Assessment of barrier potential of sediments for selection of waste disposal sites in the Kathmandu Valley

Abstract

The Kathmandu Valley is an intramontane basin filled with fluvio-lacustrine sediments of Quaternary age. The deposits of the valley comprise lake sediments that accumulated due to damming of the Bagmati River system during Plio-Pleistocene time as a consequence of rise of the Mahabharat Range. The northern and north-eastern parts of the basin are occupied by coarse sediments mainly micaceous sand and gravel derived from the Sheopuri gneiss, while in the central and southern part of the valley, these occur very fine sediments mostly black  carbonaceous  clay  with  lignite and diatomaceous earth at places. The maximum thickness of the sediments as revealed by the deepest drill hole in the valley is about 550 m at the central part. The valley floor has a highly undulating topography with buried ridges of Precambrian basement rocks.

The Kathmandu Valley with population of around 1.5 million is witnessing rapid urban growth, unplanned development and environmental degradation leading to increased pollution. Consequently, the valley is confronted with major problem of safe disposal of urban waste produced at a scale of 500 tones per day. This is largely due to lack of awareness and realization at planning and decision-making levels about the importance of geo-scientific information in identification of suitable landfill sites for solid waste management. This paper attempts to deal with the waste disposal problem by preparing a geo­scientific map showing the potential areas for selecting proper landfill sites based on assessment of subsurface sediments in combination with other relevant aspects related to waste disposal.

The assessment of barrier potential of sediments is based on the lithological description of bore holes to a depth of 7 m assuming that clay has a high barrier potential, silty to fine sandy sediments have moderate and sandy to gravelly sediments have low barrier potential. The thickness of low-permeable zone has to be at least 5 m to act as natural barrier against toxic leachate originating from waste to protect the groundwater. The effectiveness of such a barrier against migrating pollutants depends on the ability of this layer to retard or exchange pollutants and fix them into soil complex. This is evaluated by determining the Cation Exchange Capacity (CEC) values of the soil derived from laboratory analysis of samples taken down to 2 m depth. Other important parameters for an estimation of the soil barrier function are percolation rate, soil texture, grain-size and soil depth.

Using GIS ARC/INFO, a map has been prepared delineating areas of barrier potential in three classes: high, moderate and low. The high barrier potential areas are considered as the most favourable sites for waste disposal, whereas the low barrier potential zones are assessed as the negative areas that should not be considered for selecting waste disposal sites. These three groups in conjunction with other selecting criteria (e.g., infrastructures, settlements, water bodies, cultural heritage sites etc.) are presented on a map at a scale of 1:50000. This map provides a good basis for planners and decision-makers for selecting geo-scientifically viable landfill sites for urban waste management.

Based on this map, six sites were selected for further investigation by drilling up to 8 m depth. Taikabu of Chetdol (Bhaktapur) turned out to be the most promising site for development of a sanitary landfill for long-term management of urban waste of the Kathmandu Valley.