Evaluation of Solvent Plume Discharge to a Wetland Stream Using an Innovative Passive Diffusion Sampling Methodology

Abstract

This paper describes the first phase of a multi-phase project currently underway to delineate a chlorinated solvent (primarily trich loroethene [TCE]) plume in groundwater downgradient of a former industrial site. The TCE was introduced to the subsurface environment at the site, resulting in a groundwater plume extending from the on-site source to a wetland-stream complex, the primary discharge point for groundwater migrating from the site. To date, the plume has been delineated from the source to the upgradient edge of the wetland via quarterly groundwater sampling through a network of monitoring wells. At the upgradient edge of the wetland, TCE concentrations in groundwater remain elevated, while products of reductive dechlorination are present. Additionally, surface-water sampling indicates that the solvent plume is discharging to a small stream that drains the wetland.

The first phase of the plume delineation project consisted of a recently completed solvent plume discharge evaluation (SPDE), with the objective of evaluating the locations of solvent plume discharge to the wetland stream. The SPDE was cost-effectively conducted by deploying passive diffusion samplers (PDSs) in the wetland stream sediments at 45 locations along a 1,900-foot length of the stream. Once equilibrium between the PDSs and the sediment pore water was attained, the concentrations of chlorinated ethenes in the PDS samplers were measured. The analytical results were mapped as a function of stream length and indicate a definitive point of plume discharge to the stream and a possible plume fringe. The results were also correlated with sediment type, water-quality parameters, and piezometric measurements obtained during this work. The ratios of degradation products to TCE were greater in the PDS samplers than ratios observed at upgradient monitoring locations, indicating an increasing natural attenuati on potential as the plume migrates through the wetland. The results of the SPDE were used to focus the next phase of plume delineation, currently being conducted. Preliminary results of the plume evaluation compare well to the SPDE results.