Chapter 17
Evaluating Pesticide Fate and Transport: II. Mass Balance and Tracking
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M . R. Barrett, L . Liu, H. P. Nelson, N. C. Thurman, J. A. Hetrick, L . L . Parsons, J. K. Wolf, and E. Behl Office of Pesticide Programs, Environmental Fate and Effects Division (7507C), U.S. Environmental Protection Agency, Washington, DC 20460
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 M C L for a given pesticide could be much higher than 0.1 μg 1 , the European Community standard for all pesticides. The SSGWM study is designed to provide information on the level of ground-water -1
U.S. government work. Published 1998 American Chemical Society
In The Lysimeter Concept; Führ, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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226 contamination that may arise from the use of a pesticide and thereby determine what use restrictions or other measures might be needed to prevent or otherwise mitigate ground-water contamination (3). USEPA requires these studies when the basic environmental fate studies required for registration indicate that the pesticide and/or its dégradâtes of concern have a combination of mobility and persistence characteristics that could lead to enough leaching under the intended use pattern. Controlled studies conducted in chambers or closed-system monolith lysimeters readily allow for use of radiolabeled pesticide and, as a result, a ready determination of the mass balance (4, 5, 6, 7). However, recovery of the radiolabel still must be followed by extraction, separation, and identification of the residues. Success in accounting for the applied material over the course of the study provides some measure of assurance that one is correctly identifying the importance of both leaching and degradation as dissipation processes. Although the SSGWM study design does not allow for direct mass balance determinations, it does provide a three-dimensional accounting of the pesticide fate in the vadose zone and the upper boundary of the saturated zone using a combination of soil cores, soil pore water samples (from suction lysimeters nested at different depths in the vadose zone), and shallow ground-water samples. Pesticides generally cannot be radiolabeled in open field studies because of legal restrictions. However, to facilitate tracking of subsurface transport of residues, one can use a conservative tracer such as bromide to evaluate the relationship of pesticide leaching to water movement traced by the bromide. This chapter evaluates the extent to which mass accounting can be estimated in SSGWM studies and the implications of taking a mass accounting approach to evaluating the results of such studies.
Case Studies of Pesticide Mass Accounting in Small-Scale Ground-Water Monitoring Studies This section examines results from three SSGWM studies. The datasets used for this paper were generated by random drawings from normal distributions around each of the original data points from the SSGWM studies. The mass recovery data are presented here as total residue recovery (in some cases degradation products are included). In all cases either or both the parent pesticide and its dégradâtes were compounds with high soil mobility (K s much less than 1 ml/g) and high soil persistence (degradation half-lives of a few months or greater). Each of the study sites discussed in this paper had loamy sand or sand texture soils generally with
