Urban impact on water quality of a coastal catchment in Brazil

Rapid and unplanned urban development has become critical to urban water resources in developing countries. In general, water quality degradation of rivers and ecosystems are result of lack of sewage treatment and urban management failures. In Brazil, the sanitary sewage system and the stormwater system are separated, and irregular connections of sewage to the pluvial network are common. In this context, it is fundamental to monitor water quality to understand how rivers are being modified by urbanization and, in the future, to propose measures for environmental recovery and for regulation of sanitation. Conceição Lagoon (CL), situated in the south of Brazil, is the largest lagoon system (21 km2) of Santa Catarina state. Similar to other regions in Brazil, water quality is degraded as a result of the urbanization of its basin. Thus, this study analyzed the influence of urbanization on the water quality of the largest sub-basin affluent to CL, the João Gualberto (JG) Basin (9.92 km2). Five field work were conducted, with measurements in loco and laboratory analysis, in two sample sites: upstream of urbanization (contribution area equal to 0.17 km2) and downstream of urbanization (contribution area equal to 6.07 km2). Were analyzed parameters capable of indicating contamination by sewage; chlorophyll-a; total coliforms (TC); fecal coliforms (FC); biochemical oxygen demand (BOD); total phosphorus; dissolved oxygen (DO); pH; ammonia, nitrate and nitrite. In addition, flow measurements to calculate the nutrient loads arriving in the lagoon and the trophic index (TRIX), which characterizes the trophic conditions of water bodies. Results were evaluated in regards to the resolution of CONAMA 357/05, which defines the maximum concentrations of water quality parameters for water bodies in Brazil. Chlorophyll-a concentrations were generally low. In regards to fecal coliforms, high values of 2419.6 MPN for TC were found at all points, with a maximum of 21.8 MPN of FC in upstream and 1046.2 MPN in downstream. Lower values of BOD, phosphorus, nitrite, ammonia and nitrate were obtained for the most upstream monitored point, and the highest values for downstream. The downstream of the river exceeded the limits in 0.110 mg/L N-NO2, 11.548 mg/L N-NO3 and 0.18 mg/L P, considered as the main sources of nutrients for the eutrophication process. DO values decreased from upstream to downstream, remaining within the limit such as for the pH. Differences in concentration of all parameters analyzed, at the upstream and downstream points, indicate domestic sewage releases, as it passes through urbanization. In relation to TRIX, the river presented an oligotrophic state in its upstream and eutrophic state for its downstream. The research allowed to confirm the JG river, the main tributary of the lagoon, contributes to the release of loads of nutrients and, consequently, to the eutrophication process of CL, expanding the understanding of the influence of surface runoff from the basin on the hydrodynamics of the lagoon.