Summary: In surveying state officials across the country, GAO discovered that many states possess at least some of the data needed to assess the vulnerability of groundwater to pesticide contamination. Significant gaps in the data exist, however. Officials in only 15 states reported that data have been mapped in their state for all eight vulnerability factors necessary for doing valid assessments. Even when data are available, they often cover only part of the state and are not detailed enough to use in preparing valid assessments. In fact, no state has been completely mapped at a sufficient resolution for every factor. States need to collect more data if they are to conduct valid groundwater assessments. GAO found that the performance of vulnerability assessment models has been inconsistent. At best, existing models have been shown to predict groundwater vulnerability adequately only in some cases--that is, for some pesticides in some soils. Moreover, the model tests generally have not been done on the subcounty scale necessary to show whether the Environmental Protection Agency's differential strategy is either scientifically sound or economically viable. To be useful for regulatory purposes, both the models and their testing will have to be improved.
