Theme: 01. Remediation Technology Innovations

Topic Area: 01m. In Situ Technologies: Lessons Learned

Formatted Title:

Adaptive Strategies in ISTR: Navigating Groundwater Surprises in a Post-Drought California

Background/Objectives:

Amid the backdrop of California's prolonged drought, our team embarked on a complex in situ thermal remediation (ISTR) initiative at a 1,087 ft² former dry cleaner facility to target a treatment depth of 35 ft. The core task: efficacious removal of chlorinated volatile organic compounds (CVOCs), primarily PCE. Remarkably, the entire undertaking was managed indoors, ensuring minimal disturbances to the neighboring infrastructure. An unexpected surge in groundwater levels due to recent rainfall—from 25 ft bgs during design to 10 ft bgs during installation—posed unprecedented challenges. The hydrogeology of a site plays a pivotal role in ISTR, significantly impacting both the heating and extraction approaches. Notably, treatment in a saturated zone necessitates a distinctly different design and strategy compared to the unsaturated vadose zone.

Approach/Activities:

A nuanced thermal conductivity heating (TCH) strategy was meticulously crafted. The design ensured that VOC co-boiling points remained consistently around the 91~100°C marker within the treatment target zone (TTZ). As subsurface temperatures hovered around this threshold, a boosted evaporative transfer was set into motion. This exploited the contaminants' heteroazeotropic behavior, ensuring their proficient transfer to the vapor phase.

The original layout incorporated a well field consisting of 21 TCH heater wells, 11 shallow SVE wells, 6 deep SVE wells, 14 shallow SVE wells around the perimeter of the TTZ, and temperature/pressure monitoring elements. With the operation underway, the escalating groundwater levels and the deep SVE design called for a strategic pivot. Recognizing this, our team proactively shifted from deep SVE to MPE systems at operational kickoff. This led to a marked water extraction, notably 2000-5000 gallons per day, substantially surpassing the preliminary estimate of peak at 500 gallons per day, which was premised on vadose zone heating, steam creation, and condensation. A continuous 7-week MPE operation revealed the presence of an expansive perched water layer. Realizing that sole reliance on MPE extraction wouldn't eliminate this water within the projected 141 days of treatment, we reverted from MPE back to SVE systems.

Results/Lessons Learned:

Our experience emphasized the critical importance of adaptive strategies in the context of evolving hydrogeological conditions. For successful ISTR projects, real-time data interpretation, rapid decision-making, and the ability to adjust techniques based on current observations are essential, particularly when encountering unexpected groundwater anomalies.

As we approach the project's anticipated conclusion in early 2024, we are preparing for a comprehensive quantitative evaluation. This forthcoming analysis aims to elucidate the intricate interdependencies between water production rates and the temporal progression of temperatures, factoring in the variable utilization of MPE and SVE techniques. This endeavor not only progresses towards achieving the set environmental remediation benchmarks but also sets a precedent for the robustness and flexibility required in ISTR initiatives under variable geothermal scenarios.

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