Evaluating Pesticide Fate and Transport: II. Mass Balance and Tracking

Abstract

The U.S. Environmental Protection Agency (USEPA) has relied on the small-scale prospective ground-water monitoring (SSGWM) study to evaluate the ground-water contamination potential of mobile and persistent pesticides for a number of years. Unlike in monolith lysimeter studies, mass balance of the applied pesticide cannot be determined in open field studies (such as the SSGWM study) without making assumptions about the distribution of residues in the subsurface environment. However, the recommended vadose zone pore-water and saturated zone ground-water sampling scheme in SSGWM studies may facilitate an approximation of mass balance of many pesticides with high leaching potential for an extended period. In one example, the mass of pesticide residues (including degradates) in ground water and the lower part of the vadose zone nearly two years after application represented the majority of the originally applied material. This high mass balance in a field study can be attributed to a combination of adequate sampling design and a high environmental persistence of pesticide residues. Open field studies like the SSGWM study and closed-system studies like the monolith lysimeter studies can be used together to provide a more complete picture of how leaching amounts relate to the level of ground-water contamination that may occur and how much mass of the pesticide is likely to leach under a variety of conditions. In the United States, regulations on pesticide use are designed to prevent ground-water pollution and to protect human health. The most recent legislation requiring regulation of the ground-water and surface-water impact of pesticide use is the Food Quality Protection Act of 1996. This Act requires that the Agency specifically determine concentrations of pesticides that may occur in drinking water as a part of dietary exposure assessments (7); the Agency must take action to ensure that pesticide dietary exposure will not occur at toxicologically significant levels. When a pesticide is determined to be a potential ground-water (or surface water) contaminant, USEPA must set health-based limits on residues in drinking water (regulatory limits are called Maximum Contaminant Levels, or MCLs) for that pesticide (2). These standards are based on the no-effect and low-effect levels determined in toxicity tests with mammals and therefore an MCL for a given pesticide could be much higher than 0.1 μg 1 -1 , the European Community standard for all pesticides.The SSGWM study is designed to provide information on the level of ground-water U.S. government work.