SORBED CHEMICAL-TRANSPORT IN OVERLAND-FLOW .1. A NUTRIENT AND PESTICIDE ENRICHMENT MECHANISM

Mechanisms of sediment detachment by rainfall and transport by runoff were studied to seek explanation for the enrichment of sorbed chemicals in eroded sediment compared with the original soil. Experiments were carried out in which the outer coats of structurally stable aggregates were peeled away and the peeled material and inner core analyzed separately for chemical concentrations. Results showed that the material removed from the outer layers of the aggregate had higher concentrations than the inner core of organic matter, N, and six different sorbed pesticides. Aggregates of the same soil were exposed in a flume to constant-rate simulated rainfall for two periods of 30 and 50 min. As a result of such exposure, the concentration of N, organic matter, and pesticides decreased for large aggregates and increased for finer particles. It appears that upon impact, raindrops not capable of totally breaking down stable aggregates remove their outer coat, converting it into fine particles that are richer in sorbed chemicals than any size fraction, including those of similar size in the original soil. This generation of fine particles by the peeling action of raindrops yields eroded sediment with finer size characteristics than the original soil. Thus, the hypothesis examined in this article is that the differential concentration of sorbed chemicals within the soil aggregate, coupled with the peeling action of raindrops, is one reason why eroded sediment is finer and richer in sorbed chemicals than the soil from which it originates. This hypothesis was supported by the results of both peeling and flume experiments carried out in this study. Provided raindrop impact does not lead to the total collapse of aggregates, stripping by raindrops can be the major cause of chemical enrichment in the eroded sediment.