Effect of Ethanol on Water Flow in the Vadose Zone.

Ethanol is increasingly used as an automotive fuel in an effort to diminish the use of fossil fuels and to reduce the emission of contaminants into the atmosphere. This study is concerned with the potential negative impact of the (accidental) release of ethanol to the subsurface. Spillage and leakage of ethanol may occur accidentally during production of blended fuels, transport, storage, handling, or use. It will often occur in the vadose zone, i.e., from the soil surface to the water table where the soil or rock contains both water and air. Ethanol is highly miscible with water. It will reduce the air-water surface tension and alter the viscosity of aqueous solutions. Therefore, ethanol may affect the transmission and retention of water and dissolved contaminants in the vadose zone. There is an urgent need to quantify this effect. In the present study changes in the hydraulic properties of porous media due to ethanol are examined. In particular, we incorporated the surface tension of aqueous solutions as a function of the ethanol concentration in the water retention function with the “air entry” value. Furthermore the changes in hydraulic conductivity with ethanol content have been quantified through the definition of a function that relates the solution viscosity to ethanol content. The HYDRUS-1D model developed for water flow and solute transport in unsaturated soils was modified by introducing a subroutine that performed the scaling of the water retention curve and the hydraulic conductivity function according to the ethanol concentration. The code was used to model three different scenarios. In the first scenario, ponding and redistribution of pure ethanol in an ethanol-free soil column is compared with ponding and redistribution of pure water. The results show that the amount of pure water that enters the soil is larger than the ethanol and that pure water will move faster than pure ethanol. In the second scenario we compare spillage with no spillage of ethanol at the soil surface followed, in both cases, by a dry atmospheric condition, precipitation, and again dry atmospheric conditions. The ethanol spillage will lead to a higher water content in the upper part of the soil. In the third scenario, we compare water flow in columns with a 10% initial ethanol content (contaminated soil) and without initial ethanol (pristine soil). Both columns are subjected to precipitation (infiltration) followed by dry atmospheric conditions (redistribution). The higher conductivity of the pristine soil causes a rapid infiltration of water during precipitation and advancement of the liquid front during redistribution compared to the contaminated soil.