Study of the Longitudinal Dispersion Coefficient of Nonreactive Solute through the Rockfill Medium

In many flowthrough patterns from body of rockfill dams or long rock drains with free water surface, modeling of advection and dispersion of pollutants is one of most important issues. In this research with application of sodium chloride as tracer and EC sensors installed inside the rockfill media, experimental breakthrough curves were extracted. Then with analytical solution of unsteady advection- dispersion equation and none linear optimization least square technique the analytical breakthrough curves at the positions of sensors were also determined. The data acquisition instrument consisted of EC sensors, data logger and application software. Two types of rocks with diameters of 1.1 and 1.8 centimeters, two completely flowthrough discharges of 0.26 and 0.37 liters per second and 5 injection mass of sodium chloride 5, 10, 15, 25 and 50 grams were the variables of laboratory experiments. Finally it was observed that against normal distribution of mass which is special and symmetric case of equation, the observed BC curves were asymmetric in raising and falling limbs. They had longer tail in falling limb because of transient storage of tracer inside the rock media and gradually transport towards the downstream. It was concluded that the classical AD equation is suitable for predicting the general trend of BC curves. In many cases it was observed that the peak point in the analytical solution is lower than the experimental one. Average magnitudes of RMSE, CRM, MAE and ME the statistical parameters for simulated concentration data were 1.35, 0.000608, 52.9ppm and 648.8 ppm respectively which showed a good agreement between experimental and analytical data series.