CORRES: SIGNED COPY OF FINAL EXPLANATION OF … · Enclosed for your information is a signed copy ofthe Final Explanation of Significant Differences for the Chemical Sewer Remediation

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DEPARTMENT OF THE ARMY 79613O""R"" PROGRAM MANAGER FOR ROCKY MOUNTAIN ARSENAL '^U-RoSDMS

COMMERCE CITY, COLORADO 80022-1748 253444

December 5, 2000 '~ll(t>l'^0

REPLY To

ATTENTION OF:

Remedy Execution

Ms. Laura Williams U.S. Environmental Protection Agency

Region VIII Mail Code 8EPR-F 999-18 Street, Suite 500 Denver, Colorado 80202-2466

Dear Ms. Williams:

Enclosed for your information is a signed copy ofthe Final Explanation of Significant Differences for the Chemical Sewer Remediation (Section 35 and Section 26).

Ifyou have any questions, please contact the undersigned at 303-289-0246.

Sincerely,

Bruce M. Huenefeld

RMA Committee Coordinator

Enclosure

Copies Fumished:

Major M. Weslyn Erickson, Chief Counsel, Program Manager Rocky Mountain Arsenal, ATTN: AMSSB-PM-ORMA-CL, Commerce City, Colorado 80022-1748 (w/encl)

Ms. Barbara Nabors, Colorado Department of Public Health and Environment, 4300 Cherry Creek Drive South, Denver, Colorado 80246-1530 (w/encl)

Mr. Roger B. Shakley, Shell Oil Company, P.O. Box 538, Commerce City, Colorado 80037 (w/encl)

Mr. Thomas F. Cope, Holme Roberts and Owen, Suite 4100, 1700 Lincoln Street, Denver, Colorado 80203(w/encl)

Mr. L. Ronel Finley, Coordinator, U.S. Fish and Wildlife Service, Rocky Mountain Arsenal, Building 111, Commerce City, Colorado 80022-1748 (w/encl)

Mr. Thomas J. Butts, Msc, Tri-County Health Department Envirormiental Health Division, 4301 East 72nd Avenue, Commerce City, Colorado 80022-1488 (w/encl)

Program Manager Rocky Mountain Arsenal, ATTN: AMSSB-PM-ORMA-D, Document Tracking Center, Commerce City, Colorado 80022-1748 (w/encl)

Readiness is our Profession

EXPLANATION OF SIGNIFICANT DIFFERENCE FOR CHEMICAL SEWER REMEDIATION (SECTION 35 AND SECTION 26)

ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE

1.0 INTRODUCTION

1.1 The Rocky Mountain Arsenal

The Rocky Mountain Arsenal (RMA) On-Post Operable Unit 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). The RMA site is 27 square miles and is currently on the U.S. Environmental Protection Agency (EPA) National Priority List for environmental cleanup as a result of contamination released during previous RMA operations.

The Record of Decision for the On-Post Operable Unit (ROD) (Foster Wheeler 1996a), which describes the site-wide remedy for RMA, was signed by the U.S. Army, the EPA, and the Colorado Department of Public Health and Environment (CDPHE) on June 11, 1996. The selected remedy includes 31 cleanup projects for soil, structures, and treatment of ground water contamination. When the site-wide remediation is complete, most of RMA will become a National Wildlife Refuge, as provided for in Public Law #102-402.

1.2 Documenting Changes to the ROD

In implementing RODs at many sites throughout the country, the EPA realized that changes will inevitably occur and has established guidelines for documenting these changes. 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 40 CFR Section 300.435(c)(2)(i), the lead agency is required to publish an Explanation of Significant Difference (ESD) when significant changes are being considered to a final remedial action plan as described in a ROD. The lead agency for RMA is the Army. Regulatory oversight is conducted by the EPA, CDPHE, and the Tri-County Health Department (TCHD). TCHD oversees local public and environmental issues in Adams, Arapahoe, and Douglas Counties.

This ESD summarizes modifications to the remedy for chemical sewers in Section 35 and Section 26 that result from new information developed by the Army since the ROD was signed. These changes do not alter the hazardous waste management approach that was selected in the ROD. The fomner chemical sewer site (NCSA-6a) is composed of a comdor of soil and some remaining sewer pipe associated with the former chemical sewer in Sections 26, 35, and 36. The site is located in the central part of RMA and runs approximately 9,700 linear feet between South Plants and Former Basin F (Figure 2).

