Transport and distribution of manganese in tidal estuarine system in Taiwan

Based on the observed heavy metals in the Danshui River estuarine system, the concentration of manganese (Mn) exceeds the water quality standards. High concentrations of manganese in aquatic environment can cause disturbances in the sodium balance, disturb the metabolism of carbohydrates, and impair the immunological functions of fish. Therefore, a three-dimensional heavy metal transport model was developed and incorporated into the hydrodynamics, salinity, and suspended sediment transport model to evaluate the concentration distribution of the heavy metal manganese (Mn) in the Danshui River estuarine system of northern Taiwan. The model was validated with observational data for water level, tidal current, salinity, suspended sediment concentration, and heavy metal (Mn) concentration that was measured in 2015. The indicators of statistical error, including mean absolute error (MAE), root mean square error (RMSE), and skill score (SS), were adopted to evaluate the model performance. There was good quantitative agreement between the simulation results and measurements. Sensitivity analysis of suspended sediment and heavy metal transport model was carried out to understand which parameters were important to be cautiously determined. Furthermore, the validated model was used to investigate the influence of suspended sediment on the concentration distribution of heavy metals (Mn) in tidal estuaries. If the suspended sediment transport module was excluded in model simulations, the predicted results for the heavy metal (Mn) concentration underestimated the measured data. The modeling results showed that the inclusion of the suspended sediment transport module in the model simulations was critically important to the results of the heavy metal (Mn) concentration in the tidal estuarine system in Taiwan.