Galloway Site Location and History Completed

Sources/Usage: Public Domain.

The site overlies the Cohansey Sand, part of the Kirkwood-Cohansey aquifer system. The aquifer sediment is predominantly fine- to medium-grained sand with intermittent layers of dense clay.

Gasoline hydrocarbons were detected in a shallow domestic supply well at the site in 1986 (see fig. 2). The source of the contamination was determined to be a leaky 600-gallon underground storage tank. The tank was installed in the 1940’s and the amount of product lost is uncertain. A clay lens approximately 10 to15 feet below land surface has created a perched ground-water condition, which limits the downward migration of gasoline product to the regional water table 9 to 11 feet below land surface. The hydrocarbon plume, represented by dissolved benzene, toluene, ethylbenzene, and xylenes (BTEX) hydrocarbons exceeding 2.0 mg/L, extends roughly 60 feet downgradient from the excavated tank location to the edge of the perching unit. Dissolved BTEX concentrations less than 2.0 mg/L are present in the regional groundwater 75 feet downgradient. Hydrocarbons are not detected in the unsaturated zone.

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Sources/Usage: Public Domain.

The monitoring network consists of 27 vapor probe nests, 13 ground-water observation wells, 2 multilevel ground-water samplers, 5 vapor extraction wells, 2 neutron-probe access pipes, and a nest of thermistors. The vapor probes were constructed of 1/4-inch outside-diameter stainless-steel tubing and used for unsaturated zone gas sampling.  The probes were stacked as close as 2 feet in hand-augered holes, packed in coarse sand, and separated by layers of bentonite. A total of 135 vapor probes was installed during a drought when no perched water was present. Subsequently, many of the probes positioned above the clay lens became submerged, allowing for perched-water sampling. Unsaturated zone vapor samples were collected in gas-tight syringes from dry probes (located above the capillary zone) by using a peristaltic pump.  A gas chromatograph equipped with flame ionization and thermal conductivity detectors was used to measure gas concentrations.  The gas sampling and chromatography methods are discussed further by Fischer et al. (1991) and Baker et al. (1991), respectively.  The nest of thermistors, and neutron-probe access pipes were used to obtain depth-specific temperature and moisture content data, respectively, for evaluating the transport characteristics of the unsaturated-zone sediments.