Hydrogeologic framework of Pico Island, Azores, Portugal

Pico, the youngest island of the Azores archipelago, is composed of basalti c volcanic deposits less than 300.000 years old. The principal aquifer syst em consists mainly of recent lava flows that are very permeable and whose h ead is influenced by tidal fluctuations. Groundwater abstraction is almost entirely by drilled wells. The hydraulic gradient is very low, about 10(-4) , which agrees with observations made on similar volcanic islands. Groundwa ter also occurs in perched-water bodies, but the spring discharge from them is very low, about 10(-3) L/s. The transmissivity of the volcanic rocks ranges from 9.44x10(-3) to 3.05x10 (-1) m(2)/s, indicating the heterogeneity of the aquifers. The hydraulic di ffusivity, estimated from observations of the effects of tidal fluctuations , also confirms the high permeability of the aquifer system; the average va lue is higher than published values for other volcanic islands. A mixing process for fresh water and seawater, often coupled with ion-excha nge mechanisms, explains the groundwater composition, which is mainly of th e sodium-chloride type. The water salinity influences the groundwater quali ty, resulting in a chloride content that exceeds the recommended chloride l imit in 91% of the wells. Water-rock interactions are dominant in the chemi cal evolution of the perched water, which is characterized by bicarbonate-a nion type water.