Laboratory experiments were used to identify and quantify processes having a significant effect on molybdate (MoO2-4) absorption in a shallow alluvial aquifer on Cape Code, Massachusetts. 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 MoO2-4, phosphate (PO3-4), and sulfate (SO2-4) on aquifer sediment. Equilbrium constants for the model were calculated by calibration to data from batch experiments. The models was then used in a one-dimensional solute transport program to successfully simulate initial breakthrough of MoO2-4 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 MoO2-4 from the columns. The mobility of MoO2-4 increased with ionic strength and with the formation of aqueous complexes with calcium, magnesium, and sodium. Failure to account for MoO2-4 speciation and ionic strength in the model resulted in overpredicting MoO2-4 adsorption. Qualtivatively, the laboratory data predicted the observed behavior of MoO2-4 in the aquifer, where retardation of MoO2-4 was greatest of uncontaminated groundwater having low pH, low ionic strength, and low concentrations of PO3-4 and SO2-4.