1.3 Circumstances that Led to the Chemical Sewer ESD

This ESD documents the following significant changes to the Chemical Sewer remedy in Section 35 and Section 26:

• No additional soil surrounding the location of the former chemical sewer site, NCSA-6a, in Sections 35 and 26 will be excavated.

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The ROD requires that sewer lines and human health exceedence soil for chemical sewers located outside of the South Plants Central Processing Area and Complex Trenches areas be excavated and landfilled in the on-site Hazardous Waste Landfill. Most of the site NCSA-6a sewer pipe was removed as part of a separate response action in 1982. The ROD estimated that contaminated soil associated with the former sewer pipe location would extend 10 feet on each side of the sewer line and 10 feet below ground surface or 2 feet below the sewer line, whichever was deeper. Design review of the 1982 response action indicated that a large portion of the associated soil had also been removed. Additional soil sampling was conducted in April 2000 to determine the extent of any remaining exceedance soils. Analytical results for all 27 identified chemicals of concern (COC) were below site evaluation criteria (SEC) as identified in the ROD, Volume 1, Table 7.1-4 "Site Evaluation Criteria and Principal Threat Criteria for Soil." Because the analytical data shows no evidence of human health exceedance soil remaining in the area surrounding the former chemical sewer line location, no further soil excavation is required for site NCSA-6a to comply with the ROD remedy.

This ESD is being provided for public review as defined in 40 CFR 300.435(c)(2)(i). The ESD and supporting documentation will become a part of the administrative record for the RMA and is available to the public at the Joint Administrative Record Document Facility (JARDF) located at the west gate of RMA on 72"^ Avenue and Quebec Street in Commerce City, Colorado. The JARDF is open to the public on Monday, Wednesday, and Friday from noon to 4:30 p.m.; on Tuesday and Thursday from 5:00 p.m. to 9:00 p.m.; and on Saturday from 10:00 a.m. to 4:00 p.m. The telephone number is 303-289-0143. Although not required, a formal public comment period is being held for this ESD. Additional information on the submittal of comments is provided in Section 7.

2.0 BACKGROUND

2.1 Summary of the RMA Site History and Contamination Problems

RMA was established in 1942 by the Army to manufacture chemical warfare agents and munitions for use in Worid War ll. Following the war and through the eariy 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 used during these years resulted in contamination of structures, soil, surface water, and ground water.

The On-Post Operable Unit is one of two operable units at RMA. The On-Post Operable Unit addresses contamination within the fenced 27 square miles of RMA. The Off-Post Operable Unit addresses contamination north and northwest of RMA.

The contaminated areas within the On-Post Operable Unit include approximately 3,000 acres of soil, 15 ground water plumes, and 798 structures. The most highly contaminated sites are located in South Plants (the central processing area, Hex Pit, Buried M-1 Pits, and the chemical sewers). Basins A and F, Lime Basins, and the Army and Shell trenches. The primary contaminants found in soil and ground water at these sites are pesticides, solvents, metals, and chemical warfare agent byproducts.

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The sites 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, particulariy at sites where burial trenches, disposal basins, or manufacturing complexes are located.

Ground water contamination plumes are also present at RMA. The characteristics and locations of the plumes suggest that the greatest contaminant releases to the ground water have occurred from Basin A and the Lime Settling Basins, the South Plants chemical sewer, the South Plants tank farm and production area, the Amriy and Shell trenches in Section 36, and the former Basin F. Plumes flowing from the Motor Pool/Rail Yard and North Plants area are other sources of contaminant releases to the ground water.

Since 1975, the Army and Shell have undertaken several Interim Response Actions and Remedial Actions at RMA designed to remove threats to the environment, while remaining consistent with any long-term cleanup. Continuing Interim Response Actions include ground water intercept and treatment north of RMA; ground water intercept and treatment north of Basin F; ground water intercept and treatment in the Basin A Neck area; boundary ground water intercept and treatment operation; remediation of other contamination sources (Motor Pool and Rail Yard ground water treatment); asbestos removal; removal of chemical process related equipment; and others.

