OU 3 includes the ground water contamination originating and emanating from a former textile plant. The water table occurs in saprolite and is connected to the weathered and fractured metamorphic bedrock below. Permeability in the saprolite and weathered bedrock is controlled by differential weathering of mineral bands and fracture fill in the parent material; fractures largely control permeability in the bedrock.
Targeted Environmental Media:
 Fractured Bedrock
The former textile plant is located on a ground water divide and the contaminant plume extends both north and south of the source.
Major Contaminants and Maximum Concentrations:
- Perchlorate (10,000 µg/L)
- Fluid Loggings
- Flow
- Surface Seismic Surveys
- Vertical Chemical Profiling
- Other (Pneumatic permeability testing; Helium Tracer Test)
Comments:
An air sparging/soil vapor extraction (AS/SVE) pilot test was conducted in 1998 at the site prior to remedial design and included helium tracer testing, pneumatic permeability testing, and radius of influence testing. One SVE well was installed in the vadose zone at a depth of 32 feet and was screened at 20 feet. Two air sparging wells were installed within the saprolite at depths of 50 and 66 feet and screened at 2 feet. Monitoring probe clusters were installed around the air sparge and SVE wells. A series of SVE-only tests were performed to establish the potential capture zone or radius of influence of SVE. Air sparging testing was performed using compressed air from the former textile plant. A helium tracer test was conducted by injecting helium into the sparge air and monitoring the soil gas/vapor at the monitoring probes and SVE well with a portable helium detector. Results of the pilot test confirmed that AS/SVE could be effective at this site. The results also indicated that the subsurface was quite heterogeneous within both the unsaturated and saturated zones. SVE performance in the full-scale system was predicted to be asymmetrical, e.g., air flow and the later distance of influence would not be expected to be the same in all directions and would not be predictable. Sparge air was found to preferentially travel horizontally due to the higher horizontal permeability at the sparge depths. It was determined that several SVE wells would be required to capture the sparge air from one air sparging well. The observed heterogeneity indicated that an observational (or phased) approach to the design and implementation of air sparging and SVE at the Site was appropriate.
- Soil Vapor Extraction
- In Fractured Bedrock Vadose Zone
- Other (Air sparging conducted in conjunction with SVE)
Comments:
The remedial design for a full-scale AS/SVE system was prepared to accommodate the heterogeneity of the site in a cost-effective manner. The observational approach that was incorporated into the design allowed for additions and changes to the AS/SVE system configuration based on the observed performance at the site. The full-scale was implemented in two phases: Phase I included installation of the first AS/SVE system using a wider well separation, use of innovative multipurpose wells for AS/SVE; a ground water monitoring and performance testing system. Phase II was installed based on performance results of Phase I to enhance VOC mass removal
The Phase I AS/SVE system design targeted the PCE source area, which was mostly under the former textile plant. Two dual-purpose AS/SVE wells were placed about 100-feet apart in the heart of the source area. These wells were surrounded by eight SVE-only wells at separations of about 100-feet. The intent of the design was to place the surrounding SVE wells such that the sparge air would be effectively captured. In total, the Phase I system design had two air sparging locations and ten SVE locations. Several monitoring probes were incorporated into the design and were placed at strategic locations to monitor the performance of the system.
After the Phase I AS/SVE system was installed, performance testing was used to assess the parameters for Phase II. The SVE system was operated from February to May 2001. The air sparging system was tested during a two-week period while the SVE system was operating. Ground water and vapor samples were collected to check for changes in VOC concentrations in the ground water and vadose zone, respectfully, and to estimate VOC mass removal rates by the Phase I system.
Results of the helium tracer test again confirmed that the subsurface in the source area is heterogeneous with respect to both air sparging and SVE. The flow-pressure testing of the wells and monitoring probes also indicated that the subsurface was heterogeneous but that most wells could be used for SVE if needed. The SVE system removed significant VOC mass (440 kg PCE) from the vadose zone during the performance test period. Likewise, air sparging removed measurable amounts of VOCs from the ground water as measured in the increase of VOCs in the extracted vapors during the AS/SVE portion of the test. The capture zone of the SVE system appeared to be adequate for the two air sparging wells.
Following completion of the performance test, the Phase I SVE system started in July 2001 and the air sparging system started in September 2002.
The Phase II AS/SVE system expansion was designed in 2002 and constructed between March and June 2003. Upon completion of the Phase II construction, there were a total of 24 SVE locations and 5 air sparging locations. The Phase II AS/SVE system was started up in June 2003 and has operated since.
The source area has been sampled for VOCs at least semiannually since installation of the Phase I wells.
Specific remediation goals are not stated and remedial actions were ongoing at the time of publication.
As of the end of December 2003, the total estimated mass of VOCs as PCE that have been removed from the source area by the SVE system was calculated to be approximately 5,500 kilograms (kg) (± 25 percent). The source area has been sampled for VOCs at least semiannually since installation of the Phase I wells. The portion of the ground water in the source area that was targeted for treatment by AS/SVE contained PCE concentrations greater than 10,000 µg/L. The area with 1000 µg/L PCE in ground water decreased substantially between the Spring 2001 and Fall 2003 ground water sampling events. Ground water PCE concentrations have decreased by one or more orders of magnitude in about 15 of the source area wells after air sparging was initiated. In the two Phase I air sparging wells, PCE concentrations have decreased by approximately three orders of magnitude. These changes in PCE concentrations were a result of less than 1 year of air sparging operations.
Results of the pilot test indicated that the subsurface at the Site was quite heterogeneous within both the unsaturated and saturated zones. The calculated radius of influence of SVE in the vadose zone varied from 22 to 59 feet, depending on the direction from the pilot test SVE well location. Based on results of the pilot test, it was determined that several SVE wells would be required to capture the sparge air from one air sparging well. It was also deemed appropriate that an observational (or phased) approach be taken to the design and implementation of air sparging and SVE at the site.
Reference - Oma, K.H., Miller, J.P., Megehee, M.M., Prince, N.K., and Towe, R.D. 2004. An Observational Approach for Effective Removal of PCE by ASSVE. Presented in Proceedings of the Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monteray, CA. www.battelle.orgbookstore
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