Information obtained in a research survey carried oat by IDESP was used to investigate, groundwater hydrogeochemistry on Marajó island. The samples were collected in both dug and drilled wells. The results of physico-chemistry analyses indicated a wide range of concentrations in the water quality parameters which reflects the hydrogeologic complexity on the area. The largest total dissolved solids concentrations ocuured in the central and northern part of the area while the lowest levels were found in the south, southeast and northwest. The waters were predominantly of the sodium-chloride type followed by the mixed-chloride and mixed-bicarbonate types. The Schöeller semi-logarithmic diagram was used to identify waters with similar chemical characteristics. The variation in groundwater composition along the subsurficial flow path indicated a dissolution phenomenon with the dissolved ions increasing from the active groundwater flushing zone to the zone of very sluggish flow. This resulted in a variable mixture of fresh and brackish groundwater. The reduction of sulfate by oxidation of organic matter,the base exchange and adsorption are phenomena that exert an important control on the water chemistry.