Formatted Title
Using Sustainability Analysis as the Basis for Transitioning from Active to Passive Remediation
Background/Objectives
Initial remedial activities for a gasoline release included soil excavation and manual light non-aqueous phase liquid (LNAPL) recovery. Remedial efforts transitioned into an interim period of manual LNAPL skimming and pneumatic pumping of total fluids into temporary storage tanks and transfer by vacuum trucks to the facility’s wastewater treatment plant. Interim recovery was followed by long-term automated total fluids recovery, local processing by on oil-water separator and granular activated carbon, and direct conveyance to the facility wastewater treatment plant before discharge. LNAPL recovery declined after 10 years of operation, therefor an analysis was performed to quantify the environmental impact of different phases of the site remedial actions, assess the optimal time for transition from active to passive remediation and identify sustainable technologies to mitigate dissolved phase hydrocarbon transport towards the site perimeter.
Approach/Activities
A tiered LNAPL evaluation including decline curve analysis, diagnostic gauge plots, and LNAPL transmissivity was performed to evaluate the effectiveness of active LNAPL recovery. A hydrogeologic investigation consisting of electrical resistivity tomography, cone penetrometer testing (CPT) and a hydraulic profiling tool (HPT) was used to update the conceptual site model with respect to applicability of passive remedial technologies. Groundwater model simulations were performed to estimate dissolved constituent transport under different groundwater extraction and treatment scenarios including continued pump and treat, phytoremediation, a permeable reactive barrier, enhanced bioremediation, and monitored natural attenuation. Finally, the SiteWiseTM Tool for Green and Sustainable Remediation was used to quantify the environmental footprint of the different remedial phases to planned passive, sustainable technologies and compare the outputs to different indicators of remedial progress.
Results/Lessons Learned
The tiered LNAPL evaluation demonstrated that LNAPL was no longer effectively recoverable by active means. The CTP/HPT data identified an aquitard between the shallow and deeper saturated transmissive zones and indicated that the shallow interval of the saturated zone represented the majority of the groundwater and mass flux within the hydrostratigraphic layers evaluated, and therefore posed the greatest risk for dissolved phase hydrocarbon transport. Groundwater model simulations indicated that depression of the water table to within the aquitard sufficiently captured dissolved constituents resulting in downgradient concentrations below site screening levels. Based on the model simulations and remedial alternatives analysis, phytoremediation was selected as a passive, more sustainable alternative to hydraulic control. Use of the SiteWiseTM Tool showed declining LNAPL recovery after seven years of system operation and signified the turning point where adverse environmental impacts began to outweigh the benefits of active LNAPL recovery. Mapping greenhouse gas (GHG) emissions as a function of changing LNAPL transmissivity and cumulative LNAPL mass removal showed an inflection point where the transition from active to passive treatment technologies became more effective and sustainable. Overall, shutdown of active LNAPL recovery systems and implementation of passive phytoremediation would improve sustainability parameter footprints, including GHG emissions, total energy used, and accident risk, by more than an order of magnitude. Based on results of the analysis, active LNAPL recovery was transitioned under Agency approval to phytoremediation using hybrid poplar and willow trees inoculated with endophytic bacteria.