MDE MARYLAND DEPARTMENT OF THE ENVIRONMENT 1800 Washington Boulevard • Baltimore MD 21230 410-537-3000 • 1-800-633-6101

Robert L. Ehrlich, Jr. Governor

Michael S. Steele Lt. Governor

May 5, 2005

Ms. Cindy Powels Directorate of Safety, Health and Environment Environmental Conservation and Restoration Division U.S. Army Aberdeen Proving Ground Support Activity Aberdeen Proving Ground, MD 21005-5001

Kendl P. Philbrick Secretary

Jonas A. Jacobson Deputy Secretary

SDMS DocID 2049071

RE: Final Explanation of Significant Differences fi-om the Interim Action Record of Decision for the Old 0-Field Source Area (0-Field Operable Unit 2), 0-Field Study Area, U.S. Army Garrison, Aberdeen Proving Ground, April 2005

Dear Ms. Powels:

The Federal Facilities Division of the Maryland Department of the Environment's Hazardous Waste Program has no comment on the above referenced document. The report addresses written comments made by this agency on the previous revised draft dociunent.

If you have any questions, please contact me at (410) 537-3496.

Sincerely,

._ / . / ^ - v ^ /^ (/

David Healy Remedial Project Manager Federal Facilities Division

DH:mh

cc: Mr. Frank Vavra Mr. Horacio Tablada Mr. Harold L. Dye Jr.

( ^ Recycled Paper www.mde.state.md.us TTY Users 1-800-735-2258 Via Maryland Relay Service

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O-FIELD STUDY AREA Explanation of Significant Differences from the Interim Action Record of Decision for the Old O-Field Source Area (O-Field Operable Unit 2) Final Document

LU CD

April 2005 - Pc <3.eQ I i- - c / u ^ 6-A_ 'r-

U.S. Army Garrison Aberdeen Proving Ground, Maryland

DISTRIBUTION RESTRICTION STATEMENT APPROVED FOR PUBLIC RELEASE: DISTRIBUTION IS UNLLMITED. # 6346-A-6

TABLE OF CONTENTS

Section _ ^ Page

1.0 INTRODUCTION AND PURPOSE 1-1

1.1 SITE NAME AND LOCATION .• 1-1 1.2 IDENTIFICATION OF LEAD AND SUPPORT AGENCIES 1-1 1:3 CIRCUMSTANCES LEADING TO THE NEED FOR AN ESD 1-1 1.4 ADMINISTRATIVE RECORD 1-3

2.0 SITE HISTORY, CONTAMINATION PROBLEMS, AND SELECTED REMEDY 2-1

2.1 SITE HISTORY '. ....2-1 2.2 CONTAMINANTS OF CONCERN 2-1

2.2.1 Materials Identified on the Surface of Old O-Field 2-1 2.3 HAZARDS POSED BY OLD O-FIELD PRIOR TO PIU INSTALLATION 2-3 2.4 SUMMARY OF THE REMEDY AS DESCRIBED IN THE ROD..... 2-4

3.0 BASIS FOR THE DOCUMENT 3-1

3.1 IMPLEMENTATION OF THE SELECTED REMEDY 3-1 3.2 RISK ANALYSIS FOR THE OLD O-FIELD PIU 3-3

3.2.1 Explosion Risk Assessment 3-3 3.2.2 CWM Vapor Release Under Non-Explosive Scenario 3-4 3.2.3 CWM Vapor Release After an Explosion 3-4

3.3 TREATABILITY STUDIES 3-5

4.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES 4-1

4.1 NON-UTILIZATION OF THE SUBSURFACE AIR MONITORING SYSTEM 4-1 4.2 NON-UTILIZATION OF THE SPRINKLER SYSTEM FOR TREATABILITY STUDIES

AND ADDITION OF THE SUBSURFACE TRICKLING SYSTEM 4-2

5.0 SUPPORT AGENCY COMMENTS 5-1

6.0 STATUTORY DETERMINATIONS 6-1

7.0 PUBLIC PARTICIPATION COMPLIANCE 7-1

8.0 REVIEW CHECKLIST _ 8-1

9.0 REFERENCES 9-1

LIST OF FIGURES

Figure Page

1-1 Location of Aberdeen Proving Ground 1-2

2-1 Location of the O-Field Study Area 2-2

3-1 Old O-Field Permeable Infiltration Unit, Typical Cross-Section 3-2

DACA31-95-D-0083 i Explanation of Significant Differences from TERC03-167 tfie Interim ROD for O-Field 0U2 April 2005 Final Document

UST OF ACRONYMS AND ABBREVIATIONS

ACEM Automated Continuous Environmental Monitor AEL Airborne Exposure Limit APG Aberdeen Proving Ground APG-EA Aberdeen Proving Ground - Edgewood Area ASTM American Society of Testing and Materials atm atmosphere CERCLA Comprehensive Environmental Response, Compensation, and Liability Act CG. Phosgiene ON Chloroacetophenone CNS Chloroform CWM Chemical Warfare Materiel DM Adamsite DOD Department of Defense ECN Electrochemical Noise ESD Explanation of Significant Differences ft feet GA Tabun GB Sarin GO Gas Chromatography GD Soman gpm gallons per minute GWTF Groundwater Treatment Facility H Mustard HD Distilled Mustard in inch IRA Interim Remedial Action L Lewisite lbs pounds MDE Maryland Department of the Environment MIC Microbiologically Influenced Corrosion mm millimeter mpy mils per year NAPL Non-Aqueous Phase Liquid NCP National Oil and Hazardous Substances Pollution Contingency Plan OU1 Operable Unit 1 0U2 Operable Unit 2 PCA 1,1,2,2-Tetrachloroethane PIU Permeable Infiltration Unit RAB Restoration Advisory Board ROD Record of Decision TEU Technical Escort Unit U.S United States USEPA U.S. Environmental Protection Agency UXO Unexploded Ordnance VX Methylphosphpnothiolate WP White Phosphorus yr year

DACA31-95-D-0083 ii Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field 0U2 April 2005 f^inal Document

DECLARATION FOR THE EXPLANATION OF SIGNIFICANT DIFFERENCES FROM THE INTERIM ACTION RECORD OF DECISION FOR THE OLD O-FIELD SOURCE AREA

(O-FIELD OPERABLE UNIT 2)

Site Name and Location

Old O-Field.Source Area . • (O-Field Operable Unit. (OU) 2) Edgewood Area, Aberdeen Proving Ground, Maryland

Statement of Basis and Purpose

This Explanation of Significant Differences (ESD) documents changes to the Interim Remedial Action (IRA) Record of Decision (ROD) for 0U2 (Old O-Field Source Area).

