HR: 1340h
AN: H33E-1685    [Abstracts]
TI: Soil Flushing Through a Thick Vadose Zone: Perchlorate Removal Documented at Edwards AFB, California
AU: * Battey, T F
EM: todd.battey@earthtech.com
AF: Earth Tech, Inc., 655 University Ave. Suite 255, Sacramento, CA 95825, United States
AU: Shepard, A J
EM: andy.shepard@earthtech.com
AF: Earth Tech, Inc., 655 University Ave. Suite 255, Sacramento, CA 95825, United States
AU: Tait, R J
EM: robert.tait.ctr@us.af.mil
AF: Tybrin Corporation, 5 East Popson Ave. Bldg 2650A, Edwards AFB, CA 93524, United States
AB: There are currently few viable alternatives for perchlorate remediation in the vadose zone, particularly for the relatively thick vadose zones that are typical in the arid southwest where many perchlorate sites occur. Perchlorate in the vadose zone occurs in the form of highly soluble salts that may represent a risk to human or ecological receptors, and may also represent a threat to the underlying groundwater. A soil flushing treatability study was conducted at Edwards Air Force Base in the Mojave Desert of southern California at a site with a 129-foot thick vadose zone consisting primarily of clayey sand. This study utilized an infiltration gallery in conjunction with extraction, treatment, and re-injection of groundwater at the site, which contained perchlorate-contaminated soil and groundwater. The study objective was to evaluate the effectiveness of the infiltration gallery to 1) introduce treated groundwater back into the aquifer and 2) wash the perchlorate from the vadose zone soils to the aquifer. The infiltration gallery consisted of slotted PVC pipes within a highly permeable engineered bed of washed gravel. The initial water introduced into the gallery was amended with potassium bromide tracer. A downhole neutron probe was used to track the movement of the wetting front downward and outward from the gallery. Successive neutron measurements in vertical access tubes revealed that the introduced water reached the 125-foot bottom of the access tubes 14 weeks after the water was introduced into the gallery. The bromide tracer was detected in groundwater immediately below the gallery approximately 1 week later. The infiltration gallery was able to sustain an average flow rate of 2.3 gallons per minute. Prior to infiltration, the perchlorate concentration in groundwater below the gallery was 4,500 µg/L. Approximately 18 weeks after the start of infiltration, a perchlorate spike of 72,400 µg/L was detected below the gallery. The increase in perchlorate groundwater concentrations indicates the transfer of perchlorate from the vadose zone to the saturated zone, where it was readily captured by an adjacent groundwater extraction well. Continued flushing of treated water through the vadose and saturated zones resulted in a rapid decline in perchlorate groundwater concentrations. Confirmation soil boreholes documented the effectiveness of the soil treatment after perchlorate groundwater concentrations returned to their pre-soil flushing levels. This treatability study demonstrates that perchlorate can be removed from a thick sandy vadose zone by controlled infiltration with associated hydraulic control of groundwater to capture the leached perchlorate. The treatability study results also indicates that an infiltration gallery may provide 1) a cost-effective alternative to injection wells for reintroducing treated groundwater to the aquifer and 2) an effective mechanism for the delivery of amendments to the vadose zone and aquifer for promoting enhanced biodegradation of perchlorate in soil and groundwater.
DE: 1831 Groundwater quality
DE: 1838 Infiltration
DE: 1866 Soil moisture
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting