Factors Affecting Agricultural Chemical Losses to Surface and Ground Water Resources

The major chemical factors determining concentrations and losses of agricultural chemicals from cropland are their persistence and adsorption to soil. The major hydrologic factors are rate and route of infiltration. The major management factors are the rate, method, timing, and choice of applied chemicals; cropping; and tillage system. To determine the Best Management Practices (BMPs) to reduce chemical losses, their mechanisms of interactions with the soil and management practices must be understood. Depending on soil adsorption, pesticides and nutrients can be mostly lost with surface runoff water, sediment, or water percolating out of the root zone (water which may return to the surface through base flow or artificial subsurface drainage). Erosion control is a BMP for strongly adsorbed chemicals. For moderately adsorbed chemicals, soil incorporation is a BMP that reduces the amount of chemical in the thin surface soil mixing zone and reduces surface runoff losses. BMPs that enhance infiltration also reducesurface runoff losses. The route of infiltration (e.g., through macropores ) can allow chemicals to percolate through the root zone more quickly than normally expected, resulting in concentration spikes . However, for moderately adsorbed chemicals, leaching losses are usually much lower than with surface runoff. For weakly or non-adsorbed chemicals, primarily nitratenitrogen (N03-N), increased infiltration may increase losses. BMPs that reduce the rate or improve the timing of nitrogen (N) applications relative to crop requirements can reduce leaching losses. N03-N concentrations are usually much lower in surface runoff than in drainage from the root zone; however, if root zone drainage dominates surface water resources, concentrations in excess of the drinking water standard are likely. In the following sections, using atrazine and N03-N as main examples, the importance of chemical, hydrologic, and management factors on concentrations and losses with surface runoff and shallow subsurface drainage will be further discussed. A major new project underway to further our understanding and develop and refme additional BMPs will be discussed