AXIAL DISPERSION OF POLLUTANT IN POROUS MEDIA '

Abstract

Ground water is no doubt the greatest and most important natural source of water supply, but increasing industrial, nuclear and domestic wastes being discharged into the aquifers pose a serious problem to its pollution. It was generally f ..Tt that passage of polluted water through porous media would purify itself. However, it has been found that purifying action has its limitation. The pollution of underground waters has attained a serious magnitude on account of its long persistance and slow movement. In the past attempts have been made to investigate the problem of dispersion of pollutant through porous media both in the field as well as in the laboratory. AH these investigation have restricted themselves to the study of salt tracers, though scattered references are also available for the travel of bacteria as well. The results of these investigations are far from uniform. The research reported in present investigation was concerned with the dispersion of chemical and bacterial pollutants through porous medium. The problem has been attempted both analytically as well as experimentally. The experiments were conducted using a lucite column containing porous medium of uniform size, 245 cm long and 20.2 cm square in section. Three sands each having d™ = 0. 6 75 mm, 0. 590 mm and 0. 395 mm with U.C. equal to one were used. The water was supplied to the lucite column from a constant head tank and flow regulated with in the Ill Reynolds number range of 0.1 and 1.5. Sodium chloride was used as chemical pollutant and Escherichia coli as bacterial pollutant for the axial dispersion through the saturated porous media of the column. Steady flow was established in the column before the addition of chemical or bacterial pollutant into the flowing water. Four conductivity probes were located in the column to measure the variation of salt. Coefficient of longitudinal dispersion of salt tracer is an exponential function of Reynolds number, permeability Reynolds number and average velocity of flow. The concentration distribution of the tracer with time could be considered to a Gaussian distribution. These findings are in line with (35, 36, 37, 38, 51 ). However the exponents of the functional relation are somewhat different than obtained by other investigators. The velocity of chemical pollutant was also found to be the same as seepage velocity. The dispersion of bacterial pollutants was different from the salt tracer. When the bacterial poHutant was added to the flowing water, the number of bacteria at a section were found to increase initiaUy but after sometime due to their retention in the sand, the number passing through the section reduced. This was due to the mat formation. The mat formation was associated with head loss in the concerned region due to clogging of the medium. The formation of mat was most pronounced in the top layers. The clogging caused reduction in porosity of medium exponentially. IV The convective dispersion equation of salt tracer which is of a very simplified form, cannot be applied for bacterial travel as the porosity of the medium changed due to clogging. Therefore, a modified form of dispersion equation had to be used. In the unchoked zone the coefficients of dispersion as computed from the modified equation are also an exponential function of Reynolds number, permeabHity Reynolds number and average velocity of flow. The coefficient of dispersion was found to be about 1. 5 times that of salt and the two coefficients found to have a linear regression relationship. In the unchoked zone the rate of head loss was more or less constant indicating flow conditions did not change with time. Analysis similar to that for salt tracer could therefore be applied in the unchoked zone. The variations of the concentration were found to fellow reasonably the Gaussian distribution and the velocity of flow of bacteria was found to be l/l2th of the seepage velocity. The study has lead to a better understanding of travel of bacterial pollutant through the porous media for axial flow. In order to be able to use these results effectively in the field, the radial dispersion characteristics wiH also be required. Also there is a necessity of more v/ork in this field using different types of bacteria likely to occur in the field including the effect of industrial and domestic wastes.