Abstract

The geosystem approach to source-zone characterization was used at a dry cleaners contaminated with tetrachloroethene (PCE) as a dense nonaqueous-phase liquid (DNAPL) within a shallow, low-permeability aquifer. This comprehensive approach to source-zone characterization provided a design-basis level of knowledge of the site geosystem, which includes aquifer heterogeneity, and the volume and spatial distribution of DNAPL. DNAPL was found to occur beneath and adjacent to the dry cleaning building, with both residual and free-phase DNAPL in the bottom 4 ft (1.2 m) of the shallow aquifer. The bottom 2.5 ft (0.76 m) of the aquifer consists of a fining downward sequence that grades from fine sand to clayey silt, with a resulting decrease in permeability in the DNAPL zone by at least one order of magnitude. The DNAPL zone is bounded below by a clay aquitard that serves as an effective capillary barrier to downward DNAPL migration. A partitioning interwell tracer test (PITT) measured approximately 81 ± 7 gallons (280–330 L) of DNAPL in the test zone, with the majority of DNAPL located near the building and decreasing northward away from the building. Soil samples analyzed for PCE show that the DNAPL saturations are greatest vertically from 17 to 18.5 ft (5.2 to 5.6 m) below ground surface (bgs), in the fine-sand to sandy-silt portion of the DNAPL zone. DNAPL saturations generally decrease at greater depths, from approximately 18.5 to 20 ft (5.6 to 6.1 m) bgs, in a silt to clayey silt (i.e., lower permeability) zone, with little or no DNAPL penetration into the underlying clay aquitard. Postremediation soil core analyses indicate that low permeability at depth was a limiting factor in the detection of all the DNAPL within the targeted zone using the PITT. It is concluded that the partitioning interwell tracer test successfully detected DNAPL in the more sandy portions of the aquifer, while not detecting an estimated 23 ± 5 gallons (87 ± 19 L) of DNAPL in the clayey silts below 17.8 ft (5.4 m) bgs.

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