The IRA ROD for 0U2. dated October 1994, required the installation of a Permeable Infiltration Unit (PIU), or sand cover, over the Old O-Field Source Area to reduce the risks associated with unexploded ordnance and chemical warfare materiel present within the landfill. The primary purpose of this ESD is, therefore, to document the signifrcant modifications to the PIU system that was specified in the OU2 ROD. The modifications are: i) Non-Utilization of the Subsurface Air Monitoring System and ii) Non-Utilization of the Sprinkler Systerri for Treatability Studies and addition of the Subsurface Trickling System. The circumstances that precipitated the need for these changes and the rationale behind the selection of these changes are also presented.

This ESD has been prepared in accordance with the requirements of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Section 117 (c). A notice to the public explaining the significant differences from the IRA ROD for 0U2 will be published in local newspapers, in accordance with the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), Part 300.435(c)(2)(i). The Lead Agency, and owner of this site, is the United States (U.S.) Department of the Army. The Lead Regulatory Agency is the U.S. Environmental Protection Agency (USEPA) and the Supporting Regulatory Agency is the Maryland Department of the Environment (MDE).

Statutory Determinations

Considering the new information that has been developed and the changes that have been made to the selected remedy, the Army and USEPA believe that the remedy remains protective of human health and the environment, complies with federal and state requirements that are applicable or relevant and appropriate to the remedial action, and is cost-effective.

Approval

By: ^ ^ - ^ U ^ ^< Uj^-^^c/y ^J i ^ ^AJ l c^S-Johrpr. Wright / / Date C<ii<Jnel, US Amny C / Commander, US Army Garrison Aberdeen Proving Ground

By: Abe Ferdas Date Director, Hazardous Site Cleanup Division U.S. Environmental Protection Agency Regibn III

^^mm^m^ The purpose of this Explanation of Significant Differences (ESD) document is to present the

differences between the remedy described in the Interim Remedial Action (IRA) Record of Decision (ROD) for the Old O-Field Source Area (O-Field Operable Unit 2, 0U2), and the system implemented at the site.

This ESD has been prepared in accordance with the requirements of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Section 117 (c). A notice to the public explaining the significant differences from the ROD for O-Field 0U2 (dated October 1994), will be published in local newspapers, in accordance with the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), Part 300.435(c)(2)(i).

1.1 SITE NAME AND LOCATION

Old O-Field is a 4.5-acre hazardous waste disposal site located on the lower half of the Gunpowder Neck in the Edgewood Area of Aberdeen Proving Ground (APG-EA), Maryland. The site is located in eastern Maryland adjacent to the Chesapeake Bay. Old O-Field is bordered by surface water on three sides: Watson Creek to the north and east, and the-Gunpowder River to the west. The Gunpowder River is considered part of the Chesapeake Bay estuarine system. The location of the site is shown on Figure 1-1.

1.2 IDENTIFICATION OF LEAD AND SUPPORT AGENCIES

The Lead Agency, and owner of this site, is the United States (U.S.) Department of the Army. The Lead Regulatory Agency is the U.S. Environmental Protection Agency (USEPA). The Supporting Regulatory Agency is the.Maryland Department of the Environment (MDE).

1.3 CIRCUMSTANCES LEADING TO THE NEED FOR AN ESD

In October 1994, the ROD for O-Field 0U2 identified the Permeable Infiltration Unit (PIU) as the selected remedy for the Old O-Field Source Area. This interim action was selected because the PIU, which is a sand cover, would minimize the likelihood and impact of a potential explosive event by absorbing Shockwaves, filling any voids, and reducing the flow of chemical warfare materiel (CWM) vapor (APG, 1994). The PIU was also selected because it would allow infiltration of water to promote leaching of contaminants and produce an ultimate reduction in the volume of wastes. The PIU was constructed based on the specifications outlined in the 1994 ROD, except for the non-utilization of the subsurface air monitoring system, non-utilization of the sprinkler system for treatability studies, and addition of an auxiliary subsurface trickling system.

According to the ROD, the subsurface air monitoring system was intended to detect the presence of CWM within the pore spaces of the sand cover. The detection of CWM vapor by the subsurface air monitoring system would trigger activation of the sprinkler system to spray water on the PIU and suppress the CWM vapor. According to the design report for the Old O-Field Source Area (APG, 1995), the intent was to connect the subsurface air monitoring system to the existing Automated Continuous Environmental Monitors (ACEMs) located around the perimeter of the landfill. However, the presence of moisture in the subsurface would adversely affect the ACEMs and possibly generate false alarms in the air monitoring system and unnecessarily trigger activation of the sprinkler system. Application of excessive amounts of water from the sprinkler system to the PIU could cause the loss of groundwater capture and an overload of the groundwater treatment system. Due to these concerns, the subsurface air monitoring system was never commissioned. Instead, ambient air samples are collected above ground, on the perimeter of the PIU. If necessary, the sprinkler system will be manually activated by Groundwater Treatment Facility (GWTF) personnel or emergency response personnel.

Since 1994, the site has been monitored for mustard (HD) and nerve agents. In 2004, the system was modified to monitor for mustard and phosgene instead of mustard and nerve agents. To date, there has been no confirmed detection of CWM vapor at the site. All system responses have been false alarms; response to an alarm can take anywhere from 30 minutes to 2 hours or more depending on the time of day and .personnel available for response. These false alarms occur often, requiring the

DACA31-95-D-0083 1-1 Explanation of Significant Differences from TERC03-167 the Interim ROD for 0-Fleld 0U2 April 2005 Final Document

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SITE-LOCATION

1-2

Section 1.0 Introduction and Purpose

evacuation of all personnel from O-Field. The modification to monitor phosgene instead of nerve agents has had no significant effect on the frequency of false alarms. However, new communication lines were installed in the Fall of 2004, which may have reduced the frequency of false alarms. In 1998, the USEPA conducted flux chamber sampling on the surface of the landfill and did not detect-any CWM (USEPA, 1999). Their report concluded: "It appears that the cap at O-Field is effective in preventing emissions." A post-PIU risk analysis was performed to evaluate the hazards posed by the presence of unexploded ordnance (UXO) and CWM in the landfill. The results of this analysis indicate that continuous monitoring at Old O-Field may no longer be warranted because releases of CWM vapor from within the landfill to the ambient air above the PIU is no longer anticipated (APG, 2004). However, EPA believes that continued monitoring is needed to detect an acute release due to explosion or a release of phosgene gas. The Army will continue to work with regulators to develop an alternative monitoring approach that will reduce costs without endangering the public or the workers at the Old O-Field GWTF.

