Madeira is the largest island of the Madeira archipelago, with an area of 737 km2. The island was generated during Post-Miocene times until 6000-7000 years B.P. Groundwater resources are the main source of water supply for the 240 500 inhabitants of Madeira island. Groundwater exploitation is made by means of water galleries, tunnels, wells and springs, corresponding to a total volume of groundwater of 185x106 m3 by year for domestic and industrial use, as well as for irrigation and electrical power production purposes. Recharge volume due to rainfall is not sufficient to maintain the actual balance between recharge and discharge at the basal aquifer, suggesting the fog contribution to recharge.
A quadratic regression model explains the variation of rainfall due to altitude effect. River discharge is the result of rainfall as well as a significant contribution of groundwater and hypodermic discharge.
Due to the geologic setting of the Madeira island the hydrogeological conceptual model includes the following domains:
- Basal groundwater: occurs after a certain depth. Is formed by geological units of the main and ancient volcanic complexes. The Main Volcanic Complex (CP) has high transmissivity values varying between 1.16x10-2 to 2.89x10-1 m2/s, low hydraulic gradients (3x10-4 to 6.4x10-3) and groundwater with low to medium levels of mineralization (electrical conductivities between 100 e 500 µS cm-1). The Ancient Volcanic Complex (CA) has much lower transmissivity values, varying between 2.31x10-4 and, high hydraulic gradients (2x10-2) and highly mineralised groundwater (groundwater electrical conductivity varies from 600 to over 3000 µS cm-1).
- Dike impounded groundwater: In the central part of the island, sub-vertical dikes that produce the lateral impoundment of the aquifer, with adjacent compartments having different hydraulic potentials, divide the basal groundwater level.
- Perched groundwater: It occurs in high parts of the island in relation to low permeability levels. This perched groundwater may be divided in shallow and deeper levels. The former are located at higher altitudes and are characterized by chloride type waters, with the lower mineralizations (electrical conductivity between 33 and 62 µS/cm), temperatures and pH values. These levels are highly vulnerable to climate variations, with water levels varying significantly along the year depending on the rate of recharge. The later, although still located in island highlands are located at lower altitudes, containing waters with higher mineralizations and pH values.
The spatial distribution of the electrical conductivity shows an increase in the water mineralization with depth and with decreasing distances to the sea; however the water chemistry does no show signs of seawater intrusion. The evaluation of groundwater resources shows that recharge is underevaluated because fog precipitation is not included. Due to its importance deeper studies are suggested.