base realignment and closure rocky mountain …holme roberts and owens, (mr. daniel j. dunn), 1700...

DEPARTMENT OF THE ARMY 1093161

BASE REALIGNMENT AND CLOSURE ROCKY MOUNTAIN ARSENAL

5650 HAVANA STREET BUILDING 129

COMMERCE CITY, COLORADO 80022-1748

FEB - 9 2009

DAIM-ODB-RM 5 February 2009

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

SUBJECT: Explanation of Significant Differences for Basin F/Basin F Exterior Remediation Project - Part 2 (Basin F Cover) and Chemical Sewer Remediation

1. Enclosed is the approved Explanation of Significant Differences for Basin F/Basin F Exterior Remediation Project - Part 2 (Basin F Cover) and Chemical Sewer Remediation, Revision 0, as indicated by signatures on page 13 of the document.

2. The point of contact on this matter is Mr. Lou Greer at 303-853-3951.

CF: Rocky Mountain Arsenal, (DAIM-BD-A-RM-CL/Mr. M . Weslyn Erickson), Chief Counsel, 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/encl)

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

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

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 72nd Avenue, Commerce City, Colorado 80022-1488 (wo/encl)

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

Encl RMA Committee Coordinator

EXPLANATION OF SIGNIFICANT DIFFERENCES FOR BASIN F/BASIN F EXTERIOR REMEDIATION PROJECT - PART 2 (BASIN F COVER)

AND CHEMICAL SEWER REMEDIATION

ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE

Prepared by: Tetra TechEC, Inc.

Prepared for: Rocky Mountain Arsenal Committee

Department of the Army Shell Oil Company

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

Colorado 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 will be used solely for the execution or review of the engineering, remediation, and/or construction of the subject project.

Revision Prepared By / Reviewed By Approved By Date Pages Affected

0 S . A c h e ^ A. Todd l y J. Lowrey — January 6,2009 All

Rocky Mountain Arsenal Explanation of Significant Differences Basin F Cover Project Revision 0 WBS 2.06.32.02 ' January 6,2009

CONTENTS

1.0 INTRODUCTION . . . .. 1

2.0 SITE HISTORY, CONTAMINATION AND SELECTED REMEDY .. 2

2.1 RMA Operational History............ .........2 2.2 Basin F History and Contamination Svornmary 3

2.2.1 Basin F Site Use History 3 2.2.2 Basin F Contamination Svornmary 3 2.2.3 Basin F Interim Response Action . 4

2.3 Chemical Sewer History and Contamination Summary 4 2.4 Summary of the Selected On-Post Remedy 6 2.5 Summary of the Selected Remedy for Basin F/Basin F Exterior Part 2 7 2.6 Summary of the Selected Remedy for Chemical Sewers 7 2.7 Basin F/Basin F Exterior Remediation Project Implementation 8