2.2 Summary of the Selected On-Post Remedy

Under the on-post-cleanup plans, contaminated ground water plumes will be intercepted and treated at the three existing onsite treatment plants. Structures with no designated future use will be demolished and the debris placed in either the new, on-post hazardous waste landfills or the Basin A consolidation area, depending upon the degree of contamination. The contaminated soil at RMA will be addressed primarily through containment in the on-post hazardous waste landfills or under caps/covers, or through treatment depending upon the type and degree of contamination. Areas that have caps or covers require long-temn maintenance and will be retained by the Army. These areas will not become part of the wildlife refuge.

2.3 Summary of the Chemicai Sewer Remedy for Site NCSA-6a

Site NCSA-6a represents the former location of a vitrified clay, gravity flow chemical sewer line that conveyed waste from South Plants to Basin F (located in Section 26). Three pipelines exited South Plants running north under 7^ Avenue. Two of these three pipelines were east of D Street, converged into a single line in southwest Section 36 and entered Section 35 approximately 800 feet north of 7"̂ Avenue. The third pipeline entered Section 35 approximately 820 feet west of D Street and converged with the Section 36 pipeline in southeastern Section 35. The sewer continued north through Section 35 to its outfall at Basin F in Section 26.




Removal of most of the chemical sewer line in Sections 35 and 26 took place in 1982. The process included removal of the sewer line and manholes appurtenant to the line. A minimum of two feet of soil was also removed beneath the excavated lines and manholes and approximately two feet on either side of the sewer line because of suspected leakage. Two segments of chemical sewer associated pipe were not removed during this action. One remaining pipe section (Figure, 3a) extends north of the former sewer manhole MH 3-2 approximately 900 feet. This portion of sewer was never used and was plugged to allow diversion of South Plants aqueous waste to Basin F in 1956. The second remaining pipe section (Figure 3b) extends from near the visiting officer's quarters to former sewer manhole MH 1-3 in the northeast comer of Section 35. This approximately 350-feet portion of sewer is identified as a 4-inch diameter metal pipe. These segments will be remediated under the Section 35 Soil project as specified in the ROD.

Removal of contaminated soil associated with the former chemical sewer line was scheduled for implementation under three projects: Section 35 Soil, Basin F and Basin F Exterior, and Section 36 Balance of Areas Soil. The portion of the sewer located in Section 26 was originally planned to be addressed as part of the Basin F/Basin F Exterior Remediation project; however, it was reassigned to the Section 35 project to allow the former chemical sewer site to be addressed at one time.

Data reviewed for this ESD does not support a change to the remedy requirement for the segment of the former chemical sewer located in Section 36 at this time. This segment remains part of the Section 36 Balance of Areas Soil Remediation project. This ESD pertains to the chemical sewer site (NCSA-6a) in both Section 35 and 26 and as such the summary of the remedy presented herein only addresses this site. Other sites within the Section 35 Soil Remediation project remain unaffected by this ESD.

The ROD identifies the following major actions for the NCSA-6a remedy:

• 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 HWL.

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

• Finish grade and revegetate all disturbed areas.




3.0 MODIFICATIONS TO THE ROD REMEDY

This ESD summarizes the change to the excavation requirement for the former chemical sewer site, NCSA-6a, located in Sections 35 and 26. Table 1 summarizes the ROD-prescribed requirements for remediation of chemical sewer site NCSA-6a and shows the changes to those requirements for the portion of NCSA-6a in Sections 35 and 26. As shown in the table, the primary change to the remedy for this site is the deletion of further soil excavation in the area surrounding the former chemical sewer line. Portions of the line not previously removed will be removed under the Section 35 Soil Remediation project. The modifications described do not alter the hazardous waste management approach selected in the ROD and the remedy remains protective of human health and the environment.

Table 1: Changes to the Excavation Requirement for Chemical Sewer Site, NCSA-6a

ROD-Prescribed Remedy

Excavate overburden from excavation areas and stockpile.

Excavate remaining sewer line and dispose In the HWL.

No sampling requirement.

Excavate iiuman health exceedance soil to 10 feet on both sides of the sewer line to a depth of 10 feet or 2 feet below the line, whichever is deeper, and dispose in the HWL.

Backfill excavation with stockpiled overburden and other clean fill dirt.

Revegetation standards consistent with ROD requirements for disturbed areas.

