Hydrochemical data collected during a field study for a landfill leachate contaminated aquifer and corresponding laboratory experiments suggest that the natural attenuation of phenoxy acid herbicides such as mecoprop (MCPP) is confined to the transition zone between the anaerobic core of the leachate plume and the overlying aerobic water. Vertical transverse hydrodynamic dispersion and the oxidation of organic carbon is thought to control the position of this zone. Numerical modeling was carried out to evaluate this conceptual model and to quantify both the rates of physical and chemical processes as well as their interactions. The modeling study included a first step in which a three-dimensional chloride transport model was used to identify the governing flow and nonreactive transport/mixing processes and a subsequent step in which a two-dimensional, cross-sectional reactive multi-component transport model was used to simulate reactive transport.