The ROD also specified that the sprinkler system be used to conduct a series of treatability studies to evaluate the feasibility of enhanced leaching of the landfill contents. During the design phase, concerns were raised regarding the compatibility of leaching solutions with the air monitoring drainage strips, possible air releases that could result if volatile solutions were applied using the sprinkler system, and limitations including freezing during the winter (APG, 1995). As a result, an auxiliary subsurface trickling system was added to the PIU to deliver potential treatment solutions.

As presented in Section 3.3, bench-scale treatability studies were conducted to evaluate if biocorrosion and/or chemical corrosion would enhance the leaching process. Chemical corrosion was selected as the more cost-effective technique. However, an on-site chemical corrosion study, which would have required use of the subsurface trickling system, was not implemented because of unquantifiable effects of the proposed leaching solution on the landfill contents. Consequently, the trickling system has never been used, and there are no plans to actively accelerate corrosion in the subsurface.

1.4 ADMINISTRATIVE RECORD

In accordance with the NCP, Part 300.825(a)(2), this ESD document will become part of the administrative record file for Old O-Field, which can be accessed at the following locations:

Harford County Public Library, Edgewood Branch 629 Edgewood Road Edgewood, MD 21040 Phone: 410-612-1600 Hours: Monday, Tuesday, Thursday 10:00 a.m. to 8:00 p.m.

Wednesday 1:00 p.m. to 8:00 p.m. Friday 10:00 a.m. to 5:00 p.m. Saturday 10:00 a.m. to 5:00 p.m. Sunday Closed

Harford County Public Library, Aberdeen Branch 21 Franklin Street Aberdeen, MD 21001 Phone:410-273-5608 Hours: Monday, Tuesday, Thursday 10:00 a.ni. to 8:00 p.m.

Wednesday 1:00 p.m. to 8:00 p.m. Friday 10:00 a.m. to 5:00 p.m. Saturday 10:00 a.m. to 5.00 p.m. Sunday 1:00 p.m. to 5:00 p.m. (October-April)

Miller Library 300 Washington Avenue Washington College Chestertown, MD 21620 Phone:410-778-2800

DACA31-95-D-0083 1-3 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field OU2 April 2005 , Final Document

Section 1.0 Introcjuction and Purpose

Hours: Monday-Thursday 8:15 a.m. to Midnight Friday 8:15 a.m. to 10:00 p.m. Saturday 10.00 a.m. to 10:00 p.m. Sunday Noon to Midnight

Though not required as part of the public participation process under CERCLA, the contents of this ESD were presented to the public at the APG Restoration Advisory Board (RAB) meeting on January 29, 2004. After the ESD has been finalized, a notice will be published in local newspapers to alert and inform the public.

DACA31-95-D-0083 1 -4 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field OU2 April 2005 Final Document

pgr^Sj t^ jg t f f ig fgMl^^ This section provides a brief summary of site history, contaminants of concern, pre-PIU hazai-ds,

and the selected remedy as outlined in the ROD.

2.1 SITE HISTORY

Old O-Field is a 4.5-acre fenced site located in a restricted area of APG-EA (Figure 2-1). The first documented use of Old O-Field occurred in May 1941 (Yon et al., 1978), although other records suggest that disposal activities occurred as eariy as the late 1930s. Records indicate that some of the burial trenches at Old O-Field were 100 yards long, 10 feet (ft) deep, and 10 ft wide; however, most known trenches were much shorter. The existence of 35 trenches was documented in historical records (Yon et . al., 1978). Inspection of survey notes and historical aerial photographs revealed that the trenches and pits were not distinct. The last pit used for disposal of materials within Old O-Field was closed in June 1953.

2.2 CONTAMINANTS OF CONCERN

Various CWMs have been identified or are believed to be present at Old O-Field based on historic information. Mustard (H), especially in its distilled form HD, is believed to be the most abundant of the CWM at Old O-Field. It is estimated that 73 percent of ordnance items and bulk containers at Old O-Field contain H or HD (Yon, 1994). Phosgene (CG), the second most abundant of the CWM, is estimated to be present in 5 percent of the Old O-Field ordnance items and bulk containers. The following CWM are also believed to be present at Old O-Field: lewisite (L), sarin (GB), soman (GD), o-ethyl-s-(2-diisopropylaminoethyl) methylphosphonothiolate (VX), tabun (GA), chloroacetophenone (CN), chloroacetophenone in chloroform (CNS), adamsite (DM), napalm, and white phosphorus (WP) (APG, 1999). In addition, it is believed that explosives as well as munitions of British, German, Russian, and Japanese origin, whose chemical contents are unknown, are also present at Old O-Field.

Several decontamination and cleanup operations were performed at Old O-Field from 1949 through the eariy 1970s. The most notable of these efforts was carried out in 1949 when 1,000 barrels of a decontaminating agent were applied to the field in an attempt to detoxify mustard agent that had been scattered over the site by several spontaneous detonations. The decontaminating agent contained approximately 95 percent 1,1,2,2-tetrachloroethane (PCA). PCA and its degradation products have subsequently been detected at elevated levels in the groundwater at Old O-Field, indicating that the groundwater at Old O-Field may have been impacted as a result of the decontamination effort.

From the late 1960s to the eariy 1970s, the U.S. Army Technical Escort Unit (TEU) performed surface sweeps of the area. A number of suspect CWM-filled rounds were recovered from Old O-Field, temporarily stored in Conex containers at Old O-Field, and then transported and stored in the storage bunkers at N-Field. In the eariy 1980s, the U.S. Army TEU began another surface sweep. A series of aerial photographs taken in 1984 show that neariy all the vegetation on Old O-Field had recently been defoliated and burned. Historical records suggest that an ignition of WP initiated the fire.

2.2.1 Materials Identified on the Surface of Old O-Field

Prior to the PIU construction, a survey was conducted in March 1994, from the perimeter of Old O-Field to identify items visible on the landfill surface. The survey was conducted by two UXO specialists from the bucket of a cherry picker provided by APG. The cherry picker was parked on the road around the perimeter of Old O-Field, and the boom was raised and positioned approximately 20 to 30 ft inside the fence of Old O-Field. Due to vegetation cover over the surface of most of Old O-Field, only approximately 15 to 20 percent of the surface was visible, making it impossible to determine whether observed items comprised the complete list of exposed materials. In addition to this visual survey, the UXO specialists took a videotape and still photographs of the field from the bucket of the cherry picker. To supplement these observations, a remote-controlled helicopter was later used to fly over Old O-Field while videotaping and photographing the field. Items identified during this survey included: a large steel tank, cylindrical containers [approximately 3 ft long with 10- to 12-inch (in) diameter], stacks of sand/dirt-filled ammunition crates, drurns, and ordnance items. Most of the ordnance items that were visible were

DACA31-95-D-0083 2-1 • Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field 0U2 April 2005 Final Document

2-2

Section 2.0 Site History. Contamination Problems, and Selected Remedy

located in a trench on the eastern side of Old O-Field. The types of ordnance items included: 4.2-in mortars, 75-millimeter (mm) projectiles, 105-mm projectiles, 175-mm projectiles, canisters, a 750-pound (lb) bomb, and other projectiles. The contents of the items, if any, were unknown. In Fall 1996, as part of the PIU construction activities, numerous UXO items were inventoried on the surface of Old O-Field. Many of these items were originally identified during the 1994 survey, discussed previously.

