Impacto das condições de escoamento na dinâmica de coliformes em um lago tropical

Abstract

This study investigated the seasonal and interannual variation of total coliforms (TC) and Escherichia coli (EC) in an urban lake in the Brazilian semiarid. Although this lake receives large inputs of untreated sewage from the catchment, it is informally used for fishing and water collection for irrigation. Measurements of meteorological and hydrological parameters revealed that precipitation and flow rate were the controlling parameters influencing coliform dynamics in the lake. Correlations were obtained to describe the lake inlet conditions of flow rate, TC and EC, resulting in relatively high coefficients of determination (R² > 0.60). TC increased with flow rate, while EC slightly decreased. This unexpected behavior for EC was attributed to the counterbalancing impact of surface runoff dilution and additional loading from the catchment. The coliform removal efficiency of the lake was similar to that of facultative pond systems. Correlations were fitted to the experimental data (R² > 0.80) to obtain first-order decay rates of TC and EC as a function of the lake Reynolds number, assuming both the plug-flow and complete-mix models. A general mechanistic-empirical model was used to simulate the seasonal and interannual variation of TC and EC in the lake, with deviations of up to about 33% of experimental data. The results indicated that seasonal variations are more pronounced than interannual variations, with about 90% of the lake exceeding the mandatory water quality standards for EC. Eliminating the dry weather flow rate and loading yielded reductions in this exceedance to 30 and 0% in the wet and dry seasons, respectively.