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Sediment Organic Matter and Physicochemical Properties of a Multipurpose Artificial Lake to Assess Catchment Land Use: a Case Study of Kaptai Lake, Bangladesh

Md. Enamul Hoque
Department of Oceanography, University of Chittagong, Chittagong 4331, Bangladesh

Mazharul Islam
Department of Oceanography, University of Chittagong, Chittagong 4331, Bangladesh

Shyamal Karmakar
Corresponding author
Environmental Science, Institute of Forestry and Environmental Science, University of Chittagong, Chittagong 4331, Bangladesh

Md. Atiqur Rahman
Department of Geography and Environmental Studies, University of Chittagong, Chittagong 4331, Bangladesh

Md. Simul Bhuyan
Institute of Marine Sciences, University of Chittagong, Chittagong 4331, Bangladesh

Abstract/ Executive Summary:

The Kaptai lake, situated in the northeast region of Bangladesh, is the largest reservoir in southeast 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 a 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

Keywords: Organic matter, Sediment carbon, Texture, Sediment dynamics, Reservoir

Location: The Kaptai lake, situated in the northeast region of Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Sediment, Land management

Objectives:

To suggest some suitable sediment parameters as an indicator of land use patterns in the adjacent catchment areas.

Methodology:

Study area The present study was conducted in the Kaptai lake that was created for hydroelectric power generation in 1961 by damping the Karnaphuli river at Kaptai point ( Karmakar et al., 2011 ). The lake comprises two main tributaries and is separated into two different arms by hilly regions in between of this lake —the north-eastern part situated within the upper catchment composed of the Kasalong tributary. The lower south-western part was comprising of the Chengi tributary and termed as the main reservoir. Pristine natural catchment areas characterize the former with limited agriculture activities while the main reservoir area is surrounded by hefty human settlement and urbanization which also visible in the land use map of 2019. The total area of this euphotic lake comprises about 68,800-hectare water body with a mean depth of 9.0 m, and a mean annual water level fluctuation of 8.1m ( Mahmood, 1986 ). The main reservoir area that includes the catchment of Rangamati Pourashava and Rangamati area is the most populous and most dense settlement area in the catchment. On the contrary, the Karnaphuli river area comprised the least population density (55.13 person/km 2 ) and the highest vegetated area in the catchment though there are sporadic concentrated settlements. Sediment sampling points and sample collection Current year sediment deposit samples were collected by using a grab sampler by dividing the reservoir and river section into two segments viz. i) the main reservoir zone where urban areas dominate the catch- ment land-use and ii) the river section of two tributaries and main river segment of the lake. Samples are collected from a relatively straight section of the lake and river channel at ~10 km transect interval. Among the 10-sampling point, two samples were collected from the Kasalong tributary and two samples from the Chengi tributary and the main river Karnaphuli confluence. The local name and geographical location of those ten sites are (1) Beganachari (22°44'46" and 92°18'54"); (2) Dewanchar (22°45'46" and 92°19'83"); (3) Shuvolong (22°42'65" and 92°14'64"); (4) Chan Pang Restaurant (22°39'61" and 92°13'81"); (5) Reserve Bazar (22°39'77" and 92°12'73"); (6) Parachara (22°43'18" and 92°09'99"); (7) 9 No. Barunechori (22°44'37" and 92°16'05"); (8) 10 No. Barunechori (22°45'57" and 92°16'36"); (9) Parjatan (22°37'60" and 92°11'51"); (10) Borodam (22°34'52" and 92°10'69"). To be noted here that 10 No. Barunechori and 9 No. Barunechori sampling points are located in the Kasalong tributary. Dewanchar and Beganachori points are situated on the Karnaphuli river section. Shuvolong is the starting part of the reservoir as well as the confluence of Kasalong tributary with Karnaphuli river. Parachara point is in Chengi tributary and Reserve Bazar points is the confluence of the tributary with the main reservoir Sample analysis Sediment texture analysis was executed following the Hydrometer method (% of sand, silt, and clay) described by ( Huq and Alam, 2006 ), which is a modified method from ( Bouyoucos, 1936 ). The process measures the actual proportions of three sizes of primary soil particles by using a hydrometer based on their settling rates in an aqueous solution.

Source of Information: Environmental Challenges 3 (2021) 100070

Article Link (Online Journal): https://doi.org/10.1016/j.envc.2021.100070

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

This study found significant differences in the Physico-chemical parameters of sediments collected from two tributaries of Kaptai lake, Bangladesh. Those two tributaries represent two distinct sediments de- positional zone also associated with catchment land use. Hence, it can be concluded that the sediment study of this lake could provide significant clues about upland land use patterns and changes. At least a few of the sediment properties, e.g. clay, moisture, OM, and OC showed a clear connection to the land use pattern in the adjacent catchment areas settlement intensity. However, we suggest a detailed study on this lake context to discern the relationship. Intensive settlement in the catchment would have a greater impact on sediment properties, eventually on the reservoir ecology. This sorts of research on sediment are quite crucial for lake and reservoir management, especially considering the sediment de- position, organic matter contents and changes in water level and depths of a lake for sustainable fisheries management. Moreover, the change in the sedimentation process in the reservoir would affect the navigation, fisheries and livelihood of the inhabitants of the catchment, eventually, affect the sustainability of this hydropower lake.

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