Although this grading is not a requirement under the ROD, it was included as an enhancement in the North Plants Soil Design to provide a fmal surface for revegetation following remediation ofTRER area 25CC-3. As noted earlier, remediation and verification sampling of25CC-3 was completed separately from the North Plants Soil Project. Also, final grading was accomplished under the Site-Wide Traffic Management Project - Section 25 Soil Grading Plan. All North Plants project areas will be seeded to permanent vegetation by the USFWS under a USFWS-approved revegetation plan (Army 2008).

4.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES

4.1 Changes to North Plants Soil Remediation

The change identified for North Plants soil remediation is the elimination of the two-foot-thick soil cover. The addition of biota risk soil removal along with additional soil sampling ensures that the overall remedy remains protective of human health and the environment. During the North Plants Structures Project, all identified HHE and biota risk soil was removed. Verification and confirmatory sampling conducted throughout the North Plants area did not identifY any remaining HHE soil and showed that there is no evidence of soil contamination acting as a groundwater contamination source. Composite surface soil sampling for potential residual ecological risk soil was also completed. This sampling was conducted over the entire North Plants project area and showed that there were no concentrations of OCPs exceeding biota risk action levels. Investigation of the petroleum-impacted soil resulted in the conclusion that there was no further action required for soil. Additional efforts for LNAPL removal will be completed under the North Plants LNAPL Remediation Project and do not rely on the soil cover to accomplish removal or minimize the spread of contamination into previously uncontaminated zones.


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In addition, remaining elements of the North Plants Soil Project identified in the North Plants Soil 30 Percent Design have been completed or transferred to other projects. Specifically, remediation of25CC-3 was completed as part of the site-wide TRER remediation effort, final grading was transferred to the Site-Wide Traffic Management Project - Section 25 Soil Grading, and LNAPL recovery will be completed under the North Plants LNAPL Remediation Project. No additional design efforts will be completed for North Plants Soil Project and the project is considered complete. The project file will be closed following a review of existing project records. All remediation requirements for the North Plants Soil Project are documented in the North Plants Structures CCR (TtFW 2004c) and the RER CCR Part I (TtEC 2006a) and no CCR for the North Plants Soil Project is required. A summary of the I 'ortb'Plants Soil Project requirements and changes is included in Table 4.1-1.

Table 4.1-1: Changes to North Plants Soil Remediation

ROD-Prescribed Remedy Modification

No sampling required. Enhancement. Verification and confirmatory sampling were conducted during the North Plants Structures Project to verify no HHE soil remaining following demolition and excavation actiyities and to evaluate the subsurface soil for potential impacts to groundwater.

Soil sampling was conducted over the entire North Plants area to verify that no unacceptable residual ecological risk soil remained.

Soil sampling to characterize the extent of contamination associated with the fuel oil spill was conducted in support of the North Plants Soil Design.

Excavate North Plants surface HHE Change. Excavation completed under the North Plants Structures Project. soil. ch~mical sewers and associated Excavation of all ROD-defined HHE soil, chemical sewers and HHE soil HHE soil and dispose in the on-post associated with the sewers. All HHE soil was disposed in the on-post HWL. HWL.

Excavate biota risk soil outside the 2-foot soil cover area and dispose in Basin A.

Leave biota risk soil in place beneath the 2-foot soil cover area.

Change/Enhancement Excavation completed under the North Plants Structures Project. All biota risk soil in the North Plants area, including biota risk soil in the 2-foot cover area, was excavated and disposed in Basin A. Excavation volume was less than the biota risk soil volume identified in the ROD due to elimination of structure and concrete/asphalt areas'.

Continued biomonitoring for areas Enhancement. Residual ecological risk assessment resulted in cfesignation . that may pose potential risk to biota ofTRER area 25CC-3 for remediation. Remediation of this area included

in order to refine design boundaries soil excavation, soil tilling, and sampling to demonstrate no remaining for surficial soil. unacceptable risk.

Monitor for chemical agent during excavation; treat agent~contaminated

soil by caustic solution washing.

Change. Chemical agent monitoring conducted during soil and chemical sewer excavation activities was performed under the North Plants Structures Project. No chemical agent detections.