2.3 HAZARDS POSED BY OLD O-FIELD PRIOR TO PIU INSTALLATION

Prior to the PIU installation, the contaminant transport pathway that posed the highest risk to human health and the environment was a potential atmospheric release of CWM vapor. The probability of such an event was low but not insignificant, as the history of Old O-Field included a number of unplanned explosion and fire events.

The presence of both CWM and ordnance presented the possibility of an explosion and dispersion of toxic chemicals into the atmosphere. This possibility posed high risks to nearby populations and ecosystems. From numerous discussions with experts knowledgeable about the condition of Old O-Field, the following were identified as events that could have caused an explosion at Old O-Field before the PIU was installed:

FIRE

Exposed rounds at, or near, the surface may have detonated if subjected to fire. Because Old O-Field was heavily vegetated and there was a substantial amount of organic detritus on the ground, it was assumed that Old O-Field would burn vigorously and that a fire started on any side would consume the field. Although the field was surrounded by a road, the gap (approximately 12 feet on the north, east, and south sides) might not have been enough to stop a brush fire. The proximity of Old O-Field to H-Field, where active testing of combat tracked vehicles occurred and where brush fires occasionally started, increased the potentialfor a fire. The narrow access road added along the perimeter of the existing road also would not significantly reduce the spread of a fire, because in many places, there was no gap between the branches of trees on opposite sides of the road. It was also possible that a fire could start inside the field, due to the presence of WP' and other incendiary materials. Observations prior to the PIU construction suggested that items continued to be exposed through erosion, frost heave, and other mechanisms. However, the most likely stimulus for explosive release or rupture was fire.

SHOCK OR PRESSURE

Although fuzes and initiating devices are far more sensitive to shock or pressure than high explosives, most munitions at Old O-Field are believed to be un-fuzed and those munitions with fuzes would not easily self-detonate because significant external force is required to ignite the point-detonating fuze. The probability that a significant external force, such as an earthquake, would occur in the O-Field area is low and ongoing activities at nearby firing ranges should not generate enough Shockwave or thermal energy to trigger an explosion. Nevertheless, should there be enough external shockWave or pressure to trigger an explosion, sympathetic detonation could occur because many munitions are buried together in the same cell.

ORDNANCE EXPOSURE

The processes of erosion, corrosion, and waste settling resulted in the formation of voids and the structural weakening of portions of the buried waste volume. The continued action of these processes could also result in the collapse of material into voids and settling/consolidation of wastes. Holes created by erosion on the surface could have formed, exposing WP ordnance to oxygen and providing a pathway for CWM vapor release. It was also possible that movement of wastes and soil could have resulted in impact, shearing, and crushing of the buried items. This could have resulted in the release of CWM from corrosion-weakened items and could have initiated the detonation of sensitive ordnance items. However, this initiation source was less likely than the thermal ignition hazard. The other possible causes of ordnance exposure at the surface were:

When exposed to air (e.g., during trench collapse or soil shifting), WP spontaneously ignites.

DACA31-95-D-0083 2-3 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field 0U2-April 2005 Final Document

Section 2.0 Site History, Contamination Problems, and Selected Remedy

Honeycombing of Trenches: Historical aerial photographs showed that as available space became scarce at Old O-Field, the trenches began to overlap. This would have resulted in very unstable soil conditions.

Density Differences: The difference in density between some types of ordnance and soil sometimes allowed munitions to work their way up through the soil, resulting in eventual exposure to the atmosphere.

Presence of Animals: The movement of animals on the top of the filled trenches or burrowing through the subsurface could also have caused the soil to shift.

Frost/Thaw Cycles: These cycles could have aided in trench erosion and the mobilization and movement of munitions to the surface.

Time: Historical records indicated that numerous surface clearance activities took place at Old O-Field. However, as documented by the surface survey of Old O-Field in March 1994, a number of ordnance items were exposed at the surface of the site due to erosion and trench collapse. As more items became exposed to the air, the threat of WP-initiating fire and the possible consequences of a fire initiated by any source would have been heightened;

2.4 SUMMARY OF THE REMEDY AS DESCRIBED IN THE ROD

As stated eariier, the Army issued an IRA ROD for OU2 in October 1994. Based upon consideration of the requirements of CERCLA, the detailed analysis of the alternatives, and public comments, the Army with the concurrence of the USEPA and in consultation with MDE, determined that the PIU was the most appropriate remedy for 0U2. The major components of this remedy included:

• Construction of the PIU;

• Installation of a subsurface air monitoring system;

• Construction of a surface sprinkler system;

• Performance of treatability studies; and

• Verification of the effectiveness of the OU1 groundwater extraction and treatment system.

The PIU was to be constructed using sand or other granular materials to attenuate the effects from exploding munitions and reduce CWM emissions from the burial pits and trenches. In addition, these materials would tend to flow and fill in gaps if an explosion or collapse of a trench occurred, so repair of the PIU would be simpler than repair of other types of covers. The buried materials would also be insulated from the effects of surface fires by the sand. In addition, the possibility that exposed WP would serve as an ignition source was reduced by isolating the wastes from air contact.

The ROD specified that an air monitoring system be installed in the PIU to detect the presence of CWM within the pore spaces of the sand. If a CWM release was detected by the air monitoring system, it would trigger activation of the sprinkler system. The sprinklers, located on top of the PIU, would be capable of quickly spraying water or other solutions onto the landfill. The objectives of a water spray were to: i) fomri a vapor barrier within the sand to prevent an air release of CWM; and ii) hasten the hydrolysis of the CWM.

Treatability studies were also planned to evaluate the feasibility of enhanced leaching of the contaminants from soil and buried containers to the groundwater. According to the ROD, the sprinkler system would be used to apply water or other solutions to the PIU during the treatability studies. In addition, the surface of the PIU would be monitored to evaluate the rate of subsidence of Old O-Field.

The PIU was designed to work in conjunction with the GWTF, which was under construction in 1994. Therefore, the selected remedy for 0U2 specified verification of the GWTF's abilities to capture and treat the contaminated groundwater emanating from Old O-Field. In addition, the effectiveness of the groundwater monitoring program to detect changes in the site hydrogeology and groundwater chemistry was to be verified.

