An analysis of 70 wells that tap groundwater from depths of up to 260 m in and around the town of Cremona, N. Italy, shows that 50 of them contain more than 10 μg/L of arsenic. Concentrations of As >10 ppb are accompanied by concentrations of Fe ranging from <0.1 to 6 mg/L and high concentrations of NH4 and Mn (<19 and <1.3 mg/L, respectively). The associations suggest that the mechanism of mobilization of As is the reductive dissolution of Fe oxides driven by the degradation of peat, which is commonly found in the aquifer system. Groundwater in the aquifer has a component of downward flow via leakage through aquitards and flow through lateral discontinuities in them. Along these flow paths, As is released by reductive dissolution of Fe oxides in shallow and intermediate aquifers (0-85 m below surface), reaching up to 183 μg/L, and is attenuated (<95 μg/L) at greater depths (100-150 m). Coprecipitation in iron sulfides could play an important role in As attenuation at these depths. The lower As concentration (<37 μg/L) in the deepest aquifer (160-260 m) is less related to the As concentration of the overlying aquifers because the groundwater here has a component of upward flow.