A Methodological approach to assess the dissolution of residual LNAPL in saturated porous media and its effect on groundwater quality: preliminary experimental results

In this paper, we present a simple methodological approach to assess the dissolution behaviour of residual light nonaqueous phase liquid (LNAPL) sources entrapped in saturated porous media and to estimate the actual risk to human health by water ingestion related to their presence in the subsurface. The approach consists of collecting experimental data on the release kinetics through lab-scale column tests and including these data in a modified version of the analytical model used to describe the groundwater ingestion pathway in risk analysis. The approach was applied to different test scenarios using toluene as a model compound and three types of porous media, i.e. glass beads and two sandy soils with slightly different textures. The experimental results showed that the concentration of toluene in the eluted water was far from the solubility value after a limited number of pore volumes. Furthermore, different behaviour was observed for the three types of porous media. In particular, higher residual saturation and a slower dissolution rate were observed for the soil characterized by the finest texture. This behaviour suggests that the release rate is inversely proportional to the total residual saturation due to the reduction in the porosity available for water flow and the permeability of the porous media. Using these data in a modified risk-based model showed that a remarkable reduction of the hazard index related to the water ingestion pathway can be achieved for a relatively high groundwater velocity and a small contamination source.