WATER EROSION PROCESSES AND THE ENRICHMENT OF SORBED PESTICIDES .2. ENRICHMENT UNDER RAINFALL DOMINATED EROSION PROCESS

Rainfall erosion experiments were carried out on a clay soil containing high concentrations of organochlorine pesticides aldrin and dieldrin. Soil placed in a 6 × 1 m flume was subjected to simulated rainfall of rate 100 mm/h and average raindrop diameter of 2·2 mm. Samples of sediment transported out of the flume at various time intervals, and samples of soil left behind at the end of the erosion event were subjected to chemical extraction and their pesticide contents measured. Results show that the enrichment of both pesticides in the eroded sediment of all experiments was dependent upon the erosion duration and the size of the eroded particles. A major finding was that fine particles in the eroded sediment had a significantly higher pesticide content than particles of similar size in the original soil. The pesticide content of large aggregates was also reduced during erosion and sediment transportation. Such changes can be explained by raindrop stripping mechanism of large aggregates and the high pesticide concentration typical on the outer layers of stable aggregates. Pesticide concentration of the outer layer of aggregates can remain high compared to the inner part for several years after application, and, in the raindrop stripping process, these layers are broken down into fine particles rich in sorbed pesticides. Time dependency of the concentration ratio of sorbed pesticide shown in the results further supports the occurrence of this raindrop stripping process. The fine particles formed early in the erosion event are highly enriched but enrichment declines with time. The concentration ratio of different particle sizes of the eroded sediment tended, through time, to become similar to that of the soil left behind at the end of the erosion event. © 1993 Academic Press. All rights reserved.