Modelling the seasonal variation in bioavailability of residual NAPL in the vadose zone

A multiphase model (UTCHEM v.6) has been modified to account for boundary conditions present in the vadose zone, as well as adding an analytic solution for the soil temperature profile, the temperature dependence of physicochemical properties, and revised interphase mass transfer correlations. The model also includes biodegradation, which is limited by moisture content, oxygen and pollutant availability and temperature. The model is then used to study the effect of seasonal variations in soil temperature and moisture, driving by actual climatic conditions. A similar Non-Aqueous Phase Liquid (NAPL) spill is simulated under two climates, one appropriate for Mediterranean conditions (mild rainy winters and dry summers) and another one typical of temperate climates with higher rainfall spread throughout the year and longer, colder winters. Based on these modifications, the water table is allowed to rise and fall following seasonal variations in rainfall infiltration and atmospheric temperature. We also allow the exchange of chemicals across the soil-atmosphere interface, which has a significant effect on the predicted rate of disappearance. The results indicate that although biodegradation is an important process, the water table fluctuations and diffusive processes also produce significant transfer to the atmosphere. There is considerable difference in the rate at which pollutant disappears between these two climates, requiring more than double the time for natural attenuation in the temperate climate relative to the Mediterranean climate.