Residue-based interpretation of toxicity and bioconcentration QSARs from aquatic bioassays: polar narcotic organics

Bioconcentration and toxicity estimation for a group of substituted phenols often categorized as "polar narcotics" can be confounded by pH-dependent ionization. Two methods of correction for ionization were applied to toxicity data obtained by U.S. EPA-Duluth for fathead minnows exposed to 30 different phenols in 37 bioassays. Toxicity QSARs with corrected data were substantially different from those obtained with raw toxicity data. When ionization-corrected toxicity data were used in the critical body residue (CBR) estimation process previously successful with neutral narcotic organics, several categories of CBR, apparently related to different modes of toxic action, resulted. Published data on lethal CBR for substituted phenols were in general agreement, although such information is limited. Elimination half-life rate constants, estimated from nonlinear curve fitting to time-toxicity information, were relatively constant for the Duluth bioassay data, averaging 0.3 days. Half-life information for small aquatic organisms, both from toxicity- and bioconcentration-based tests in the literature, was in a similar range. Much of the relatively high variability encountered experimental data for substituted phenols may in large part be due to differences in metabolic degradation between chemicals and species.