Assessing the impact of Floodplain Landfill on Hydrology and Water Quality in the Lower Mekong River Basin by Numerical Modeling and Field Survey

Abstract:

The rapid watershed development, such as landfilling the floodplains, has dramatically influenced the Lower Mekong River Basin. Since the floodplains are of considerable importance for livelihoods and environment in the Lower Mekong River Basin, understanding the effect of the landfill is necessary for future basin management. This study aims to quantitatively evaluate the impact of floodplain landfill on the hydrology and water quality in the Lower Mekong River Basin. To achieve our goal, we identified the landfilled areas using MNDWI, NDVI, and NDSI derived by Landsat data. We then developed the multiple logistic regression model for estimating the probability of landfill occurrence in the study area. The developed model describes the probability of landfill occurrence as a function of the distance from the Phnom Penh city and distance from the river channels. The goodness-of-fit test, such as the Hosmer-Lemeshow test or Nagelkerke coefficient, showed high adaptability (R²=0.89) of the developed model. Finally, we incorporated the effect of the landfill into the flood-inundation and the total phosphorus (TP) transportation model, changing the corresponding elevation and phosphorus loading. As a result of the numerical simulation, the inundation water level increased by 3.8m around Phnom Penh city when 10% of the floodplain in Phnom Penh and Kandal province are landfilled. Moreover, the TP concentration increased by 0.71 mg/L around Phnom Penh city when 10% of the floodplain in Phnom Penh and Kandal province are landfilled. In the water quality field survey in Cambodia, TP reached 2.05 mg/L in the floodplain where the large-scale landfill was carried out using waste, with iron at the factory site 6.54 mg/L. Furthermore, the field survey in Thailand implied that water quality management corresponds to the economic development level.