3.0 BASIS FOR THE ESD 8

4.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES 10

5.0 SUPPORT AGENCY COMMENTS 12

6.0 PUBLIC PARTICIPATION COMPLIANCE ; 12

7.0 STATUTORY DETERMINATIONS 13

8.0 REFERENCES ; 14

TABLES

Table 4.0-1 Changes to the Basin F Cover Project and Chemical Sewer Remediation

FIGURES

Figure 1.0-1 Rocky Mountain Arsenal Regional Reference

Figure 3.0-1 Basin F Chemical Sewer Alignment

Figure 4.0-1 Revised Basin F Cover Configuration

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ACRONYMS AND ABBREVIATIONS

B B M Biota Barrier Material

CAR Contaminant Assessment Report

CDPHE Colorado Department of Public Health and Environment

CERCLA Comprehensive Environmental Response, Compensation and Liability Act

CFR Code of Federal Regulations

CPA Central Processing Area

DCN Design Change Notice

EPA U. S. Environmental Protection Agency

ESD Explanation of Significant Differences

HHE Human Health Exceedance

HWL Hazardous Waste Landfill

IRA Interim Response Action

JARDF Joint Administrative Record Document Facility

NCP National Contingency Plan

NPL National Priorities List

OU Operable Unit

RAB Restoration Advisory Board

RCRA Resource Conservation and Recovery Act

RI Remedial Investigation

RMA Rocky Mountain Arsenal

ROD Record of Decision

SEC Site Evaluation Criteria

TCHD Tri-County Health Department

Rocky Mountain Arsenal Basin F Cover Project WBS 2.06.32.02

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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 Basin F/Basin F Exterior Remediation Project - Part 2 (Basin F Cover) and the chemical sewers remedy of the Rocky Mountain Arsenal (RMA) Federal Facility Site. Specifically, this ESD documents changes resulting from conditions encountered during implementation of Part 2 of the project. The RMA On-Post Operable Unit (OU) is a federally owned facility located in southern Adams County, Colorado, approximately 10 miles northeast of downtown 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) National Priorities List (NPL) for environmental cleanup as a result of contamination released during previous RMA operations. The Basin F Cover Project area is 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 11, 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 of RMA 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 Basin F Cover Project and chemical sewer remedy that result from new information developed by the Army since the ROD was signed. In June 2008, while excavating a channel east of the Basin F cover area, vitrified clay pipe was encountered. An extensive review of RMA records revealed that this pipe was part of the original chemical sewer constructed in 1957 to convey liquid waste to Basin F. In 1975, a portion of the sewer was replaced and the segment adjacent to Basin F was abandoned in place without any grouting or other mitigation conducted.

Following discovery and identification of the sewer line, the ROD requirements and project conditions were reviewed to determine appropriate remedial action for the sewer. Sewer location, excavation and disposal options, potential cover configuration, and cost were evaluated to determine the appropriate remedial actions consistent with the ROD. This evaluation resulted in identification of containment as the remedial action for the chemical sewer and associated soil. The containment remedy, included as part of the Basin F Cover Project through Design Change

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Notices, includes plugging the sewer void space with a concrete mixture and extending the Basin F Resource Conservation and Recovery Act (RCRA)-equivalent cover over the contaminated soil area associated with the abandoned sewer. The addition of the chemical sewer remedy increases the project cost by $485,000, an approximate 2 percent increase compared to the ROD baseline estimate. These changes, while resulting in the need for an ESD, do not alter the overall hazardous waste management remedy that was selected in the ROD.

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 Noon 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 of RMA. 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 [CPA], Hex Pit, Buried M - l 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, 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 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

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South Plants chemical sewer, the South Plants tank farm 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 Basin F History and Contamination Summary

2.2.1 Basin F Site Use History Construction of the Basin F (NCSA-3) surface impoundment occurred between July and December 1956. The impoundment was created by constructing a dike around a natural depression and lining it with a 3/8-inch asphalt membrane and a 1-foot-thick soil protective layer. The impoundment had a surface area of approximately 93 acres and a capacity of approximately 243 million gallons. The impoundment was to be used to contain liquid wastes from Army and Shell chemical operations, including the Chlorine Plant, Shell Manufacturing Area and the Sarin (GB) complex (North Plants).

Basin F was used continuously between December 1956 and December 1981 for the solar evaporation of contaminated liquid wastes. Liquid wastes were conveyed to Basin F from South Plants and North Plants through the chemical sewer system, described in Section 2.3. Following the termination of all waste discharges to the chemical sewer in December 1981, the Army implemented a series of measures designed to accelerate the evaporation of the remaining liquids in the basin, prevent sewer-transported flows from infiltrating both ground and surface waters, and prevent surface runoff from generating additional liquid waste volumes contained in the basin. These measures included: 1) removal of the chemical sewer trunk line and lateral connection to Basin F from South Plants and North Plants; 2) construction of a pipe trickier system in the basin to enhance natural solar evaporation; 3) installation of a dike in the basin separating the 'wet' from 'dry' areas; and 4) construction of a north-south surface runoff interceptor ditch along the eastern basin perimeter. The basin was preliminarily closed by the removal of all conveyance systems into the basin on July 14, 1982.

2.2.2 Basin F Contamination Summary The Remedial Investigation (RI) for Basin F was conducted in two phases. Phase I was performed in the fall of 1985 and the summer of 1986 and included soil sampling and field observations. Results are presented in the Phase I Contamination Assessment Report (CAR) (ESE 1988b). The Phase II program began in February 1988 to complement the Interim Response Action (IRA) by indicating the lateral and vertical extents of contamination remaining at the site.

The Phase I results showed samples with elevated concentrations of organic contaminants at depths down to 20 feet in areas where the physical integrity of the liner was poor. The concentrations in these locations remained relatively uniform with depth, and high concentrations of many contaminants occurred in the soil at or above the water table elevation. In contrast, moderate to low contaminant concentrations were detected in most samples taken where the liner was still intact and concentrations decreased with depth.

The Phase II program was conducted in two stages. Phase Ha consisted of sample collection outside the basin area, conducted to assess both the lateral and vertical extent of soil

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contamination outside the Basin F fence. Phase lib consisted of sample collection inside the basin during the IRA after the overburden, liner and some of the underlying soils were removed. The results of Phase Ha and lib are presented in separate CARs (ESE 1988a and Ebasco 1989).

