Soil temperature effect in calculating attenuation and retardation factors

The effect of annual variation of daily average soil temperature, at different depths, in calculating pesticides ranking indexes retardation factor and attenuation factor is presented. The retardation factor and attenuation factor are two site-specific pesticide numbers, frequently used as screening indicator indexes for pesticide groundwater contamination potential. Generally, in the calculation of these two factors are not included the soil temperature effect on the parameters involved in its calculation. It is well known that the soil temperature affects the pesticide degradation rate, water–air partition coefficient and water–soil partition coefficient. These three parameters are components of the retardation factor and attenuation factor and contribute to determine the pesticide behavior in the environment. The Arrhenius equation, vanʹt Hoff equation and Clausius–Clapeyron equation are used in this work for estimating the soil temperature effect on the pesticide degradation rate, water–air partition coefficient and soil–water partition coefficient, respectively. These dependence relationships, between results of calculating attenuation and retardation factors and the soil temperature at different depths, can aid to understand the potential pesticide groundwater contamination on different weather conditions. Numerical results will be presented with pesticides atrazine and lindane in a soil profile with 20 °C constant temperature, minimum and maximum surface temperatures varying and spreading in the soil profile between −5 and 30 °C and between 15 and 45 °C.