Formatted Title
In Situ Treatment of Chlorinated Solvent Source Zone: Injection Design and Treatment Performance Assessed with Compound-Specific Isotope Analysis
Background/Objectives
When conducting engineered in situ treatment of volatile organic compounds (VOCs) contaminated groundwater, proper establishment of the intended mass removal process requires specific attention to ensure optimal performance of the remediation technology. To address this challenge, the use of compound-specific isotope analysis (CSIA) to characterize the VOC mass removal process during remediation treatment is becoming an indicated tool for field practitioners. By tracking the change of the VOC isotopic composition caused by either biotic or abiotic processes, VOC mass destruction process initiated by the remediation treatment can be distinguished from co-occurring diluting process. Since the fate of VOC and the treatment cost-effectiveness are closely related to a successful field implementation of the selected remedial technology, such information obtained via CSIA becomes valuable. Furthermore, when CSIA application is designed to site-specific conditions, additional information other than VOC destruction can be obtained, such as mass displacement. The current site conditions involved a low-permeable silt unit containing the CS mass above a permeable sand unit. To evaluate whether the injection process is resulting in a downward CS mass movement, CSIA was additionally used as a fingerprint method.
Approach/Activities
A pilot scale followed by a full-scale treatment was carried out to evaluate the performance of an in situ reduction strategy that combined both biological processes and abiotic pathways to reduce chlorinated compounds into harmless end products. A zero-valent iron (ZVI) colloidal suspension composed of ZVI particles in glycerol and dispersants was co-injected with a water-mixable vegetable oil based organic substrate. Direct push injection method was used to deliver the amendments at pre-determined depths inside the contaminated silt unit. To avoid creating preferential pathway to the sub-unit sand, the latter unit was not reached by the injections. Instead, the ZVI colloidal proportion was increased for the injections carried out at the lower part of the contaminated silt unit. Monitoring for CS concentration and δ13C was performed through monitoring wells with screen portion located either within the silt or the sand unit. The VOCs evaluated were trichloroethene (TCE), cis-dichloroethene (cis-DCE) and vinyl chloride (VC). A baseline and at total of five post-injection sampling events were carried out.
Results/Lessons Learned
The information gained through the application of CSIA (δ13C) during the treatment was twofold. By measuring positive isotopic shifts up to 5‰ and 7‰ for TCE and cis-DCE respectively, the injected products proved to be effectively degrading those two chlorinated compounds. In contrast, while VC was found to be produced (concentration increase), negative isotopic shifts up to 15‰ were measured. Such a concentration-isotope pattern strongly suggests a VC stall in the aquifer. The isotopic mass balance nevertheless suggests full dechlorination process, which might be attributed to b-elimination of TCE (via ZVI) rather than VC biodegradation. Accordingly, the full-scale treatment was implemented to consider this potential VC stall. Secondly, δ13C for TCE in the sand sub-unit also showed more positive values combined with a decrease in concentration following the injection event. This concentration-isotope pattern supports the fact that no un-impacted TCE was pushed down into the sand unit during the injection process and suggests a decreased TCE mass (impacted by the treatment) leaking down to the sand aquifer. The presentation will present field data results and discuss CS degradation process documented by CSIA.