Abstract

Anatomy of a Sinking Chlorinated Solvent Plume

Presented at the 4th Annual Washington Hydrogeology Symposium, April 8–10, 2003, by Dan Matthews and Chip Goodhue

At a chlorinated solvent spill site in southwestern Washington, several factors have led to the formation of a sinking trichloroethene (TCE) plume in the absence of an apparent dense non-aqueous phase liquid (DNAPL). This article summarizes the evidence for a sinking plume, identifies the hydrogeologic conditions at the site that cause downward plume migration, and discusses the implications for site assessment, monitoring, and evaluating remedial alternatives.

During a 2-year remedial investigation completed in the fall of 2002, a TCE plume approximately 2,200 feet long was identified. Water quality measurements indicate that the highest TCE concentrations occur near the water table in the suspected source area and occur at progressively greater depths north of this area. The chlorinated ethene plume appears to reach the bottom of the roughly 40 foot thick saturated interval within a horizontal distance of approximately 100 feet from the source area. Throughout most of the plume, TCE impacts are present only in the lower third of the saturated interval in the aquifer. DNAPL accumulations have been ruled out based on the lack of TCE detections in saturated soil samples from the base of the aquifer, the relatively low levels (sub-part per million) of dissolved phase contaminants, and the absence of DNAPL accumulations in site monitoring wells.

Groundwater elevation measurements at the site indicate a consistent downward vertical gradient and northerly horizontal gradient at the site. Evaluation of groundwater flow, contaminant transport modeling, and assessment of chlorinated ethene degradation activity were performed to assess the relative importance of these factors on formation of the apparent sinking chlorinated ethene plume. Of these factors, a strong downward vertical gradient driven by high precipitation recharge appears to be the primary factor in driving development of the sinking plume.

Site assessment techniques that facilitated identification and characterization of the sinking plume included multi-level depth discrete water sampling during auger drilling, and installation of monitoring wells clusters completed at two or more depth intervals. Evaluation of remedial alternatives for the site required assessment of performance and restoration time frames using a site hydrogeologic model that addresses the vertically heterogeneous contaminant distribution.