Flood Water Hydrochemistry Patterns Suggest Floodplain Sink Function for Dissolved Solids from the Songkhram Monsoon River (Thailand)

Abstract

Although important for the eco-hydrological functioning of the floodplain, the interactions between river and floodplain are not well understood, especially for rivers in the tropical monsoon region. To explore the floodplain functioning of a tropical monsoon river system the longitudinal and transverse floodwater hydrochemical characteristics were studied in the Songkhram river (Thailand) and its floodplains. Water samples were taken during the monsoon period from 61 locations in the river and on its floodplain. Analysis of floodwater hydrochemistry revealed a significant decreasing longitudinal trend for most dissolved solids, attributable to geological differences. We also observed a significant decreasing transverse trend from river to floodplain. Nutrient concentrations revealed lower dissolved nitrogen, phosphate, and potassium concentrations on the floodplain than in the river channel, which suggests the floodplain functions as a sink for nutrients and chemical species in the river floodwater. This sink function may be related to the presence of a belt of bamboo separating the river from the floodplain, which seems to acts as a sediment trap and nutrient filter, but this needs to be verified by additional dedicated research.