Abstract

A regional soil-gas study centered on the U.S. Department of Energy Savannah River Site (SRS), located in the southeast Atlantic Coastal Plain, was conducted between 1993 and 1996. The study involved 2870 samples in three separate sampling events and covered an area of ~3200 km². The primary purpose of this study was to determine background values for various chemical constituents to aid in waste-unit screening at SRS. In addition, the study was conducted to support the evaluation of regional geology and possibly provide input from methane data into climatological models. Both free soil-air and soil-plug samples were obtained from the near-surface environment.

Data distribution of soil-air and soil-plug values were generally analogous but differed in magnitude. Soil-plug C₂–C₄ and toluene showed sparse but clustering patterns most likely related to geological sources of hydrocarbon associated with the Riddleville and Dunbarton Triassic basins. Oxygen data suggest that the near-surface soils (to a depth of 1 m) were in full contact with the near-surface atmosphere. For all analytes, except methane and ethane/ethylene, modal values approached zero or were less than instrument detection limits.

The presence of anomalous soil-gas constituents occurring naturally within the region, notably toluene and the light saturates ethane through butane, suggests that regional values must be considered when evaluating localized geochemical characterization. Additionally, this study confirms that there is a variation in soil-gas results as affected by different soil-air and soil-plug techniques. Regional values for C₁ through C₁₀ and halogenated hydrocarbons are presented.