Utilities that treat water containing high concentrations of arsenic (As) face the challenge of preventing As mobilization from residuals. Some treatment plants prepare residuals for disposal to a landfill by placing them in a lagoon and allowing the solids to dry naturally over months or even years. Other utilities use mechanical dewatering to remove water from residuals. In either case, the As in drying ponds must be immobilized to protect public health and prevent recontamination of water sources with As through leakage or infiltration. Furthermore, before residuals can be disposed of in a landfill, they must meet individual state requirements. This study provides insight into the mechanisms of As immobilization, specifically the mechanism by which calcium (Ca) prevents As release. Bench-scale lagoon tests were conducted on residuals from treatment plants. Results indicated that many parameters contribute to soluble As mobilization, but Ca in the form of lime may prove to be an effective method that water suppliers can use to immobilize As in ferric and alum residuals. Includes 15 references, tables, figures.