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Over six hundred sites in the state of Iowa were evaluated for hydrocarbon contamination associated with underground storage and delivery systems. Driven probe technology was used to obtain the soil vapor samples which where analyzed on-site using a field mobilized, laboratory grade gas chromatograph. A hydrocarbon concentration of 1000 (μg/l in the soil gas had been established as an action level for state assisted cleanup. This action level proved to be conservative. Approximately 72% of the sites were at or above this action level. The pattern of hydrocarbon distribution was found to be affected by three physical site properties. As was expected, concentration readings increased with depth and as groundwater was approached. Higher readings were obtained under pavement vs under unpaved areas. High hydrocarbon concentrations were not a reliable indicator of leakage. Of the sites which passed a tightness test, hydrocarbon concentrations in the soil gas approached saturation at 70% of the sites. However, low hydrocarbon concentration was a better indication of system tightness. Only 8% of the sites with low levels of hydrocarbons failed a system tightness test. Relative soil permeability remained fairly constant down to approximately 7 feet (2.1 meters) where it began to increase due to the proximity to groundwater in the capillary fringe.
soil gas, site assessment, UST, hydrocarbon contamination, field analytical methods, Iowa
Special Projects Director, Tracer Research Corporation, Tucson, Arizona
Research Assistant, Tracer Research Corporation, Tucson, Arizona