WATER EROSION PROCESSES AND THE ENRICHMENT OF SORBED PESTICIDES .1. ENRICHMENT MECHANISMS AND THE DEGRADATION OF APPLIED PESTICIDES

Enrichment mechanisms and disappearing rates of organochlorine pesticides were studied using well-aggregated clay soils with a past history of pesticide applications. Heptachlor labelling experiments revealed that a concentration gradient within the soil aggregates is established upon the application of a pesticide, with the concentration in the surface layers being substantially higher than in the inner parts. Antecedent water content of treated aggregates did not change this feature, the concentration gradient being similar whether pesticide was applied to wet or dry aggregates. Removal of such a chemically rich outer layer of aggregates by rain erosion through the raindrop peeling mechanism resulted in the enrichment of pesticides in the eroded sediment, and this enrichment was specially pronounced for the first few erosion events after pesticide application. Such a peeling process results in an overall decline with time in the pesticide content of the soil. However, because the concentration gradient within the remaining parts of aggregates still prevails, the pesticide enrichment ratio of eroded sediment, though decreasing, continues to be higher than unity for several years after pesticide application to soil. Disappearing rates of dieldrin and aldrin from the soil were measured and found to follow a first-order rate equation. The equations fitted to the collected data predict longer persistence periods in the soil for both dieldrin and aldrin than previous estimates given in the literature. Results of aldrin and dieldrin analysis carried out on some other soils of the region are in agreement with the values obtained by using equations obtained in this study. The relative contribution of organic and non-organic constituents of soil to its sorbed pesticide content was measured. Organic matter adsorbed higher concentrations of aldrin and dieldrin than the non-organic fraction, but this fraction released its pesticide content at a faster rate than the non-organic fraction. Organic matter as a pool for sorbed pesticides in the soil is more important in the short term than in a longer duration after pesticide application. Organic matter shows a higher affinity to adsorb aldrin than dieldrin, thus affecting long-term degradation and persistence of these two pesticides. © 1993 Academic Press. All rights reserved.