PROJECT OVERVIEW 7

Source Area Cleanup and Monitored Natural Attenuation
Conducted for a Hoyt Manufacturing Corporation
Westport, MA

Introduction

AET conducted the subsurface site investigation for this site and submitted the Phase II Comprehensive Site Assessment to MA DEP prior to conducting remedial operations. It was determined that groundwater and soils at this site were contaminated with chlorinated compounds including 1,2 dichloroethene, trichloroethene and tetrachloroethene. Very low levels of benzene, toluene, ethylene, and xylene (BTEX) contamination were also detected in groundwater. It was determined that a plume of dissolved chlorinated compounds extended approximately 500 feet downgradient of the release area. Several wells exhibited concentrations of groundwater contaminants exceeding the applicable MA DEP GW-1 Groundwater Cleanup Standards. The site is located on the bank of the Noquochoke River, however, no surface water contamination was documented.

Remedial Design Parameters

Using monitoring data gathered during the Phase II Site Investigation, AET prepared a MA DEP Phase III Remedial Action Plan (RAP) in which various remedial technologies were evaluated with respect to cost and anticipated effectiveness. For this site, the following remedial technologies were evaluated for use:

1) Air Sparge (AS) and Soil Vapor Extraction (SVE)
2) Dual Phase Extraction
3) Active Bioremediation.

Prior to preparation of the Phase III RAP, various field pilot studies were conducted including a SVE Pilot Study and a Bioremediation Bench Test. For the bioremediation bench test, contaminated groundwater samples from the site were delivered to the Micro-Bac Corporation laboratory. At the Micro-Bac laboratory, various tests with combinations of bacteria and nutrients are conducted to determine if the contaminated groundwater is suitable for bioremediation.

Results of the Phase III field tests were outlined in the Phase III RAP that was submitted to MA DEP on April 1, 2000. AET concluded that the most cost-effective solution to site contamination for this release was to operate a Dual-Phase Extraction operation in the source area followed by some bioremediation and finally natural attenuation (for the furthest downgradient wells). The dual-phase system was proposed for the most highly contaminated areas where there was also heavy soil contamination in the vadose zone. Bioremediation would be conducted in the monitoring wells just downgradient of the source area where there was groundwater contamination only. The wells on the fringe of the plume would be monitored for the natural degradation of contaminants as allowed by MA DEP.

Description of AS/SVE Process

A Phase IV Remedial Implementation Plan (RIP) was submitted for the site in April 2000. The Phase IV RIP contained the design specifications for the dual-phase extraction system and designated the wells for bioremediation application and monitored natural attenuation. Dual-phase extraction is the process whereby groundwater and vapors are removed from a well simultaneously. By lowering the groundwater table at the point of vapor extraction, the most highly contaminated soils in the vadose zone become accessible to the SVE process thus removing an ongoing source of groundwater contamination and completing soil remediation simultaneously. For this site, based on the contaminants of concern and the concentrations present, granular activated carbon (GAC) treatment was selected for both soil and groundwater treatment. Contaminated vapors were treated with vapor phase GAC prior to discharge to the atmosphere and contaminated groundwater was treated with liquid phase GAC prior to discharge to the Noquochoke River. Discharge to the river was conducted under a US EPA NPDES Discharge permit prepared by AET.

Treatment System Installation and Operation

AET supervised the installation of two dual phase extraction wells and associated piping and trenching between May 1 and May 5, 2000. Because this work was conducted within 100 feet of the river, AET was required to file a Notice of Intent with the Westport Conservation Commission prior to conducting this subsurface work. As is standard in Massachusetts, AET was required to lay haybales along the riverbank to prevent siltation of the river. All transmission lines from the extraction wells to the equipment area were buried below the frost line to prevent freezing.

Operation of the dual-phase system was conducted between June, 2000 and May, 2001. At this time, influent vapor concentrations were determined to be negligible and groundwater contaminant concentrations were approaching GW-1 cleanup criteria. In May, 2001, the system was shut down and AET began the bioremediation process in the source area for polishing and in the area immediately downgradient of the release. Soil is assumed to be sufficiently cleaned as influent system concentrations were zero at shutdown.

Results of Remediation Process

The site is currently under Monitored Natural Attenuation. All gross contamination at the site has been removed and there are no ongoing sources of dissolved groundwater contamination. AET has applied for, and been granted, Remedial Operating Status (ROS) for this site. ROS is a condition for a site granted by the MA DEP whereby annual MA DEP fees are greatly reduced and DEP oversight is eliminated. This site is very close to achieving a condition of No Significant Risk in accordance with the MCP and, currently, the annual cost of maintaining MA DEP compliance for this disposal site is very low.