Orion was retained to design and implement an in situ chemical oxidation (ISCO) treatment program at a former gasoline service station in southern California currently occupied by an active medical facility. Groundwater was primarily impacted with fuel oxygenates methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) at concentrations over 100,000microgams per liter (µg/l). Lithology at the site was heterogeneous and generally consisted of semi-consolidated, partially-cemented silts, silty sand, and clayey sands, with discontinuous clay lenses.
Orion conducted an ISCO pilot test near the former source areas to evaluate the effectiveness of the technology. The pilot test was completed at night to eliminate impacts to daily medical facility operations. Borings were advanced using a direct-push type rig and pH-activated sodium persulfate was injected at 12 temporary points. Reductions in MTBE and TBA concentrations indicated that ISCO was effective at treating dissolved-phase oxygenates. However, reagent application challenges were encountered including a nonuniform radius of influence (ROI), excessively high injection pressures, and low flow rates. The application challenges indicated that lithology was more complex than initially assumed and necessitated a modified injection approach.
Orion conducted a second full-scale ISCO injection that utilized pneumatic enhancement (PE) to attempt to address issues related to complex lithology. The full-scale injection was completed over the weekend to eliminate business disruption as well as reduce nuisances to nearby residences by night work. Borings were advanced using a sonic drill rig with nitrogen gas injected into discrete intervals before chemical injection to improve reagent distribution and contact with contaminant mass. pH-activated sodium persulfate was injected at 11 temporary points in similar dosages and quantities as during the pilot test. Field water quality and post-ISCO groundwater monitoring indicated that PE improved the lateral extent and uniformity of the injection ROI, increased injection flow rates, and decreased injection pressures. A final injection event with PE to treat a focused residual hot spot was completed 1 year later.
Improvements in chemical application with PE resulted in greater reductions in contaminant concentrations and overall ISCO effectiveness. Oxygenate concentrations decreased by an average of 94 percent at source area monitoring wells following PE-enhanced ISCO injection events. MTBE and TBA concentrations decreased below cleanup objectives throughout the plume following the final injection event and the regulatory agency subsequently approved case closure. Orion was able to accelerate a recalcitrant site to closure by optimizing chemical injection delivery to treat petroleum hydrocarbons within complex lithology and reduce the project lifecycle as a cost benefit to the client.
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- Rotosonic drill rig with packer system.
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- Illustration of Cascade packer system for isolating pneumatic enhancement zones.
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- Compressed nitrogen delivery system.
