The Problem
An Above Storage Tank (“AST”) fuel spill occurred at the site on July 19, 2010, of which 3,581 litres of 10% blended ethanol gasoline leaked into a containment dyke. Over the course of 2015 and 2016, 32 boreholes were drilled to evaluate the extent of contamination establishing the following:
- 20 locations – Excess benzene concentrations of benzene
- 7 locations – Petroleum Hydrocarbon (PHC) fraction F1
- 3 locations – PHC fraction F2
- 1 location – Total xylenes PHC
- Groundwater – Excess benzene – PHC F1 and PHC F2
- Soil across the site – Excess PHC impacts (Figure 1)
Substances Of Potential Concern (“SOPCs”) were found to exceed guidelines at the subsurface down to 9 m bgl, with the highest concentrations at 2-5 m bgl. Subsequent drilling events achieved vertical delineation and indicated PHC impacts exceeding the applicable guidelines in groundwater. No Light non-aqueous phase liquid (“LNAPL”) or sheen was observed during sampling.
The Implemented Solution
A drive-point network, consisting of 36 injection wells, was installed in 2015 (Figure 1). The BioLodestone, a proprietary amendment solution, was delivered to the contaminated area for 1 month in 2015 and 3-4 months for 2016, 2017, 2018, 2019, 2020.
The Results
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.
Soil benzene
Between 2015 and 2020, mean saturated soil benzene concentrations decreased by 89% (8 to 1 ppm; P < 0.05) and by 90% (13 to 1 ppm; P < 0.05) in the vadose zone (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 5 years of our remediation system running, we reduced the estimated volume and mass of benzene (SEQG Tier 2 guidelines, benzene concentrations > 11 ppm) in soil by 99% (Figure 1). Specifically, we reduced benzene volume in soil from 77 m3 to 1 m3 and estimated mass of benzene from 58 kg to 1 kg (Table 1).
Groundwater benzene
Mean concentrations of dissolved benzene in groundwater remained unchanged (6 to 2 ppm; P > 0.05) but below SEQG Tier 2 guidelines.
Figure 1. Above: Aerial view of the site, indicating the site boundary, former concrete dyke and cardlock, and the location of the remediation system, and groundwater wells that had dissolved benzene concentrations below Tier 2 guidelines (green dots); no wells exceeded applicable criteria (Tier 2 guideline: 19 mg/L) in 2020. Soil benzene concentrations at the site prior to remediation treatment (2015; left) and post-treatment (2020; right). The aerial view of the benzene plume is at 4 m bgl, to show the depth with the highest concentration and extent. The cross-sections (centre) are through the borehole with the highest concentration. Black points indicate boreholes sampled.
Figure 2. Mean soil benzene concentrations in the saturated and vadose zones (left), and dissolved benzene concentrations in groundwater(right) with 95% confidence intervals at the site during 2015 and 2020. Canadian Council of Ministers of the Environment (CCME) and applicable Saskatchewan Environmental Quality Guidelines (SEQG) Tier 2 benchmarks of environmental quality are presented.
Lithology | Volume (m3) | Mass (kg) | Reduction (%) | |||
Pre-treatment | Post-treatment | Pre-treatment | Post-treatment | |||
Clay | 75 | 1 | 57 | 1 | 98 | |
Sand | 2 | 0 | 1 | 0 | 100 | |
Total | 77 | 1 | 58 | 1 | 98 |
|
Table 1. Pre- and post-treatment volume (m³) and mass (kg)
of soil benzene within the dominant soil lithologies on site. The reduction (%)
of volume and mass of soil benzene is also shown.