Formatted Title
Data Driven Approach to Construction and Adaptive Management of a 3,000-foot In Situ Reactive Zone for Cr(VI) Treatment in Groundwater
Background/Objectives
Pacific Gas and Electric (PG&E) is implementing remedial actions to treat hexavalent chromium (Cr[VI]) in groundwater at the PG&E Topock Compressor Station in San Bernardino County, California (the Site). One component of the groundwater remedy is an in situ reactive zone (IRZ) created through groundwater recirculation and ethanol amendment. The IRZ is oriented perpendicular to groundwater flow and was designed to target treatment along a length of 3,000 feet and a saturated thickness ranging from 10 to 350 feet. The approved IRZ design was to be comprised of 28 recirculation wells, including 24 injection and 4 extraction wells, and to be operated at 300 gallons per minute with ethanol delivered at an average of 100 milligrams per liter of total organic carbon. A data driven approach was taken to minimize drilling and construction disturbance within a cultural and biologically sensitive area and to optimize operations.
Approach/Activities
During the design phase of the project, the dataset describing the Cr(VI) distribution in the subsurface was limited due to site-specific constraints on the number of wells that could be installed. To overcome this constraint, a data collection approach was employed during remedy installation to efficiently collect high-resolution Cr(VI) concentration data as remedial wells were drilled and the remediation wells were designed with multiple corrosion resistant and long-lasting screened intervals, rather than a traditional approach with each screened interval in a separate borehole. The data collection approach consisted of continuous coring using roto-sonic drilling, vertical aquifer sampling for Cr(VI) concentrations using temporary wells, and collection of a high density of samples for grain-size analyses throughout potential screen zones. These data was entered in real-time into a 3-D model to optimize design and screen placement of the remediation wells. Construction of the IRZ was completed and operations began in December 2021. Operational data for remedial wells and analytical and water level data are routinely analyzed together in dashboards to evaluate distribution of ethanol in the subsurface and creation of reducing conditions for Cr(VI) treatment and to guide operational decision making. Key operational changes to improve performance include increasing flowrates in a subset of injection wells and modifying ethanol injection volumes to balance ethanol distribution, Cr(VI) treatment and byproduct formation.
Results/Lessons Learned
The high resolution Cr(VI) data collected and 3-D model built during IRZ well installation revealed a smaller Cr(VI) plume than had been understood during remedy design. In the area of the IRZ, the plume was 20 percent shorter than previously understood and, in the northern half, the plume was absent from a 75- to 100-foot thick layer in the middle of the saturated thickness of the aquifer. The characterization of a smaller plume in real-time during the full-scale drilling campaign allowed a 35% reduction in the recirculation well network footprint and conveyance infrastructure and a smaller and simpler remedy produced water conditioning facility for initial operations. The remainder of the planned infrastructure was deferred to the future, if it becomes necessary. Adaptive operations of this optimized recirculation well network resulted in 40% less groundwater recirculation and 40% less ethanol to successfully establish an IRZ to treat Cr(VI) in groundwater and establish a cut-off barrier to manage the Cr(VI) plume. This paper will present the adaptive approach and consequent adjustments made to the full-scale design along with operational data collected during the first 2 years of operation to demonstrate the system success and efficiencies.