During the past 5 years, Orion has managed a northern California site containing two active bulk fuel terminals and a former retail gasoline station. Historical releases from each property resulted in a 15-acre commingled groundwater plume of fuel constituents including benzene and methyl tert-butyl ether (MTBE). Two water-bearing zones with heterogenous soil within the shallow interval lie beneath the site. Seasonal water table fluctuations result in changes in contaminant concentrations and mass discharge that can vary orders of magnitude.
Many remediation technologies had been implemented at the site including excavation, light non-aqueous phase liquid (LNAPL) recovery, air sparge, soil vapor extraction, dual-phase extraction (DPE), and oxygen injection (OI). When Orion began managing the site in 2016, three remedial systems (one DPE and two OI) were operating near source areas and at downgradient portions of the groundwater plume. In addition, LNAPL monitoring and recovery were being conducted monthly. Sampling of approximately 90 groundwater monitoring wells occurred at various frequencies, from quarterly to annually.
Stakeholder Management: Upon taking over site management, Orion engaged the regulatory agency through frequent in-person meetings. Early meetings focused on reviewing the conceptual site model and understanding the agency’s primary concerns at the site. Meetings continued regularly to maintain transparency regarding ongoing remediation and keep an open line of communication and dialogue regarding site closure objectives.
Sustainability-Driven Optimization: Orion evaluated the effectiveness of each system using various assessment tools as part of a sustainability effort to optimize the remedial management strategy. Existing groundwater monitoring data were used to (1) calculate mass discharge rates over time at multiple transects and (2) evaluate the ongoing benefit of remedies that mainly relied on mass transport through treatment areas.
After calculating mass discharge and evaluating long-term concentration trends, Orion concluded that natural attenuation rates in groundwater exceeded mass discharge and were sufficient to achieve remedial objectives. Furthermore, remaining contaminant mass is predominantly stored in fine-grained lithology with low risk of transport to nearby sensitive receptors. In addition, Orion demonstrated that remaining LNAPL was not recoverable in significant volumes by physical removal processes and had been removed to the maximum extent practicable. Through evaluation of existing data, rebound testing of pilot DPE and OI system shutdowns, and transparency with the regulatory agency, each of the remediation systems were approved for shutdown by the end of 2018, while LNAPL monitoring and recovery was suspended in 2020.
Recently, Orion teamed with EarthCon Consultants of Memphis, Tennessee, to conduct a comprehensive plume stability analysis to optimize the monitoring program. Based on these results, the regulatory agency approved an approximately 40 percent reduction in the monitoring well network and a decrease in monitoring frequency to annual sampling without compromising monitoring program objectives. The well network reduction further reduced the project carbon footprint during the post-remediation lifecycle.
Orion used the SiteWise tool to evaluate the effects of remedy and post-remediation monitoring optimization on project sustainability. The results showed that sustainability indicators including greenhouse gas and air emissions, water and energy usage, and accident risks decreased by over 99.9 percent by shutting down the active remedial systems and reducing sampling locations and monitoring frequency.
Key Benefits: Orion’s site management approach shows that using effective stakeholder communication and innovative analysis of existing data resulted in an optimal transition from active to passive remediation as well as an increase in overall project sustainability