Results from Phase Ha demonstrated that the highest contaminant concentrations were located on the east side of the basin, primarily on the surface. These surface areas, located outside the Basin F boundary, were part of the Basin F Exterior project area. The Phase lib sampling results generally paralleled the results collected during the Phase I sampling effort. The Phase lib sampling again showed that the greatest concentrations of contaminants were found in the eastern and southern portions of the basin with organic contaminants exceeding the site evaluation criteria.

2.2.3 Basin F Interim Response Action In 1986 the Army, Shell Oil Company, and the EPA Region VIII decided that an accelerated remediation should be conducted to address concerns regarding liquid and soil contamination in and under Basin F. This IRA for the Basin F hazardous liquid waste, sludge, and soil was undertaken pursuant to CERCLA and was performed between 1988 and 1996.

During May 1988, the Army began transferring Basin F hazardous liquid waste into the three lined tanks at the Tank Farm for interim storage. Additional liquid retention capacity in two surface ponds (double-lined impoundment identified as Ponds A and B) was used because of unexpected liquids found perched between a false bottom (salt lenses) and the asphalt basin floor, and seasonal precipitation which increased the volume of the liquid beyond the initial estimate. The hazardous liquid was subsequently incinerated by Submerged Quench Incineration between 1993 and 1995.

Approximately 480,000 cubic yards of contaminated soil, crystalline sludge, sludge, overburden, and asphalt liner were stripped, partially dried by piling and turning, and then transferred to the Basin F Wastepile. The depth to which contaminated materials were removed from Basin F varied depending on field observations. However, because it was impractical to remove all of the contamination down to depths approaching 40 feet, Basin F soil removal was halted at a depth of 6.5 feet below the asphalt liner elevation. After the designated contaminated material had been consolidated into the wastepile, the Basin F surface area was covered with a clay cover, topsoil, and vegetative cover.

2.3 Chemical Sewer History and Contamination Summary The chemical sewer was constructed in stages as RMA facilities were built or expanded. The first part of NCSA-6a was constructed in 1942. This original line was 12-inch clay tile and connected the Chlorine Plant with what was to be the caustic waste basin. Although this line was never used for caustic waste disposal, the southern portion of the line eventually became part of the chemical sewer system connecting South Plants to Basin F.

Between 1944 and 1946, the Army constructed a 12-inch clay sewer line that received waste from South Plants and transported the waste to Basin A. In 1953, a second 12-inch line was constructed to segregate Hyman waste flows from Army waste. This line crossed the existing





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Army sewer line and discharged waste into a stilling basin in Section 36. Effluent from the stilling basin entered Basin A via an open ditch.

After the completion of Basin F in 1956, the original chemical sewer lines leading from South Plants to Basin A were modified and a 10-inch vitrified clay gravity flow chemical sewer line was extended to Basin F. The new sewer line intercepted flows from both 12-inch lines and carried the waste to Basin F. This sewer was identified during the RI/Feasibility Study as NCSA-6a. The southern part of the 12-inch line from the Chlorine Plant to the unused Caustic Waste Basin was connected to the 10-inch main line using an 8-inch clay tile pipe. The sewer line from North Plants to Basin A was also modified and connected with the 10-inch main in the northeast comer of Section 35.

In 1960, an investigation of the chemical sewer line was conducted and pipeline flow measurements were recorded. The results indicated that a considerable amount of fluid had been lost through the pipeline and the chemical sewer was considered a potential source of groundwater contamination. However, because the chemical sewer was located within areas of groundwater contamination due to RMA. waste disposal in South Plants and Section 36, a specific relationship could not be determined.

In 1975 the Army replaced a portion of the sewer near Basin F in response to discovery of contaminants in groundwater contamination north of RMA. Records show that this pipeline segment adjacent to Basin F was leaking and possibly deteriorating and the sewer was identified as a potential source of groundwater contamination (Army 1975). Therefore, this segment was abandoned in-place, without any grouting or other mitigation, and replaced with a new sewer pipe approximately 50 feet south of the original sewer alignment (Watson 1975). Although Army correspondence estimated this sewer segment at approximately 800 feet, inspection of record drawings from the original sewer construction show this segment to be closer to 700 feet (ACOE 1957).

Removal of the majority of chemical sewer line NCSA-6a took place in 1982 (ESE 1988c). The process included removal of the sewer line and manholes appurtenant to the line. Soil within two feet (at least) of any point of the excavated lines and manholes was also removed because of suspected leakage. Additional soil could also be removed at the direction of the Contracting Officer. Sewer pipe and potentially contaminated soils were disposed within Basin F. All trenches created during the chemical sewer removal were backfilled with clean soil from a borrow area.

