Sorption behavior and competition of bromacil, napropamide, and prometryn

Groundwater contamination by pesticides is a problem of major and increasing concern. Detailed field measurements reveal that leaching of pesticides from soils is often faster than predicted using simple models. This discrepancy is likely due to a combination of physical effects (e.g., preferential pathways) and complexities in the chemical sorption process. To address the issue of sorption complexity, a series of laboratory batch sorption studies have been conducted to determine the detailed sorption behavior of three herbicides, bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4(1R,1H)pyrimidinedione], napropamide [N,N-diethyl-2-(1-naphthalenyloxy)propanamide], and prometryn [N,N'-bis(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine], onto a sandy loam soil. The studies were designed to determine the extent of nonideal sorption behavior; specifically non-equilibrium, nonlinearity, sorption hysteresis, and solute-solute interactions. Results showed all three herbicides reached sorption equilibrium quickly, within 2 h. Both napropamide and prometryn were linearly sorbed, while bromacil sorption followed a Freundlich isotherm, with n = 0.92. All three herbicides displayed apparent sorption hysteresis. Bromacil and napropamide sorption decreased in the presence of the other herbicides, while prometryn sorption was unaffected. Contrary to most literature data, napropamide was more strongly sorbed than prometryn in these studies, due to the high pH of the soil. Results suggest that sorption nonlinearity and competition may increase field-scale transport velocities by 10% or more.