SFUND RECORDS CTR

2154180

Operable Unit 2 (ST012)

Former Williams Air force Base, Arizona Draft Final Explanation of Significant Differences

JUNE 2004

United States Air Force Real Property Agency

OU-2 WILLIAMS AFB, MARICOPA COUNTY, ARIZONA DRAFT FINAL EXPLANATION OF SIGNIFICANT DIFFERENCE

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Draft Final Explanation of Significant Differences Operable Unit - 2 (OU-2)

Former Williams Air Force Base, Arizona

I. INTRODUCTION

This Explanation of Significant Differences (ESD) is issued ip accordance with Section 117(c) of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended (CERCLA), 42 U.S.C. § 9617(c), and Section 300.435(c)(2)(i) of the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), which requires the United States Air Force (USAF) to issue such a document where a remedial action will differ in any significant, but not fundamental, respect from that selected by the USAF and described in the Record of Decision.

This ESD relates to the remedial action selected by the USAF for the cleanup of fuel contamination from the Liquid Fuel Storage Area (also referred to as Site ST012) at the former Williams Air Force Base (AFB) in Maricopa County, Arizona. .Site ST012 was initially designated as Operable Unit 2 (OU-2) among several OUs being investigated at the former Williams AFB in the early 1990s. During the Feasibility Study for OU-2, the deep soil (soil below 25 feet down to groundwater) was removed from OU-2 and aligned under OU-3 for further study. After the investigation of deep soil at ST012 was completed, it was realigned back into OU-2 in a Record of Decision Amendment (RODA) issued in August 1996. The 1996 OU-2 RODA selected SVE, bioventing, and natural attenuation to clean up fuel in the deep soils.

The Proposed Plan for OU-2 (April 1992) evaluated four alternatives for the groundwater and soils for the site. Alternative A evaluated no action monitoring to be performed to verify changes in the contaminant levels in designated areas to determine if there have been reductions below the action levels due to natural degradation of contaminants. Alternative B evaluated Institutional Actions and Controls, Alternative C evaluated Groundwater Extraction, Air Stripping, and Injection plus Soil Vapor Extraction with In Situ Bioremediation and Alternative D included Groundwater Extraction, Air Stripping and Injection plus On-Site Soil Incineration. The OU-2 Record of Decision issued in December 1992 addressed the clean-up of shallow soils (top 25 feet of soil) and groundwater at ST012 and chose Alternative C for the remedial action.

This ESD describes a modified approach for targeting the reduction of chemicals of concern (COCs) in the groundwater and saturated zone by the extraction of free phase NAPL, the extraction of fuel components dissolved in groundwater, and the extraction of fuel vapors liberated from the dewatered saturated zone. Due to complicated site conditions, the pump and treat technology initially selected has proved to be ineffective by itself for remediating fuel contaminants in groundwater. Thermal Enhanced Extraction (TEE) enhancements to the pump and treat technology will improve the recovery and reduce toxicity of light non-aqueous phase liquids (LNAPL) in the saturated zone at Site ST012. The use of natural attenuation will follow TEE treatment for further reduction of groundwater concentrations to meet cleanup goals.

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Incorporation of monitored natural attenuation as the final remedy for groundwater will be proposed in a ROD amendment and is not a component of this ESD. The original cleanup standards for groundwater in the 1992 ROD are unchanged.

This ESD has been prepared to provide the public with an explanation of the nature of the modification to the groundwater component of the selected remedy set forth in the ROD and to summarize the information that supports this modification. A copy of this ESD, together with information supporting the changes described'herein, will be included in the Administrative Record for the Site. Public access to these documents is discussed in Section IV of this ESD. A brief summary of this ESD will also be published in the local newspaper.

