Background/Objectives.  An understanding of a site’s conceptual site model (CSM) in achieving a phased treatment train approach is critical in remediation to achieve long-term goals.  A site in Southern California (SoCal) comprised of over 400 acres was systematically characterized by our consultant in achieving stepwise treatment of both on and off-site contamination.  Multiple remedies were selected to address on-site trichloroethene (TCE) “hot spot” areas including in-situ chemical oxidation, phytoremediation, and in-situ thermal destruction.  Off-site TCE plume migration was captured using an extraction system with an on-site water treatment plant.  After low and sporadic recoveries were observed in the extraction wells, GeoSierra conducted pneumatic fracturing to enhance formation permeability and interconnect new borings with existing extraction wells.  These locations were further retrofitted to extraction wells.

The off-site plume is being channeled by an existing bedrock outcrop, resulting in a kidney bean shape plume traveling through a residential neighborhood.  A passive remedy was selected based on the plume shape and site accessibility.  GeoSierra installed an in-situ zero valent iron (ZVI) permeable reactive barrier mid-plume to reduce the downgradient TCE concentrations.  However, another challenge faced by the team was extending the lifetime of the passive system due to the existing geochemistry which could accelerate the degradation of the ZVI.

Approach/Activities.  PRBs have been widely accepted and installed as passive long-term remedies.  The preferred location of a PRB is at the plume tail preventing off-site migration of contaminants; however, many sites have accessibility issues so a mid-plume PRB is more common as in this SoCal site.  An additional challenge at this site included the presence of a ZVI passivator, nitrates, in existing groundwater, which could reduce the lifetime of ZVI.  GeoSierra worked in conjunction with the consultant to understand lithologic changes, variations in hydraulic conductivity, contaminant hot spots and flux of existing nitrates in order to construct an effective remedy and prolong the lifetime of the PRB using GeoSierra’s trenchless installation method.

The site is comprised of colluvium to 50 feet below ground surface (bgs) from the existing bedrock outcrop changing to a decomposed granite.  The target interval of the PRB is 25 to 90 feet bgs, resulting in two installed PRB thicknesses (i.e., 4-inches and 2-inches) based on an order of magnitude change in conductivities between lithologies.  Additionally, due to the presence of elevated nitrates, a parallel line of injection points along the southern half of the PRB were installed in the same borehole with GeoSierra’s resistivity strings to reduce overall drilling costs.  The objective of the upgradient treatment barrier is to reduce the nitrates by the addition of an emulsified vegetable oil (EVO) using the injection points which will also promote and maintain a reductive environment.  The northern half of the PRB was inaccessible for upgradient treatment due to access restrictions but also had reduced nitrate concentrations.  Therefore, the northern PRB was not pre-treated with EVO but injected with additional ZVI to increase the thickness.

Results/Lesson Learned.  While an understanding of this site and the consultant’s longevity on the project resulted in a valuable CSM for a long-term treatment train, this is not always the case.  Poor characterization and lack of insight for long-term remedial goals typically results in remedies that are perceived to be non-performing.  A discussion of lessons learned from installed PRBs and their relation to the CSM will provide insight for future applications.