CONTRIBUTIONS OF MACROPORES AND CRACKS TO PREFERENTIAL FLOW AND CONTAMINANT MOVEMENT

The use of no-till and reduced-tillage management practices for crop production can greatly decrease runoff and losses of soil and agrochemicals in overland flow compared to conventional tillage practices. The enhanced infiltration, however, increases the potential for ground water contamination. Earthworm populations also frequently increase with a reduction in tillage intensity, which suggests that their effects on soil structure and porosity may contribute to the decrease in runoff. In particular, the size and number of Lumbricus terrestris (L.) burrows suggest that they can have a major impact on hydrology. Field research indicates that the amount of rainfall transmitted by L. terrestris burrows increases with storm intensity and is as much as 10% of total rainfall. Laboratory studies indicate that if a heavy, intense storm occurs shortly after surface application of agrochemicals, the water transmitted to the subsoil by earthworm burrows may contain significant amounts of applied chemical, up to a few per cent, regardless of the affinity of the chemical for the soil. Transport can be reduced by an order of magnitude or more with the passage of time or if light rainstorms precede the first major leaching event. Because of movement into the soil matrix and sorption, solutes normally strongly adsorbed should only be subject to significant transport in earthworm burrows and other macropores in the first few storms after application. In the case of fields with subsurface drainage, however, close association of earthworm burrows to the drains may substantially increase the risk of surface water contamination by surface-applied agrochemicals and animal wastes. Likewise, earthworm burrows can connect to subsoil fractures and contribute to rapid water and chemical movement to drains and ground water.