"The dissolution of calcium arsenates in residues released on the premises of an inactive smelter has caused high levels of arsenic pollution in the adjacent down-gradient 6 km of a perched aquifer, reaching up to 158 mg/L of dissolved arsenic, and releasing a total of 7.5 tons of arsenic in a year. We characterized the morphology of the calcium arsenates responsible of this pollution and determined their thermodynamic solubility products by (1) following the dissolution of arsenic from soil samples in batch reactors until equilibrium was reached and (2) modeling dissolution data using the geochemical code PHREEQC. Solubility product calculations took into account soil solution chemistry. Our results provide critical thermodynamic data to better understand interactions between water and arsenic containing minerals in the environment."