DACA31-95-D-0083 2-4 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field 0U2 April 2005 Pinal Document

PL^i^i&S!s^j^@gaiHigi§j5i«igg^^ This section summarizes the information that prompted and supports the significant changes from

the Selected Remedy that was presented in the 1994 ROD.

3.1 IMPLEMENTATION OF THE SELECTED REMEDY

Construction of the PIU was initiated in Summer 1996 and completed in Fall 1997 (with the exception of the water storage tank which was completed in 1998). The PIU consists of an initial 2 ft layer of sand (minimum); geotextile support material (geogrid); subsurface trickling system; subsurface air monitoring system; final 3 ft layer of sand; erosion control mat; 6 in gravel erosion control layer, and sprinkler system. As a result of the installation of the PIU, the land-surface elevations now range from 20 to 26 ft above sea level at Old O-Field (APG, 1997). A cross-section of the PIU is presented on Figure 3-1.

Placement of the initial sand layer was accomplished using low-ground-pressure teleoperated equipment. Cameras mounted on the equipment allowed workers to operate the machinery from approximately 1.5 miles away, further reducing potential risks due to explosion or CWM vapor release. The initial sand layer ranged from 2 to 14 ft thick depending on the original surface elevation of Old O-Field; the final sand layer was 3 ft thick. The 5 ft minimum thickness of sand was selected based on the containment of a blast from detonation of up to 5 lbs of explosives^.

The sand, designated as "Number 2 Blended Concrete Sand", was a blend of a gravelly soil and a medium to fine sandy soil. The measured permeability of the sand using a constant head test procedure (American Society of Testing and Materials [ASTM] D-2434) varied from 1.7 x 10'̂ to 6.4 x 10"* centimeters per second. A pooriy graded sand was selected to better absorb and dissipate the energy from a potential ordnance explosion. The permeable sand cover provides the following advantages:

• Reduces the hazards associated with the explosion of a round within Old O-Field by reducing the explosive force and the release of shrapnel and contaminants.

• Provides a self-healing cover as the sand flows into voids in the event of an explosion or during settling.

• Restricts the migration of CWM vapor and reduces the possibility of CWM vapor's release to the atmosphere in the event that an agent filled munition explodes.

• Restricts airflow to th.e subsurface, thereby, significantly reducing the risk of WP ignition and subsequent explosion.

• Absorbs shock applied to the top of the PIU (such as the impact of falling trees) and pressure exerted due to movement of vehicles. These shock and pressure hazards could potentially detonate fuzes, busters, initiating devices, and other shock sensitive materials.

• Prevents exposure of munitions and reduces the potential for human and animal contact with exposed ordnance.

• Allows infiltration of rainwater and future application of appropriate solutions to achieve in-situ treatment of the buried materials.

A geogrid was installed beneath the subsurface trickling and air monitoring systems to provide support and to allow flow through of sand from the top layer to the bottom layer in the event of an explosion or during settlement. The geogrid material contains large open spaces, or apertures, which allow the sand to pass through. In addition, the geogrid provides temporary support for the subsurface trickling and air monitoring systems in the event of potential large-scale settlement.

The subsurface trickling and a subsurface air monitoring systems were installed between the two sand layers. The subsurface trickling system was designed to allow application of appropriate solutions

^ According to historical disposal and recovery records, 99.6% of ordnance items believed to be within the site contain a mass of explosive less than this amount (APG, 1995).

DACA31-95-D-0083 3-1 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field 0U2 April 2005 Final Document

_ ^ ^ Sprinkler System

Geogrid Reinforcement

System

Existin Site Soils

and Waste Material

Figure 3-1. Old O-Field Permeable Infiltration Unit, Typical Cross-Section

Section 3.0 Basis for the Document

for in-situ treatment of buried materials. This system consists of six independent grid systems of piping with 4 in headers running east west and % in pipes placed on 20 ft centers north to south. A main header has been installed to feed each system with a shut-off valve located on the perimeter road of Old O-Field. The piping is a perforated high-density, extra-high molecular weight polyethylene pipe capped at each end. Each system was designed to deliver a meiximum of 200 gpm.

The subsurface air monitoring system is designed for eariy detection of any CWM vapor before it is released to the atmosphere. The air monitoring system consists of 12 in wide filter fabric-wrapped synthetic drainage strips. The strips were first placed on 50 ft centers along the longitudinal axis of the site and then on 50 ft centers perpendicular to that axis. A strip was also placed around the perimeter of the field into which all grid strips were tied.

The final 3 ft sand layer was applied after installation of the subsurface air monitoring system. This sand layer was then covered with a permeable erosion control mat and a 6 in gravel erosion control layer. Following the completion of the erosion-control layer, the sprinkler system was installed. The intent of the water was to reinforce the vapor barrier properties of the sand and reduce the migration of CWM, while also enhancing the hydrolysis of CWM. The sprinkler system has 14 sprinkler heads, each capable of distributing 50 gallons per minute (gpm) of water over a 100 ft radius across the site. The system also includes a 1,000 gpm pump system and a piping system to convey water from a 200,000 gallon dedicated water stofage tank (40 ft diameter and 24 ft cylinder height).

3.2 RISK ANALYSIS FOR THE OLD O-FIELD PIU

After the installation of the PIU, a risk analysis was conducted to evaluate the hazards posed by the presence of UXO and CWM at the Old O-Field Source Area (APG, 2004). The potential risks include explosives safety risks and risks from CWM vapor. The baseline explosives safety risk was qualitatively assessed based on the type and condition of the UXO disposed in the landfill. The potential release of CWM vapor through the PIU was evaluated for both non-explosive (leaking munitions) and explosive scenarios. For the first scenario (non-explosive), chemical properties of the agents and environmental factors were assessed to determine the potential for a CWM vapor release. For the second scenario, an explosion model was developed utilizing blast prediction equations and mass and energy balances. The initial conditions predicted by the explosion model were then used to simulate CWM vapor transport through the PIU using the numerical model A-T2VOC. This model was also used to simulate CWM vapor transport for the non-explosive scenario. The findings of these assessments, which are summarized hereafter, suggest that the likelihood of an explosion at Old O-Field is relatively low. It is also highly unlikely that CWM vapor would be released through the PIU to the atmosphere under either explosive or non-explosive scenarios.

3.2.1 Explosion Risk Assessment

The three key elements for estimating baseline explosives safety risk at Old O-Field are: i) the presence and characterization of a source, ii) identification and reasonableness of a potential pathway, and iii) availability and frequency of a receptor; without these, there is no risk. The primary source of an explosion and CWM vapor release at Old O-Field is the CWM-filled UXO; the exposure pathway is transport through the PIU sand cover to the atmosphere; and the potential receptors are workers on the PIU and in the immediate vicinity of Old O-Field, people in transit, boaters on the Gunpowder River, or people in nearby residences and offices.