No MEC clearance activities required.

Enhancement. MEC sweep performed prior to demolition and excavation. activities. Oversight provided during demolition and excavation to respond to and clear anomalies. No MEC recovered.

Backfill HHE soil excavations with Change. Backfill completed under the North Plants Structures Project. clean soil. (Priority I soil allowed All HHE excavation areas were backfilled. per ROD-defined BAS Additional site grading will be completed in association with removal of recommendation) the North Plants haul road.


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Table 4.1-1: Changes to North Plants Soil Remediation (continued)

ROD-Prescribed Remedy Modification

Construct two-foot-thick soil cover over soil posing a potential risk to biota and the footprint of the '!'iorth Plants processing area.

Elimination. A two-foot-thick soil cover will not be constructed.

Change'. Vegetation will be consistent with general site remedy requirements. Revegetation of the North Plants area is documented in the RER CCR, Part I (TtEC2006a).

Revegetation standards consistent with ROD requirements for cover systems.

Institutional controls consistent with ROD requirements for cover systems (e.g., delineation and access control).

Change'. Institutional controls for a noncover area will be implemented (e.g., controlled access via roads and trails).

Elimination. Groundwater monitoring to assess the effectiveness of the soil cover is no longer needed.

Long-term groundwater monitoring.

'previously documented in Nor.h Plants Structures Demolition Project ESD (TtFW 2004d).

2ROD requirements for soil cover revegetation and instirutional control standards are no longer applicable since a cover will not be constructed. Revegetation and institutional controls will be consistent with general site remedy requirements.

4.2 Summary of Cost Change

The baseline estimated cost for implementation of the North Plants Soil Project was $2.74 million based on cost estimates presented in the ROD (FWENC 1996). The baseline estimate represents original ROD estimated costs reorganized to reflect implementation project descriptions in the RDIS (PMRMA 2008). Although the project cost decreased approximately $726,000 for elimination of the soil cover, implementation of project activities to support cover deletion including biota soil removal, sanitary sewer removal, petroleum-contaminated soil removal and additional soil sampling resulted in cost increases totaling approximately $1.16 million. Additional cost savings (approximately $625,000) were generated due to implementation efficiency of combining the North Plants structures demolition and soil remediation into one design and implementation effort. The final cost for implementation of the North Plants Soil remedy components is estimated at $2.55 million. This cost represents North Plants soil remediation activities that were conducted and documented under the North Plants Structures Project and sampling conducted in support of the North Plants Soil Design. In addition, the ROD included an estimated $1.24 million cost for short-term and long-term maintenance costs for the soil cover. Elimination of the soil cover also results in elimination of these maintenance costs.

A review of the range of alternatives evaluated in the ROD for the North Plants Soil Project indicates that the other four site-wide alternatives (not selected) would likely have experienced similar cost change. All site-wide alternatives presented in the ROD included excavation of HHE soil and construction of a two-foot-thick soil cover over soil posing a potential risk to biota and the footprint of the North Plants processing area. Assuming implementation in all cases


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would have been combined with the North Plants Structures Project, all would have been subject to the same implementation efficiency.

5.0 SUPPORT AGENCY COMMENTS

The EPA, CDPHE, and TCHD have reviewed this ESD. Comments from these Agencies have been incorporated into the document.

6.0 PUBLIC PARTICIPATION COMPLIANCE The Army published a public notice in the Rocky Mountain News andDenver Post on October 30,2008, making the Draft North Plants Soil Project ESD available for public review and comment. Notices were also published in the Commerce City Beacon, Brighton Blade and Gateway News. A presentation explaining the proposed changes contained in the ESD will be provided to the RMA Restoration Advisory Board (RAB) at a future meeting. The RAB is a community group that meets periodically to receive information and provide input on the cleanup being conducted at the RMA. The public comment period closed on December I, 2008 and no comments were received. The requirements set out in the National Contingency Plan. Section 300.435(c)(2)(ii). have been met.

