TR(BR)-7/96-97 : Parameter characterization for solute transport in groundwater

Abstract

Scarcity of surface water and increasing threat of pollution to the surface water bodies, have given rise to more pressure on the groundwater resources. Resulting effects are being experienced in many spheres; such as: threat of continuous depletion of groundwater tables, intrusion of saline water in costal aquifers, more pronounced entry of contaminants from surface to the groundwater reservoirs etc. When the demand of water is on the rise and availability is a limiting factor, the planning strategy should be scientific and efficient management of available resources.

Management of groundwater resources is becoming a complicated task due to overwhelming pressure on the groundwater and changing scenario of groundwater quality (physical, chemical and biological). True picturization of spatial and temporal variation of groundwater both in terms of quantity and quality can only be made once causes of the problem are well understood and addressed properly. Groundwater flow and transport modelling can assist as an aid to attempt such kind of problems.

Hydraulics of groundwater is governed by many bounded and un- bounded factors. To obtain an accurate groundwater flow model, appropriate skill of the modeller is required. Modelling of the fate of contaminant's transport in groundwater further needs more expertise, particularly in understanding the kinetics of flow and transport behaviours of contaminated water. Mixing of constituent in groundwater is another important factor which shape the occurrence of constituents in any precise location at a given time.

A mathematical model for the transport of a solute in groundwater can be developed by taking the mass balance of the dissolved pollutant over a static elementary volume in three Cartesian co—ordinates. Solutions of equation both analytical and numerical are also well established. Analytical solution is only available for one-dimensional case while numerical solutions are available for I-A 2-D and 3-D cases. Despite well established theory of contaminant's transport in groundwater, there is a common tendency of simplification of a 2-D or a 3-D problem to a l-D problem. As a consequence, a good agreement between the observed and computed values do not comprehend in many cases.

The present study is an attempt to evaluate the behaviour of constituents /pollutants moving with groundwater. Three hypothetical examples having relevance to the field conditions have been demonstrated to satisfy the study. Dispersivity, adsorptivity and decay which are the primary parameters affecting the transport phenomena have been considered. The dispersivity which is the main characterizing parameter in transport problems, have been considered for sensitivity analysis. It is also attempted to quantify errors involved when a 3-D problem is simplified to a 2-D problem and a 2-D problem to a 1-D problem.

On comparison of solution of a 1-D transport problem involving advection, dispersion, adsorption and decay solved through analytical procedure and numerical flow/transport model MODFLOW/MT3D, an excellent match was found between the two solution. The effect of varying dispersivity (longitudinal, transverse and vertical) on the concentration profile was also attempted by taking a 2-D and a 3-D case example.