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

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Abstract
A natural‐gradient tracer experiment and laboratory batch tests were conducted to study attenuation and transport of atrazine and picloram in an alluvial gravel aquifer. An analytical transport model, AT123D, was used to analyse the field data. Data analysis suggests that there was no retardation and 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 duration (194 days), atrazine concentrations significantly decreased and picloram 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 partially 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 picloram. Rates of atrazine degradation estimated from the data after 2 days (slow rate of decrease) are 3.4 × 10‐3 per day (equivalent half‐life 204 days), and 3.0 × 10‐3 per day (equivalent half‐life 231 days), for total and chemical degradation, respectively, assuming that there was no adsorption after 2 days. No degradation rate could be determined from the picloram data of the batch tests because of its highly noisy data and insignificant decrease in concentrations. The significantly higher attenuation parameters derived from the batch tests compared to those derived from the field data suggests that attenuation of pesticides in the field is affected by other factors, such as aquifer heterogeneity and preferential flow, and flow hydrodynamics etc., which should be taken into account when applying laboratory‐derived values to the field conditions.