The concept of stratified remediation for petroleum hydrocarbons (PHCs) refers to the application of alternative remediation standards for varying soil depths at upstream oil and gas facilities in Alberta. The surface soil criteria apply from the soil surface down through the soil profile to a depth of either 1.5 m or 3 m depending on the area of consideration. The subsoil criteria which are less stringent can be used below the 1.5 m or 3 m depth. Use of the “subsoil” definition and related hydrocarbon criteria in the Alberta Tier 1 Soil and Groundwater Remediation Guidelines (AENV 2007, 2008, and 2009) need to be validated by additional research. To demonstrate the validity of the stratified remediation criteria outlined in the Alberta Soil and Water Quality Guidelines, a field based assessment was conducted between 2007 and 2009. The objective of the study was to confirm if the use of the subsoil criteria below 1.5 m is protective of crop quality and health, which was identified as a priority in ‘Task 4 in the Alberta Environment Recommendations to the Minister of Environment, 2004’. Although the primary objective of the study was to focus on agricultural crops as they are affected by PHC concentrations in subsoil, resources were also allocated for a literature review on the possible effect on invertebrate ecology and invertebrate community structure at the study sites. Soil invertebrates are often used to characterize hydrocarbon eco-contact as an integral component of soil quality evaluation.
Site requirements to validate stratified remediation criteria outlined in the Alberta Soil and Water Quality Guidelines included: 1) sites that were subjected to stratified remediation at the 1.5 m depth and were being used for crop production; 2) PHC concentration in the subsurface soil no greater than the values provided in the Tier 1 Guidelines for fine and coarse grained soils; 3) no co-contaminants (e.g. salts, metals, etc.); and 4) soil textural class (i.e., fine- and coarse-grained). A field survey approach which focused on the evaluation of crop performance and invertebrate ecology on wellsites subjected to stratified remediation was used. Six sites were provided for evaluation which had been reclaimed and were used for crop production however, these sites did not directly represent stratified remediation because no PHC was found in the well centre at any of the sites, although sump pits containing PHCs were found close to the well centers at two sites. Evaluations had to be made on the basis of sump pits and flare pits, which did not completely satisfy the initial conditions for site selection.
A site specific sampling scheme was developed for each location based on the available PHC data and site characteristics. In general, two soil cores were taken in both the area of contamination and an undisturbed control location near the edge of the lease to a 2 or 3 meter depth for root mass and distribution. Plots were established in the same locations for above-ground biomass and yield evaluations. Data analysis was performed depending on the specifics of the selected sites.
Invertebrate populations were measured in subsamples from soil cores. Specimens were properly prepared and examined under a dissecting microscope. Analyses were carried out at several levels including density, relative abundance, and community structure.
The above-ground biomass and grain yields measured at the six sites in this study suggest that the crops were not affected by PHC and other contaminants at levels higher than the critical values stated in the stratified remediation guidelines. Moreover, PHC contamination at depths shallower than 1.5 m did not seem to have any detrimental effect on the crop at the Taber site. Although root mass and distribution of canola, alfalfa mix, and wheat decreased with depth, there was no significant PHC effect on roots at any of the six sites evaluated.
Invertebrate species were found at greater depths than previously thought in both undisturbed and disturbed soil. Community structure and density was obviously affected by soil disturbance and PHC presence at the well centre resulting in lower densities for most faunal groups. How this impacts invertebrate ecology or soil quality at this point requires further examination.
Although these observations may suggest that crop health and productivity are sufficiently protected by current Alberta Environment Tier 1 stratified remediation guidelines for PHC (AENV 2007, 2008, and 2009), we note that the tested sites were inadequate to reach a different conclusion. Additional sites with PHC contamination levels would be needed to verify the AENV claim. It is recommended that additional research be conducted to further substantiate the stratified remediation guidelines under various soil and climatic conditions.