Technology Status Report: In Situ Flushing - CLU-IN

In Situ Flushing Project Summaries
GWRTAC Case Study Database
The magnitude of Cr+6 impact in the upper water-bearing zone has been reduced, off-site
migration of the plume has been prevented, and the dischage of impacted groundwater to local
surface drainage ditches has been reduced. Over a three year duration, 9.7 MM gallons of
impacted groundwater containing 26,732 lbs of Cr+6 have been removed. A total of 5.2 MM
gallons of city-supplied water was recharged through the two infiltration basins and one infiltration
trench. (An additional 30% of the extracted volume was estimated to be due to precipitation.)
Cr+6 concentrations have been reduced from the overall average of 1,923 mg/L (August 1988) to
87 mg/L (June 1991). The maximum Cr+6 concentration of 19,000 mg/L was reduced to 530
mg/L. The site cleanup goal of 10 mg/L was achieved in four of 23 extraction wells and ten of 11
monitoring wells tested in April 1991.
Remediation progress is highly variable, and dependent upon the well location; wells near
infiltration basins have yielded the largest volume of groundwater and display the greatest Cr+6
decline. The rate of Cr+6 decline began to exhibit a tailing effect as the project progressed, likely
due to more apparent effects of variable length contaminant flow paths, slow diffusion of Cr+6 from
fine-grained sediments, hydrodynamic isolation, and desorption of Cr+6 from soils/dissolution of
solid phase Cr+6. A pore volume (PV - volume of groundwater within the plume) at this site
contains approximately 2.6 MM gallons of water (using 10 mg/L Cr+6 as the definition of the
plume). The time required to remove a PV at this site (PV (gal)/pumping rate (10 gpm)) is
approximately every six months, if the 10 gpm pumping rate is sustained. However, due to highly
variable well yields, some areas are flushed much more rapidly than others, and within areas of low
well yield, cleanup rates are reduced. It was planned to alter the operation of the delivery and
extraction system to increase contact time of infiltration water and solid/sorbed Cr+6, deliver
recharge area to other areas within the extraction well network, and assist in preventing short
circuiting, and thereby maximize Cr+6 recovery per gallon of groundwater extracted.
Cleanup duration estimates for this site to achieve the 10 mg/L Cr+6 remediation goal have ranged
from five to 15 years. Cleanup duration is difficult to predict due to uncertainties related to
sediment heterogeneity, distribution of contaminant mass, variable groundwater extraction rates,
and seasonal weather patterns limiting groundwater availability.
Cost estimates indicate that treatment costs were estimated at $40/lb of Cr+6 removed for the first
25,000 lbs. As Cr+6 concentrations drop in later phases of operation, it is estimated that costs will
double to $80/lb of Cr+6 removed.
Report(s)/Publication(s) (Additional Information Sources):
Internet URL http://www.epa.gov/superfund/index.htm
Mann et al., 1993: Innovative Site Remediation Technology Soil Washing/Soil Flushing, Vol. 3 of
8, WASTECH, William C. Anderson, P.E., DEE, Ed., American Academy of Environmental
Engineers, 1993
U.S. EPA, Nov. 1996: Inn. Trtmt. Techs.: Ann. Status Rpt. (8th Ed.), EPA 542-R-96-010, No. 8,
U.S. EPA OSWER (5102G) TIO, Wash., DC
U.S. Environmental Protection Agency, 1991: Engineering Bulletin In Situ Soil Flushing,
EPA/540/2-91/021, U.S. EPA Office of Emergency and Remedial Response (OERR), Washington,
DC 20460, Office of Research and Development (ORD), Cincinnati, Ohio 45268, 8 pp., October
Ground-Water Remediation Technologies Analysis Center
Operated by Concurrent Technologies Corporation
Appendix - Page 91 of 164
Copyright GWRTAC 1998
Revision 1
Tuesday, November 17, 1998