Modification

No Change. Overburden will be excavated and stockpiled in areas that are excavated.

No Change. Excavation of all remaining sewer line. Dispose in the HWL.

Remedy Support. Collected 41 soil samples to assess if human health exceedance soil remained along the previously removed or yet to be removed sewer line.

Reduction. Eliminate excavation of additional soil associated with sewer line. Removal of soil associated with sewer was previously perfonned throughout much of Sections 35 and 26. Figure 4 illustrates the differences between the ROD described excavation and the previous response action. Samples collected in both excavated and non-excavated areas do not indicate any human health exceedance soil remaining in place.

No Change. The same backfilling standards will be used in excavation areas.

No Change. The same revegetation standards will be used, consistent with refuge policy, in disturbed areas.

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4.0 BASIS FOR THE ESD

The ROD requires removal of all human health exceedance soil associated with the former chemical sewer site, NCSA-6a. Although analysis of soil borings collected during the Remedial Investigation (Rl) (Ebasco 1992) along the former chemical sewer indicated COC concentrations were less than the SEC, the ROD estimated that the soil along the former chemical sewer was human health exceedance soil due to suspected leaks. Calculations for the volume of human health exceedance soil associated with NCSA-6a are presented in the Soil Quantity Calculation Summary Report (Foster Wheeler 1996b). These calculation show that the soil 10 feet to both sides of the sewer line, to a depth of 10 feet or 2 feet below the line, whichever is deeper, was to be removed as human health exceedance soil.

During design of the Section 35 Soil project a review of existing literature, data and documentation relevant to site NCSA-6a indicated that a large portion of the soil to be removed as required by the ROD had already been removed as part ofthe 1982 sewer response action. A position paper on the Former Chemical Sewer in Section 35 (Foster Wheeler 1999) was provided to Regulatory Agencies demonstrating that a large portion of the soil to be excavated as part of the ROD had been previously removed from below the sewer in Sections 35 and 26. The position paper provided a review of as-built drawings from the previous sewer line response action and documented the differences between the ROD-required excavation and the previous sewer and soil removal.

The position paper also presented the request that the remaining soil within the ROD-prescribed depth and width parameters (Figure 4) be eliminated from remedial requirements. Boring logs from the Rl sampling activities supported 1982 project plans that showed that soil, in addition to the sewer pipe, was removed to a depth of at least 2 feet below the sewer line and approximately two feet on either side of the sewer line. However, Rl boring log data from Section 36 was insufficient to support confirmation of soil depth removal provided by logs for borings in Sections 35 and 26. No modification or exemption from the ROD-prescribed remedy is proposed for the former chemical sewer in Section 36. Remediation of the Section 36 portion of the chemical sewer site will be completed as specified in the ROD under the Section 36 Balance of Areas Soil Remediation project.

The infonnation reviewed for the position paper indicated that the Rl for this site was conducted in one phase. Phase I was performed in the spring of 1986 and included soil sampling and field observations. A surface geophysical exploration was not warranted because the chemical sewer line had been removed in 1982 and there was no historical evidence of buried metal along the pipeline. Results from soil sampling and field observations are presented in the Phase I Contamination Assessment Report for this site (Environmental Science and Engineering, Inc., 1988). The Phase I analytical results indicate that the removal of the chemical sewer line and associated soils removed the vast majority of the contamination. Twenty-six of the total 28 samples retrieved from 24 borings detected no organic COCs and detected no inorganic COCs at concentrations above their indicator levels. Two samples were retrieved from below the trench backfill at boring 4063. One sample was determined to be residual sewer bedding material and the second was obtained immediately below the first. OCPs were detected in these two samples; however, the concentrations do not exceed human health SEC.




On July 22,1999 comments received from the U.S. Environmental Protection Agency (EPA) and the Colorado Department of Public Health and Environment (CDPHE) on the position paper submittal requested that additional soil sampling and analysis be performed to verify that human health exceedance soil does not exist. Samples were requested at areas identified as not sufficiently evaluated by the Rl sampling efforts and in areas where the ROD-designated soil was not previously removed. Consequently an additional sampling program was developed and conducted under the "Former Chemical Sewer Sections 35 and 26 - Sampling and Analysis Plan for the Former Chemical Sewer" (Foster Wheeler 2000a). The requested sampling effort resulted in a total of 41 subsurface soil samples collected using Direct Push Technology.

