Utilização dos índices do estado trófico (IET) e de qualidade da água (IQA) na caracterização limnológica e sanitária das lagoas de Bonfim, Extremóz e Jiqui-RN.

Abstract

This work evaluated the sanitary conditions of three surface water bodies: Bonfim, Extremóz and Jiqui, located in the Oriental coast of Rio Grande do Norte State-Norheast Brazil. The results correspond to the period from december/96 to december/97. Three sampling stations were chosen in each pond for a monthly program of sample collection. Some parameters were analyzed to determine the Water Quality Index (WQI): dissolved oxygen, fecal coliforms, pH, BOD5 , total nitrogen, total phosphorous, turbidity, total solids and temperature, and also total coliforms and heterotrohic mesophilic bactérias. The WQI(addictive and multiplicative) classified the Bonfín water as excellent, the Extremóz and Jiqui between excellent and good. The multiplicative method was more restrictive and consequently, more sensitive to reflect values of lower quality in relation to the different parameters. In this study, the variables which influenced the calculation o f WQI negatively were fecal coliforms, BOD5 , total nitrogen, total solids and turbidity. According to thesc indexes, the waters of the three reservoirs could be used for the human consumption, being Bonfim adequate after simplified treatment (simple desinfection), while Extremóz and Jiqui need conventional treatment processes. According to the CONAMA Resolution nr. 20/86, Bonfim pond (2-80 MPN fecal coliforms/lOOmL) would fít in class 1(< 200 fecal coliforms/ lOOmL), since samples presented less than 80 fecal coliforms/lOOmL. The ponds of Extremóz (4-900 MPN fecal coliforms/lOOmL) and of Jiqui (2-2.200 MPN fecal coliforms/lOOmL) they would be classified in the classes 2 (< 1.000 fecal coliforms lOOmL) and class 3 (< 4.000 fecal coliforms;lOOmL), respectively. The Trophic State Index o f Carlson (TSI) and its modified version (MTSI) were also applied in orderâ» evaluate the limnological conditions of these three lakes. MTSI classified Bonfim as oligomesotrophic, Extremóz as mesotrophic and Jiqui as mesoeutrophic. TSI classified Bonfim as mesotrophic, Extremóz as eutrophic-hipereutrophic and Jiqui as mesoeutrophic. These indexes presented temporal and spacial variations, according to the considered parameter. The chlorophyll "a", reduced the índex values in Bonfim and Jiqui and the total phosphorous generated the highest indexes in ali three ponds. The two indexes present variations in relation to the trophic state of a same aquatic body. The TSI can be considered as more restrictive and conservative, resulting in higher numbers, while MTSI absorbs higher values of the parameters, in particular of the total phosphorous. The modifíed version of the index showed more adequate for the trophic state determination of tropical lakes.