Estudo comparativo das características físicas e químicas e da qualidade sanitária de dois ambientes lacustres costeiros do município de Paraipaba-CE, nordeste do Brasil (lagoas das Almécegas e Canabrava)

Abstract

This study was developed at two coastal lakes in the municipality of Paraipaba (on the western coast of Ceará State), which differ for their location: one located in the urban municipality perimeter (Canabrava Lake), and the other one (Almécegas Lake) located in the rural zone (Pedrinhas District). The main objectives of the study were to establish a comparative profile of the physical, chemical, and biological characteristics of those ecosystems, as well as to evaluate the sanitary qualities of the waters and their bathing conditions according to the environmental legislation, associating them to the different degrees of antropic impacts they are submitted to. Thus, the environmental impacts due to the usage and soil occupation of the respective surroundings have been surveyed, and their influence on the spatial-temporal distribution of the parameters analyzed under the rainy season effect. The sample collections were accomplished from January to June, 2004 at four strategically selected places for each one of the lakes. A set of chemical and physical variables was analyzed in order to establish the limnological and metabolic behavior characterization of those two aquatic environments, according to methodologies from APHA et al. (1989 and 1998), and RODIER (1975). Under the bacteriological point of view, fecal contamination indicators (total coliforms – TC, thermal-tolerant coliforms – TTC, and Escherichia coli – EC) have been analyzed, as well as an estimate of the heterotrophic and aerobic bacterial action (heterotrophic and mesophile bacteria – BHM). The Canabrava and Almécegas Lakes are considered of small and medium size, respectively. Both of them are shallow (varying between 2,0 and 4,0 m deep), and have shown great similarity in relation to most of the analyzed parameters, especially water the temperature, pH, turbidity, transparency, “a” chlorophyll, and phytoplanktonic diversity. The largest instabilities have been observed concerning the DQO, DBO5, and total volatile solids variables, in higher values for the Canabrava Lake, evidencing a remarkable contribution of organic matter coming upon a larger soil occupation in the basin. Such behavior is also reflected on the average content of dissolved oxygen (5 mg/L). From the point of view of macro-nutrients (nitrogen and phosphorus), a certain availability of total phosphorus can be observed in both lakes (0,055 mg/L – Almécegas / 0,062 mg/L – Canabrava), however the low values of ionized ammonia and nitrate (2,07 and 0,511 mg/L for Canabrava, and 1,91 and 0,078 mg/L for Almécegas), seem to act as a limiting factor of the euthrofication advance. Regarding the sanitary quality, the results obtained from the coliform counts, have demonstrated that the Canabrava Lake presents lower sanitary quality in relation to the Almécegas Lake, for being located in the urban municipality perimeter, although in a discontinued way; with total and thermal-tolerant coliform values above those allowed by the effective legislation for class 2 water (CONAMA Resolution No. 20/86). Yet, the Almécegas Lake, came up in accordance with the referred resolution, presenting thermal-tolerant coliform values below 1000 NMP/100mL, and total coliforms below 5000 NMP/1000mL. Regarding bathing conditions, both lakes are classified as proper with their waters considered as of excellent quality for primary contact use, once the thermal-tolerant coliforms and a Escherichia coli values were never up to 250NMP/100mL and 200 NMP/100mL respectivelly (CONAMA Resolution No. 274/00). In a general way, the obtained results verify that the pollution level is closely linked to the type and handling of the activities developed in the influence area, and point to the need of measures in order to preserve the Almécegas Lake, and recover the Canabrava Lake, being considered the irregular usage, and the punctual and diffuse pollution that is beginning to reach the latter lake nowadays.