VA002 – Pneumatic Fracturing, SVE
Abandoned Tank Farm – Richmond, Virginia
The site was underlay by a stratum of stiff, over-consolidated clay in the vadose zone. At the time of the field test, previously existing VOC storage tanks had been removed, leaving only a six-inch-thick concrete slab over the site. Soil samples from the vadose zone beneath the slab identified two principal VOCs in the clay: methylene chloride and 1,1,1-trichloroethane (TCA). The cause appeared to have been leakage from sumps, resulting in a localized contamination. VOC concentrations, as determined by headspace analysis of soil samples, ranged up to 8,500 ppmv and 485 ppmv.
After site characterization, a vapor extraction system was installed and briefly operated to establish baseline flow characterization against which the Pneumatic Fracturing could later be evaluated. Six (6) fracture injections were made in two boreholes at the depth range of 6.8-10.7 ft. below the slab surface. In addition, groundsurface heave measurements were recorded to assess fracture radius. Pneumatic connection with surrounding monitoring wells was minimal.
Baseline testing established that the VOC removal rate form the formation was very low. The methylene chloride concentration peaked at 17 ppm after several minutes and reached a nondetectable level after 35 minutes. Typical TCA concentrations were significantly lower.
Following the Pneumatic Fracturing operation, substantial increases in both VOC concentrations and extracted air flow were observed. Effluent VOC concentration increased to a peak of 8,677 ppm for methylene chloride and 4,050 ppm for TCA. After the fourth injection, the concentrations leveled off at approximately 432 ppm and 21 ppm. Comparison with the baseline behavior shows the magnitude of the concentration surge following fracturing by 200 folds. Post-fracture air flows also increased by more than a thousand-fold over the pre-fracture air flow rate. Observed effects on the slab by the fracturing were minimal and were limited to minor widening and extension of existing cracks, and slight relative movement at expansion and contraction joints.
1. Schuring, J.R., P.C. Chan, T.M. Boland, 1995. “Using Pneumatic Fracturing for In Situ Remediation of Contaminated Sites,” Remediation, Spring 1995.