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 80.9% (39.0 to 7.5 ppm; P < 0.05) and by 45.8% in the vadose zone (0.9 to 0.5 ppm; P > 0.05) (Figure 2). 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 > 1.6 ppm) in soil by 85.5% (Figure 1). Specifically, we reduced benzene volume in soil from 988.1 m3 to 143.6 m3 and estimated mass of benzene from 741.1 kg to 107.7 kg (Table 1).
While most wells are clean, with PHC values below guidelines, there were two wells that had dissolved benzene concentrations exceeding applicable criteria in 2019 (Figure 1). Mean concentrations of dissolved benzene in groundwater (3.7 to 5.2 ppm) remained unchanged (P > 0.05).