II. SUMMARY OF THE SITE HISTORY, SITE CONDITIONS, AND SELECTED REMEDY

The former Williams Air Force Base Site is located in Maricopa County, Arizona, approximately 30 miles southeast of Phoenix. OU-2 is the former Base's Liquid Fuel Storage Area (LFSA), which is also referred to as "ST012". The former Williams AFB, constructed on 4,127 acres, was commissioned as a flight training school in 1941. Pilot training was the primary mission throughout the history of the base. Jet aircraft training started at the base as early as 1949. A site vicinity map is presented as Figure 1.

Liquid fuels were stored at ST012 from 1942 until the tanks and distribution lines were removed in 1991. Cathodic protection was installed on the underground storage tanks (UST) in 197Q. Aboveground storage tanks (ASTs) at Facilities 556 and 557 were constructed in 1962 and 1954, respectively. These tanks were emptied and are no longer in service. One 17,000-gallon UST located at Facility 548 was decommissioned in 1960 and filled with sand. In 1966, the USAF updated the fuel delivery system and abandoned in place approximately 3,600 linear feet of 4- and 6-inch diameter delivery pipe. Four major spills and/or leaks, as well as several minor leaks, were reported at ST012 from 1977 to 1989. The fuel delivery lines were removed along with the storage tanks in early 1991, eliminating any source of fuel leaks.

The Installation Restoration Program (IRP) was implemented by the U.S. Department of Defense (DoD) in 1980 to identify and control environmental contamination from past hazardous materials use and disposal activities at U.S. Air Force (USAF) installations. Federal facilities are cleaned up under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), as amended by the Superfund Amendments and Reauthorization Act (SARA).

On November 21, 1989, the Williams AFB was added to the U.S. Environmental Protection Agency's (EPA) National Priorities List (NPL).

On September 21, 1990, a Federal Facilities Agreement (FFA) was signed which established a cooperative and participatory framework among the federal and state agency members, defined their roles and responsibilities, and developed a process to

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resolve any disputes that may arise during the study and execution phases of the IRP. Parties of the FFA include the following: USAF, EPA, the Arizona Department of Environmental Quality (ADEQ), and the Arizona Department of Water Resources (ADWR). As a result of DoD downsizing, Williams AFB was recommended for closure, and the base closed on September 30, 1993.

Site investigations began at ST012 in 1984. The first seven, monitoring wells, LI-01 through LI-04 and LI-06 through LI-08, were installed by Aefo Vironment, Inc. (AV) in, 1986 and 1987. IT installed monitoring wells, SS01-W01 through SS01-W29, during various investigation activities between August 1989 and September 1991. CDM installed monitoring wells SS01-W30 through SS01-W33 between January 1992 and March 1994 in support of remedial pilot studies and additional investigation activities. In support of a treatability study, BEM installed eight monitoring wells (N-l through N-8) in September and October 1996, and an additional seven monitoring wells (N-9 through N-l5) from November 1997 though January 1998 (BEM, 1997). Because of the rising water level, the majority of the older monitoring well screens are completely submerged beneath the water table. These wells have been replaced by the N-series wells, which were screened across the water table.

Between 1991 and 1994, the USAF performed a demonstration conceptual design, which then resulted in a pilot study/demonstration study (PS/DS) for groundwater extraction and treatment at ST012. This PS/DS was conducted to determine how to best implement the selected remedy specified in the OU-2 ROD. Two horizontal wells were installed within the saturated subsurface at respective depths of 219 and 235 feet below ground surface. Pump tests performed on these wells concluded that the formation would not yield adequate pumping rates for groundwater treatment. These wells proved ineffective for their intended purpose of remediation of groundwater and called into question the ability of the pump and treat technology alone to effectively remediate groundwater at ST012.

Between 1992 and 2002, groundwater elevations have risen approximately 39 feet, or nearly four feet per year. Historical groundwater elevation data show that the horizontal hydraulic gradient is consistently from west to east. A low permeability zone beneath the site separates two saturated zones. The vertical hydraulic gradient is downward across the low permeability zone as measured between the two saturated zones.

