Caracterização hidrogeológica da Bacia do Ribeirão Sobradinho - Brasília (DF)

Abstract

The Sobradinho River basin comprises an area of 144 km² in the north central portion of the Distrito Federal. Groundwater has become an effective alternative water supply for at least 22,500 persons in the basin. The studied area is composed of metasedimentary Proterozoic covered by a thick weathered mantle. Hidrogeologicaly, a Porous Domain and a Fractured Domain have been individualized. The Fractured Domain is classified as the Paranoá and Canastra systems. The Paranoá System is subdivided into four aquifer systems: Metarritmito Arenoso (sandy metarhythmite with a mean yield 26.7 m³/h), Quartzito (quartzite with a mean yield 8.7 m³/h), Metarritmito Argiloso (argillaceous metarhytmite with a mean yield 4.6 m³/h) and Psamo-Pelito Carbonatado (carbonate sandy pelite with a mean yield 15.7 m³/h). As there are no wells in the Canastra System it remains undivided. The main producing fractures of the wells are limited to a depth of 100 m. The Porous Domain is related in the area to the weathered mantle (soil and weathered rock), which reaches more than 100 m in thickness, and is associated with unconfined aquifers of great lateral continuity. The thickness and permeability of the weathered mantle are strongly influenced by the geological basement. The Porous Domain is exploited in the studied area by excavated wells. Of the mean annual rainfall in the area (1,442.5 mm), approximately 28% is involved in the aquifer recharge. The main use of the groundwater, which presents good physical-chemical qualities, is domestic. Bacteriological contamination in water samples from the weathered mantle is related to technical deficiencies in construction of the excavated wells and the proximity of cesspools. Problems with sanitary protection and procedures of deactivation and abandoning of the deep groundwater wells have been recognized.