Evaluating pesticide fate and transport: II. Mass balance and tracking

The U.S. Environmental Protection Agency (USEPA) has relied on the small-sc ale prospective ground-water monitoring (SSGWM) study to evaluate the groun d-water contamination potential of mobile and persistent pesticides for a n umber of years. Unlike in monolith lysimeter studies, mass balance of the a pplied pesticide cannot be determined in open field studies (such as the SS GWM study) without making assumptions about the distribution of residues in the subsurface environment. However, the recommended vadose zone pore-wate r and saturated zone ground-water sampling scheme in SSGWM studies may faci litate an approximation of mass balance of many pesticides with high leachi ng potential for an extended period. In one example, the mass of pesticide residues (including degradates) in ground water and the lower part of the v adose zone nearly two years after application represented the majority of t he originally applied material. This high mass balance in a field study can be attributed to a combination of adequate sampling design and a high envi ronmental persistence of pesticide residues. Open field studies like the SS GWM study and closed-system studies like the monolith lysimeter studies can be used together to provide a more complete picture of how leaching amount s 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 .