Compromised Chemical Toxicity in Year-Weathered Soils: Implications for Compromised Toxicity Factors

With contaminated terrestrial sites always being multiple decades old before they first submit to health risk assessments for humans and ecological receptors, there is great opportunity for soils to elicit markedly lesser chemical toxicity than would be expected. Soil aging and weathering foster various physico-chemical processes that reduce the toxic potency or bioavailability of sequestered chemicals. Because only brand new and unadulterated chemicals with seemingly maximum potencies are used in animal dosing that supports toxicity factor derivation, measured chemical concentrations in soil can be misleading, producing exaggerated risk and hazard outcomes. We sought to determine the extent to which toxicity reduction occurs in experimentally amended soils, working with large soil volumes exposed to the unimpeded ambient condition for a calendar year. A broad toxicity testing matrix for two chemicals (i.e., multiple test species, endpoints, effect level concentrations, and soil types), found species’ responses in contaminated soils to be indistinguishable from those in control soil 80% and 98% of the time for the inorganic and organic compounds used, respectively; a case in point was lead with a soil concentration of 11,000 mg/kg. The results suggest that incorporating a toxicity reduction term is an indispensable task when deriving toxicity factors.