U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Hydrogeology, Contaminant Distribution, and Biodegradation Processes at a Gasoline-Spill Research Site in Galloway Township, New Jersey

Abstract

The hydrogeology of the Toxic Substances Hydrology Program research site in Galloway Township, New Jersey, was characterized to determine factors controlling hydrocarbon-contaminant distribution and biodegradation. Sediments beneath the site are predominantly sand and clay. A localized zone of perched water forms above the regional water table. The direction of regional ground-water flow in the unconfined aquifer has varied from slightly north of east to east-southeast. Horizontal hydraulic gradients have been less than 0.005 since 1988 and estimates of average horizontal linear velocities range from 0.04 to 0.36 feet per day. A monitoring well provides a conduit for water to flow down from the perched water table zone to the unconfined part of the regional aquifer. The direction of ground-water flow near this well differs from regional flow directions.Gasoline as a separate-phase liquid is floating on the regional water table in the leaky monitoring well and in the perched water zone near the location from which the tank was exhumed. Dissolved hydrocarbons are present in the perched-water zone within a stable elliptical plume about 80 feet long and 40 feet wide in the 11 to 14 foot depth interval. Hydrocarbon contamination in the unconfined aquifer was detected downgradient from the leaky well, but contaminant location varied with changes in the direction of ground-water flow. The concentration of total aromatic hydrocarbons decreased downgradient as a result of dilution, dispersion, and biodegradation. Ground-water flow affects the types of hydrocarbon-degradation processes and, thus, the locations in which hydrocarbon contaminants are found.