Samples were selected to evaluate areas along the sewer where existing Rl data and boring logs did not sufficiently characterize the ROD-prescribed soil removal depth. Samples were also located offset from the sewer centeriine where soil was not removed to the ROD-prescribed width. Figure 4 illustrates the location of samples placed offset to the former sewer centeriine. Sampling was also conducted near former manhole MH 1-9 (the tocation of boring 4063) where Rl data indicated evidence of leakage and contamination. Additionally, samples were collected along the sewer segments that were not previously removed. The locations of samples collected during both the Rl and design sampling effort are shown on Figures 3a and 3b.

A total of 41 samples (two of which were split for duplicate sample analysis) were collected and analyzed for volatile organic compounds (VOCs), organochlorine pesticides (OCPs), metals, mercury and chloroacetic acid. All samples except one had detections of lead, about two-thirds of the samples had detections of arsenic, and about half of the samples had detections of cadmium and chromium; however, all metals concentrations were below background levels for the site. Mercury was not detected in any samples. One sample reported a low level detection of dieldrin (6.35 ppm). This is well below the human health SEC for dieldrin of 41 ppm. No other detections were reported for OCPs or VOCs. The complete results of the design sampling effort are presented in the Former Chemical Sewer Sections 26 and 35 Data Review and Summary Report (Foster Wheeler 2000b).

The design sampling combined with Rl data provides a data set of 110 samples associated with the former chemical sewer. No evidence of soil contamination in excess of the human health SEC was identified. Based on the analytical results and review of the as-built records from the chemical sewer removal project, additional excavation along the former chemical sewer line is not necessary to protect human health and the environment, or meet Applicable or Relevant and Appropriate Requirements. The reduction in soil volume to be excavated will also result in some cost savings.

5.0 SUPPORT AGENCY COMMENTS

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




6.0 STATUTORY DETERMINATIONS

Considering the changes described in this ESD and the new information, the Amny, EPA and CDPHE believe that the remedy remains 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.

Signatures

For U.S. Environmental Protection Agency

Date Max H. Dodson Assistant Regional Administrator Office of Ecosystems Protection and Remediation

For U.S. Army

Date Charies T. Scharmann Program Manager for Rocky Mountain Arsenal

For State of Colorado

\ \ - \ 0 - c r O Date

Howard Roitman Director, Hazardous Materials and Waste Management Division

7.0 PUBUC PARTICIPATION COMPLIANCE

The Army published a public notice in the Rocky Mountain News and Denver Post on August 3, 2000, making the Draft Chemical Sewer ESD available for public review and comment. A presentation regarding the ESD was made to the Restoration Advisory Board (RAB) on August 3, 2000. The RAB is a community group that is open to the public and meets monthly to receive information and provide input on remedial activities being conducted at RMA. The public comment period was closed on September 5, 2000. No comments from the public 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 available at the Joint Administrative Record Document Facility (JARDF) and the EPA Region 8 Superfund Record Center.

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8.0 REFERENCES

Environmental Science and Engineering, Inc. 1988 (May) Final Phase I Contamination Assessment Report: Site 35-2/26-9, Chemical

Sewer, Final, Version 3.1.

Ebasco Services Incorporated. 1992 (Jan.) Remedial Investigation Summary Report, Final, Version 3.2.

Foster Wheeler Environmental Corporation (Foster Wheeler).

1996a (June 11) Record ofDecision for the On-Post Operable Unit, Version 3.1.

1996b (Sept.) Soil Quantity Calculation Summary Report.

1999 (Apr. 2) Position Paper on the Former Chemical Sewer in Section 35. 2000a (Feb. 18) Former Chemical Sewer Sections 35 and 26 - Sampling and Analysis Plan

for the Former Chemical Sewer. Final.

2000b (May) Former Chemical Sewer Sections 26 and 35 Data Review and Summary Report.

U.S. Environmental Protection Agency (EPA). 1999 (July) A Guide to Preparing Superfund Proposed Plans, Records ofDecision, and Other

Remedy Selection Decision Documents. EPA 540-R-98-031. Washington D.C.

1989 (Feb. 17) Federal Facility Agreement Pursuant to CERCLA Section 120. Docket No. CERCLA VIII-89-13.


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