Master thesis presented by Alberto Lima , 1994, 202 p.
Abstract
Braga area is caracterized by the preponderance of granitic rocks, forming the subject of this hidrogeologic study. These granitoid rocks are biotite essentially rich, present a porphiritic texture or porphiritic tendency and a coarse to medium-fine matrix. The study area corresponds basically to the River Este basin, a tributary of River Ave.
Weathering and fracturaction studies of the granitic rocks are presented and the geomorphology of the area is described. Small morphologic unities which are extremely important for the aquiferous recharge are identified, as well as, the definition of the areas with interesting hydrogeological, potential through the elaboration of maps of fracturation density and drainage density.
A climatic study led to the determination of the area's mean annual values of the precipitation (1723 mm) and temperature (14,2 C), as well as, the estimation of the actual evapotranspiration (613 mm), using the data obtained in the Braga/Posto Agrário climatic station (P=1514 mm).The aquiferous recharge is quantified by the hidrologic and chloride balances. It was obtained a deep mean infiltration rate of 18 %.
The interpretation of the pumping test and the hidrograms concerning eight springs, with recession coeficients between 10-3 and 10-2 per day, allows the definition of a hidrogeologic model of the area, typical of fissured media. The productivities of the impoundings are modest (1-4 l/s) and the transmissivity values (10-30 m2/day) have been obtained by pumping tests.
The analysed water samples are very low mineralized and basically belong to the sodium chloride and calcium chloride facies. The responsible processes for the mineralization are essentially the concentration by the evaporation of rain water and by the dissolution of silicate rich minerals and the introdution of some artificial salts. The equilibrium balance water/rock shows a subsaturation of the waters relatively to all the minerals excluding the silica polymorphs. Concerning these polymorphs the analysed water samples are supersaturated or mean equilibrium. In same places, the anthropic impact in the quality of the underground waters for domestic use assumes high levels, wich are revealed by the amounts of nitrates much higher than the admissible values.