UXO explosion risk exists when a military munition detonates. The detonation is usually triggered by physical forces including thermal transfer and Shockwave. Other explosion triggering factors include the type and quantity of energetic material, the presence of a fuze, movement by natural forces, and intrusive activities. After taking into account these factors, the overall explosion risk at Old O-Field is found to be low. Most munitions at Old O-Field are believed to be uh-fuzed and those munitions with fuzes would not self-detonate because significant external force is required to ignite the point-detonating fuze. The self-detonation of the Japanese munitions within the landfill that contain picric acid would not occur because the subsurface environment is not conducive to the re-crystallization of picric acid. Intrusive activities that could penetrate below the sand layer or generate Shockwaves or thermal energy should not be allowed. The potential for an earthquake to occur in the O-Field area is low and ongoing activities at nearby tiring ranges should not generate enough Shockwave to trigger an explosion. Despite

DACA31-95-D-0083 3-3 • Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field OU2 April 2005 Final Document

Section 3.0 • . Basis for the Document

the foregoing, should an explosion occur, the potential for sympathetic detonation exists because of the proximity of many of the munitions to each other. EPA believes that continual monitoring, for this unknown event is necessary. EPA and APG are working towards a more cost effective approach.

3.2.2 CWM Vapor Release Under Non-Explosive Scenario

In the absence of a catastrophic event at Old O-Field, such as an explosion, CWM could be released from leaking munitions or containers. Upon release, the CWM could remain as a free phase in the vadose zone, adsorb to the soil, migrate to the bottom of the aquifer, dissolve in the water phase, or migrate as a vapor through the overburden soil and sand to the atmosphere. The likelihood of subsequent atmospheric CWM vapor release would then be governed primarily by two factors: i) the properties of the agent, and ii) the release environment. The physical and chemical properties of an agent determine the agent's tendency for natural migration, while environmental factors facilitate or retard the agent's mobility.

A vapor release of HD or CG, represents the worst case scenario at Old O-Field. HD and CG are the two most abundant CWM, accounting fiDr approximately 73% and 5% of the total CWM at the site, respectively. The properties of HD and CG, relevant to the transport of their vapors from the subsurface include: physical state, adsorption, viscosity, vapor pressure, volatility, vapor density, moleculai* diffusion, solubility, hydrolysis, and oxidation. In general, these properties do not favor the upward mobility of HD and CG vapors through the PIU, nor their persistence in the subsurface environment under ambierit conditions.

The nature of the subsurface environment also plays a significant role in mitigating the release of CWM vapor. The low temperature environment (groundwater temperature between 12°C and 14°C) would reduce volatility. The presence of at least five feet of sand cover would act as a physical barrier to vapor transport as well as provide the residence time and water for the solvation and hydrolysis processes. Unless heated, the. vapor density of CWM is higher than air. As such, the vapors would tend to stay below the surface of the PIU where they would react with soil moisture and be degraded. The use of sand in the PIU also provides a "self-healing cover" because sand will move to fill voids or cracks, which could act as conduits for gas flow to the atmosphere during periods when barometric pressure falls below the ambient gas pressure in the burial zone. The low pH environment (pH of groundwater less than 6) is also favorable to the solubility, hydrolysis, and oxidation processes. The results of numerical simulation confirm that the vapors of HD and CG, and other CWM vapors at Old O-Field, would not be detected above the PIU.

3.2.3 CWM Vapor Release After an Explosion

The instantaneous or gradual release of CWM after an underground explosion depends on whether the explosion results in the formation of a crater or a camouflet. When the force of the explosion is sufficient to rupture the surface of the overburden material, a crater is formed and the end products of the explosion are vented to the atmosphere along with the CWM present in the ruptured munition. If, however, the munition is buried deep enough, such that the force of the explosion is not sufficient to rupture the surface of the overburden material, an underground cavity called a camouflet is formed.

At Old O-Field, calculations show that an explosion would result in the formation of a camouflet because the munitions are covered by at least 5 ft of sand and relatively small energetics are associated with chemical munitions. The subsequent release of CWM vapor from the camouflet to the atmosphere would be subject to gas phase transport through the overburden material. Under this scenario, the quantity of CWM and energetics in the munition become the dominant factors in determining the maximum release of CWM to the atmosphere. The amount of energetics control the energy release in the explosion and the corresponding increase in temperature and pressure, factors that determine subsequent gas phase transport to the atmosphere. Following an explosion, the cavity of the camouflet would be filled with the end products of the explosion and the CWM. The heat generated by the explosion would cause an elevation of the temperature and pressure of the contents within the camouflet.

Because of the very small quantity of explosive in CWM-filled munitions relative to the quantity of CWM, the product of combustion is rapidly cooled by the CWM. The pressure of the gas is also lowered to a few atmospheres above ambient level. Under these conditions, CG would remain in the gas phase and HD would be a warm liquid. As a result of the rheological properties of the PIU sand, any cracks or

DACA31-95-D-0083 3-4 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field 0112 April 2005 Pinal Document

Section 3.0 Basis for the Document

fractures caused by the explosion would be healed as the cracks develop. Therefore, fluid venting from within the camouflet would not occur and dispersion forces would govern vapor transport. The results of numerical simulation show a zero concentration of CG and HD above the surface of the PIU, which is in line with inferences made from physical and chemical considerations. However, due to the uncertainty of the contents of the PIU, monitoring forCWM is still justified. Although modeling shows minimal risk from an air release, air monitoring will continue along the perimeter of the PIU, until a more cost effective method is determined to detect a significant subsurface explosion and vapor release.

3.3 TREATABILITY STUDIES

Since completion of the PIU, activities have been initiated to evaluate integration and optimization of the two interim remedial actions at Old O-Field: the PIU and GWTF systems. As a result of a brainstorming meeting in 1996 and subsequent evaluation, enhanced corrosion was selected for further study. The objective of enhanced corrosion at Old O-Field was to degrade the metallic casings of the munitions buried in the landfill, in an attempt to further reduce the risk of explosion. The degradation of the casings would also cause the CWM in the munitions to be released; thus, facilitating their removal from the environment, through the solvation/hydrolysis processes and subsequent extraction and treatment at the GWTF. Two types of enhanced corrosion were studied, biocorrosion (also referred to as microbiologically influenced corrosion [MIC]) and chemically enhanced corrosion.

