Caracterização da vulnerabilidade intrínseca do Aquífero Alter do Chão na cidade de Manaus - AM

Abstract

This work presents the evaluation of the intrinsic vulnerability of the Alter do Chão Aquifer in the Manaus city. The selected method for the determination of intrinsic vulnerability is the DRASTIC, which embraces and correlates the main physical and hydrogeological parameters of the study area. The DRASTIC is defined by a weighted sum of depth to water (D), recharge (R), aquifer media (A), soils (S), topography (T), unsaturated zone (I) and hydraulic conductivity (C). An extensive literature review of existing data and studies of the Alter do Chão Aquifer in Manaus and region was performed, gathering information about geology, geomorphology, pedology, borehole profiles and climate. The average depth to water in the Alter do Chão Aquifer in Manaus exceeds 15 meters; smallest depths are observed close to drainages, whereas deeper levels occur where there is greater groundwater extraction by boreholes. The proposed conceptual model for groundwater recharge indicates natural sources as the main contributors, mainly by infiltration of rain water in areas with dense vegetation cover (963 mm/year), and the anthropic recharge by infiltration of water from mains leakage cesspits (between 153,5 and 652,4 mm/year). For the induced anthropic recharge, it is estimated that about 68% of extracted water that is distributed by the public supply becomes artificial recharge for the aquifer. The sand layers of Alter do Chão Formation are the preferential pathway for groundwater percolation in the saturated zone. The soil types of the area are predominantly sandy loam, in the valleys and slopes, and clay loam, in the plateaus. The unsaturated zone is characterized as intercalations of sandy, silty sand or clayey sand, and pelitic layers. The hydraulic conductivity of the aquifer is mainly lower than 4,1 m/day. The slopes of the area are mainly lower than 21%; high slopes (superior to 12%) occur at restricted areas along ramps in the transition of plateaus and valleys. The obtained results for intrinsic vulnerability show that low and slight moderate vulnerability have predominance in the area and moderate and high vulnerability occur restrictedly in the northwest region of the area. The main factors that generate a higher grade of natural aquifer protection are the deep water level, the soil layer with significant amounts of clay and the lithologic substrate of the unsaturated zone that has a wide range of silt and clay layers. Therefore, despite the low slopes and high recharge rates that could significantly increase the vulnerability, the clay substrates and the deep water levels provide significant intrinsic protection.