2006 Ecotoxicity Evaluation of Composted Site and Reference Soils Contaminated Predominantly with PHCs and Arsenic

Komex International Ltd. (Worley Parsons) contracted Stantec Consulting Ltd. (Stantec) to conduct chronic ecotoxicity assessments of composted reference soils and composted site soils contaminated with petroleum hydrocarbons and arsenic. The contaminated site soils are subsurface soil (clay till) that originated from flare-pit plume material in a site located in northwest-central Alberta; therefore, it was predominantly contaminated with weathered petroleum hydrocarbons and arsenic. The reference soil was collected from the same vicinity and had physico-chemical characteristics similar to those of the contaminated site soil, but was free of contamination. Both the site and reference soils were amended with compost to promote degradation of the PHC residuals in soil. An ecotoxicity assessment of these soils was conducted between January and April, 2006.

Test species included one invertebrate earthworm species (Eisenia andrei) and five plant species, alfalfa (Medicago sativa), barley (Hordeum vulgare var. Chapais), northern wheatgrass (Elymus lanceolatus), red clover (Trifolium pratense) and red fescue (Festuca rubra). A field- collected reference soil and an experimental negative control soil were used as control treatments for this assessment. The field-collected reference soil had physical and chemical characteristics similar to those of the site soils under investigation, but was free of contamination. The reference soil delineated effects that might be attributable to the soil independent of the contamination. The experimental negative control soil was an artificial soil recommended for testing by OECD (1984), ASTM (1995), and EC (2005a,b). It serves primarily as a quality control for test organism health, test conditions and procedures, and technician proficiency.

Plant toxicity tests were conducted with three soil treatments; the artificial control soil, the reference and the contaminated site soil. The earthworm toxicity test design also included two additional soil treatments; the reference soil and the contaminated site soil amended with a soil conditioner. As subsurface clay till soils, the reference and site soils are fine-textured with high bulk density and little organic matter. These characteristics are known to provide suboptimal environments for reproduction of Eisenia andrei. Eisenia andrei normally inhabits soils with high organic matter content and moderate to low bulk density. Since soil physical and chemical characteristics can have a strong influence on earthworm reproduction, a soil conditioner (Sphagnum peat) was added to both the reference soil and the site soil to render soil conditions more amenable to earthworm reproduction. Testing peat-amended and unamended reference and site soils was conducted in an attempt to discriminate the effect on reproduction attributable to soil petroleum hydrocarbon contamination from that of soil characteristics. Because of the nature of the soil conditioner (Sphagnum peat is minimally used as a food source by E. andrei and is well-tolerated by this species) and the fact that the PHCs were well weathered and aged (i.e., tightly bound to soil particles), the soil conditioner was thought to minimally affect redistribution of the PHCs in the contaminated site soils and influence PHC toxicity to earthworm reproduction.

Petroleum hydrocarbon concentrations in the site soils were also measured at the beginning and the end of the earthworm ecotoxicity tests conducted by Stantec.