Dual Phase Liquid/Vapor Extraction System Retail Service Station Facility

Project Description

The project site is a retail service station facility located in Veneta, Oregon. Below-ground releases of gasoline at the subject property dating back to 1979 resulted in on-site and off-site soil and groundwater impact beneath a broad area.

In 1991, two (2) independently operated groundwater pump-and-treat systems were installed to process groundwater impacted by the release and control plume migration. Although considered the most appropriate remediation option at the time, pump-and-treat techniques are considered less effective in low yielding aquifers characterized by high silt- and clay-rich soils such as are present beneath the site. Beginning in 1995, BB&A Environmental conducted a series of investigations to evaluate the effectiveness of several alternate remediation options to accelerate the rate of remediation. These options included air sparging, soil vapor extraction (SVE), dual-phase extraction, and bioremediation.

In November 1995, BB&A Environmental conducted a pilot study to evaluate the potential for implementing SVE techniques at the site. A vacuum of 45 inches of water was applied to a SVE well. Measurements of the exhaust gases confirmed the presence of combustible gases at concentrations ranging up to three (3) percent of the lower explosive limit (LEL) of gasoline. Results of the test generally indicated that SVE may be feasible remedial technology.

In 1997, a feasibility study was performed to evaluate the potential for effective implementation of air sparging and SVE techniques for site remediation. Air sparging is process in which compressed air is introduced below the water table. Air sparging benefits the remediation process two (2) ways. First, the introduction of compressed air promotes the volatilization of sorbed-phase petroleum hydrocarbons which can subsequently be recovered by a secondary mechanical system (e.g., SVE). Secondly, air sparging increases the level of dissolved oxygen (DO) in groundwater which can promote in-situ (i.e., in-place) biodegradation.

The air sparging feasibility study included six (6) air sparging points placed around an existing monitoring well. An off-site groundwater monitoring well was used as a control monitoring point. Groundwater samples were collected from the observation and control wells to establish baseline concentrations of dissolved benzene, toluene, ethylbenzene, xylene (BTEX), DO, biological oxygen demand (BOD), and aerobic hydrocarbon degrading bacteria. The wells were sampled periodically throughout the sparging study to monitor responses in groundwater quality.

Reductions in benzene (approximately 15%), toluene (approximately 30 to 50%), ethylbenzene (approximately 27%), and xylene (approximately 12 to 20%) were documented in the test well relative to baseline values. Levels of DO increased by approximately 260 percent relative to baseline values. Levels of BOD and aerobic bacteria plate counts decreased by approximately 18 and 45 percent, respectively, during the study.

Based on the findings of the feasibility studies, BB&A Environmental installed an SVE and air sparging system at the subject property in February 1999. The system consisted of 26 SVE points and 24 air sparging points.

In August 2000, BB&A Environmental conducted a study to determine the potential for biodegradation within the groundwater contaminant plume. Baseline levels of DO, dissolved iron, sulfate, and nitrate were determined at one (1) well located outside the groundwater contaminant plume and five (5) wells within the groundwater contaminant plume. The study identified decreased DO levels within the perimeter of the plume and concluded that these levels could not support aerobic degradation of organic compounds. Furthermore, alternative electron acceptors (e.g., NO3-, SO42-) were absent within the groundwater contaminant plume which indicated minimal potential for future anaerobic degradation. The study concluded that increased airflow in unsaturated soils and/or the introduction of dissolved oxygen within the groundwater contaminant plume would be required to promote biostimulation.

In August 2000, BB&A Environmental installed a dual-phase remediation system designed to recover soil and groundwater contaminants from beneath an impacted portion of the adjacent off-site property. Dual-phase remediation uses vacuum-enhanced recovery techniques to simultaneously recover soil vapor and groundwater from single or multiple recovery wells. Vacuum-enhanced recovery techniques are well suited in hydrogeologic environments characterized by low permeability soils and low transmissivity aquifers such as are encountered at the project site. In these environments, the application of mechanical vacuum techniques can increase the rate of horizontal groundwater movement within the aquifer thus enhancing the rate of groundwater production relative to “conventional” pump-and-treat systems. In addition, use of dual-phase extraction techniques can increase the effective porosity of the soil matrix by removing pore moisture present in “unsaturated” soils. The removal of pore moisture results in higher airflow rates and more effective recovery of vapor-phase contaminants.

The dual-phase remediation system consists of two (2) 15-horsepower liquid ring pumps connected to 20 remediation wells. Groundwater recovered by the dual-phase extraction system is processed using air-stripping techniques. The rate of groundwater production from the dual-phase extraction system is up to seven (7) times higher than groundwater recovery rates from conventional pump-and-treat techniques. Furthermore, aggressive groundwater recovery methods result in higher levels of dissolved contaminants in groundwater recovered by the dual-phase extraction system.

In January 2001, BB&A Environmental installed a liquid propane gas-fired catalytic oxidizer to process soil vapor recovered by the dual-phase extraction system. The catalytic oxidizer converts vapor-phase petroleum hydrocarbons into more stable, fully oxidized compounds of carbon and hydrogen (i.e., CO2 and H2O) through the process of chemical oxidation. Operation of the catalytic oxidizer reduces petroleum hydrocarbon emissions by more than 99 percent.