Investigations carried out in the region of Santarém, in an area of 900 km2, outlined the hydrogeological conditions, and the technical and economic aspects for a wise use of groundwater The region within equatorial humid climate has a mean annual precipitation of 1,911.2 mm. The mean annual discharge of São Brás stream in 147.78 km2 of drainage area was 2.17 m3/s and the base flow has the annual volume of 37.0 x 106 m3. The aquifers consist of Quaternary alluvial sands of little areal distribution, and of Cretaceous sands, sandstones, and gravel of the Alter do Chão Formation, which are widely distributed in the whole area. The thickness of the hydrogeological system of the Alter do Chão Formation is 600 m. In the upper part the aquifer is unconfined, with an average thickness of 50 m, and underlying it, confined aquifers with a total thicknesses of 430 m, interbeded with aquicludes and/or aquitards. The hydraulic characteristics have been determined through pumping tests. The hydrodinamic parameters reveals that the transmissivity values range from 130 to 790 m2/day, showing large capacity of groundwater yield. The groundwater quality has shown very low concentrations of chemical constituents in solution, within the standards of potability; whereas the hydrogeochemical characterization classifies the waters into sodium-chloride type, and no dominant type, both for cations and anions. The concentration of the main ionic species in solution show the following relationship: rNa>rCa>rMg and rCL>rHCO3>rSO4. The vulnerability of groundwater range generally from moderate to low, being the lowest values found in the southern part of the area, in the plateau. The groundwater reserves are significant, 226 x 106 m3 as renewable reserves, and 86 500 x 106 m3 as permanent reserves. Besides the magnitude of the reserves, the exploitable resources bear a high potential for induced recharging. The unitary cost of groundwater exploitation in the Santarém region indicates that they decrease with the wells depth in the unconfined and confined aquifers, ranging from R$ 0.0657/m3 to R$ 0.0727/m3, for wells down to 300 m. A comparison between exploitation costs shows that the Santarém groundwater cost ranges from 38.4 % to 42.5 % of the cost for Manaus superficial water, and from 64.5 % to 71.3 % for that in Oriximiná. The confined aquifers are naturally well protected, with excellent conditions for a public water system. There are no restrictions as for the location of wells in urban areas, and no large wellhead protection areas are necessary. The wells must be adequately isolated with upper part of the annular space cemented down to the confining layer. Even considering the favorable superficial water exploitation conditions, the initial expenditures and time spent for the building of the facilities in groundwater exploitation, are much lower. Another advantage of groundwater use is that as the water demand increases the system can be adjusted accordingly to that demand.
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