The influence of water table fluctuations on the volatilization of contaminants from groundwater

Passive volatilization of contaminants from groundwater has to be considered as a possible release process in the quantitative assessment of natural attenuation in aquifers. Previous studies in experimental systems with a stagnant water table suggest that volatilization is limited by slow diffusion and negligible vertical dispersion in the water phase. Water table fluctuations increase vertical mixing and facilitate volatilization by increasing the contact area between contaminated water and soil air. We investigate the influence of water table fluctuations on the volatilization of halogenated compounds in column studies. Diffusive fluxes are calculated from concentration profiles in the soil gas and compared to the decrease in the contaminant load in the groundwater. Results show an order of magnitude increase of the contaminant concentrations in the soil gas above a retreating water table. Up to several percent of the contaminant load transported by advancing water into the zone above the lowest position of the water table will be retained in the capillary fringe and eventually volatilize. This enhanced volatilization should be considered for shallow contaminant plumes in unconfined aquifers.