Modeling the Transport of Organic Compounds in Heterogeneous Soil Matrix

ABSTRACT
Heterogeneous unsaturated soil layers comprise both mobile and immobile regions as characterized by different flow velocities of fluid. The movement of pollutants is governed by advection and dispersion in mobile regions, and by diffusive mass transfer between mobile and immobile regions.
In this study, the aim is to estimate the distribution of the first-order exchange rate coefficients that reveals the presence of soil-matrix heterogeneity, and to develop an advection-diffusion mass transport model that includes the distributed first-order exchange coefficients in unsaturated soils. The study also simulate the experimental results from laboratory soil columns and a soil-vapor-extraction (SVE) field site by using different organic compounds as tracers and under different test conditions.
Modeling analyses and experimental results indicate that the including a log-normal distributed or a gamma distributed first-order exchange rate coefficient could explain the organic compounds transport in the vadose zone. Experimental results and model simulation could explain that the tailing phenomenon in chemical breakthrough curves, which signifies a non-equilibrium transport, resulting mainly from a rate-limiting exchange into and out of immobile (stagnant) regions. Model simulation could also explain the rebounding phenomenon and fit the data well by only adjusting the volume fraction of the immobile regions, the averaged radius of the aggregates and the standard deviation of the logarithmic first-order exchange rate coefficient.
The problem of the lack of local geological information while modeling chemical transfer in immobile regions can be solved by using a set of first-order exchange rate coefficients in log-normal or gamma distribution, in which the related parameters are functions of the system length scale, the soil moisture content, and the chemical partition coefficient. Among these properties the system length scale is found the most important site-specific property. The findings from this study enable one to better predict the transport of trace pollutants in a heterogeneous vadose zone based on the properties of chemicals and the soil system. The model will be able to predict the organic compounds moving and distribution situation in soil and groundwater. Also the model will be able to assist the designing of the SVE system and operational strategy, and the prediction of the performance of the clean-up actions.