In January 1999, BEM decommissioned 20 monitoring wells at ST012. Various other types of wells, such as piezometers, injection wells, and dual-phase extraction wells have also been installed at ST012 over the course of site investigations. There are currently a total of 31 monitoring wells installed at ST012. Groundwater sampling and monitoring is currently being conducted at the site.

A SVE system for remediation of deep soils was installed at the site in 1999. The SVE remedy will be significantly enhanced with the expansion of the SVE system associated with thermal enhanced extraction described below.

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III. DESCRIPTION OF SIGNIFICANT DIFFERENCES

A. Description of the Modification

The ROD calls for groundwater pump & treat to remediate groundwater and LNAPL. The USAF is modifying the groundwater component of the selected remedy set forth in the ROD to provide for implementation of the TEE system as part of the pump & treat remedy. Application of TEE in areas with significant accumulations of LNAPL and residual soil saturation will afford a more rapid removal of the COCs compared to the use of groundwater pumping alone. TEE is also expected to improve the recovery and reduce toxicity of light non-aqueous phase liquids (LNAPL) in the saturated zone at Site ST012.

TEE consist of injection of steam, co-steam and air, and air into the saturated zone through injection wells followed by extraction of groundwater, LNAPL, and vapors (steam, volatile organic hydrocarbons, and air) through a series of extraction wells. Waste streams from the process are treated and discharged or disposed of in accordance with local, State, and Federal regulations.

B. Summary of Supporting Information and Data

A successful full-scale demonstration of the TEE process was conducted at Hill Air Force Base, Utah with an independent assessment of the technology performed by the EPA Superfund Innovative Technology (SITE) Program. A phased approach of this technology will be implemented at ST012 by treating portions of the site sequentially to meet cleanup objectives. Workplans, reports, technical memoranda and supporting documentation will be included as part of the Administrative Record.

Section 300.430(e)(9)(iii) of the NCP identifies nine criteria to be used to evaluate remedial alternatives presented in the Feasibility Study (overall protection of human health and the environment; compliance with applicable and relevant and appropriate requirements [ARARs]; long-term effectiveness and permanence; reduction of toxicity, mobility or volume through treatment; short-term effectiveness; implementability; cost; state acceptance and community acceptance.) While consideration of these criteria is not required in this ESD, an analysis of those criteria most likely affected by the proposed remedy change follows:

Compliance with Applicable or Relevant and Appropriate Requirements (ARARs): The selected remedy, as modified by the changes identified in this ESD, is expected to comply with all chemical-specific, location-specific, and action-specific ARARs identified in the ROD. No new ARARs are triggered by the changes identified in the ESD.

Reduction of Toxicity, Mobility, or Volume through Treatment: TEE provides a reduction in toxicity, mobility and volume through treatment. Because TEE would be an addition to. the existing remedial alternative, it would remove the more volatile

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component in the subsurface such that selected clean-up levels can be met in a reasonable timeframe.

Short-Term Effectiveness: TEE is expected to reduce contaminant mass in the saturated zone more rapidly than groundwater pumping alone. The targeted volatile compounds will be remediated at a higher rate with implementation of TEE.

State Acceptance: The Arizona Department of Environmental Quality (ADEQ) has reviewed and commented on this Explanation of Significant Differences and concurs with this ESD.

IV. PUBLIC PARTICIPATION

This ESD and the information upon which it is based will be included in the Administrative Record for this Site. The Administrative Record also includes the ROD and all documents that formed the basis for USAF's selection and EPA and ADEQ's concurrence of the remedial action for the Site. The Administrative Record is available for public review at the locations listed below:

Arizona State University (ASU) East Library 7001 E. Williams Field Road Mesa, Arizona 85212 480/727-1037

U.S. EPA Region IX Superfund Record Center 95 Hawthorne Street San Francisco, California 94105 415/536-2000

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Figure 1

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