Seasonal variations on characteristic hydraulic times in Guanabara Bay, Rio de Janeiro, Brazil – Part 1: Water Age vs. Time of Renewal Rate of 50%

Urban development has modified the water quality of Guanabara Bay, one of the most important estuarine systems of Brazil, located in Rio de Janeiro, because most of its tributary rivers receive considerable quantities of domestic sewage. Therefore, it is important to understand how the water renewal capacity of the system behaves for different scenarios. This knowledge is relevant for environmental monitoring programs of environmental agencies.

In this concern, this work aims to study the influence of environmental seasonal changes on water renewal at Guanabara Bay through the analyses of three characteristic hydraulic times widely used in the literature: Residence Time, Time of Renewal Rates and Water Age. Altough all of them are space and time varying functions, the Water Age concept was adopted as a reference for comparison with the others because of its versatility.

This Part 1 addresses the Water Age (W_A) and the Time of Renewal Rate of 50% (T_RR50%). We discuss the methodologies to calculate these functions through a well-tested environmental hydrodynamic modelling system; and present respective analyses in Guanabara Bay. The simulations considered two scenarios, a rainy summer and a dry winter. Both include external effects of astronomical tides, storm surges, coastal currents, local winds, salinity gradients, and variable discharges from tributary river. The influences on water exchanges of each external effect are analyzed.

Comparing the two scenarios, the T_RR50% in the central area of the bay ranges from 60-90 days during summer-winter conditions, whereas for the W_A varies from 60-105 days. Both analyses represent the same water renewal conditions, indicating that the central part of the system is more vulnerable to kinetic reactions associated to water pollution. The divergence between the values points out to the existing differences between the W_A and T_RR50% concepts and computation methods. A good interpretation of the results of the W_A and the T_RR50% shows that during the summer, the northern part of the system presents a higher water renewal due to the highest river flows. During the winter scenario, the southern part presents a higher water renewal due to the presence of higher and more frequent storm surges with stronger coastal currents.

This work contributes to the understanding of water mixtures in Guanabara Bay. It demonstrates how characteristic hydraulic times in general, and the concept of Water Age in particular, together with computational modeling, can assist the environmental management of Guanabara Bay.