LABORATORY DETERMINATION OF WATER AND PESTICIDE PARTITIONING

Laboratory studies provide the best opportunity for studying water and pesticide partitioning for a range of soil conditions while maintaini ng reproducible conditions. We developed and evaluated a laboratory te chnique for determining pesticide partitioning among infiltration, run off, and sediment. Simulated rainfall (44 mm h(-1)) was applied to a s andy clay loam 14 days after pesticide (atrazine, chlorpyrifos, and 2, 4-D) application. The laboratory technique provided controlled test co nditions and reproducible results on the partitioning of rainfall, run off and pesticides at the soil surface and on sediment and pesticide t ransport. Average atrazine and chlorpyrifos half-life (t(1/2)) values determined from corresponding concentrations in the 0- to 1-cm soil la yer were approximately 30 and 10 d, Atrazine, chlorpyrifos, and 2,4-D concentrations remaining in the 0- to 1-cm surface layer decreased log arithmically with rainfall duration. Atrazine and 2,4-D concentrations in the 0- to l-cm layer were correlated positively with those associa ted with splash (S), runoff (R), and sediment yield (E) (R-2 = 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 (R? = 0.10-0.63) were found between chlorpyrifos lo sses and corresponding concentrations in the surface layer even though positive correlations were found between R and E (R-2 = 0,91) and E a nd S-s (R-2 = 0.77), Measured effective partitioning coefficients (K-d eff) increased gradually for each event, and were at least 31 times gr eater than reported K-d 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 lay er are influenced by soil, rainfall, and pesticide characteristics, wh ich will enhance the development of process-based models to predict pe sticide fate and transport for event-based storms.