A field study of the effect of soil structure and irrigation method on preferential flow of pesticides in soil

Recent experimental evidence indicating extreme movement of pesticides in the vadose zone raises concerns about possible groundwater contamination. A series of comprehensive field and laboratory experiments with three herbicides (atrazine, prometryn, and napropamide), two dyes and two water tracers were performed on a Tujunga loamy sand to isolate and examine the transport processes causing the "preferential flow" of pesticides in soil.; The laboratory leaching experiments, which were conducted on repacked field soil, did not reveal any preferential flow, suggesting that solution complexing with soluble organic matter or particulates was not contributing to extreme mobility.; In the second part of the study, a number of field-plot experiments were performed to characterize the downward flow of the three pesticides under various soil water regimes and soil structure. Each experiment consisted of the uniform application of a 0.5 cm pulse of a pesticide mixture solution to the surface of a 1.5 x 1.5 m plot. The plot was then irrigated with 12 cm of water and soil samples were collected and analyzed to a depth of 150 cm. In all, 64 different plots were employed, to study individual as well as interactive effects of such variables as irrigation method, pesticide formulation method and tillage on pesticide transport. A similar field-plot study was also performed with two types of water-soluble and dispersed dyes to examine the flow patterns of solutes under flood vs. sprinkler irrigation and disturbed vs. undisturbed soil surface treatments.; While all three pesticides were expected to remain in the surface layer of the soil, when averaged over all the treatments, 18.8% of atrazine, 9.4% of prometryn, and 16.4% of recovered mass of napropamide were found between the 30-150 cm depth. There were occurrences of preferential flow of pesticide under every experimental condition except where the pesticides in wettable powder form were applied to disturbed plots. This argues strongly that there may be fine preferential flow pathways through which solution may move but particulates may not. As evident by the deep pluming of the water soluble dye in the field, the channeling of water caused by the formation of unstable flow regions is also capable of rapid transmission of a fraction of the pesticide through the soil matrix.