Formatted Title
ADEQ Experience with Remediation of Deep Tetrachloroethylene (PCE) Plume Using In Situ Micro-Diffusion Ozone Treatment in Phoenix, Arizona
Background/Objectives
The Arizona Department of Environmental Quality (ADEQ) conducted a pilot study to evaluate the effectiveness of in situ micro-diffusion ozone (MD-O3) treatment for remediating PCE contamination in groundwater at a ~1/2-acre former dry cleaner site. This property is in a mixed commercial/residential setting at the 24th St and Grand Canal Water Quality Assurance Revolving Fund site in Phoenix, AZ. The PCE plume is found in a layered alluvial aquifer approximately 90 to 130 feet below ground surface (bgs). Post-soil vapor extraction (SVE) concentrations of PCE in source area groundwater were up to 55 micrograms per liter (ug/L), likely adsorbed in fine-grained layers. Previous work suggested that MD-O3 sparging could treat a larger aquifer volume compared to injection of conventional liquid chemical oxidants. The pilot test objectives were to remediate the residual source area to below the aquifer water quality standard (AWQS) of 5 µg/L, map the distribution of O3 treatment, and evaluate cost-effectiveness.
Approach/Activities
The test ran from May 2020 to June 2022, employing a rental O3 generator capable of producing up to 46 pounds (lb) of O3 per day. O3 was directed to five specially designed ozone saturation injection (OSI) wells. These wells were screened from 120 to 125 ft bgs, enabling targeted delivery of ozone-saturated groundwater, followed by a mixture of gaseous ozone and compressed air, into a coarse-grained lithologic layer (predominantly gravel) underlying a less permeable layer (silts and clays). This facilitated diffusion into an overlying fine-grained layer. Monitoring included testing for volatile organic compounds, redox parameters, and metals. O3 dispersion was measured using a field gas meter and O3 test strips for water. O3 delivery was pulsed in 5-minute intervals to the OSI wells. The system was deactivated when temperatures exceeded 110°F. OSI wells were periodically acid washed. O3 diffusers were replaced as required. Cost-effective measures were applied, including the use of a hollow stem auger for drilling and using existing SVE infrastructure.
Results/Lessons Learned
Approximately 13,500 lb of O3 was delivered to the source area aquifer through the pulsed MD-O3 system. PCE concentrations in the "hotspot" declined by 95%. There is evidence that treatment extended beyond the anticipated area, reaching ~250 feet downgradient from the OSI wells. The absence of near-term rebound suggests that the O3 diffused into the fine-grained layers and treated the adsorbed fraction. Pulsed injections at 5-minute intervals was determined to be optimal. Dissolved chromium (Cr) was liberated proximal to select OSI wells. Long-term rebound monitoring will evaluate concentrations of PCE and dissolved Cr in groundwater. Based on geochemical data, Cr transport is not anticipated. The system will not be expanded because the pilot test effectively reduced PCE in the source area.
ADEQ is using this site as the basis for evaluating the cost effectiveness of MD-O3. The overall cost for treating the 1/2-acre source area (130 feet deep) was ~$710,000. The cost for delivered O3 was ~$53 per lb, based on 9,300 operational hours.
ADEQ determined that this technology offers a more feasible strategy than closely spaced injection wells in situations where well drilling costs are high (due to deep aquifer) and access is limited (due to an urban setting). As with many technologies, concentration and favorable geochemical conditions are critical to success. The costs are relatively high compared to the mass of PCE treated so the expectations and objectives, including risk to human health, need to be considered. The mass of PCE treated was small, but addressing the residual source was an objective of the Agency in an area where groundwater resources are highly valued.