Sediment organic matter and physicochemical properties of a multipurpose artificial lake to assess catchment land use: a case study of Kaptai lake, Bangladesh

Abstract

The Kaptai lake, situated in the north-east region of Bangladesh, is the largest reservoir in south-east Asia, built by impounding an artificial earth-dam on the Karnaphuli river in 1961 for hydropower generation. It now serves in various ways, including fisheries, inland navigation, tourism, seaport operation, and municipal water supply. This study has been carried out to know the physicochemical characteristics of sediment, focusing on the organic matter of sediment of Kaptai lake. With the assumption of sediment characteristics being a predictor of catchment land-use, the spatial change of sediment properties across the reservoir also discussed. By doing hydrometrical analysis of sediment physical properties, we found that sediment sand, silt and clay contents are 35%-67.5%, 11.75-39.5%, 5.75-45.75% respectively. The bulk density, organic matter (OM), organic carbon (OC), inorganic matter (IOM), pH ranged for 1.72 - 2.48g/cm3, 4.25 - 8.18%, 2.23 - 4.30%, 91. 82-95.76%, and 6.25 - 6.62 respectively. Sediment texture (e.g. sand, clay, and silt) showed significant variations at different sites (p<0.05). Catchment areas with higher urban land-use showed a higher concentration of clay, moisture, OM, and OC contents to the lake's sediment compared to areas with a little settlement. Contrary to these, areas with limited settlement and land use activities have a higher concentration of sand, silt, IOM, pH, bulk density, particle density and porosity. A strong positive correlation (p<0.05) was found between OM to moisture (0.907), OC to moisture (0.907), porosity to particle density (0.799), which indicate their origin of the source is identical. This study suggests that lake sediment can indicate land-use change in the catchment area. The difference in sediment properties reveals that sediment transport from upstream to the main reservoir is not significant, which may reduce the usable lifespan of this reservoir and the navigability of the rivers. Sediment characteristics signify that major sediment trapping/deposition occurred in the main reservoir part as well as in the Chengi tributary.