A 2-D STUDY OF DISPERSION OF POLLUTANTS THROUGH POROUS MEDIA

Abstract

A 2-D study of dispersion of pollutants through porous media. The ever increasing magnitude of deliberate or inadver tent subsurface discharge of wastes poses a threat to the quality of ground waters, which often constitute important resources of fresh water supplies. Considerable research has been done on the dispersion of pollutants through porous media. Yet a satisfactory method for the prediction of dispersion of pollutants, underground, is still elusive. Hence there is a good need for extensive laboratory and field studies, as well as development of theoretical models. In this study, the two dimensional dispersion of pollutants through a uniform flow field in a homogeneous, isotropic, porous medium has been taken up. The pollutants for this study have been Sodium chloride, often considered to be an ideal tracer for such studies, and a pure bacterial culture, Echerichia coli Var. I, which is a popular indicator of sewage pollution. The organisms have an inherent decay and are also absorbed on the soil matrix. The experimental facility comprised of a 77.5 cm long, 80 cm high, and 5 cm wide perspex box with suitable inlet and outlets and a supporting structure. The box was filled with sand (d^Q = 0.055 cm, Uc • 1.3) to a porosity of O.46. Horizontal uniform seepages were maintained within a Reynold Ill number range of 0.6 to 2.0, for different runs. Pollutants were injected at constant rates, with step function inputs, at a point, into the model, and the concentrations were monitored with time, from samplings at selected tapping points. The concentrations of Sodium chloride were measured on a conductivity meter, while E. coli concentrations were estimated by the standard M.F. technique. Steady state concentrations and break through curves (concentration vs. time) were obtained at selected tapping points. The available analytical solutions for 2-D dispersion have been used for evaluation of apparent dispersivities for the two pollutants, studied. Theoretical break through curves have been computed with the evaluated dispersivities. Numerical solutions have been obtained by a fortran algorithm, developed with the adoption of ADIP (alternating direction implicit procedure), which ensured good conver gence. Solutions have also been worked out from discretized pulse technique. The experimental, analytical and numerical solutions for the two dimensional dispersion of the pollutants have been compared, for the selected parameters of this study.