This paper examines the possible use of information on the relative marginal costs of point and nonpoint source water pollution abatement to assess the efficiency implications of shifting a greater portion of the burden for water quality protection to nonpoint sources. The inherent uncertainty about the effects of changes in resource allocation for nonpoint pollution abatement on nonpoint pollution loads is recognized in the analysis. This uncertainty is shown to result in significant limitations on the use of marginal cost comparisons even when point and nonpoint pollutants are perfect substitutes.