Evaluation of Site Suitability for Disposing of Solidwaste Using Waste Aquifer Isolation Principle

The primary concern in selecting waste site is the need for isolating the waste site from the fresh water zones, e.g., water table. It implies that there should be an impermeable zone for pollutant migration between any waste site and groundwater regime. The unsaturated zone, otherwise called the vadose zone, that lies between the topsoil and water table forms the most important natural attenuating zone for the migration of leachate. Thus, there is a need to select a site, which has very low permeable and absorbing vadose zone. Selection of an ideal site needs rigorous field investigations, followed by mass transport modelling. In case of scarce resources of money and time, one needs a few faster methods based on empirical approach to identify the disposal site. One such empirical method is to identify a site, which has good attenuation potential for the leachate produced by the waste pile. This is termed as waste aquifer isolation principle (WAIP). This will restrict the contaminant migration to reach the water table within short span of time. This principle makes use of the hydrogeologic criteria using the data related to type of waste and area to be disposed, quantum of pollutant, effective porosity, thickness of unsaturated zone, and vertical permeability of the medium. A computer code WASP, based on the above principle, was used to compute an index to decide upon the suitability of the site.

The above approach was tested in an existing site near Ranipet (125 km southwest of Chennai) in Vellore district. Tamil Nadu. Tamil Nadu Chromates and Chemicals Ltd., (TCC) at Ranipet Industrial Estate produces sodium dichromate and disposes of its solid wastes containing soluble hexavalent chromium (Cr⁶⁺), which is toxic, for the last 20 years in an area of 35000 m² and has contaminated the groundwater regime with high level chromium concentration. It was found through this study that the site is unsuitable for disposing toxic wastes. This approach may be followed as a first step, wherever quick solution is needed with minimum resources, but this cannot and does not replace the need for quantification of pollutant migration through mass transport modelling. This quick method is based on hydrogeological criteria with minimum computational effort, as such it can be employed easily by any field hydrologist.