The RI for this site was conducted in the spring of 1986 and included soil sampling and field observations. Results from soil sampling and field observations are presented in the Phase I CAR for this site (ESE 1988c). A Phase II program was not recommended for the site, because further soil investigations would not provide a more accurate determination of potential leakage points along the sewer line.

The Phase I program consisted of 20 soil borings in Sections 26 and 35, yielding 22 soil samples. Using as-built drawings from the sewer line removal and other documents, boring locations were placed as close as possible to the centerline locations. Thirteen of the borings were located in

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Section 35 and yielded 15 samples. The remaining 7 borings were located in Section 26 and yielded 7 samples. Samples were taken immediately below the base of backfill in undisturbed alluvium, unless field conditions required an adjustment in the intervals. The depths of the interface of backfill placed after the sewer removal and underlying undisturbed native soil indicate that at least 2 feet of soil from beneath the sewer, and significantly more at many locations, was removed with the sewer line.

The Phase I analytical results indicate that the removal of the chemical sewer line and associated soils removed the vast majority of the potential contamination in Sections 35 and 26. Samples retrieved from 8 of the 13 borings in Section 35 detected no analytes at concentrations above their indicator levels. Samples retrieved from the 7 borings in Section 26 detected only metals, all of which were also below the human health site evaluation criteria (SEC). In addition, the ROD identified remaining chemical sewers as potential chemical agent sites. However, NCSA-6a was not included as a potential chemical agent site since the sewer had been previously removed.

During the Section 35 Soil design process, information was discovered that documented removal of soil beneath and adjacent to most of this sewer (FWENC 2002). Although removal of the majority of this sewer was known to have occurred in 1982, the ROD remedy required removal of soil immediately beneath and adjacent to the former sewer location. No contamination exceeding SEC was detected during RI soil sampling. However, soil beneath and adjacent to the former sewer location was inferred to be Human Health Exceedance (HHE) soil based on sewer-associated soil contamination within South Plants. In addition, two segments of chemical sewer associated with NCSA-6a that had not been removed in 1982 were identified in Section 35. These segments were added to the Section 35 Soil Project and were removed in 2002 (TtFW

Additional soil sampling was conducted in April 2000 to augment the previous RI sampling along the centerline of the former, removed chemical sewer and to evaluate potential lateral contamination at locations between where soil had been removed in 1982 and the ROD-prescribed limit of remediation (10 feet from the former sewer centerline). The analytical results, documented in the Former Chemical Sewer Sections 26 and 35 Data Summary Report (FWENC 2000c), showed no evidence of contaminated soil remaining requiring excavation. Because the potentially contaminated soil nearest the sewer had already been removed and no contaminants exceeding the human health SEC were found in the remaining nearby soil, the requirement for further soil excavation was eliminated. The changes to the chemical sewer remediation requirements for NCS A-6a were documented in the Explanation of Significant Differences for Chemical Sewer Remediation Section 35 and Section 26 (FWENC 2000a).

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

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

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

• 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 Army. 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

2.5 Summary of the Selected Remedy for Basin F/Basin F Exterior Part 2

The ROD identifies the following major remedial actions for the Basin F Cover Project:

• Construct subgrade to support cover construction.

• Construct a RCRA-equivalent cover over the former Basin F.

• Revegetate consistent with ROD requirements for covers.

• Provide institutional controls consistent with ROD requirements for covers.

• Provide long-term maintenance of the soil cover.

• Provide long-term groundwater monitoring.

2.6 Summary of the Selected Remedy for Chemical Sewers The ROD included two distinct remedial actions for chemical sewers based on their location.

For chemical sewers located within the South Plants CPA and Complex Army Trenches, the ROD identifies the following for sewer remediation:

• Plug sewer void space with a concrete mixture to prohibit access and eliminate them as a potential migration pathway for contaminated groundwater.

• Contain plugged sewers beneath the soil covers in their respective sites.

For chemical sewers located outside South Plants CPA and Complex Army Trenches, which includes NCSA-6a, the ROD identifies the following for sewer remediation:

• Remove overburden (soil above the top of the pipe) from excavation areas and stockpile.

• Excavate remaining sewer line and dispose in HWL.

• Excavate contaminated soil 10 feet to both sides of the remaining sewer and the former sewer line to a depth of 10 feet or 2 feet below the line whichever is deeper and dispose in

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• Backfill excavation with stockpiled overburden and other clean fill dirt.

