Floodplain sedimentation in nutrient and heavy metal transfer in Jamuna and Padma rivers

Four sites in the Jamuna and the Padma rivers and their floodplains were selected for detailed characterization of river water, suspended sediments and soil samples, collected before, during and after the flood season of 2002. Results suggest that river water quality changes significantly with time. Total solids and suspended solids as well as turbidity of river water reached their peaks during flood season. However, total dissolved solids, electrical conductivity and hardness of river water samples collected after flood were higher than those collected during the flood season. River water samples were found to be more alkaline after flood. Sedimentation during flood appears to play an important role in regulating floodplain soil characteristics. Floodplain soils became more alkaline at top layers after the flood season, which indicated new sedimentation. In floodplain soil, all the micronutrients either decreased or remained virtually unchanged after flood season. Among the macronutrients, nitrogen, potassium and magnesium in floodplain soil decreased by 60%, 4.1% and by 4.2%, respectively; while phosphorus and calcium increased by 41% and 83%, respectively after flood season. Copper, iron, manganese and zinc in floodplain soil decreased by 17%, 2%, 5% and 7%, respectively. Increase in phosphorus content at the topsoil may be attributed to sedimentation, since suspended sediments during flood showed very high phosphorus contents. Arsenic, chromium and lead contents of the floodplain soil increased after flood by 31%, 25% and by 30% on an average, respectively, possibly due to partitioning of these heavy metals from the aqueous (river water) phase to soil e.g., by adsorption. Results of batch experiments, which were conducted to simulate the reducing condition in the floodplains during inundation by floodwater, suggest that such reducing condition may have a significant impact on soil as well as river water quality. For example, decrease in nitrogen content at the topsoil of floodplains and increase in dissolved nitrogen in river water were found to be consistent with the results of batch experiments. Estimates made in this study suggest that huge quantities of nutrients and heavy metals are transported through the Jamuna and the Padma river systems, both in dissolved and suspended forms.