Theme: 01. Remediation Technology Innovations

Topic Area: 01l. Innovative and Optimized Amendment Delivery and Monitoring Methods

Formatted Title:

Optimizing an In Situ Bioremediation Injection Program for CVOCs Using a Robust Conceptual Site Model

Background/Objectives:

Groundwater in situ remediation programs can fail if the relationship between geologic heterogeneity and contaminant distribution is not properly characterized or considered during remedial design. High resolution site characterization (HRSC) programs, combined with interpretation of environmental sequence stratigraphy (ESS), can be used to develop a robust conceptual site model (CSM) that can, in turn, be used to guide and support the design of an effective groundwater remedy. This presentation provides a case study of how to design an effective in situ bioremediation (ISB) injection program for chlorinated volatile organic compounds (CVOCs, principally trichloroethene [TCE]) in a complex, heterogeneous geologic environment.

Approach/Activities:

An ISB injection program was designed and implemented to remediate a TCE source area and resultant groundwater plume at a site overlying unconsolidated fluvial deposits within the Santa Clara Valley. HRSC tools (e.g., membrane interface hydraulic profiling tool [MIHPT] and Waterloo APS™) were used to define the three-dimensional distribution of CVOCs in saturated aquifer zones with geologic heterogeneities. ESS was considered when interpreting the resultant dataset to develop a robust CSM. The CSM was used to identify and define treatment areas and intervals to optimize dosing and delivery of the ISB reagents. The injection material consisted of a bacterial inoculate (Dehalococcoides [DHC]-containing microbial consortium [SDC-9™]) with known abilities to biodegrade TCE and Emulsified Lecithin Substrate (ELS) solution, a food-grade carbon used to enhance anaerobic bioremediation.

Results/Lessons Learned:

The heterogenous geology at the site was anticipated to be challenging for designing and implementing an effective injection program, however our robust CSM aided in focusing on target areas with elevated concentrations and higher permeability geology. Post-injection sampling was completed 7-, 10-, and 30-days after the injections. Preliminary post-injection TCE results were variable across 15 remedy performance monitoring wells. A reduction greater than 99 percent was observed at four wells, and the remaining monitored wells have indicated a reduction between 13 and 40 percent. A Mann-Kendall trend analysis was also conducted and indicated a decreasing to stable trend of CVOC concentrations post injections at 14 of the 15 performance monitoring wells. A three- and four-dimensional Leapfrog model will be presented, showing groundwater flow and TCE transport within the complex hydrogeologic environment. Results from performance monitoring will be provided to show the progression of the treatment and effects of matrix back diffusion on remedial performance. Lessons learned and best practices gleaned from the design and implementation of the injection program will be shared.

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Nina Allred