Equilibrium Environmental Inc.
February 2019
Executive Summary
Selenium (Se) toxicity and interaction with sulphate (SO4) was assessed for potentially updating the soil Se ecological direct contact guideline in Alberta and Canada. Se is an essential nutrient for plants but can cause toxicity at higher concentrations. The current soil Se guideline of 1 mg/kg is based on two studies (conducted in 1991 and 1974) of limited in scope that involved a small number of plant species. Research activities conducted under funding by the Petroleum Technology Alliance Canada (PTAC), combined with previously published work, demonstrates elevated soil SO4 concentrations (a common occurrence in the Western Canadian Sedimentary Basin) can ameliorate Se toxicity. This deficiency can potentially occur at Se background concentration ranges in Canada (up to 4.7 mg/kg) when SO4 concentrations are relatively high.
Six plant species (alfalfa (Medicago sativa), barley (Hordeum vulgare), carrot (Daucus carota), cucumber (Cucumis sativus), northern wheatgrass (Elymus lanceolatus), red fescue (Festuca rubra)) and two invertebrate species (earthworms (Eisenia andrei) and springtails (Folsomia candida)) were tested in coarse and fine soils under control, low, medium, and high Se and SO4 concentrations, to evaluate Se toxicity as a function of variable SO4 concentrations. Se concentrations ranged from <0.3 to 31 mg/kg (administered as selenate) and SO4 concentrations ranged from 28 to 1,482 mg/kg. Benchmark Dose Software (BMDS) was used to characterize and test dose-response curves and calculate 25% Effect Concentration (EC25) values for use in guideline derivation. Plant toxicological enpoints included germination percent, root and shoot length, and dry weight. Intertebrate endpoints included number of live adults, juveniles, and dry weight of juveniles for invertebrates. EC25 values were plotted to derive a Species Sensitivity Distribution (SSD) as a function of soil texture and SO4 concentration, following a detailed statistical analysis of model fit. The 25th percentile from each SSD was used for ecological guidelines development, as a function of SO4 concentration.
Results for coarse textured soils suggest that under variable SO4 concentrations, the existing Se guideline of 1 mg/kg may be appropriate and protective. However, of all the experiments conducted, those involving coarse soils were associated with the highest response variability, and lowest relative confidence, particularly with increasing SO4 concentrations. It was postulated that this was due to difficulties in dosing plant roots growing in a readily drainable coarse soil with a water soluble substance (i.e., Se as selenate), given the requirement of regular water addition for plant growth. Results for fine textured soils suggest a more appropriate minimum baseline Se soil quality guideline of 2 mg/kg is appropriate. At higher SO4 concentrations (i.e., > 1000 mg/kg), a guideline of 5 mg/kg would be more appropriate due to the amelioration of Se toxicity at higher SO4 concentrations. The analysis by nature was complex given: Se induced toxicity at high Se concentrations combined with low SO4 concentrations; SO4 ameliorated Se toxicity at higher SO4 concentrations; higher Se concentrations overcome SO4 induced low Se dose nutritional deficiency; and, SO4 may induce a distinct toxic effect at higher SO4 concentrations. Further research work with coarse soils may improve the accuracy of the Se-SO4 interaction and may allows for a more variable guideline to be developed as a function of SO4 concentration.