• Finish grade and revegetate all disturbed areas.

For NCSA-6a where the chemical sewer had been previously removed, the ROD requirement applied only to excavation of the potentially contaminated soil associated with the sewer. As discussed in Section 2.3, additional information developed during the Section 35 Soil Remediation Project design resulted in an ESD eliminating the requirement for further soil excavation along the previously removed sewer line (FWENC 2000a). Because the ESD was based on removal of the chemical sewer and surrounding contaminated soil, it is not applicable to sewer segments remaining in place.

2.7 Basin F/Basin F Exterior Remediation Project Implementation The Basin F/Basin F Exterior Remediation Project was implemented in two parts, each with a separate design. Part 1 was implemented in two phases starting in December 2001. Part 1, Phase 1 included excavation of contaminated soil from Basin F Exterior areas with disposal in the HWL or Basin A depending on level of contamination (FWENC 2000b). Remediation of all Phase 1 areas included in the final design was completed in January 2003. Additional soil excavation was completed in fall 2004. The additional excavation addressed soil at depths greater than 1 foot that exceeded the acute human health SEC. Al l remediation work completed under Part 1, Phase 1 of the project is documented in the Part 1, Phase 1 Construction Completion Report (TtEC 2005a).

Part 1, Phase 2 of the Basin F/Basin F Exterior Remediation Project consisted of excavation of biota soil located in the northern part of the Basin F Exterior that was designated for consolidation within Basin F. This was completed in 2008 during the Basin F Principal Threat Soil Remediation Project to allow use of the biota soil for backfill of principal threat soil excavations within Basin F. Remediation efforts completed under Phase 2 are documented in the Part 1, Phase 2 CCR (TtEC 2008a).

Part 2 of the Basin F/Basin F Exterior Remediation Project, or the Basin F Cover Project, includes construction of the Basin F RCRA-Equivalent cover as well as gradefill placement below the cover, excavation of soil and surface grading around most of the perimeter of the cover area, and installation of engineering controls on and around the cover. The Part 2 design was completed in January 2008 and construction started in June 2008.

In June 2008, while excavating a drainage channel east of the Basin F cover area, vitrified clay pipe was encountered. An extensive review of RMA records revealed that this pipe was part of the original chemical sewer constructed in 1957 to convey liquid waste to Basin F. The location of the pipe aligns with the segment of chemical sewer abandoned in 1975 after construction of the bypass line, as discussed in Section 2.3.

3.0 BASIS FOR THE ESD Discovery of the abandoned chemical sewer requires modification of the Basin F Cover Project to include remediation of the chemical sewer. As discussed in Section 2.3, the majority of the chemical sewer was removed in 1982. Records from that excavation activity show that the





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newer replacement segment was removed along with the rest of the chemical sewer. However, the original abandoned segment was not removed with the exception of a small portion of sewer line encountered during construction of a drainage ditch near the southeast comer of Basin F. Most of the abandoned segment still exists from the location of the 1982 drainage ditch upgradient to a point where the 1982 sewer excavation occurred. The remaining abandoned segment is approximately 630 feet long. The original chemical sewer alignment, the replacement sewer segment and the location of the discovered pipe are shown on Figure 3.0-1.

Because the original, abandoned segment was not identified on the initial RI map of the sewer, no samples were collected along the abandoned segment. Also, the RI samples collected along the removed chemical sewer alignment were collected after sewer and soil excavation and do not represent potential contamination levels associated with the abandoned segment. However, based on the use history as a conveyance line for liquid waste disposal in Basin F, soil contamination is assumed to be consistent with contamination present in Basin F. Potential soil contamination limits for remediation are defined consistent with the ROD as 10 feet deep or 2 feet below the pipe whichever is deeper and 10 feet laterally on each side of the pipe. In addition, the ROD identified remaining chemical sewers as potential chemical agent sites. Although NCSA-6a was not included as a potential chemical agent site since the sewer had been previously removed, this remaining segment is considered a potential chemical agent site consistent with the ROD (TtEC 2008b).

Following discovery and identification of the sewer line, the ROD requirements and project conditions were reviewed to determine appropriate remedial action for the sewer. As discussed in Section 2.6, there are two ROD-identified remedial actions for chemical sewers, which were applied based on their location relative to required soil covers. For sewers located within cover areas, the ROD-identified remedy is plug and contain beneath the soil cover. For sewers located outside soil cover areas, the ROD-identified remedy is excavation and disposal in the on-site landfill. Although this sewer segment is outside the design boundary of the Basin F cover, its proximity adjacent to the cover and expected soil contamination similar to Basin F resulted in evaluating both the containment and excavation remedial actions.

