EPIC Greensheet
Fall 1994, No. 14
This is a fact sheet produced by the EPIC alliance.
Council:
McClellan AFB
U.S. EPA, Region 9
Cal-EPA
EPIC is the Environmental Process Improvement Center, an alliance between McClellan Air Force Base, the U.S. Environmental Protection Agency, Region 9, and Cal-EPA to promote effective environmental protection through innovative management, education, communication and action.
McClellan Air Force Base (AFB) is applying a new technology called 2-PhaseTM extraction, an innovative technique for remediation of low-permeability formations where volatile organic compounds (VOCs) are present in soils and groundwater. 2-PhaseTM uses a high vacuum to remove contaminants from above and below the water table simultaneously. The name comes from the two phases of contaminants the system extracts: both aqueous phase (in the groundwater) and vapor phase (in the soil vapor above the water table).
During preliminary testing, 2-PhaseTM has proven to be highly efficient at removing contamination and preventing polluted groundwater from migrating offbase. The novel system can also increase groundwater flow in low-yielding wells and extract contaminants from the soil vapor at the same time. The 2-PhaseTM system cuts cleanup costs by an order of magnitude, simplifies the extraction of both contaminated water and vapor, and shortens remediation times. The system works best in low permeability soils in wells screened across the water table. If wells have already been installed, the 2-PhaseTM extraction system can be easily retrofitted into the existing well.
At a depth of 100 to 110 feet below ground surface, groundwater contaminated with chlorinated solvents and Freonş from a former plating shop was migrating slowly toward a primary water supply well at McClellan AFB. Prior to demonstrating the 2-PhaseTM extraction system, McClellan installed a Pump and Treat extraction system in the southwest portion of the base and a Soil Vapor Extraction (SVE) system in the northwestern section. In Figure 1, these techniques are compared to the 2-PhaseTM system.
The Pump and Treat groundwater extraction system, consisting of two extraction wells (EW 233 and 234) and a carbon treatment system, had been installed to contain the plume before it reached the supply well. However, the EWs proved to be low producers: one well averaged only 1.4 gallons per minute (gpm), and the other averaged about 3.5 gpm while it was operating. Together they removed only 120 lbs of contaminants per year. One of the EWs was also eventually shut down due to low flow rates. Although Pump and Treat serves a critical purpose by containing contaminated groundwater on base, as a treatment process it is slow and expensive.
SVE, on the other hand, is effective and regularly implemented at McClellan. A remediation technology designed to treat only soil vapor, SVE uses a relatively low vacuum to draw out the air from between soil particles, which literally strips the VOCs out of the soil. The extracted contaminants are then destroyed in a treatment system. In the first few months of use alone, SVE removed more pounds of VOCs than the groundwater treatment plant had since its installation eight years ago. One limitation of the SVE system, however, is that it cannot be used below the groundwater table, resulting in the need for another technology to treat the groundwater. To date, the traditional method has been Pump and Treat.
Taking SVE a step further, 2-PhaseTM extraction offers an alternative to Pump and Treat by targeting both the soil vapor and the groundwater. The 2-phase system applies a vacuum through a specially sized and positioned extraction tube. The tube is installed within a four to six-inch well or a modified conventional recovery well. A high vacuum (18 to 25 inches of mercury at the source) is used to remove groundwater by entrainment into the flow of recovered soil vapors, supplemental atmospheric air, or a combination of the two.
Groundwater and soil vapors drawn into the well by the vacuum are removed from the well casing through the extraction tube. The vacuum causes vapor to be drawn into the tip of the extraction tube at high enough velocity to entrain water and convey a water/vapor spray up the tube and to the surface. This in turn increases the soil vapor and groundwater flow from the formation by enhancing pressure gradients. The increased flow also means the extraction well doesn't need to be turned off because of dry conditions, thus increasing its effectiveness in containing migration.
The vapor and water phases are separated at the surface in a knock-out tank prior to treatment. The turbulence caused as the entrained groundwater moves up the extraction tube has been reported to effectively transfer more than 90% of the VOCs from the water to the vapor phase; the separated water phase then only requires carbon polishing before discharge. The contaminated soil vapor can be more efficiently treated than the liquid phase.
A demonstration of 2-PhaseTM extraction is underway at McClellan as part of the US EPA Superfund Innovative Technology Evaluation (SITE) program and McClellan's Public/Private Partnership. McClellan is partnered with seven private companies, including Xerox, Dow, AT&T, Monsanto and Southern California Edison, to share comprehensive cost and performance data.
The 2-PhaseTM extraction demonstration focuses on EW-233, a well located in the southwest portion of the base. This well was converted from a standard Pump and Treat well and is the primary focus of the demonstration, since it captures relatively high levels of contaminants from a groundwater plume with a source nearby. A second converted extraction well also located in the same area, EW-234, is being investigated only as a secondary objective since it is located away from the main plume area2-PhaseTM system is skid-mounted and connected to existing wells and piping (see Figure 2).
The base's southwest area encompasses approximately 325 acres and has historically contained storage yards, warehouses, a chemical laboratory, an aircraft fueling area, a woodshop, an instrument repair facility, a painting facility, two industrial waste treatment plants, and a plating shop. Materials handled at various locations within the area include solvents, dimethyl ether, low-level radioactive wastewater, and waste chemicals generated during plating activities. The soil and groundwater in the area contain significant concentrations of trichloroethane (TCE), tetrachloroethane (PCE), and Freon 113. Measured groundwater concentrations range from 37 parts per billion (ppb) to nearly five parts per million (ppm), and measured soil gas concentrations range from 220 ppb to 11 ppm.
The demonstration began in July 1994 and will extend to February 1995. The primary objectives of this demonstration are:
Pilot-scale test results indicate 2-PhaseTM extraction is effective in the low permeability silts at McClellan AFB (see Figure 3):
[Figure -- refer to source document]
Installation of the 2-PhaseTM extraction system at other McClellan sites will be relatively easy. Areas with soil conditions of low air permeability and water tables at depths of approximately 100 feet are good candidates. Most of McClellan's targeted sites match these soil characteristics, so the potential for widespread use of 2-PhaseTM onbase is great. If success continues, McClellan should meet its goals of increasing the contaminant removal rate, containing groundwater on the base, and removing sources of groundwater contamination.
Project Manager: Kevin Wong, SM-ALC/EMR, McClellan AFB, (916)643-0830 ext. 159.
Sources: Gordon Kingsley, Radian Corporation, 10389 Old Placerville Road, Sacramento, CA 95827, (916)362-5332.
Greensheet Author: Onnaca Heron, California State University Intern, McClellan AFB Environmental Management, SM-ALC/EM.
Greensheet Editor: Linda Geissinger, SM-ALC/EM, 5050 Dudley Blvd., Suite 3, McClellan AFB, CA 95652-1389, (916)643-0950.
Last Updated: January 16, 1996