This ESD and all documents that support the changes and clarifications are part of the Administrative Record and are available at the JARDF and the EPA Region 8 Superfund Record Center. The JARDF is open Monday through Friday between Noon and 4 pm or by appointment. The telephone number for the JARDF is 303-289-0983. The EPA Superfund Record Center can be reached at 303-312-7287. Hours of operation are Monday through Friday from 8:00 am to 4:00 pm.

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Rocky Mountain Arsenal Explana[ion ofSignifieant Differences Nonh Plants Soil Revision 0 wes 2.05.27.02 December 10. 2008

7.0 STATUTORY DETERMINATIONS

Considering the new infom1ation presented in this ESD, the Army, in consultation with EPA and CDPHE, believes that reailocation of the North Plants Soil remedy components within the overall RMA remedy, with the modifications described, satisfies the requirements of CERCLA Section 121 and is protective of human health and the environment, complies with federal and state requirements that are legally applicable or relevant and appropriate to the remedial action, uses a permanent solution through proper disposal and containment of the wastes in the on-post HWL or Basin A, and is cost effective.

Signatures

For U.s. Environmental Protection Agency

Date I , I ,

(;

Carol L. Campbell Assistant Regional Administrator Office of Ecosystems Protection and Remediation

For U.S. Army

Date Charles T. Scharmann Program Manager for Rocky Mountain Arsenal

For State of Colorado

DateS::!-¥JL2~ Director, Hazardous Materials and Waste Management Division Colorado Department of Publie Health and Environment


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Rocky Mountain Arsenal Explanalion of Significant DifTc:rcncc:s North Plants Soil Rc:vision 0 WBS 2.05.27.02 December 10.2008

8.0 REFERENCES

Army (DepaltIllent of the Army)

2008 (May 8) Vegetation Certification Leiter in Support ofthe Explanation of Significant Differencesfor the North Plants Soil Remediation Project. Memorandum from U.S. Fish and Wildlife Service to U.S. Environmental Protection Agency.

BAS (Biological Advisory Subcommittee)

2006 (Mar.) Terrestrial Residual Ecological Risk Soil Tilling Demonstration Study Report. Revision I.

2002 (Apr.) Assessment ofResidual Ecological Risk and Risk Management Recommendations at the Rocky Mountain Arsenal. Part I: Terrestrial Pathways and Receptors. Final Report.

CDLE (Colorado Department of Labof and Employment)

2005 (Oct.) Colorado Department ofLabor and Employment Petroleum Storage Tank Owner/Operator Guidance Documenr.

2004 Colorado Code ofRegulations. 7 CCR 1001-14.

Ebasco (Ebasco Services Incorporated)

1992 (Jan.) Remedial Investigation Summary Report. Version 3.2.

1988a (Sept.) Phase I Contamination Assessment Report North Plan/s Complex. Final. Version 3.2.

J988b (Sept.) Contamination Assessment Report Chemical Sewers - North Plants and South Plants. Final. Version 3.2.

EPA (U.S. Environmental Protection Agency)

1990 (Mar. 8) Na/ional Oil and Hazardous Substances Pollution Contingency Plan. Final Rule. 40 CFR Pan 300. Federal Register 55 (46): 8666-8865.

FWENC (Foster Wheeler Environmental Corporation)

2003a (Oct. 30) North Plants Soil Remediation Project Sampling and Analysis Plan. Revision O.

2003b (Mar. 4) Nor/h Plants Demolition Project Sampling and Analysis Plan Addendum. Revision O.

2003c (Jan. 27) North Plants Structure Demolition and Removal Project Design Change Notice DCN-NPD-018.

2002a (Aug. 19) Amendment No.1. North Plants Structure Demolition and Removal Project Soil Sampling and Analysis Plan (DCN-NPD-013).


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Rocl..-y Mountain Arsenal Explanarion ofSignificanr Diffcrcncl."S NOl1h Plants Soil Revision 0 WBS 2.0;.27.02 December 10. 2008

2002b (Aug. 9)

2002c (Feb.)

200la (Aug. 28)

2001 b (July 13)

200lc (July 13)

200ld (Aug. 23)

1996 (June)

1995 (Oct.)

Nor:h Plams Groundwater .Monitoring Project. Data Summary Report. Revision O.

