Land disposal of sewage effluent resulted in contamination of a sand and gravel aquifer (Cape Cod, Massachusetts) with zinc (Zn). The distribution of Zn was controlled by pH-dependent adsorption; the Zn extended 15 m into the 30-m-thick sewage plume within approximately 100 m of the source but only 2--4 m into the plume between 100 and 400 m downgradient. A two-dimensional vertical cross section model coupling groundwater flow with solute transport and equilibrium adsorption is used to simulate the influence of pH on Zn transport. Adsorption is described using semiempirical surface complexation models (SCM) by writing chemical reactions between dissolved Zn and mineral surface sites. SCM parameters were determined in independent laboratory experiments. A 59-year simulation with a one-site SCM describes the influence of pH on Zn transport well, with greater mobility at the low pH values near the upper sewage plume boundary than at the higher pH values deeper in the sewage-contaminated zone. Simulation with a two-site SCM describes both the sharpness and approximate location of the leading edge of the Zn-contaminated region. Temporal variations in pH of incoming groundwater can result in large increases in Zn concentration and mobility. The influence of spatial and temporal variability in pH on adsorption and transport of Zn was accomplished much more easily with the semiempirical SCM approach than could be achieved with distribution coefficients or adsorption isotherms. ¿ 2000 American Geophysical Union