Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4a. Adaptive Site Management: Lessons Learned for Site Characterization and Remedy Implementation

Formatted Title:

Getting the Last 1% for Site Closure

Background/Objectives:

Background/Objectives. We all know the phrase in remediation…”you spend 80% of the money on the last 20% of cleanup.” We will share lessons learned in finding, and subsequently removing, the proverbial needle in a haystack. That last 1% to achieve closure. A project example of a chlorinated organics impacted compliance well at the property boundary will be shared. The well would continually rebound above the trichloroethene groundwater quality standard within 6 to 9 months after an air sparge and soil vapor extraction system was shut down. The system was expensive to operate and off-site cleanup had been demonstrated. Finding and removing the needle required non-traditional techniques.

Approach/Activities:

Approach/Activities. Operations at a former manufacturing site in Ohio had resulted in chlorinated organic compound (VOC) impacts to on-site soil, groundwater and a 2-mile long off-site groundwater plume. A perimeter soil vapor extraction (SVE)/air sparging (AS) system was installed in 1998. It was subsequently modified in 2007, 2010, and 2018 with certain points being taken offline as groundwater was cleaned up and new points added to intensify the remedial efforts in the increasingly smaller area. Focused source investigations (e.g., soil, groundwater, MIP-HPT) of the unsaturated soil and groundwater were undertaken during each optimization step to find the source(s) of the rebound and direct the addition and design of new AS & SVE wells. However, these investigations never found a true source area. In 2018, the AS well spacing had shrunk from the original 90 feet to 10 to 15 feet. Throughout these optimization steps, the design of the AS and SVE wells were modified by adding microporous tips to the AS wells and varying the depths of sparging and vapor extraction to create new sparge air pathways and focus vapor extraction closer to the top water table fluctuation elevation. The aggregate of these optimization steps reduced the area of concern from the original 1300 linear feet of property boundary to less than 100 feet. However, groundwater would still continue to rebound at a single well within 15 feet of the property boundary which was also the compliance point. This occurred repeatedly within 6 to 9 months following AS/SVE system shutdown.

Additional, high density, vertical profiling (every 2 feet) in 2018 identified a pocket of high tricholorethene concentrations (0.1 to 1.0 mg/L) 10 feet upgradient of the compliance well. This location was within an area of numerous previous vertical profiling points. The “source area” was estimated to be 10 ft by 10 ft. This now identified “source area” allowed the design of a targeted injection program. A pre-design sampling of soil and groundwater refined the vertical and horizontal extent of residual mass for the subsequent 400 square foot injection area. The distribution of mass was used to design the injection loading both vertically and horizontally for a proprietary product consisting of activated carbon impregnated with nano-scale metallic iron (BOS-100).

Results/Lessons Learned:

Results/Lessons Learned. A triangular, 7.5 ft gird injection layout and vertical injections every 2 foot in the saturated zone and seasonally saturated zone was successfully implemented. Post-injection sampling at 30, 60 and 90 days were all below 5 µg/L and predominantly non-detect. Ohio EPA subsequently approved site closure, AS/SVE system removal and abandonment of the remaining wells in 2022. No further monitoring is requirement for the site.

Guessing and hoping is not a strategy for success. Multiple technologies at different stages of a project are often needed to achieve closure. It is important to maximize the operational life of remediation technologies that require large capital investments, as well as know their limits. Maximization can be achieved through the implementation of good O&M practices. Making a switch in technology can be difficult for many reasons but should not be disregarded. It is critical for PRPs and contractors to constantly explore and educate themselves on new technologies to achieve final site closure, the last 1%!

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Adam Doubleday