Groundwater is the major source of urban water supply for Belem metropolitan region, which is the second largest capital of the Brazilian Amazon with a population of approximately 2,300 million inhabitants. Because hydrochemistry assessment of groundwater is critical for Belem region urban water planning, over 160 tubular wells were used for the metropolitan water quality study, encompassing two aquifer units - Barreira and Pirabas. The unconfined Barreiras aquifer consists of Pliocene-Pleistocene/ Late Miocene siliciclastic sediments overlying the calcareous Pirabas Aquifer of Miocene age. Salinity is less than 100 μS/cm and pH does not exceed 5,5 in the Barreiras aquifer, whereas those of Pirabas are greater than 100 μS/cm (up to 400 μS/cm) and pH above 7. The contrast in chemical composition can be clearly observed in the Piper and Stiff diagrams. Barreiras aquifer groundwater is dominantly sodium-chloride type and calcium-carbonate is the dominant type of Pirabas Aquifer. Presence of nitrate in the Barreiras shallow aquifers is closely associated with the population density and the scarcity of sewage system. High concentrations of dissolved iron are commonly found in the upper portions of Barreiras aquifer and are related to reduction of iron coating minerals by contaminated waters.