Atlantic Environmental Technologies, Inc.

PROJECT OVERVIEW 6

Source Area Cleanup w/ Natural Attenuation
Conducted for a Municipality in Southeast Massachusetts
Air Sparging/Soil Vapor Extraction w/ Catalytic Oxidation

Introduction

As previously described in Project Overview 2, AET conducted the subsurface site investigation for this site and submitted the Phase II Comprehensive Site Assessment to MA DEP. It was determined that heavy petroleum contamination on-site, including the presence of free-phase petroleum product, required remediation. It was also determined that there was a downgradient plume of dissolved phase petroleum contamination that extended approximately 1500 feet from the source area. The release affected private drinking water wells downgradient of the site and the Municipality installed a new municipal water line into the affected properties.

Remedial Design Parameters

In preparing the remedial plans, AET reviewed all assessment documentation, including groundwater data, site plans and subsurface soil conditions. Due to the expansive area of contamination and the prevailing GW-1 groundwater category, AET determined that it was infeasible to remediate the entire dissolved plume to the applicable GW-1 standards. In these circumstances, AET knew that the most realistically attainable solution is that of remediating the highly contaminated area directly beneath and adjacent to the release (the "source" area) and the use of Monitored Natural Attenuation (MNA) for lower level petroleum concentrations that had migrated from the site.

AET was aware that there were sandy soil conditions in the source area and that the contaminant of concern was gasoline, a substance of high volatility. Atlantic Environmental Technologies (AET) has had great success in such situations using Soil Vapor Extraction (SVE) in conjunction with Air Sparging (AS). AS/SVE is an aggressive process that eliminates the need for continuous treatment and sampling of large quantities of groundwater prior to discharge. Due to the elevated concentrations of Volatile Organic Compounds (VOCs) expected in the influent in the first few months after system start-up, AET chose Catalytic Oxidation as the method of off-gas treatment.

Description of AS/SVE Process

Air Sparging (AS) and Soil Vapor Extraction (SVE) are aggressive technologies designed to increase airflow in the subsurface and cause the volatilization of dissolved hydrocarbons in groundwater by creating a low-pressure zone above it. Increasing airflow through the subsurface media is achieved by forcing air into groundwater by AS and drawing air through the subsurface via SVE. Oxygenation of the subsurface will also induce volatilization of dissolved petroleum contamination in groundwater and stimulate the activity of naturally occurring microorganisms. Pressurized air is injected below the groundwater surface using an appropriately sized compressor and bubbling devices. Volatized hydrocarbons are collected by a regenerative blower through vapor extraction wells that are installed at precise locations around the air injection points. Recovered vapors are treated using either thermal destruction or carbon adsorption. As described above, in this situation, catalytic oxidation of the off-gas vapors was selected.

AS/SVE is most effective with homogeneous, high permeability soils in unconfined aquifers, all conditions present at the Lakeville DPW site. Specifically, AS/SVE is not recommended for subsurface materials with hydraulic conductivity (K) values less than 10e-5. The estimated K value for the site based on the medium sand anticipated and using Freeze and Cherry, is 10e-4. In addition, the contaminant of concern at the site, gasoline, is extremely volatile, making it ideal for treatment via AS/SVE. As a result, AS/SVE was the remedial alternative selected for treatment in the source area. After the source area has been addressed, monitored natural attenuation should begin to demonstrate the natural biodegradation of lower level petroleum concentrations off-site.

Treatment System Installation and Operation

AET oversaw the installation of the subsurface and above ground portions of the AS/SVE system in August and September 2000. The system was installed under an Immediate Response Action (IRA) Plan that was submitted to MA DEP in January, 2000. Thirteen AS wells and ten SVE wells were installed and connected to a common central area via horizontal piping that had been laid down in subsurface trenches. Above ground equipment including a regenerative blower, a compressor and a catalytic oxidizing unit were installed, wired and connected to the AS and SVE wells via the above referenced trench/transmission pipes. AS wells were installed to approximately fifteen feet below the water table surface, which was below the vertical limits of the plume in the source area. Operation of the AS/SVE system began on September 27, 2000.

AET monitors system operation through periodic site visits and through the use of remote equipment operation monitors and automatic dialing equipment. By determining the off-gas temperature before and after catalytic incineration, AET can calculate the rate of gasoline removal from the subsurface. Over the first four years of remediation, approximately 14,500 lbs. of gasoline compounds have been removed from the subsurface soils and groundwater. This is equivalent to 2,350 gallons of gasoline in the liquid phase. During the first two years of system operation, free-phase product in the source area has been eliminated.

Conclusions and Results of AET Recommendations

AET and the MA DEP have agreed that after the source area is remediated to MA DEP GW-1 Groundwater Cleanup Standards, that Monitored Natural Attenuation (MNA) can be utilized for the remainder of the dissolved plume downgradient of the site. Although substantial reductions in groundwater contaminant concentrations have been realized in the source area, additional AS/SVE treatment is required to achieve the above referenced cleanup standards.