A shallow aquifer which is very important for the development of the Ponta de Pedras region has been found and studied. Most of this aquifer is semiconfined, excepted in some areas where he can be considered as beeing free. In various wells drilled in the western part of the area and also near Igarapé-Vilar, the aquifer could not be detected. The maximum depth of the top of the aquifer reaches 14m. The depth of the base of the aquifer and its thickness exceeds respectively 34m and 17m in the well at Mangabeira. Its recharge is principally by means of meteoric water and the infiltration rate was estimated to be 106 to 107 m3/day/km2 for the month of February 1977. The effective porosity was estimated to be between 25% and 37% permiting the calculation of a volume of stored ground-water of 250x109 liters. The average Darcy coefficient (K) of the aquifer system is approximately 200/l/day/cm2. The studied ground water has the following physical and chemical characteristics: pH always acid, between 2,4 and 6,7; electrical conductivity between 13 and 2000 μmhos/cm, the most conductive waters being from the region of the basin of the Tijucaquara River; silica with average concentration of 10,4 mg/l; total iron with very low concentration implying soft water in most of the samples; manganese with a mean concentration of 0,15 mg/l; nitrogen and phosphorous with low concentrations. Sodium and potassium showed the best correlation with seasonal precipitation. At the end of the rainy season the ground water shows extremely low concentrations of nearly all elements. During the period of infrequent rainfall and until the beginning of the rainy season, the salinity increases in some areas and this causes the water to be non-potable. The domestic use of these waters is normally limited by the following characteristics: pH too acid; total iron much over 0,3 mg/l with a maximum of 4,0 mg/l; lack of fluorides; excess of chloride in one well but generally very low contents of most chemical elements. The use of these ground waters in agriculture may be limited in some parts of the area where the pH is less than 5; the conductivity too high during the dry season and when the sodium content reaches 83% to 97% (SAR).
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