A Complexation-Adsorption Model Describing the Influence of Dissolved Organic Matter on the Mobility of Hydrophobic Compounds in Groundwater

Natural dissolved organic matter found in groundwater can bind hydrophobic contaminants to form “complexes” and possibly affect their transport in the subsurface. The mobility of trace organic contaminants in soil originating from non-point source pollution may thus be affected by complexation reactions in the unsaturated zone. In order to predict the possible impact of such interactions on contaminant retention in soil, simulations have been made with a three-site sorption model. Two kinetic rate equations and an equilibrium Freundlich equation are used to describe possible sorption of two species of the contaminant (free or bound with dissolved organic matter) in a soil-water system. The equations governing the contaminant adsorption and the transport are simultaneously solved using an explicit-implicit finite difference technique, under steady-state water flow conditions. An analysis of the model's sensitivity to variations in the “complex” sorption rate constants and the complexation constant, Kc, demonstrated the relative importance of these processes and their effects on the vertical movement of the contaminant in soil. The influence of slow sorption kinetics for the “complex” varies as a function of residence time in the soil column, i.e. the pore-water velocity. Sorption of the “complex”, when it does occur, diminishes the “carrier-effect” of the complexation. If the “complex” is non-adsorbable on soil, the transport of contaminants with Kc values greater than 105 (mol/L)−1 will be significantly affected by dissolved organic matter concentrations typically encountered in soil-water systems. The possible application of the model to chemical transport studies in soil is discussed with respect to our present knowledge of complexation processes that can occur in field situations.