Client Successes

Case Study - High Vacuum Extraction (HVE) Design/Build

The subject site is a retail gasoline station. A Phase II Comprehensive Site Assessment was conducted to define the nature and extent of petroleum hydrocarbons in soil and groundwater.

Site Hydrogeology and Likely Remedial Alternatives

The site is underlain by the Rhode Island Formation, which is comprised of sandstone, graywacke, shale and conglomerate. Bedrock was not encountered in any borings advanced on the site. The deepest well was installed to a depth of approximately 27 feet below grade. Overburden stratigraphy consists of fine silt and sand with increasing percentages of silt and clay toward a depth of 10 feet below grade. Dense basal till was encountered at depths between 10 to 27 feet below grade. Based on historical gauge data, groundwater flow direction is to the north, and depth to groundwater is approximately 7 to 10 feet below grade. The estimated groundwater flow velocity (v) at the site was calculated to be approximately 1.3 feet per year, which correlates well with the geologic materials observed during subsurface investigation.

Since low-permeability soils such as silts and clay were observed during the assessment phase of the project, high vacuum extraction (HVE) technology was selected as the alternative that would most likely achieve a Permanent Solution at the site. The observed soil types are not conductive to soil vapor extraction, air sparging or conventional pump and treat technologies.

HVE Pilot Testing

To further evaluate HVE technology, a high vacuum extraction test was performed using a liquid ring pump as the extraction mechanism. A sustained airflow of 10 to 12 cubic feet per minute was achieved in test well CEA-1, at an applied vacuum of 20 to 22 inches of mercury (Hg). A PID concentration of 388 parts per million (ppm) was measured prior to termination of the test. The maximum %LEL was measured 90 minutes into the test (70%), decreasing to 21 % after 240 minutes. Pneumatic influence was not observed in any of the monitoring wells that were monitored. Hydraulic influence was observed in CEA-4D (DTW increase of 0.06 feet) and PZ-1 (DTW increase of 0.04 feet), located approximately 37 feet from CEA-1. Approximately 237 gallons of hydrocarbon-impacted groundwater and one gallon of NAPL were recovered during the test and drummed for future treatment or disposal.

System Design and Remedy Implementation Plan

Based on the outcome of pilot testing, a full-scale HVE system was designed for implementation at the site, to address petroleum constituents in soil and groundwater. It was determined that eight HVE wells were required to treat the affected areas of the site. Using the number of wells and the anticipated air flowrate of 10 cfm per well, a liquid ring pump system designed to process 100-cfm airflow at 27.5–inches of mercury was specified. A maximum influent concentration of 100 ppm total volatile organics is anticipated in the water stream, therefore, liquid phase granular activated carbon adsorption (GACA) was selected as the treatment alternative. Influent MTBE concentrations are likely to be less than 1 ppm. A coalescing oil/water separator was also specified to promote the recovery of NAPL.

Vapor phase GACAs were selected to maintain compliance with DEP Policy WSC-94-150 (Off-Gas Treatment of Point Source Remedial Air Emissions). It is estimated that carbon consumption will be greatest during the initial stages of system start-up, but will decrease as influent vapor phase concentrations also decrease to asymptotic levels. The design details are being documented in a Phase IV Remedy Implementation Plan.