Topic Area: 3b. Bioremediation Case Studies

Formatted Title:

Remedial Design Considerations for In Situ Enhanced Reductive Dechlorination as a Follow-Up Remedial Technology to In Situ Thermal Treatment

Background/Objectives:

A 1.75-acre Site located in the San Francisco Bay Area is impacted by chlorinated volatile organic compounds (CVOCs) in shallow and deeper groundwater, primarily trichloroethene (TCE). The highest concentrations of TCE are detected in deeper groundwater (20 to 50 feet deep) and are greater than 100 milligrams per liter (mg/L) across 19,000 square feet (sf) (0.44-acres) of the Site. Light and dense non-aqueous phase liquid containing TCE is also present in the subsurface. Given the magnitude of TCE impacts in the subsurface and the clay- and silt-rich matrix with lenticular or tabular sequences of coarse-grained deposits that comprise the subsurface, the approved remedy combines multiple remedial technologies to achieve stated remedial action objectives for the Site. This remedial technology combination includes in situ thermal treatment (ISTT) followed by post-thermal in situ groundwater polishing via enhanced reductive dechlorination (ERD). After ISTT reduces subsurface CVOC concentrations to the extent where other in situ technologies are effective for CVOC reduction, more easily implementable, and more cost effective, in situ groundwater polishing by ERD will further reduce concentrations of CVOCs in groundwater to achieve Site remedial goals for groundwater. The post-thermal warm subsurface conditions are also conducive for ERD microbial growth. This presentation focuses on the design considerations during ISTT design to facilitate an easily implementable and cost-effective transition to post-thermal ERD.

Approach/Activities:

Preparation of the remedial design and implementation plan for ISTT is currently underway with approximately 75% design completion. During ISTT design review in light of post-thermal in situ groundwater polishing by ERD, the following design issues surfaced for consideration and additional evaluation:

Should the 125 ISTT electrodes be abandoned in place or overdrilled?
- Will the magnitude of reducing conditions created by ERD result in potential metals leaching from the conductive backfill of the 125 ISTT electrodes if they were to be abandoned in place?
- Will the density of 125 ISTT electrodes abandoned in place within the 25,000 sf ISTT area create short-circuiting pathways and hinder delivery of ERD amendments?
Can remaining ISTT subsurface infrastructure be utilized to facilitate more cost and technically effective post-thermal ERD amendment delivery?
- Can the ISTT electrode borings be utilized for ERD amendment delivery?
- Is there an engineering solution to balance the design effectiveness and health and safety needs for ISTT infrastructure with post-thermal ERD amendment delivery needs?
- Is direct-push injection on an off-set grid from the 125 ISTT electrodes a more technically effective post-thermal ERD amendment delivery method?

These design questions are currently being evaluated via collaboration between the environmental consultant, ISTT vendor, injection contractors, potential ERD amendment vendors, and permitting agencies as the remedial design and implementation plan for ISTT is being finalized.

Joy Su

Affiliation

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(Group 2, Poster Board #70) Remedial Design Considerations for In Situ Enhanced Reductive Dechlorination as a Follow Up Remedial Technology to In Situ Thermal Treatment