The biocorrosion study was completed in three stages: i) surface soil analyses to determine if microorganisms capable of promoting MIC were present in the soil; ii) bench-scale tests to evaluate various amendments for increasing microbial activity; and, iii) field monitoring tests using Electrochemical Noise (ECN) technique to evaluate baseline corrosion activity at Old O-Field. Laboratory study on the chemical corrosion of munition casings was also conducted to evaluate in-situ chemical corrosion at Old O-Field. The chemical corrosion study was conducted in two stages: i) immersion tests were performed to evaluate varying concentrations and combinations of candidate solutions; and, ii) the four preferred solutions from the immersion tests were tested in soil columns to simulate field conditions at Old O-Field. The results of both the biocorrosion and chemical corrosion studies were evaluated for potential application at Old O-Field. Both studies were successful, and showed that the degradation of the metal contents of the landfill can be achieved biologically or chemically.

The biocorrosion study revealed the presence, in Old O-Field soil, of diverse communities of microorganisms known to be involved in the biocorrosion of steel. The bench scale corrosion tests and the results of monitoring corrosion in the field (using the ECN method) confirm the dominant presence of pitting corrosion at Old O-Field. Pitting corrosion is particularly associated with microorganisms and is the more desired corrosion mechanism for the denaturing of munition casings at Old O-Field because of its ability to create a more controlled release of the munitions' contents. The microbes would establish these rapid sustained pitting corrosion sites, while other environmental factors such as the presence of chloride ions would accelerate the corrosion rate. However, the measured biocprrosion rates were relatively low, which correlated to corrosion duration of hundreds of years. Accelerating the biocorrosion process would be difficult given the fact that controlling the process variables in the subsurface environment is very difficult and field application of this technique is not proven. Biocorrosion was not considered a cost-effective technique to facilitate the denaturing of the buried munitions, and therefore, was not evaluated further.

The potential for chemically enhanced corrosion at Old O-Field was investigated using various acid solutions. The test results show that corrosion rates could be enhanced at Old O-Field by exposing the buried carbon steel materials to a low pH solution of nitric acid (pH < 2.5) and by adding a strong oxidizing salt such as ferric chloride. The average corrosion rate of 100 mils per yr [mpy, or 0.1 inches per year (in/yr)] was realized from these tests, suggesting that the casing materials could be corroded in 4 to 15 years. On the contrary, using Old O-Field groundwater without a pH-lowering amendment (average corrosion rate of 1.4 mpy or 0.0014 in/yr) would require 270 to 1,070 years to corrode the casings.

Enhanced chemical corrosion was judged as a potentially cost-effective technique for accelerating the denaturing of the munition casings at Old O-Field. Metal corrosion is a proven and predictable technique, which can proceed well in the oxygen deficient subsurface of Old O-Field. The drawbacks of this technique include: i) short-term risks to workers during construction; ii) inability to quantify the effectiveness of the corrosion process and thus determine the end-point; iii) potential tor

DACA31-95-0-0083 3-5 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field OU2 April 2005 Final Document

Section 3.0 Basis for the Document

uncontrollable subsidence of the PIU; iv) increase in the amount of groundwater to be extracted, which would have a corresponding increase in the operating cost of the GWTF; v) possible shift eastward of the hydrologic divide along Watson Creek Road, which could cause contaminated groundwater to migrate westward; and, vi) potential for an increase in the number of extraction and monitoring wells. Because of these reasons, the enhanced chemical corrosion technique was not implemented.

DACA31-95-D-0083 3-6 Explanation of Significant Dillerences from TERCp3-167 the Interim ROD for O-Field OU2' April 2005 Final Document

The following significant differences from the 6U2 ROD have been identified:

• Non-Utilization of the Subsurface Air Monitoring System; and

• Non-Utilization of the Sprinkler System for Treatability Studies and addition of the Subsurface Trickling System.

A description of each of these changes is presented in this section.

4.1 NON-UTILIZATION OF THE SUBSURFACE AIR MONITORING SYSTEM

In 1994, prior to construction of the PIU, five ACEMs were in-place around the perimeter of Old O-Field to monitor for potential releases of low level toxic chemicals. This system was designed to provide continuous real-time measurements, which would trigger an audible alarm if CWM concentrations exceeded specified Airborne Exposure Limits (AELs). The subsurface air monitoring system installed within the PIU was designed to be compatible with the ACEM system; but it was not commissioned due to concerns regarding moisture in the sutisurface. However, monitoring of ambient air has continued. The ACEMS were subsequently replaced by three Miniature Continuous Air Monitoring System (MINICAMS). In 2004, the system was modified to monitor for mustard and phosgene instead of mustard and nerve agents. Also in 2004, the communications lines were replaced to reduce the frequency of false alarms.

Based on the findings of the Risk Analysis and monitoring data gathered since 1994, it can be concluded that the risk of an explosion or the release of CWM vapor at Old O-Field is low. The supporting rationales for this conclusion are summarized as follows:

• The fuzes of the majority of the UXO are likely to be in an unarmed condition. Also, external forces (intrusive activities, thermal transfer, overpressure, or secondary impact), needed to activate these point-detonating fuzes are absent. Firing activities at surrounding ranges have not been observed to set off the munitions at Old O-Field and this should not change in the future. Historically, there has been no documented explosion at Old O-Field since 1950; the four explosions that occurred in 1949-1950 were primarily due to cleanup activities at the site. A fire occurred in 1985, which was attributed to the spontaneous ignition of WP. Hazards from fire, shock, pressure, and exposure have been mitigated by the PIU.

• Real-time monitoring for CWM vapor has been in place at Old O-Field since 1994. To date, there has been no confirmed detection of CWM vapor. In addition, all system responses have been false alarms. False alarms occur often, requiring the evacuation of all personnel from O-Field. USEPA also conducted flux chamber sampling in December 1998 and did not detect any CWM (USEPA, 1999). Their report concluded: "It appears that the cap at the O-Field is effective in preventing emissions". [A replacement of the communication lines in 2004 appears to have reduced the frequency of false alarms.]

• The vapors of HD, CG, and other CWM at O-Field do not have the natural tendency to migrate toward the atmosphere because of a combination of physical properties including high vapor density, high hydrolysis rate, low vapor pressure, high solubility, and low diffusion coefficient. Upon release, the CWM vapor is trapped close to the release point and is subsequently dissolved and hydrolyzed. Also, the results of numerical simulation confirm the inferences made from consideration of the physical and chemical properties of the CWM at the site (APG, 2004). Thus, at Old O-Field, a vapor release of these agents, either as a result of a leaking munition or a subsurface detonation, is highly unlikely.

Despite the low risk of an explosion or the release of CWM vapor at Old O-Field, the Army will continue ambient air monitoring at the surface along the perimeter of the PIU. If necessary, the sprinkler system will be manually activated by GWTF personnel or emergency response personnel. EPA, MDE, and the Army will continue to work together to find a more cost effective monitoring program, which would still detect a significant explosion or vapor release above the PIU.

