Fractured rock sites, impacted with chlorinated solvents such as tetrachloroethene (PCE) or trichloroethene (TCE), remain a significant environmental challenge for the DoD. Efforts to apply in situ remedial technologies, such as chemical oxidation or bioaugmentation, have often proved challenging and/or unsuccessful with respect to attaining remedial objectives in fractured rock aquifers. Contaminant rebound typically is observed due to processes such as dense non-aqueous phase liquid (DNAPL) dissolution, matrix back-diffusion, and/or release of contaminants from low permeability/bypassed fracture zones. Unfortunately, the ineffectiveness of these remedial technologies is typically not recognized until after substantial time and resources have been expended via in situ pilot testing, and the mechanism(s) controlling the observed contaminant rebound often remain unidentified. This lack of understanding in the conceptual site model hinders effective site management, particularly with respect to designing an appropriate remedial approach and identifying the practical limits of remediation.
The overall objective of this project was to develop and evaluate a novel “Push-Push” remedial assessment technique, coupled with compound specific isotope analysis (CSIA), for use as a rapid and cost-effective means to assess the limits of in situ remediation in fractured bedrock systems.
In this project, a rapid assessment (RA) protocol was developed to assess the potential effectiveness of in situ treatment such as chemical oxidation or bioaugmentation. The RA protocol assessed chlorinated ethene rebound, the potential of naturally occurring dechlorination reactions in low permeability zones, and the remedial effectiveness of using a pair of closely spaced bedrock wells. The RA technique involved identifying hydraulically conductive fracture zones, flushing contaminant from the fracture zones using water, then evaluating contaminant rebound within this zone while hydraulically isolating the zone from the surrounding contaminated aquifer (thereby preventing re-introduction of dissolved contaminant from the surrounding aquifer). The rate, composition, and isotopic signature of contaminant rebound is then used to evaluate the limits of remedial effectiveness, identify the local source/cause of any observed rebound, and provide improvement to the site conceptual model.
The demonstration of the RA protocol was performed in shallow bedrock at Calf Pasture Point (CPP) in Rhode Island, where TCE was the primary groundwater contaminant. While nearly 99% of the TCE was removed from the conductive fracture zone during the initial flushing, substantial contaminant rebound (up to approximately 5% of the baseline TCE concentration) was observed over the ensuing five-month rebound period. The rate and extent of observed contaminant rebound was reasonably described using a matrix back-diffusion model, thus serving as a line of evidence that the observed rebound was due to matrix back-diffusion. The back-diffusion model further predicts that over a decade of treatment likely would be needed to reduce TCE concentrations by 99% in the conductive fractures.
In addition to the back-diffusion model, CSIA on carbon for TCE and cis-1,2-dichlorethene (DCE) further confirmed the source of the observed rebound. The molar-average sum of TCE+DCE was isotopically (13C) heavier at the end of rebound than at baseline conditions, thereby indicating that the “source” of the observed rebound could not be explained by any migration of contaminants from upgradient. The isotopic shift was consistent with TCE and DCE that had undergone abiotic dechlorination in the rock matrix; abiotic dechlorination of TCE in the rock matrix was confirmed in a separate bench-scale batch test using collected rock core. Thus, the CSIA testing not only served as a line of evidence demonstrating that the rock matrix was the source of the observed rebound, but also served as a useful tool for confirming that abiotic dechlorination of TCE and DCE were occurring within the rock matrix.
Based on the findings attained from this demonstration, a Rapid Assessment Protocol has been developed for potential future applications of this technique in fractured rock. This protocol is meant to serve as an incremental approach for planning and implementation of this testing method, but is not meant to serve as an exhaustive or constrictive guidance under the wide range of site specific conditions that may be encountered.
The complexity of the fracture flow path will impact the length scale of the RA test (i.e., the distance between the injection well and monitoring well). If this length scale is insufficient to capture a representative zone of the bedrock hydrogeology, then additional RA tests (using additional well pairs) may be needed to assess the site.
This demonstration provides the DoD and stakeholders with a management tool to improve the evaluation and treatment of contaminated plumes within fractured bedrock. This will allow remedial strategies to be optimized, limits of remediation to be identified cost effectively, and improved estimates of natural attenuation time frames to be achieved.
Schaefer, C., D.R. Lippincott, H. Klammler, K. Hatfield. 2018. Evidence of Rock Matrix Back-Diffusion and Abiotic Dechlorination using a Field Testing Approach. Journal of Contaminant Hydrology, 209:33-41.