Groundwater Protection:

Measurement of Relative Vulnerability to Pesticide Contamination

PEMD-92-8: Published: Oct 31, 1991. Publicly Released: Oct 31, 1991.

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Pursuant to a congressional request, GAO evaluated the feasibility of differentially protecting groundwater from pesticide contamination based on the relative vulnerability of different geographic areas, focusing on the degree to which: (1) states and counties are uniform in their susceptibility to groundwater contamination; and (2) two common measures of relative vulnerability diverge in identifying areas that are susceptible to contamination.

GAO found that: (1) although the Environmental Protection Agency (EPA) has taken the position that the variation in statewide sensitivity to pesticide contamination of groundwater is too large for uniform state-level management of pesticides, it has not taken a stand on how large the within-state areas to be managed differently should be; (2) since the variability in hydrogeologic vulnerability does not become significantly smaller when moving from the national level to the state and to the county levels, it generally makes no more sense to make distinctions between counties than it does to treat an entire state as a uniform area; (3) the lack of uniformity in vulnerability both between and within counties indicates that EPA risks undermining its differential protection philosophy if it permits states to differentially protect groundwater on the basis of county-level differences in vulnerability; (4) the system EPA endorses would target manifestly different areas for differential protection than a system that included data on population exposure; (5) techniques which provide valid vulnerability assessments at the subcounty level are expensive when applied across an entire county; and (6) it is important for states to explicitly consider the number of groundwater users as a factor in the development of differential pesticide management plans.

Recommendations for Executive Action

  1. Status: Closed - Implemented

    Comments: EPA has provided states with technical guidance documents which will, among other things, address the issue of the geographic scale for vulnerability assessments.

    Recommendation: EPA should provide explicit guidance on how the states should determine the geographic scale at which vulnerability assessments must be conducted to achieve an adequate level of protection.

    Agency Affected: Environmental Protection Agency

  2. Status: Closed - Implemented

    Comments: EPA has incorporated into its guidance documents that population factors are important to consider in targeting geographic areas for protective measures.

    Recommendation: EPA should incorporate a measure of population use as a risk factor in determining which sources require special protection, thus removing the ambiguity on this point which currently exists in the proposed Pesticides and Ground Water Strategy.

    Agency Affected: Environmental Protection Agency

  3. Status: Closed - Implemented

    Comments: EPA will still proceed with its differential management strategy.

    Recommendation: EPA should reevaluate its differential management strategy in light of effectiveness considerations raised by these findings.

    Agency Affected: Environmental Protection Agency


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