R. Don Wauchope

Pesticides in runoff: Measurement, modeling, and mitigation

Abstract Agricultural pesticide runoff has been investigated at four scales: laboratory research on processes, microplot and mesoplot studies under simulated rainfall, and field/watershed studies under natural rainfall. Because rainfall and hydrology may be controlled, the intermediate scales (meso‐ and microplots) are becoming accepted techniques for efficiently estimating the risk of runoff of a pesticide as a function of weather, soil, pesticide properties and cropping practice, and as a way to calibrate simulation models. The prediction desired—the probability distribution of both chronic and acute levels of pesticides in aquatic ecosystems and drinking water sources‐depends on a large number of interacting processes and state variables. A complex computer simulation model is required to sort out the possibilities and to define the most important controlling factors. Thus, runoff experiments at different scales and model development and validation are mutually dependent enterprises.

Sources of the Data

The original “ARS pesticide database” was compiled by ARS Soil Scientist Ralph Nash for research purposes but was never published. The data were compiled on paper forms by Nash and, after conversion to electronic records, became the nucleus of this database. Hornsby and Rao and their colleagues of the University of Florida have collected a large amount of data [129,223,234–237]. In addition to the primary literature, excellent compilations of some of the parameters are available, and these publications remain the only source of some values. The Weed Science Society of America Herbicide Handbook [325–327], a result of voluntary industry submissions of information on herbicides, has data on solubilities, vapor pressures, and, in some cases, persistences. The Royal Society of Chemistry Agrochemicals Handbook [245,246], and the British Crop Protection Council Pesticide Manual [32–34] continue the British tradition of pesticide science with a physical-chemical emphasis, giving high-quality solubilities, vapor pressures, Chemical Abstract Service Reference Numbers, molecular weights, and formulas for most pesticides. Trademark, formulation, and detailed use information are available from the Crop Protection Chemicals Reference [47], which is a collection of product labels. However, all manufacturers are not included. The Farm Chemicals Handbook [195,196] is the most complete cross-referenced listing of pesticides new and old and their uses and properties. These handbooks also contain much toxicological, chemical, and other information not covered here.

Database Limitations: Other Information Needs

It must be said that the “screening” procedures that are being used with these data are trustworthy only if they determine that the pollution potential for a specific pesticide site use situation is extremely high or extremely low. More accurate predictions can be made using computer simulation modeling to integrate a much more detailed process description, which includes more information about the properties of the chemical and the use and site situation of concern. Considerable progress is being made in this area [49,61], and it is clear that the adequate characterization of a pesticide’s behavior in the environment (not to mention its toxicology) requires more than six parameters. The Beltsville ARS database [116], which contains the six parameters compiled here plus heats of vaporization, phase transition temperatures, hydrolysis and photolysis rate constants, and specific soil sorption coefficients, is a step in the right direction. Although many of those data are missing, much of that will become available as part of the reregistration process, and the quality and completeness of reporting of that data should be improved as a result of Good Laboratory Practice (GLP) compliance.

Notes on the Database Fields

Generic names have been developed by the pesticide science societies to refer to active ingredient compounds without naming specific products or trade names. Generally we used the International Union of Pure and Applied Chemistry (IUPAC) common name if more than one existed.