Laboratory study on the effects of tracer concentration changes on longitudinal and transverse dispersion coefficients in saturated porous media

Details are given of an experimental study of the effects of changes in concentration of pollution on longitudinal and transverse dispersion coefficients in saturated porous media. A 2D physical model was constructed to enable observations and measurements to be made of pollution transport through a saturated horizontal layer of sand of 0.18 m thickness. Water soluble sodium chloride salt with 5 different concentrations, i.e. 2.5, 5, 7.5, 10 and 12.5 g/l, was used as a tracer. Tracer concentrations were estimated in different time steps by means of an equation, which was derived from the analytical solution of the original 2D Advection-Diffusion equation, involving optimizing longitudinal and transverse dispersion coefficients. The estimated results were compared to the measured laboratory data. According to the results, increasing concentration leads to decrease in longitudinal dispersibility values and increase in transverse dispersibility values. It was found that tracer's diffusion speed in the longitudinal direction is more than its speed in lateral direction. In fact, contribution of advection in transmission of pollution is greater than contribution of diffusion. Based on the results, the usable value of the tracer's longitudinal distribution coefficient in measured conditions and for all the concentrations, mentioned above, is equal to 1/53*1013 meters and the values of the transverse dispersibility coefficients of the above-mentioned concentrations are 27800, 26400, 25800, 25200, 24900 meters. Meanwhile, decrease in longitudinal dispersion was negligible by taking more distance from the tracer release source. However, transverse dispersion values increased with taking more distance from the tracer release source.