Our intensive incremental sampling of soil was only done in 2015 and 2016 following the delivery of the second BioLodestone, as such we are only presenting soil results from one-year after this date. We focused our results on benzene, because it is the most recalcitrant of PHCs, and it was found to exceed criteria guidelines the most out of all PHCs present.
Between 2015 and 2016, mean saturated soil benzene concentrations decreased by 78.2% (19.5 to 4.3 ppm; P < 0.05) and remained unchanged in the vadose zone (1.0 to 0.8 ppm; P > 0.05) (Figure 2). Mean concentrations of dissolved benzene in groundwater (12.2 to 7.5 ppm) 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 1 year of the second remediation system running, we reduced the estimated volume and mass of benzene (SEQG Tier 2 guidelines, benzene concentrations > 1.2 ppm) in soil by 40% (Figure 1). Specifically, we reduced benzene volume in soil from 74.5 m3 to 44.3 m3 and estimated mass of benzene from 55.8 kg to 33.3 kg (Table 1).
Most wells were clean in 2021, with PHC values below guidelines, however there were four wells that had dissolved benzene concentrations exceeding applicable criteria (Figure 1). Given the high concentrations present in the utilidor, our hypothesis is that the utilidor is acting as a reservoir of pollutant to diffuses back into the site.