In August 2013, Orion began managing an active retail gasoline station in southern California impacted by petroleum hydrocarbons released to the subsurface by former fueling infrastructure. Previous investigations indicated that benzene was the primary contaminant of concern and risk driver for the site. Light non-aqueous phase liquid (LNAPL) was observed intermittently in onsite monitoring wells. Dissolved benzene concentrations at offsite wells were near the effective solubility of this compound in gasoline, which typically indicates the nearby presence of LNAPL above residual saturation.
The existing remedy included air sparging and soil vapor extraction (SVE) connected to a network of 41 remediation wells. The system was originally designed to treat soil and groundwater upgradient of the former source area, but was subsequently expanded in early 2013 by others to treat source area impacts.
Upon taking over site management, Orion evaluated post-expansion performance monitoring data to assess the efficacy of the treatment system. The evaluation indicated that air sparging in the source area was not successfully enhancing SVE system influent vapor concentrations and mass removal rates. In order to assess whether air sparging was a viable source area treatment technology, Orion implemented a performance monitoring program from August 2013 through November 2014. The program objectives were the following:
- Assess whether the vertical distribution of dissolved-phase impacts was stratified, which would indicate the need for a more targeted treatment approach
- Evaluate whether preferential flow pathways and/or air entrapment were limiting delivery and subsequent removal of sparged air from impacted zones
- Determine whether the “breakthrough threshold” required for air sparging to successfully volatilize petroleum hydrocarbons was being achieved.
To accomplish these objectives, Orion performed the following activities:
- Conducted depth-specific groundwater sampling passive diffusion bag samplers (PDBs) and field parameter monitoring at several source area wells
- Installed pressure transducers in several monitoring wells to evaluate changes in water levels before, during, and after active air sparging
- Monitored wellhead pressure and vapor concentrations using a photoionization detector at selected wells before, during, and after active air sparging.
Field and analytical data collected during the evaluation supported the following observations and conclusions:
- Groundwater data indicated that hydrocarbon impacts across the water column were consistently elevated throughout the source area treatment zone.
- Changes in groundwater elevation measurements indicated that the treatment zone was highly channelized and air distribution was likely controlled by preferential flow pathways.
- Water levels at multiple observation wells increased significantly during air sparging and remained elevated for an extended period of time, which indicated that air may be entrapped beneath low permeability soil lenses.
- Operational data confirmed that the air sparging system was able to achieve “breakthrough threshold” conditions required to volatilize hydrocarbon mass. However, preferential air flow pathways observed during air sparging limited the effective distribution of air within the source area target treatment zone.
Based on the results of the evaluation program, Orion concluded that air sparging with SVE was not effectively treating dissolved-phase source area impacts and subsequently shut down the system. Additionally, Orion recommended the collection of high‑resolution site characterization (HRSC) data to evaluate alternative treatment technologies.
During August 2015, Orion implemented an HRSC program to identify the depth and location of (1) elevated hydrocarbon mass treatment zones and (2) high permeability soil lenses that were likely controlling contaminant migration to offsite wells. Orion characterized the source area using a membrane interface probe and hydraulic profiling tool (MiHPT) and supplemented the data with soil and grab groundwater sampling.
Based on HRSC results, Orion identified the zones of highest permeability controlling contaminant migration and designed an alternative remedy including pneumatically‑enhanced in situ chemical oxidation. Orion achieved site closure objectives after implementing the alternative remedy and is scheduled to decommission the site during third quarter 2020.
Through thoughtful analysis of site data, Orion was able to optimize the remedial management strategy and expedite progress to site closure.