contaminant transport

Laboratory and field experiments have shown that dispersivity is one of the key parameters in contaminant transport in porous media and varies with elapsed time. This time-dependence can be shown using a time-variable dispersivity function. The advantage of this function as opposed to constant dispersivity is that it has at least two coefficients that increase the accuracy of the dispersivity prediction. In this study, longitudinal dispersivity values were obtained for the conservative NaCl solute transport in a laboratory porous medium saturated with tap water. The results showed that the longitudinal dispersivity initially increased with time (pre-asymptotic stage) and eventually reached a constant value (asymptotic stage). Four functions were used to investigate the time variations of dispersivity: linear, power, exponential and logarithmic. In general, because of the linear increase of dispersivity during a long time of transport, the linear function with R2=0.97 showed better time variations than the other three functions; the logarithmic function, having an asymptotic nature, predicted the asymptotic stage successfully (R2=0.95). The ratio of the longitudinal dispersivity to the medium length was not constant during the transport process and varied from 0.01 to 0.05 cm with elapsed time.

In this paper, a modeling tool for risk assessment analysis of the movement of hydrocarbon contaminants in the vadose zone and mass flux of contamination release into the groundwater table was developed. Also, advection-diffusion-reaction equations in combination with a three-phase equilibrium state between trapped air, soil humidity, and solid particles of unsaturated soil matrix were numerically solved to obtain a one dimensional concentration change in respect to depth of soil and total mass loading rate of hydrocarbons into the groundwater table. The developed model calibrations by means of sensitivity analysis and model validation via data from a site contaminated with BTEX were performed. Subsequently, the introduced model was applied on the collected hydrocarbon concentration data from a contaminated region of a gas refinery plant in Booshehr, Iran. Four different scenarios representing the role of different risk management policies and natural bio-degradation effects were defined to predict the future contaminant profile as well as the risk of the mass flux of contaminant components seeping into the groundwater table. The comparison between different scenarios showed that bio-degradation plays an important role in the contaminant attenuation rate; where in the scenarios including bio-degradation, the contaminant flux into the ground water table lasted for 50 years with the maximum release rate of around 20 gr per year while in the scenarios without including bio-degradation, 300 years of contaminant release into groundwater table with the maximum rate of 100 gr per year is obtained. Risk assessment analysis strongly suggests a need for bioremediation enhancement in the contaminated zones to reduce the contaminant influx to groundwater.