PARENT SESSION
PW7 - Biotransformation / Metabolisms / Degradation
Wednesday, 20 November 2002
8:00 AM to 6:30 PM
Exhibit Hall

(P787) Estimating Degradation Rate of Chemical Substances Using Effective Temperature Sums (ETS).

Seppala, Timo^*,1, ¹ Department of Applied Chemistry and Microbiology, Soil and Environmental Chemistry Division, Unversity of Helsinki, Helsinki, Finland

ABSTRACT- Temperature is one of the key factors affecting the degradation rate of chemical substances in the environment. In ecotoxicological risk assessment the degradation rate of a chemical under low temperature conditions (e.g. 10° C)is usually calculated by extrapolating degradation rate from laboratory studies with Arrhenius equation, as empirical data is scarce. A simple method to estimate the persistence of a substance in soil based on the use of Effective Temperature Sum (ETS) is presented. The same principle has earlier been used for agricultural purposes to estimate the growing season heat summation. The heat summation needed for total degradation of a substance (ETS_tot) is calculated by summing the degree-days (dd = daily mean temperature exceeding the defined temperature under which no degradation is assumed) from a respective laboratory study. The amount of remaining substance in soil in a given time is calculated by substracting the degree-days (dd) from daily mean soil temperatures from laboratory based ETS_tot. The degradation is assumed to follow first-order kinetics. The results will be compared to persistence obtained with Arrhenius equation and limited experimental degradation study data from 10° and 20° Celsius. The same summation methodology could in principle be used to estimate photolytic or hydrolytic degradation.

Key words: degradation rate, risk assessment, Arrhenius equation, pesticides