Caracterização hidrogeológica da região a norte da cidade de Manaus, com base em informações geofísicas (resistividade elétrica), geológicas e geomorfológicas

Abstract

The area covered by this work lies in the region north of the Manaus city, between the coordinates 2o00’ and 3o00' south latitude and 59o30'and 60o30' west longitude, which predominantly exhibits sedimentary rocks of the Alter do Chão Formation, represented by fluvial-lacustrine, clastic and pelitic rocks, from late Cretaceous age. The relief in this region is considerable dissected by drainage, which contrast the hills of rounded tops (platôs), broad valleys, usually limited by steep slopes, where are located small streams. The drainage system, with dominant sub-dendritic pattern, is formed by the basins of rivers Cuieiras, Branquinho, Puraquequara and Preto da Eva, and streams Tarumã Mirim and Tarumã Açu, extending and oriented according to the directions NW-SE to NE-SW and the main tectonic lineaments of the region. The soil texture is ranged from clayey in the plateaus to sandy in the shallows, on which has grow a lush vegetation (Amazon Rainforest), in hot and humid local climate, with average rainfall exceeding 2,300 mm/year. This forest has a major environmental influence, especially in the balance of the regional hydrological cycle, in addition to its role in the process of recharge of the Alter do Chão aquifer, the most important strategic reserve of fresh water in the region. The tectonic processes that influence the local morphology and conditioned the drainages, has also influence on the storage and groundwater flow. The results of the geoelectrical survey revealed the existence of intercalate and discontinuous layers of pelitic and clastic material, which extend and taper north to its boundary with the Paleozoic rocks of the Trombetas Group. The groundwater level monitoring conducted in the Preto da Eva River, over a hydrological year period (2010/2011), changed the order of 7.0 meters (volumetric variation rate of 2.9 mm/year), where the maximum occurred one month after the peak of the rainy season, while the minimum occurred during the rainy season, with a lag of up to 4 months of peak rainfall. This long delay period corresponds to the period in which the aquifer is recharged, when the drains are fed by basis flow. A preliminary estimate points to that portion of the Alter do Chão aquifer, a stock of about 130 km3 of water of good quality, available for use. However, maintaining this value depends basically on the sustainable use and occupation of these lands, considering the limits of deforestation, the local geomorphological conditions and the sustainable use of that portion of the Amazon rainforest.