Environmental remediation at a former gasoline service station in the Southwest United States was determined to be necessary after unacceptable levels of petroleum hydrocarbons were detected in unconsolidated sediments and groundwater. Significant petroleum hydrocarbon mass was indicated to be present throughout much of the 100 feet deep vadose zone and, based on the depth and apparent low permeability of the sediments, a decision was made to pursue in-situ chemical oxidation treatment. An evaluation was conducted during which a Site Conceptual Model (SCM) was developed and a remedial technology consisting of direct ozone injection into the vadose zone was selected for pilot testing.

The pilot test design was driven by the SCM, which pointed to the presence of a laterally extensive smear zone of weathered gasoline non-aqueous phase liquid (NAPL) in silty clay at approximately 70 feet below grade. This horizon was indicated to be the historical position of the water table at the time of inadvertent fuel releases. Since that time the water table dropped over 30 feet.

The pilot test goals were to confirm the presence of the NAPL layer, improve understanding of the sediment lithology/geochemical make-up as well as gas-phase flow dynamics (e.g., intrinsic permeability, preferential flow paths), observe the transport and fate of ozone and oxygen using specially-constructed multi-purpose wells, and observe the nature of petroleum hydrocarbon destruction via ozone and hydroxyl radical oxidation. An integrated test sequence was carried out that produced data that satisfied these goals. The NAPL layer was confirmed, pneumatic injection and conservative tracer test data demonstrated fracture-like preferential flow within a low permeability unconsolidated sediment matrix, the nature of oxygen distribution and ozone utilization was defined, and post-injection performance assessment sampling indicated significant hydrocarbon mass destruction in proximity to the NAPL layer.