Dinâmica qualitativa das águas superficiais da bacia do Acaraú e uma proposta de classificação para fins de irrigação

Abstract

During a period of three years surface waters from the Acaraú basin, in the North of Ceará State, were studied for an evaluation of the quality dynamics of the water used in the perenization of Acaraú river and in the irrigation schemes Araras Norte, Ayres de Souza, Forquilha, and Baixo Acaraú. A classification according the hydrochemical type and quality for irrigation, following criteria adopted by the United States Department of Agricultural - USDA and the University of California Commitee of Consultants – UCCC, was made for ten sampling points along the basin. Isotope measurements (18O/16O), electric conductivity (CE), and Cl- concentration were used for studying the influence of evaporation on the waters stored in dams during the dry season and the mixing dynamics of the waters of renewal with water stored in the five major dams of the basin (Edson Queiroz, Araras, Ayres de Souza, Forquilha, and Acaraú Mirim) during the rainy period. A Water Quality Index for irrigation was designed considering the combined risks of salinization, soil infiltration reduction and toxicity for plants. Hydrochemical classification showed predominantly waters of the bicarbonate/sodium type. The dams studied presented differences in the seasonality of the parameters CE and δ18O due to influences from local orography and climate. δ18O showed a strong enrichment during the dry season in dam waters due to volume losses of 30-40% through evaporation. The reservoirs exhibit different behavior, during the period of renovation, with respect to the mixing of waters of renewal with those stored. The dams Edson Queiroz (right margin of Acaraú river) and Araras (left margin), both located in the upper basin, showed strong stratification in CE and δ18O during the rainy season, with the water of renovation submerging to the bottom of the reservoirs due to the lower temperature of the new waters in relation to the stored. Different from other tropical regions, the stratification observed was of long duration (seasonal). Classification of waters for irrigation with the UCCC diagram is more sensitive to the risk of soil permeability reduction than the USDA diagram. According to the Water Quality Index here proposed, the waters of the Acaraú basin present a potential risk, on the long run, for causing problems of sodicity and toxidity, if an adequate management of soil and plants should not be adopted.