Properties and behavior of pesticides in soil

Abstract

The behavior of pesticides in soil and water is regulated by the properties of the compounds and the media, and by climatic conditions. Important properties include ionizability (pKA), water solubility (Ksp), vapor pressure (VP), soil retention (Koc), and longevity (T-l/2). Quaternary N pesticides, organic As and P acid pesticides, organometallic fungicides, dinitroaniline herbicides and growth regulators, metabolites of organophosphate insecticides, pyrethroid pesticides, and very low water soluble nonionic pesticides are strongly retained by soil colloids and relatively immobile in soils. The chemicals have half-lives that range from very short to very long in soils. It is their high retention by soil that keeps them from getting into ground water. Carboxylic acid herbicides and growth regulators, hydroxy acid pesticides, aminosulfonyl acid herbicides, amide and anilide herbicides, carbamate and carbanilate pesticides, fumigants, and highly water soluble nonionic pesticides are weakly retained by soil colloids and are relatively mobile in soils. The compounds have very short to moderate longevity in soils and this lessens their potential for getting into ground water. Basic pesticides, chlorinated hydrocarbon pesticides, phenylurea pesticides, and thiocarbamate pesticides are retained by soil in low to high amounts and have very short to long half-lives in soil. Koc vs. Ksp relationships differ for different chemical families, depending on the ionizing properties and the types of functional groups present. Soil pH regulates the amount of binding and the rate of degradation of ionizable compounds but normally only the rate of degradation of nonionic compounds. Longevity and soil reactivity of the chemicals determines whether or not the chemicals contaminate ground water.