Groundwater ubiquity score: A simple method for assessing pesticide leachability

Abstract

Government agencies at both the state and federal levels now face increasing pressures to assess the likelihood of pesticide occurrence in well‒water supplies. Screening methodologies are required in order to determine which pesticides now in use should receive the greatest attention with respect to groundwater, and in order to determine whether elaborate and expensive groundwater testing should be required in order to register a new pesticide. Several screening techniques have been proposed recently, some based on threshold values for critical physical properties of the pesticide, and others based on mathematical models of the leaching process. A different approach is taken in this paper, whereby an index is derived based entirely on the physical properties of those pesticides that have been found either leachable or essentially immobile. The index is based on graphical examination of a plot formed by two widely available pesticide properties: half‒life in soil (tsoil1/2) and partition coefficient between soil organic carbon and water (Koc). Other physical properties, such as water solubility, octanol/water partition coefficient, and volatility from soil, have often been invoked as indicators of leachability, but they are found in this paper to have no useful power in discriminating between “leachers” and “nonleachers.” Scores assigned with the new screening index agree with the results of several recent well‒water monitoring programs, even though point‒source events are thought to be responsible for some of the observed contamination. A nomogram is given that reduces the task of calculating the index to simply placing a straight edge on a diagram.