HYDROGEOLOGY AND HYDROCHEMISTRY OF A PRECAMBRIAN KARST AQUIFER IN A SEMI-ARID REGION FROM BAHIA, BRAZIL

The karst limestone aquifer of the Irecê region in Bahia state is one of the most important agriculture areas in Brazil. In these areas the groundwater is the main source of water supply for agriculture, domestic and industry uses. The karst aquifer extends 9,380 km2, is extensively unconfined, and lies in structurally complex Precambrian carbonate rocks (0.7-1.1 Ga) exposed at ground surface. Well productivity in the Irecê karst quifer ranges from 1 to 120 m3/h (~4 to ~480 gpm) but about 30 percent of the inventoried wells (~1,200) now lie above the water table and are dry.

Groundwater recharge is from several sources, including (a) downward percolation from the limestone surface across the karst plain, (b) cross-formational flow from underlying Mesoproterozoic silicate rocks (1.5-1.7 Ga) exposed in low mountains surrounding the karst basins, and (c) upward cross-formational flow from beneath the karst plain. A few major rivers and their tributaries cross the karst plain and receive much of the natural discharge. The rivers and groundwater drain to the north where the karst basin is bounded by the San Francisco River valley. Variation in precipitation rates and phreatic levels in the aquifer show that withdrawals for irrigation, livestock, and municipal uses account for drawdown of the aquifer's hydrostatic level during the last 20 years.

Data on chemical and stable isotopic composition of groundwater in the karst aquifer suggest that evaporative concentration is significant, but the water budget of the aquifer remains to be worked out. In other hand, high and rapidly increasing concentrations of nitrate and chloride show that water composition is being altered, most likely owing to antiquated waste disposal practices, expansion of agricultural cultivation, and rapid urbanization. Recent aquifer management is not sustainable because of the increasing storage depletion due to groundwater withdrawal and increasing solute concentrations.