Environmental fate of alachlor and metolachlor

Decision-makers, scientists, and the interested public should be informed what future research and education is needed if a strong pesticide regulatory program is imposed. Recommendations are intended to highlight research gaps. Some may be of general concern and apply to many pesticides. A situation that calls into question the value of many of our management decisions, is the lack of good field-scale experimentation and of logical mechanisms for translating and extrapolating laboratory data to field-scale dimensions. Many experiments were not designed to allow application of basic statistical criteria. High costs often preclude sufficient replication in field-scale experiments so that researchers must make the "no-win" choice between doing one investigation well or doing two or three poorly. The following observations about alachlor and metolachlor are provided: Pysicochemical properties are accurately determined. The herbicides' modes of action and plant selectivity have received a great deal of attention, but gaps remain in defining which of three modes of action are most important. Geographic distribution and extent of residue contamination of surface waters is documented, but groundwater contamination is poorly defined. Any groundwater monitoring protocol should limit the investigation based on sound scientific judgment since a nationwide monitoring network cannot be economically justified. Enough data are needed, however, to allow mathematical model development, verification and validation for a diversity of soil, geographic, climatic, and agricultural management conditions. In view of the importance of adsorption in determining the fate of pesticides, improved methods of determining adsorption coefficients (KD) are needed particularly for very low concentrations. The impact of soil aggregation on adsorption/desorption needs to be examined. The role of temperature and water content in adsorption/desorption processes needs clearer definition. Although volatilization is probably of limited concern for herbicides with Henry's Law constants less than 10(-5), better field-scale methods of estimating volatilization are needed. Lack of clear relationships between laboratory and field investigations is particularly acute in leaching studies. Differentiation between transport rates in the root and vadose zones are limited. Methods of sampling water in the vadose zone await innovative technology. Techniques to confine pesticides to the root zone or to retard their movement beyond the root zone are needed. Management strategies must protect groundwater by curtailing movement or reducing pesticide use. Losses of chloracetanilide herbicides through surface runoff and erosion is poorly documented. When erosion control was linked to soil productivity, use of small plot experiments was probably justified. If improved surface water quality is a goal, small plot findings must be extrapolatable to large watersheds.(ABSTRACT TRUNCATED AT 400 WORDS)