North Plants Structure Demolition and Removal Project Groundwater Monitoring SAP (DCN-NPD-006).

North Plants Structure Demolition and Removal Project Soil Sampling and Analysis Plan. Revision 1.

North Plants Structure Demolition and Removal Project J00 Percent Design Analysis. Revision O.

NorTh Plants Structure Demolition and Removal Project Groundwater MONitoring Sampling and Analysis Plan. Revision O.

North Plams Structure Demolition and Removal Project Groundwater Monitoring SAP (DCN-NPD-002).

Record ofDecisionfor the On-Post Operable Unit. Version 3.1. (3 v).

Final Detailed Analysis ofAlternatives. Version 4.1. (7 v).

PMRMA (program Manager Rocky Mountain Arsenal)

2006 (Nov.) Remediation Design and Implementation Schedule.

RMA Committee

2003 (June) Refinement ofRemediation Areas for Swjicial Soil and Reduction of Residual Biota Risk.

1997 (May) Design Refinement ofExcavation Boundaries for Surficial Soil and Reduction ofResidual Biota Risk.

TtEC (Tetra Tech EC, Inc.)

2008 (Apr. 18)

2007 (Dec. 21)

2006a (Mar. 13)

2006b (Feb. 2)

2006c (Jan. 12)

2005 (June 23)

North Plants Petroleum Release Evaluation and Action Plan for LlI'APL Associated with Groundwater. Revision O.

North Plants Soil Remediation Project Interim Free Product and Groundwater Characterization Data Summary Report. Revision O.

Residual Ecological Risk Soil Remediation - Part J Construction Completion Report.

Data Summary Reportfor Residual Ecological Risk Siles. Revision 1.

North Plants Soil Remediation Project Western Parcel PI Soil Remediation Verification Data Summary Report. Revision O.

Data Summary Report Borrow Areas Managemem Residual Risk Soil Concentration Verification Soil Tilling Demonstration Study. Revision O.

NOl1h Planrs Soil ESD.doc

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Rocky Mountain Arst:nal Explanation of Significant Differences North Plants Soil Revision 0 was 2.0;.27.02 Dcc~mbcr 10.2008

TtFW (Tetra Tech FW, Inc.)

2005a (Apr. 18) North Plants Soil Remediation Project Interim Free Product and Groundwater Characterization Sampling and Analysis Plan. Revision O.

2005b (Jan. 25) North Plants Soil Remediation Project 30 Percent Design Analysis.

2004a (Dec. 7) Data Summary Report Borrow Areas Management Residual Risk Soil Concentration Verification/or PI Borrow Used in North Plants Chemical Sewer and Section 36 Existing (Sanitary) Landfill Projects. Revision O.

2004b (Dec. 3) North Plant Soil Remediation Project Petroleum Release Evaluation Report. Revision O.

2004c (Sept. 28) North Plant Structure Demolition and Removal Remediation Project and Destruction 0/Equipment in the GB Production and Fill Facilities Project Construction Completion Report. Revision I.

2004d (Sept. 24) txplanation o/Significant Differences/or North Plants Structure Demolition and Removal Remediation Project. Revision O.

2004e (June 29) North Plants Soil Remediation Project Petroleum-Impacted/Stained Soils Data Summary Report. Revision O.

2004f (Mar. 25) North Plants Structure Demolition and Removal Project Verification Soil Sample Data Summary Report. Revision 2.

2004g (Feb. 5) North Plants Structure Demolition and Removal Project Data Summary Report. Revision 1.

USFWS (U.S. Fish and Wildlife Service)

2000 (Oct. 12) Biological Advisory Subcommittee Residual Risk Soil Concentration Verification Sampling and Analysis Plan. Revision 1.


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ROCKY MOUNTAIN ARSENAL

North Plants Verfication and Confirmatory Soil Sample Locations

\\:ril'bian !;oIl !i.:mopic Loc::uicm

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ROCKY MOUNTAIN ARSENAL

North Plants Petroleum-Impacted Soil Sample Locations

• Petroleum Soil Investigation Location ..... , ~~} Estimated 2004 LNAPL Extent

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