Groundwater behaviour in Madeira, volcanic island (Portugal)

Madeira Island is a hot-spot originating from a mantle plume. K-Ar age determinations indicate that the emerged part of the island was generated during Post-Miocene times 6000–7000 years B.P. Groundwater occurs in perched-water bodies, spring discharge from them is high, about 3,650 l/s; in dike-impounded water and basal groundwater. Basal groundwater is exploited by tunnels (1,100 l/s) and wells (1,100 l/s). Hydraulic gradients range from 10⁻⁴ to 10⁻² and transmissivity ranges from 1.16×10⁻² to 2.89×10⁻¹ m²/s, indicating the heterogeneity of the volcanic aquifers. Water mineralisation is variable, and electrical conductivity ranges from 50 to 3,300 μS/cm. There is a difference between groundwater discharging from perched-water bodies (43–201 μS/cm) and from basal groundwater (109–3,300 μS/cm). Groundwater average pH is 7.37, with waters acid to slightly alkaline (6.13–8.4), and generally cold. Nevertheless, a few samples associated with fault zones can be classified as thermal waters.