Processos determinantes da qualidade das águas subterrâneas da Formação Jandaíra na Chapada do Apodi/CE

Abstract

The Jandaíra Aquifer, located in the Apodi Plateau in Quixeré, Ceará, is used predominantly for irrigated agriculture and is an aquifer of nature karst and is unconfined. In the area was found contamination by human activities and due to high heterogeneity and anisotropy of permeability, rain infiltration differs from one point to another, resulting in different concentrations in the same aquifer. Was realized monitoring of the static level and the volume produced in 11 wells using datalogger. Six campaigns were realized for sampling 17 wells for analysis of major ions and measurements of electrical conductivity and pH; and 15 wells for measuring the presence of pesticides. Geochemical modelling was performed using PHREEQC to determine the molar ratios and saturation transfer to identify the processes which occur in the interaction of water and rock. The results indicate: i) static levels usually accompany seasonality and have different behaviors location to another with respect to recharge, due to the heterogeneous characteristics of the aquifer; ii) the hydrochemical analyzes show that predominate the relations HCO3- > Cl- and Ca2+ > Na+ > Mg2+ > K+, ions of natural origin; iii) NO3- is of anthropic origin, as well as part of K+; iv) 12 different types of pesticides were identified, mostly in the months of May, July, September and November/11, mostly in wells located in the discharge area; v) the Ca2+ ratio versus HCO3- indicates dissolution of carbonate minerals; vi) the geochemical modeling showed negative saturation indices of anhydrite minerals, gypsum and halite, indicating subsaturation solution in all waters of the sampled wells; aragonite and calcite had similar behavior, ranging from subsaturated to saturated; dolomite had a higher tendency to precipitation, especially in the second half, helped by the weather; vii) the increasing concentration of Na+ is due to the cation exchange process which reduces the saturation levels of carbonate minerals; viii) the molar transfers of calcite and dolomite in nine of the simulations indicate in five of them, dissolving condition; anhydrite and halite show evidence of dissolution in two simulations; ix) the simulated results show that the direction of flow is one of the dominant factors in the dissolution process and the cationic and gas exchange.