DACA31-95-D-0083 4-1 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field OU2 April 2005 Final Document

Section 4.0 Description of Significant Differences

4.2 NON-UTILIZATION OF THE SPRINKLER SYSTEM FOR TREATABILITY STUDIES AND ADDITION OF THE SUBSURFACE TRICKLING SYSTEM

The ROD also specified that the sprinkler system be used to conduct a series of treatability studies to evaluate the feasibility of enhanced leaching of the landfill contents. During the design phase, concerns were raised regarding the compatibility of treatment solutions with the air monitoring drainage strips, possible air releases that could result if volatile solutions were applied using the sprinkler system, and limitations including freezing during the winter (APG, 1995). As a result, an auxiliary subsurface trickling system was added to the PIU to deliver potential treatment solutions.

As presented in Section 3.3, bench-scale treatability studies were conducted to evaluate if biocorrosion and/or chemical corrosion would enhance the leaching process. Chemical corrosion was selected as the more cost-effective technique. However, an onsite chemical corrosion study, which would have required use of the subsurface trickling system, was not implemented because of unquantifiable effects of the proposed leaching solution on the landfill contents.

DACA31-95-D-0083 ~ 4-2 Explanation of Significant Differences from TERC03-167 Ih^ Interim ROD for O-Field OU2 April 2005 -Final Document

' The USEPA (Lead Regulatory Agency) and MDE (Supporting Regulatory Agency) have reviewed the ESD and provided comments. All comments from both agencies have been incorporated into the final document. The USEPA and MDE support the actions, as specified in this ESD.

DACA31-95-D-0083 TERC03-167 April 2005

5-1 Explanation of Significant Differences from the Interim ROD for O-Field 0U2

Final Document

Considering the new information that has been developed and the changes that have been made to the selected remedy, the Army and USEPA believe that the remedy remains protective of human health and the environment, complies with Federal and State requirements that are applicable or relevant and appropriate to the remedial action (including those outlined in CERCLA §121), and is cost-effective. In addition, the revised remedy uses permanent' solutions and alternative treatment (or resource recovery) techniques to the maximum extent practicable for this site.

DACA31-95-D-0083 TERC03-167 April 2005

6-1 Explanation of Significant Differences from the Interim ROD for O-Field OU2

Final Document

The contents of this ESD were presented to the public at the APG RAB meeting on January 29, 2004. After the ESD has been finalized, a public notice explaining the significant differences from the 0U2 ROD will be published in local newspapers , in accordance with Part 300.435(c)(2)(i) of the NCP.

DACA31-95-D-0083 TERC03-167 April 2005

7-1 Explanation of Significant Differences from the Interim ROD for O-Field 0U2

Final Document

Introduction to Site and Statement of Purpose

Site Name & Location •

Lead & Support Agencies

Date of ROD Signature

Circumstances that led to the need for an ESD

Administrative Record File (including locations & hours of availability)

• Section 1.1

• Section 1.2

• Section 1.3

• Section 1.3

• Section 1.4

Site History, Contamination, and Selected Remedy

Summary of contamination problems and site history

Selected Remedy, as originally described in the ROD

Sections 2.1, 2.2 & 2.3

Section2.4

Basis for the Document

Summary of information prompting and supporting significant differences from the Selected Remedy

Section 3.0

Description of Significant Differences

Description of the significant differences between the remedy as presented and the action now proposed

Section 4.1: Non-Utilization of the Subsurface Air Monitoring System; and

Section 4.2: Non-Utilization of the Sprinkler System for Treatability Studies and addition of the Subsurface Trickling System

Support Agency Comments

Summary of Support Agency comments on the ESD

• Section 5.0

Statutory Determinations

Statement that the modified remedy meets statutory requirements

Section 6.0

Public Participation Compliance

Statement that public participation requirements have been met or will be met

Section 7.0

DACA31-95-D-0083 TERC03-167 April 2005

8-1 Explanation of Significant Differences from the Interim ROD for O-Field OU2

Final Document

Aberdeen Proving Ground (APG). 1991. Interim Action Record of Decision Old O-Field Site. U.S. Army Aberdeen Proving Ground, Maryland. Final Report. Prepared by ICF Kaiser Engineers. September 1991.

Aberdeen Proving Ground (APG). 1994. Interim Remedial Action - Aberdeen Proving Ground Old O-Field Source Area, Record of Decision. Final Document. Prepared by ICF Kaiser Engineers. October 1994.

Aberdeen Proving Ground (APG). 1995. Design Report for the Old O-Field Source Area. Final Document. Prepared by ICF Kaiser Engineers. January 1995.

Aberdeen Proving Ground (APG). 1997. 'Top of Stone Survey Compared to Final Sand Survey". O-Field, Aberdeen, Maryland. Prepared by Roy F. Weston and the U.S. Army Corps of Engineers. July 26, 19i97.

Aberdeen Proving Ground (APG). 1998. Memorandum from Kenneth P. Stachiw, Chief, Environmental Conservation and Restoration Division. Subject: Response to Old O-Field Automated Continuous Environmental Monitor (ACEM) Air Monitoring Alarms.

Aberdeen Proving Ground (APG). 1999. Remedial Investigation Report for the O-Field Area - Phase I. Aberdeen Proving Ground. Revised Draft Final Document. Prepared by ICF Kaiser Engineers. March 1999.

Aberdeen Proving Ground (APG). 2004. Risk Analysis for the Old O-Field Permeable Infiltration Unit. Final Document. Prepared by Shaw Environmental. October 2004.

U.S. Environmental Protection Agency (USEPA). 1999. Trip Report for Flux Chamber Sampling at Aberdeen Proving Ground, Edgewood, MD. From David B. Mickunas, Chemist, EPA Environmental Response Team Center, To Cindy Powels, Environmental Engineer, DSHE, APG. January 1999.

U.S. Geological Survey, U.S. Department of the Interior (USGS). 1991. Ground-Water, Surface-Water, and Bottom-Sediment Contamination in the O-Field Area, Aberdeen Proving Ground, Maryland, and the Possible Effects of Selected Remedial Actions on Ground-Water. Open-File Report 89-399.

Yon, R., D.J. Wenz, and C. Brenner. 1978. Information Relevant to Disposal of Hazardous Material at O-Field, Aberdeen Proving Ground, Maryland. Record Evaluation Report 1978-1, Chemical Systems Laboratory, Aberdeen Proving Ground, Maryland.

Yon, R. 1994. Various Written and Personal Communications including a Memorandum Concernirig Rational Percentages of CWM and Other Materials in O-Field. SciTech Services, Inc., May 5, 1994.

DACA31-95-0-0083 9-1 Explanation of Significant Differences from TERC03-167 the Interim ROD for O-Field OU2 April 2005 . Final Document