Several factors were considered in evaluating which ROD-identified remedial action to apply to this sewer segment. For the excavation option, disposal would be required off site since the RMA on-site landfills are full and cannot receive additional waste. Off-site disposal would also likely require incineration since the expected contamination levels would not meet Land Disposal Restrictions for off-site disposal facilities. Air and odor momtoring would be required to ensure emission and odor control during excavation. Also, chemical agent monitoring would be required since the sewer is a potential chemical agent site. Since the RMA on-site agent laboratory has already been decommissioned, an appropriate alternative to the on-site laboratory support would need to be identified. In addition, the off-site disposal would require transportation outside of RMA, introducing potential risk for the community.

The containment option would require modification of the Basin F RCRA-equivalent cover since the sewer is adjacent to the current extent of the cover. Consistent with other RMA RCRA-equivalent cover designs, the cover extension would include the extent of the contaminated soil,


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in this case the 20-foot wide corridor along the abandoned sewer alignment. In addition, biota barrier material (BBM) would extend another 50 feet beyond the design limits of the cover extension. The cover extension would add less than one-half acre to the overall Basin F cover area. The increase in cover project area, including the 50-foot B B M run out, would be less than 2 acres, representing an approximate 2 percent increase in project area. The cover extension area would also require long-term operations and maintenance consistent with RMA RCRA-equivalent covers as described in the Long-Term Care Plan (RVO 2008).

Implementation cost was also considered during remedy evaluation. Cost to implement the excavation remedy with off-site disposal is estimated at $1.17 million based on a contaminated soil volume consistent with the ROD assumptions for NCSA-6a. This option has considerable cost uncertainty due to unknown subsurface conditions and the potential for the volume to increase beyond the excavation assumptions. Cost to implement the containment remedy is estimated at $485,000. There is little cost uncertainty with this option since the cover extension area is known and construction of the cover extension would be added to the existing Basin F cover construction effort allowing use of the existing construction equipment and personnel.

Considering these factors, the containment remedy was chosen for the Basin F chemical sewer segment. The remedial action includes grouting the sewer void with a cement mixture and extending the Basin F RCRA-equivalent cover over the potentially contaminated soil associated with the sewer. The containment remedy provides protection of human health and the environment by isolating the sewer and contaminated soil and minimizing percolation of precipitation through the soil, thereby reducing the potential for groundwater contamination. To take advantage of schedule efficiency, the pipe grouting effort was completed prior to design and approval of the cover extension. In preparation for sewer grouting activities, the sewer pipe was breached and agent screening was performed to determine the potential for chemical agent. Results of the screening were negative, indicating that sewer grouting could be performed with no credible potential for encountering chemical agent. The pipe grouting was completed on August 29,2008.

In addition, a Design Change Notice (DCN) was completed detailing the changes necessary for the Basin F cover to extend over the chemical sewer area. This DCN was approved by the Regulatory Agencies as a modification to the Basin F Closure Plan following completion of a public comment period as discussed in Section 6.0. Construction of gradefill and placement of B B M for the cover extension began in November 2008 following approval of the DCN. Although these construction activities were initiated prior to finalization of the ESD, these activities do not preclude excavation of the sewer or associated contaminated soil.

4.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES The change to the Basin F/Basin F Exterior Remediation Project is the addition of chemical sewer remediation. An abandoned segment of chemical sewer was discovered during soil excavation around the perimeter of Basin F. Evaluation of the sewer, project conditions and ROD requirements resulted in selection of a containment remedy for the sewer segment consistent with the containment remedy identified in the ROD for other chemical sewers. As a result, the sewer void has been plugged and the Basin F RCRA-equivalent cover is being


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extended oyer the contaminated soil area associated with the abandoned sewer. A summary of project remediation changes is included in Table 4.0-1.

The RCRA-equivalent cover extension will be constructed consistent with all requirements contained in the Basin F Cover Project design package (TtEC 2008c). The resultant RCRA-equivalent cover limits, along with the 50-foot B B M runout limits, are shown on Figure 4.0-1. The cover extension adds less than 2 acres to the project area representing an approximate 2 percent increase. The cover maintenance road encompassing the Basin F cover will also be extended to include the cover extension area. In addition, the road, which defines the limits of the Army Maintained Area, will be expanded along the east side of the cover to incorporate the drainage channel to the east of Basin F.

The cost for construction of the Basin F RCRA-equivalent cover as presented in the 100 percent design package is approximately $19.8 million. The chemical sewer remedy, including pipe grouting and extension of the cover, adds approximately $485,000, or 2 percent, to the project cost. Additional costs for long-term operations and maintenance are minimal since the cover extension only represents an approximate 2 percent increase in project area.

