1e:T508,Abstract A mathematical screening model of the pesticide leaching process is used to estimate the potential for a pesticide to reach groundwater at significant concentrations. The model assumes steady water flow, equilibrium linear adsorption, and depth‐dependent first‐order biodegradation and predicts groundwater travel times and residual concentrations that depend on soil and environmental conditions as well as pesticide adsorption and decay constants. When groundwater protection is expressed as a condition that the residual undegraded pesticide mass remaining below the surface layer of soil must be less than a specified fraction of the initial mass added in a pulse application at the surface, the model prediction is shown to reduce to a linear inequality between the organic C partition coefficient K oc and the biochemical half‐life, τ . The screening model is illustrated on 50 pesticides and two scenarios representing low and high potential for groundwater contamination. The calculations reveal a significant dependence on site‐specific soil and environmental conditions, suggesting that regulations restricting pesticide use should take soil and management factors as well as chemical properties into account when screening for groundwater pollution potential.1f:T503,Laboratory experiments have been conducted to study the sorption of chlorinated phenols by sediments and aquifer materials. It is shown that sorption not only of the nondissociated phenols but also of their conjugate bases (phenolates) can occur. At typical ambient concentrations, sorption equilibrium can be described by the equation S = DC, where S = concentration in the solid phase, D = overall distribution ratio, and C = concentration in the liquid phase. In natural waters of low ionic strength (i.e., I ≤ ~10^-3 M) and of pH not exceeding the pK_a of the phenol by more than one log unit, phenolate sorption can be neglected and the overall distribution coefficient may be expressed by D = K_pQ, where K_p= partition coefficient of the nonionized phenol and Q = degree of protonation. K_p may be estimated from the octanol/water partition coefficient of the compound and from the organic carbon content of the sorbent. In the case of tetra-and pentachlorophenol, phenolate sorption usually has to be considered. It is strongly influenced by the organic carbon content of the sorbent and by the ionic strength of the aqueous medium.20:T58a,

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