Factors influencing entry of pesticides into soil water

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Abstract
Leaching of pesticides and hence the risk of contamination of ground‐water depends on the physicochemical properties of the pesticide, the properties of the soil and the weather. Lipophilicity is the most important physicochemical property influencing the movement of un‐ionised pesticides through soil. Water solubility is usually only an important factor in leaching for a few moderately polar solids with high melting points. Organic matter content is the most important property of the soil for un‐ionised pesticides whilst the mobility of weak acids depends on soil pH. Permanent anions and weak acids can be very weakly adsorbed and hence might easily reach groundwater. Applications in autumn are more likely to reach groundwater than those in spring because soil temperatures are low and rainfall exceeds evaporation in winter, enabling mobile pesticides to penetrate to subsoils where degradation rates can be very slow. Concentrations of pesticide in water entering subsoils cannot be reliably simulated to an accuracy of better than an order of magnitude because the complex patterns of water flow and the slow diffusion processes of the pesticide are insufficiently understood. The consequences of applying a mobile pesticide to soil where drainage is impeded or where the water table is near the surface need to be anticipated before it is registered for treatment of the soil.