Table 4.0-1: Changes to the Basin F Cover Project and Chemical Sewer Remediation

ROD-Prescribed Remedy Modification

No chemical sewer remedy for Basin F project (chemical sewer in Section 26 identified in the ROD as previously removed).

For chemical sewers outside of cover areas, excavate sewer and contaminated soil and dispose in on-post landfill.

Addition. Chemical sewer remedy required for remaining abandoned sewer segment.

Change from excavation/landfill to containment in place. Plug sewer void to prevent access and eliminate as a potential groundwater migration pathway. Contain beneath Basin F RCRA-equivalent cover.

Construct RCRA-Equivalent cover over Basin F. Increase. Extend Basin F RCRA-equivalent cover over contaminated soil area associated with abandoned chemical sewer.

Revegetation standards consistent with ROD requirements for cover systems.

No Change. Vegetation standards required for RCRA-Equivalent covers are included in the Basin F Cover design and will be applied to the cover extension as well.

Institutional controls consistent with ROD requirements for cover systems (e.g., delineation and access control) as modified by the ROD Amendment for the Section 36 Lime Basins Remediation and Basin F Principal Threat Soil Remediation (TtEC 2005b).

No Change. Institutional controls are included in the Basin F Cover design and will be applied to the cover extension as well.

Long-term Operations and Maintenance. No Change. Operations and maintenance requirements will be applied to the cover extension as well.

Long-term groundwater monitoring. No Change.


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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 and Denver Post on December 1,2008, making the Draft Basin F Project ESD available for public review and comment. Notices were also published in the Commerce City Beacon, Brighton Blade and Far NE Reporter. Presentations explaining the proposed changes contained in the ESD were provided to community groups, including the RMA Restoration Advisory Board (RAB) on September 11, 2008. 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 31,2008 and no comments were received. The requirements set out in the National Contingency Plan, Section 300.435(c)(2)(ii), have been met.

In addition, a DCN describing the changes to the Basin F Cover Project Design was prepared to provide the details for the cover extension. The DCN was submitted by the RVO as part of a notification requesting modification of the CDPHE approved Final Closure Plan for the Closure/Post-Closure of the Basin F Surface Impoundment and Closure of the Basin F Wastepile - Rocky Mountain Arsenal (Basin F Closure Plan) (HLA 1996) as amended. The CDPHE issued a public notice for modification of the Basin F Closure Plan with a public comment period starting on September 30, 2008 and closing on October 30,2008. No comments were received and the modification to the Basin F Closure Plan was approved on October 31, 2008. Subsequently, the DCN was approved on November 10, 2008.

This ESD and all documents that support the changes and clarifications are part of the Admimstrative 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.





January 6,2009

7.0 STATUTORY DETERMINATIONS Considering the new information presented in this ESD, the Army, in consultation with EPA and CDPHE, believes that the Basin F/Basin F Exterior Remediation Project - Part 2 (Basin F Cover) and the chemical sewer remedy, with the modifications described, satisfy the requirements of CERCLA Section 121 and are protective of human health and the environment, comply with federal and state requirements that are legally applicable or relevant and appropriate to the remedial action, use a permanent solution through proper disposal and containment of the wastes in the on-post landfills or Basin A, and are cost effective.

Signatures

For U.S. Environmental Protection Agency

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

For U.S. Army

Date Charles T. Scharmann Program Manager for Rocky Mountain Arsenal

For State of Colorado

Gary W. Baughman Director, Hazardous Materials and Waste Management Division Colorado Department of Public Health and Environment


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Rocky Mountain Arsenal Explanation of Significant Differences Basin F Cover Project Revision 0 WBS 2.06.32.02 January 6,2009

8.0 REFERENCES

ACOE (U.S. Army Corps of Engineers)

1957 (Feb. 1) Drawing 71-17-01 Plan - Profile Line 1A Sta. 0+00 To 15+00. Revised to show As-Built Conditions.

Army (U.S. Army)

1975 (June 23) Environmental Assessment Statement for the Off-Post Contamination Control Plan at Rocky Mountain Arsenal.

Ebasco (Ebasco Services Incorporated)

1989 (Jan.) Final Phase lib Data Addendum Site 26-6: Basin F. Version 3.1.

EPA (U.S. Environmental Protection Agency)

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

ESE (Environmental Science and Engineering, Inc.)

1988a (Sept.) Final Phase II Data Addendum Site 26-6: Basin F. Version 3.1.

