The volume of groundwater resources in São Miguel is equal to 370 hm3/yr, approximately 21% of the groundwater resources of the archipelago (Cruz, 2004). Despite differences in the volcanic successions observed in the islands, resulting from volcanic eruptions of various types, groundwater occurrence can be generally described in terms of two main aquifer systems: (1) the basal aquifer system, which corresponds to fresh-water lenses floating on underlying salt water, and (2) perched-water bodies, which are usually drained by springs located in the volcanic edifice slopes (Cruz and Silva, 2001). In São Miguel 1100 springs and 26 wells have been identified, the former associated to perched phreatic or leaky aquifers, made of basaltic lava flows or pyroclastic deposits, and the latter associated to basaltic lava flows.