Orion was retained to design and implement an in situ chemical oxidation (ISCO) treatment program at an active gasoline service station in Southern California impacted with petroleum hydrocarbons. Historical groundwater monitoring results indicated that gasoline range organics and benzene were below State Low-Threat Closure Policy (LTCP) guidance requirements for 3 years. However, results of trend analysis indicated that oxygenate (methyl tert-butyl ether [MTBE] and tert-butyl alcohol [TBA]) concentrations were not stable or decreasing at a rate sufficient to achieve closure in a reasonable period of time. Historically, the highest MTBE and TBA concentrations have been reported at source area well MW-4 with maximum concentrations of 11,000 and 50,000 micrograms per liter (µg/l), respectively. Orion selected ISCO treatment using high pH-activated persulfate to reduce residual oxygenate concentrations and expedite progress towards site closure.
Orion conducted an ISCO pilot test and concluded that ISCO by pH-activated persulfate was effective for reducing oxygenate concentrations at the site. During late 2018, Orion teamed with Cascade Environmental (Cascade) of Santa Ana, California, to conduct a full-scale ISCO injection targeting the remaining onsite dissolved-phase oxygenate plume. Cascade advanced 23 temporary injection points to approximately 52 feet below ground surface (bgs). The temporary points were spaced approximately 10 feet apart to provide adequate reagent distribution in the target treatment area. A total of 8,140 gallons of injection solution (9,600 total pounds of activated persulfate) were injected into the 23 temporary points over 5 days. The solution was radially injected in the saturated zone from 44 to 52 feet bgs in 2-foot increments. Discrete 2-foot injection intervals were selected because of complex lithology and higher risk of short-circuiting. Orion coordinated field activities with the service station operator to not only reduce impact to daily operations, but also increase the safety of field activities.
Orion used results of field parameters and analytical data collected throughout the injection to assess the effectiveness of ISCO and confirm persulfate was effectively distributed throughout the target treatment area. Changes in water quality parameters, including increases in pH and persulfate, were observed at groundwater monitoring wells located within the treatment area during injection.
Results of groundwater samples collected approximately 4 weeks after injection indicated that the lateral extent of the MTBE plume decreased approximately 55 percent immediately following ISCO injections. In addition, concentrations of MTBE and TBA at source area MW-4 decreased as much as 99 percent to below LTCP requirements. Concentrations remained below LTCP requirements following five quarterly post-injection groundwater monitoring events. The Regional Water Quality Control Board subsequently approved site closure and Orion completed well abandonment and site restoration activities in second quarter 2020.
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- Injection in source area with direct push rig and exclusion zone.
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- Working safely and maintaining access.
