Hydrogeologic framework of Pico Island, Azores, Portugal
2001
Cruz, VirgílioJ. | Silva, OliveiraM.
Pico, the youngest island of the Azores archipelago, is composed of basaltic volcanic deposits less than 300,000 years old. The principal aquifer system consists mainly of recent lava flows that are very permeable and whose head 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. Groundwater 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 9.44×10–³ to 3.05×10–¹ m²/s, indicating the heterogeneity of the aquifers. The hydraulic diffusivity, estimated from observations of the effects of tidal fluctuations, also confirms the high permeability of the aquifer system; the average value is higher than published values for other volcanic islands.A mixing process for fresh water and seawater, often coupled with ion-exchange mechanisms, explains the groundwater composition, which is mainly of the sodium-chloride type. The water salinity influences the groundwater quality, resulting in a chloride content that exceeds the recommended chloride limit in 91% of the wells . Water–rock interactions are dominant in the chemical evolution of the perched water, which is characterized by bicarbonate-anion type water.
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