Volatile Organic Compounds in the Saturated–Unsaturated Interface Region of a Contaminated Phreatic Aquifer

Abstract

Three volatile organic compound (VOC) field profiles were obtained during a period of 13 mo with a passive multilayer sampler (MLS) from a monitoring well located in the VOC‐contaminated sandy phreatic Coastal Plain aquifer of Israel in the Tel Aviv area. The profiles presented here are unique in that they span both the saturated and unsaturated zones, through the saturated–unsaturated interface region (SUIR), and represent VOC concentrations from a single borehole. In groundwater just below the water table, the major contaminant, trichloroethylene (TCE), was present in concentrations up to 260 000 μg/L water, and in the unsaturated zone just above the water table, in concentrations up to 124 000 μg/L air. Other contaminants detected in high concentrations (as high as several thousands of μg/L) included tetrachloroethylene (PCE), cis‐1,2‐dichloroethylene (cis‐1,2‐DCE) and 1,1‐dichloroethylene (1,1‐DCE). In the three profiles, TCE and PCE concentrations were greatest at the water table and decreased with increasing distance from the water table both into the saturated and unsaturated zones. Temporal variations in maximal TCE vapor concentrations ranging from 44 000 to 124 000 μg/L air were also observed between profiles. The passive diffusion sampling characteristic of the MLS makes it possible to obtain unmixed vertical samples such that, for example, differences as great as 24 000 μg TCE/L air can be measured in consecutive samples located only 12 cm apart in the unsaturated zone. The vertical detail is unique compared with other field sampling methods. Vertical detail is of utmost importance in interface regions, such as the SUIR, where water content in both the unsaturated and saturated zones varies significantly with depth, time, and space.