Attenuation and transport of atrazine and picloram in an alluvial gravel aquifer: a tracer test and batch study

A natural-gradient tracer experiment and laboratory batch tests were conduc ted to study attenuation and transport of atrazine and picloram in an alluv ial gravel aquifer. An analytical transport model, AT123D, was used to anal yse the field data. Data analysis suggests that there was no retardation an d degradation of atrazine and picloram in the aquifer over a distance of 90 m within a period of 49 h. In the batch tests with a much longer time dura tion (194 days), atrazine concentrations significantly decreased and piclor am concentrations showed a small decrease. It is considered that the rapid decrease in atrazine levels in the first 2 days was sorption-dominated and the slow decrease subsequently was mainly because of degradation and partia lly the result of slow sorption. If it is assumed that the decrease in the first 2 days was all the result of sorption and it had reached equilibrium, the estimated Kd values are 0.04 ml/g for atrazine, and 0.02 ml/g for picl oram. Rates of atrazine degradation estimated from the data after 2 days (s low rate of decrease) are 3.4 x 10(-3) per day (equivalent half-life 204 da ys), and 3.0 x 10(-3) per day (equivalent half-life 231 days), for total an d chemical degradation, respectively, assuming that there was no adsorption after 2 days. No degradation rate could be determined from the picloram da ta of the batch tests because of its highly noisy data and insignificant de crease in concentrations. The significantly higher attenuation parameters d erived from the batch tests compared to those derived from the field data s uggests that attenuation of pesticides in the field is affected by other fa ctors, such as aquifer heterogeneity and preferential flow, and now hydrody namics etc., which should be taken into account when applying laboratory-de rived values to the field conditions.