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Groundwater salinization in Graciosa and Pico islands (Azores archipelago, Portugal): processes and impacts

Cruz, J.V., Andrade, C.
Journal of hydrology 2017 v.12 pp. 69-87
electrical conductivity, groundwater, hydrochemistry, islands, mixing, normal values, oxygen, seawater, silicate minerals, sodium, stable isotopes, water quality, water salinization, water supply, watersheds, wells, Azores, Portugal
Graciosa and Pico islands, Azores, PortugalSeawater intrusion is a main driver of groundwater salinization in the Azores archipelago. In order to characterize the associated geochemical processes through major-ion and determination of stable (δ18O; δ2H; δ11B) and radiogenic isotopic ratios (δ87Sr) a total of 46 wells were sampled in Graciosa and Pico, as well as 51 wells from other islands.The overall groundwater chemistry is mainly controlled by Cl and Na, which account respectively for 10.4%–46.9% and 16%–39.7% of the relative major-ion content. Mean electrical conductivity (EC) in Graciosa is in the range 308 − 3462μS/cm, while hand-dug wells in Pico are highly mineralized (1758–9732μS/cm). Drilled wells in Pico are in the range of 186μS/cm to 5625μS/cm. Besides mixture with seawater, groundwater chemistry is also influenced by dissolution of silicate minerals which also contributes to water composition. Moreover, 18O and 2H stable isotope data show that a few samples depict an evaporative effect, resulting in heavier isotopic compositions besides mixing with a marine source. About 70% of the wells in Graciosa and Pico exceed the 200mg Cl/L and the EC Portuguese reference values, severely constraining water supply. The impact on water quality is also shown by exceedances of the groundwater threshold values derived for the Azores River Basin District (89%).