We focused our results on benzene, because it is the most recalcitrant of PHCs, and found to exceed criteria guidelines the most out of all PHCs present.
Between 2016 and 2019, mean saturated soil benzene concentrations decreased by 96.7% (103.0 to 3.4 ppm; P < 0.05) (Figure 2). Both mean vadose soil benzene (5.8 to 9.3 ppm) and dissolved benzene groundwater (3.4 to 5.9 ppm) concentrations remained unchanged (P > 0.05). While the mean benzene concentrations give an overall site-level assessment of contamination and highlight the reduction of high concentrations, soil volume and mass estimates (below) better portray the spatial extent of contamination and areas above guidelines pre- and post-remediation treatment.
Soil volume and mass estimates
After 4 years of our remediation system running, we reduced the estimated volume and mass of benzene (SEQG Tier 2 guidelines, benzene concentrations > 7.9 ppm) in soil by 88.2% (Figure 1). Specifically, we reduced benzene volume in soil from 261.8 m3 to 31 m3 and estimated mass of benzene from 196.4 kg to 23.3 kg (Table 1).
The extent of dissolved benzene in groundwater has been delineated with clean wells surrounding the plume, however there were four wells that had dissolved benzene concentrations exceeding applicable criteria in 2020 (Figure 1).