Soil sterilants were commonly used from the 1960s to late 1990s to control weeds on rangelands, pipeline right of ways, oil and gas wells, railways, sawmills, pulp mills, and other industrial sites across Alberta. Sterilants are typically non-selective, residual, and persistent; they control all plants they contact and because they persist in soil, vegetation growth on sterilant-impacted sites is often inhibited. Sterilants tend to be highly mobile, which can result in off-site contamination via leaching, surface runoff, and wind. Two of the most used soil sterilants in Alberta were bromacil and tebuthiuron.
This report provides lessons learned and a summary of the remaining knowledge gaps derived from the research projects undertaken through the 5-year Soil Sterilants Program. Recommendations arising from the Program include:
Improving sterilant identification and sampling
• To reduce overall site management costs, emphasize quality environmental sites assessments (ESAs) to inform conceptual site models (CSMs). This is required for both risk-based and remediation management approaches.
o Leverage guidance and support tools to identify sites and areas on a site most likely to be impacted by sterilants. Assume sterilants could be present if site was operational prior to 2000, particularly in agricultural regions of Alberta.
o Leverage best practices described herein and associated SSP documents to inform CSM through Phase 1 and 2 ESA.
o Do not rely on vegetation assessments as indicators of sterilant presence/absence.
• Install groundwater monitoring wells in initial Phase 2 ESA; data is required for CSM’s and in Tier 2 Risk Assessment approaches.
• Develop strong QA/QC program.
• Collect soil and water samples in a manner that reduces potential cross-contamination.
o Focus on how and what to sample to manage costs rather than application of field screening technologies.
Improving detection and delineation of soil sterilants in soil and groundwater
• Phyto-accessible (soluble) analytical method may provide useful data and “line of evidence” when applying a Tier 2 risk assessment approach and the Freshwater Aquatic Life (FAL) pathway can be eliminated OR if immobilization technologies are being (or have been) applied as a management tool.
o Demonstrate the difference between total (non-soluble and soluble) and phyto-accessible (soluble) concentrations to develop a site-specific guideline to accompany Phase 2 data.
• Use accelerated solvent extraction (ASE) for soil samples and the solid-phase extraction method with an Autotrace 280 SPE for water samples. Analyze the extractant by LC-MS/MS or LC-Orbitrap-MS when low-level precision and accuracy are essential to project success.
Improving risk assessment and management
• Compare Phase 1 and 2 ESA and CSM to Tier 1 Guidelines as a first step to determine management options.
o Always plan to assess groundwater when bromacil or tebuthiuron are present; install groundwater monitoring wells during the initial Phase 2 ESA in areas representative of “worst-case” scenario.
• Exclude pathways where applicable and determine if additional site-specific non-chemical-specific data (Db, foc, porosity, etc.) or chemical‐specific parameters (half-life, Koc) would be beneficial for calculation of modified Guidelines through a Tier 2 approach.
o Exclude FAL pathway if no surface water bodies are present within 300 m by (1) assessing the groundwater and monitoring sterilant plume characteristics; and (2) applying a conservative degradation half-life justified through literature or site-specific data.
o Exclude Irrigation Water pathway if outside agricultural regions.
• Consider where the site is located and if agronomic or native species are intended for reclamation.
• To reduce risk to receptors, if equivalent land capability requires native grass species for reclamation consider using the alternate soil quality guidelines for Tier 1 ecological direct contact surface soil quality guidelines for fine-grained soil in non-commercial and non-industrial land uses.
Improving remediation (unless otherwise specified, the recommendations apply to both bromacil and tebuthiuron)
Sterilants within surface soil (≤ 0.5 metres below ground surface (mbgs))
• Conduct bench-scale tests with impacted soil to assess treatment application rates based on soil properties and Phase 2 ESA data for sterilant concentrations.
• Destructive and immobilization technologies must be applied in slurry form to optimize in-situ soil contact (recommended moisture content – 80% of soil water holding capacity).
• Apply immobilization technologies (i.e., Activated Carbon) in-situ.
• For destructive remediation technologies excavate impacted soil for treatment in a constructed treatment cell (on- or off-site).
o Construct treatment cell to enable impacted soil depth within the cell to be ≥0.6 m.
• Bromacil: In-situ Chemical Reduction (ISCR) technology DARAMEND®
o Monitor soil moisture during treatment to maintain moisture conditions and reducing soil environment.
o Activated Carbon
• Tebuthiuron:
o In-situ Chemical Oxidation (ISCO) H2O2 with catalyst and surfactant (requires further investigation).
o Activated Carbon
Sterilants at depths greater >0.5 mbgs in unsaturated soil
• Excavate un-impacted soil to expose impacted zone and store on-site for reclamation.
• Excavate impacted soil and treat in a constructed treatment cell (on- or off-site) in the same manner as described above.
Saturated fine-grained till soils and groundwater impacted by sterilants
• Ex-situ – technologies and recommended practices described above apply.
Groundwater impacted by sterilants
• Applicable technologies for groundwater treatment described in Levy et al. (2021).