Extreme Short-Range Variability in VOC-Contaminated Soils

To accurately characterize volatile organic compound (VOC) distribution in contaminated soils for the purpose of ascertaining the need for site remediation, horizontal and vertical concentration fluctuations must be taken into account when designing the sampling program. Soils at two Superfund sites were collected to examine VOC concentration and soil property variability over extremely short distances (≤15 cm). Differences in VOC concentrations between the upper and lower core sections (15 cm apart) were generally on the same order (1 to 4 times different) as the differences found for the TOC, sand, and clay contents; however, several notably larger variations in VOC concentrations were found. Relative percent differences (RPDs) up to 161, 96, 117, and 191% were found for TOC, sand content, clay content, and VOC concentrations between upper and lower core sections, respectively. These elevated RPDs correspond to maximum concentration differences between upper and lower core sections of 25.5 and 72.5%, 5.8 and 22.0%, 0.29 and 2.71%, and 78 and 3371 ng/g for TOC, sand content, clay content, and VOC concentrations, respectively. The large differences in soil properties and VOC concentrations that can occur over extremely short distances must be taken into account during site characterization. Without knowing the magnitude of the extreme short-range variability, erroneous interpretations of contaminant concentrations and distributions can lead to costly remediation when it is not necessary (i.e., false positives) or the leaving behind of a significant threat to human health and the environment when not detected (i.e., false negatives).