Mangroves are extremely important from the ecological and socio-economic perspectives, but are degrading at an alarming rate nowadays. Besides direct clearance, hydrological alterations or climatic changes, chemical pollution could be a significant contributor of mangrove degradation. To understand chemical pollution in mangroves, the input and output (mass balance) of pollutants within a region need to be assessed. To identify critical situations that cause high concentrations of contaminants in the mangrove wetland, residence time was defined as the time required for flushing out pollutants from a determined region, and evaluated through numerical modeling. This concept was applied to a pollutant mass balance in the Sungei Buloh mangrove system in Singapore, using actual measured inflow concentrations and hydrodynamic modelling of tide in- and outflows. Delft3D was used to model the hydrodynamics and pollutants (as conservative substance) transport. The model was forced at its offshore boundaries by harmonic constituents and residual flows and also by multiple canal discharges at the landward boundaries. The pollutant transport model provided comprehensive data to analyse the behaviour of flow dynamics with pollutants. Several scenarios (i.e. high and low tides, spring and neap conditions) were assessed for their influence on pollutant dynamics. Residence time was confirmed low during high tide and spring period, and high during low tide and neap period. This study provides a better understanding on the residence time and transport of pollutants due to tidal variation within the mangrove.