Laboratory experiments were used to identify and quantify processes having a significant effect on molybdate (MoO₄²⁻) adsorption in a shallow alluvial aquifer on Cape Cod, assachusetts. Aqueous chemistry in the aquifer changes as a result of treated sewage effluent mixing with groundwater. Molybdate adsorption decreased as pH, ionic strength, and the concentration of competing anions increased. A diffuse-layer surface complexation model was used to simulate adsorption of MoO₄²⁻, phosphate (PO₄³⁻), and sulfate (SO₄²⁻) on aquifer sediment. Equilibrium constants for the model were calculated by calibration to data from batch experiments. The model was then used in a one-dimensional solute transport program to successfully simulate initial breakthrough of MoO₄²⁻ from column experiments. A shortcoming of the solute transport program was the inability to account for kinetics of physical and chemical processes. This resulted in a failure of the model to predict the slow rate of desorption of MoO₄²⁻ from the columns. The mobility of MoO₄²⁻ ncreased with ionic strength and with the formation of aqueous complexes with calcium, magnesium, and sodium. Failure to account for MoO₄²⁻ speciation and ionic strength in the model resulted in overpredicting MoO₄²⁻ adsorption. Qualitatively, the laboratory data predicted the observed behavior of MoO₄²⁻ in the aquifer, where retardation of MoO₄²⁻ was greatest in uncontaminated roundwater having low pH, low ionic strength, and low concentrations of PO₄³⁻ and SO₄²⁻.