1988b (May) Final Phase I Contamination Assessment Report Site 26-6: Basin F. Version 3.3.

1988c (May) Final Phase I Contamination Assessment Report Site 35-2/26-9: Chemical Sewer. Version 3.1.

FWENC (Foster Wheeler Environmental Corporation)

2002 (Feb. 14) Section 35 Soil Remediation Project 100 Percent Design Package. Revision 0.

2000a (Nov. 7) Explanation of Significant Differences for Chemical Sewer Remediation (Section 35 and Section 26) Rocky Mountain Arsenal Federal Facility Site.

2000b (Aug. 4) Basin F/Basin F Exterior Remediation Project Part 1100 Percent Design Package.

2000c (Aug. 1) Former Chemical Sewer Sections 26 and 35 Data Summary Report.

1996 (June) Record of Decision for the On-Post Operable Unit. Version 3.1. (3 v).

HLA (Harding Lawson Associates)

1996 (Apr.) Final Closure Plan for the Closure/Post-Closure of the Basin F Surface Impoundment and Closure of the Basin F Wastepile, Rocky Mountain Arsenal, Commerce City, Colorado. Revised by Tetra Tech EC, Inc. on Jan. 26,2006.



Rocky Mountain Arsenal Explanation of Significant Differences Basin F Cover Project Revision 0 WBS 2.06.32.02 January 6,2009

RVO (Remediation Venture Office)

2008 (Sept. 9) RCRA-Equivalent, 2-, and 3-Foot Covers Long-Term Care Plan. Revision 1.

TtEC (Tetra Tech EC, Inc.)

2008a (Oct. 28) Basin F/Basin F Exterior Remediation Project - Part 1 Phase 2 Construction Completion Report. Revision 0.

2008b (Aug. 22) Former Basin F Principal Threat Soil Remediation Project and Basin F/Basin F Exterior Remediation Project - Part 2. Recovered Chemical Warfare Materiel and Ordnance and Explosives Hazard Evaluation. Revision 1.

2008c (Jan. 31) Basin F/Basin F Exterior Remediation Project - Part 2 (Basin F Cover). 100 Percent Design Package. Revision 0.

2005a (Nov. 1) Basin F/Basin F Exterior Remediation Project - Part 1 Construction Completion ReporU Revision 0.

2005b (Oct. 20) Amendment to the Record of Decision for the On-Post Operable Unit, Rocky Mountain Arsenal Federal Facility Site Section 36 Lime Basins Remediation Basin F Principal Threat Soil Remediation. Revision 0.

TtFW (Tetra Tech FW, Inc.)

2004 (Apr. 13) Section 35 Soil Remediation Project Construction Completion Report. Revision 1.

Watson (Gerald G. Watson, Col.)

1975 (May 27) Letter to Edward G. Dreyfus, Colorado Department Health, regarding the matter of unauthorized discharge ofpollutants by Shell Chemical, Inc., and/or United States Army, Rocky Mountain Arsenal.


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N

1 2 3 4

Miles

ROCKY MOUNTAIN ARSENAL Regional Reference

Figure 1.0-1

2181750 2182250

2181750 2182000 2182250

ROCKY MOUNTAIN ARSENAL Basin F Chemical Sewer Alignment Chemical Sewer Manholes

O Final Manhole Location

Original Manhole Location

RCRA Equivalent Cover

i I I Basin F Cover Boundary

l—I—'— Basin F Cover Runout

Chemical Sewers

Final Chemical Sewer, NCSA-6a Removed in 1982

Original Sewer, Abandonded in 1975

Exposed VCP Pipe, 2008

RI Site Boundary forNCSA-6a

/:

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f—Y — r " l " (. 4 ' 3 ' 2

Lambert Conic Conformal Projection State Plane Coordinate System

Colorado North Zone - NAD 1927

Figure 3.0-1

2180000 2181000 I

2182000

+ + +

+ +

2180000 2181000 2182000

ROCKY MOUNTAIN ARSENAL Revised Basin F Cover Configuration

• I RCRA-Equivalent Cover

| | 50' BBM Runout

1 Cover Maintenance Road

/ / 2 2

A21 23 24 19 20 i

/ / 2 2

A21 25 30 29 | (33 ! 34 35 36 31 "i f« i »

1 1

2 1 6 •I 11 12 7 • i 1

IN

4-Lambert Conic Conformal Projection

State Plane Coordinate System Colorado North Zone - NAD 1927

Figure 4.0-1

base realignment and closure rocky mountain …holme roberts and owens, (mr. daniel j. dunn), 1700...

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