This presentation addresses the challenges of characterizing a 3-mile long TCE plume in the Ogallala Aquifer of west Texas. Characterization has been performed during the ongoing expansion of a pump and treat system (PAT) and the implementation of an enhanced reductive dechlorination (ERD) remedial approach. The most significant challenge has been the inaccuracies associated with the initial conceptual site model (CSM), and the resulting impact on remedial design and implementation.
Initial delineation of the TCE plume used approximately 20% of all the wells installed and as a result, the plume was depicted as a homogeneous mass moving straight along the groundwater flow path. Due to the limited understanding of high permeability flow paths, PAT wells were installed where access was convenient, in many cases away from the main body of the TCE plume and resulting in artificial widening of the plume that further obscured an accurate understanding of its geometry.
Well logs and concentration data were re-assessed in order to discern the plume distribution. One significant finding was that the contaminant distribution and its movement are highly structured, following a complex network of channelized pathways. By installing remediation wells in an organized sequence after careful review of surrounding well data, the locations of the high permeability zones were identified and remedial efforts were focused on the areas that would impact the greatest mass. Operation of strategically placed wells, optimization of remedial systems, and an adaptive design have combined to reduce the plume size by more than 50% and have allowed for greater operational flexibility and subsequently, greater efficiency.