Challenges and Objective
The primary challenges were ensuring effective distribution of suspended slurries in the coarse-grained soil where numerous underground utilities were present and demonstrating degradation of the heavy hydrocarbon mass on the activated carbon sorption platform. The primary objective of the remedial effort was to reduce the LNAPL mobility and to enhance the reduction of hydrocarbon mass utilizing the BOS 200+® biological processes to degrade the TPH mass from which the mobile LNAPL was emanating.
Approach
The 2021 remedial design characterization (RDC) data were instrumental in the development of a quantitative conceptual site model (CSM) for the LNAPL source and solute plume. The RDC included the installation of 23 soil borings, 23 temporary monitoring wells, and the collection of over 125 soil and 30 groundwater samples for hydrocarbon analyses. Bench testing determined the density, solubility, carbon range, and adsorption characteristics of the LNAPL. These parameters guided the calculations to determine the quantity of BOS 200+ needed to achieve project goals. Field pilot testing was performed to determine injection point spacing and fluid volumes required to distribute the BOS 200+ in the subsurface. The full scale BOS 200+ emplacement design was developed by combining all available data from historical assessments, the RDC, and the bench and field pilot testing. Full-scale injection consisted of 59,250 pounds of BOS 200+® emplaced in segregated injection areas based on lithology, contaminant mass concentrations, and contaminant mass distribution.
Results
The RDC data demonstrated that saturated hydrocarbon sorbed mass varied with depth and lithology. Significant contaminant mass was found in the saturated sand and gravel regime. Monitoring well gaging before and after pilot testing and full-scale implementation demonstrated substantial reductions in measured LNAPL thicknesses. Genomic sequencing determined that native bacteria were present to support the biological degradation of the predominant petroleum compounds prior to injections; however, the richness of the microbial population was relatively limited. Post-implementation microbial testing demonstrated an increase in microbial abundance and richness. At 6 and 12 months after full-scale injection, an abundant and rich population of degraders and supporting microbes persists. A 91-100% reduction in measurable LNAPL has been achieved to date.
