Hydrogeologic Setting

The highly fractured Ordovician quartzites that form the flanks of the Castelo de Vide Syncline are present in two divergent branches. A group of few wells screened in this aquifer supply a private water plant. The waters have a very low TDS

(always lower than 50 mg/l) with Ca-Na-Cl facies. The recharge of this aquifer occurs trough the quartzites and arkoses. The characterization of their hydraulic parameters is presented in Oliveira [13].

The Silurian and Devonian schists, sandstones and quartzites (schists are largely predominant), with a thickness of more than 200 m, are in direct contact with the underlying Ordovician fractured aquifer. In hydrogeological terms these rocks are of very low permeability, defining both the confining layer of the Ordovician fractured aquifer and the "impermeable" substratum of the carbonate aquifer of Escusa, which represents the uppermost aquifer in this sequence, in the nucleus of the syn-cline (top of the Devonian sequence). A very simplified schematic cross section showing the aquifers in the Castelo de Vide System is presented in Fig. 26.2. The hydrogeology of the Escusa carbonate aquifer was described in Monteiro [9, 10].

The artesian well presented in Fig. 26.2 was built by the private water plant and the quartzite aquifer was found at a 100 m depth and the arkoses at 205 m. Screened only over the entire thickness of the Ordovician quartzites, the hydraulic head at the time of drilling was 15 m above the ground level.

As also can be seen in Fig. 26.2, the carbonate aquifer is located in the bottom of a "U" shaped valley. This is important for the control of the recharge processes related with swallow holes developed near the contact with the schists that are responsible by concentrated recharge. Another recharge process is related to lateral diffusive infiltration from the colluvium deposits existing in some areas near its lateral limits (frequently referred as "allogenic drainage"). This is a secondary process due to the limited extension of these deposits.

In the rare zones where other lithologies contact with the carbonate rocks at lower altitudes, the water transfers towards the adjacent hydrostratigraphic units. These conditions occur only at the NW extreme of the aquifer. During precipitation events, the stream water infiltrates when the limits of the carbonate aquifer are met.

_| Colluvium

[j1^ Carbonate aquifer pj^-J Schists with very low permeability

1^1 Fractured quartizites (aquifer)

Arkoses

Q-J Nisa Granites

Portalegre granites

_| Colluvium

[j1^ Carbonate aquifer pj^-J Schists with very low permeability

1^1 Fractured quartizites (aquifer)

Arkoses

Q-J Nisa Granites

Portalegre granites

Fig. 26.2 Schematic N-S cross-section of the aquifers present in the Castelo de Vide Syncline. The carbonate rocks correspond to the aquifer of Escusa. Note that the scales are approximate and aspects as the thickness and depth of the formations are completely fictive

This is true for the entire stream network, except for the Sever River, which corresponds to the main discharge area of the aquifer. The drainage density over the carbonate aquifer is almost zero, and, in practical terms, infiltration equals the average precipitation values minus the evapotranspiration in addiction to the runoff generated laterally in the low permeability slopes surrounding the aquifer. It seems also that the NW limits of the aquifer, where the streams diverge from the limits of the carbonate rocks, correspond to a zone where water transferences take place from the carbonate aquifer toward the adjacent lithologies. In this area, the carbonate rocks are in contact with granites and tectonized hornfels, having a higher permeability than the schists limiting the aquifer in almost its entire extension (Fig. 26.3).

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