Why can sediment washed away by rain be richer in pesticides and nutrients than the soil it came from?

Details

The study investigated how rainfall detaches sediment and runoff transports it, seeking to explain why eroded sediment can be richer in sorbed chemicals—chemicals held on soil material—than the original soil.

In peeling experiments, researchers removed the outer coats of structurally stable soil aggregates and analyzed chemical concentrations separately in the peeled material and the inner cores. The outer-layer material had higher concentrations of organic matter, nitrogen (N), and six different sorbed pesticides than the inner cores.

In flume experiments, aggregates of the same soil were exposed to constant-rate simulated rainfall for two periods of 30 and 50 minutes. Following this exposure, concentrations of N, organic matter, and pesticides decreased in large aggregates and increased in finer particles.

The results suggest that when raindrops strike stable aggregates without completely breaking them down, they remove the outer coat and turn it into fine particles. These particles are richer in sorbed chemicals than any particle-size fraction of the original soil, including particles of similar size. Producing fine particles through this peeling action yields eroded sediment that is finer than the original soil.

The hypothesis tested was that differences in sorbed chemical concentrations within soil aggregates, combined with raindrop peeling, are one reason eroded sediment is finer and richer in sorbed chemicals than its source soil. Both the peeling and flume experiments supported this hypothesis. Provided raindrop impact does not cause aggregates to collapse completely, stripping by raindrops can be the major cause of chemical enrichment in eroded sediment.