Applications of toxicity curves in assessing the toxicity of diazinon and pentachlorophenol to Lumbricus terrestris in natural soils

Toxicity tests were conducted with Lumbricus terrestris exposed to diazinon in three different natural soil types (Brookston Clay, Fox Sand and Guelph Loam) and to pentachlorophenol (PCP) in Fox Sand. Mortalities were monitored at 2, 4, 8, 16, 24, 48, 96 h, and 5, 7, 14, and 21 d. Toxicity curves (lc50/time) were fitted to a one-compartment first-order kinetics (1CFOK) model using non-linear regression analysis to determine incipient lethal levels (ILLs). The 1CFOK model provided a reasonable fit to the observed data for diazinon in all soil types. Toxicity was markedly influenced by soil type, with lowest toxicity in Brookston Clay. The 1CFOK model provided a reasonable fit to the toxicity curve for PCP in Fox Sand up to 14 d, but not for the entire 21-d test period due to increased mortality between 14 and 21 d. The results of this study suggest that toxicity curves and ILLs can be generated from the standard soil toxicity testing protocol by increasing the number of observations taken during the course of a 14-d lethality test.