Laboratory Determination of Water and Pesticide Partitioning1

Laboratory studies provide the best opportunity for studying water and pesticide partitioning for a range of soil conditions while maintaining reproducible conditions. We developed and evaluated a laboratory technique for determining pesticide partitioning among infiltration, runoff, and sediment. Simulated rainfall (44 mm h−1) was applied to a sandy clay loam 14 days after pesticide (atrazine, chlorpyrifos, and 2,4-D) application. The laboratory technique provided controlled test conditions and reproducible results on the partitioning of rainfall, runoff, and pesticides at the soil surface and on sediment and pesticide transport. Average atrazine and chlorpyrifos half-life (t1/2) values determined from corresponding concentrations in the 0- to 1-cm soil layer were approximately 30 and 10 d. Atrazine, chlorpyrifos, and 2,4-D concentrations remaining in the 0- to 1-cm surface layer decreased logarithmically with rainfall duration. Atrazine and 2,4-D concentrations in the 0- to 1-cm layer were correlated positively with those associated with splash (S), runoff (R), and sediment yield (E) (R2 = 0.69 to 0.99). Relationships for all pesticide concentrations in the 0- to 1-cm layer and infiltration (INF) were correlated negatively. Relatively poor correlations (R2 = 0.10-0.63) were found between chlorpyrifos losses and corresponding concentrations in the surface layer even though positive correlations were found between R and E (R2 = 0.91) and E and Ss (R2 = 0.77). Measured effective partitioning coefficients (Kdeff) increased gradually for each event, and were at least 31 times greater than reported Kd values for atrazine, chlorpyrifos, and 2,4-D. The methodologies and experimental design described provide a means of evaluating how changes in pesticide concentrations in the surface layer are influenced by soil, rainfall, and pesticide characteristics, which will enhance the development of process-based models to predict pesticide fate and transport for event-based storms.