Volume estimation of free LNAPL in groundwater: comparison of Pancake Model and Vertical Equilibrium Model results and identification of critical points

LNAPLs (light non aqueous phase liquids) are potential long-term sources of contamination and are the most common and harmful contaminants in groundwater. In the subsoil, 99% of LNAPL is presented as free and residual phases, so it is fundamental estimate the volume of free phase through the application of two different conceptual models: Pancake Model (PM) and Vertical Equilibrium Model (VEM). According to the PM, the LNAPL creates a buoyant pool with uniform and constant saturation on the water table and the thickness measured in the monitoring well is an apparent thickness, which must be correct through the exaggeration factor deduced by baildown tests, in order to obtain the real thickness. The VEM instead, assumes that the LNAPL can penetrate below the water table and the pore fraction occupied by LNAPL is less than 100% due to the presence of air and water, and its saturation varies with the depth. The saturation profiles can be obtained with software as LDRM (distributed by API) which requires information about the features of the LNAPL and the affected aquifer. The two models are been used, together with Thiessen polygons and at grid at regular square mesh (100 x 100 m and 200 x 200 m), to estimate the volume of the free phase existing in an Italian contaminated site. The volume estimation has been carried out considering the type of free product detected (diesel, gasoline and mixtures of gasoline and diesel) and its features, the type of soil and relative effective porosity. The comparison between the results obtained using the two models and different methods to calculate areas, shows that there is a difference of thousands of cubic meter between the estimated volumes. In particular, the VEM estimates lower volumes than the PM probably due to the LNAPL saturation considered in the two models (100% in PM and less of 100% in VEM). In addition, it was found that for both models the estimated volume vary with the different types of area suggesting that the area delimitation is a critical point, which influences the estimation. Other critical points in the volume estimation are the presence of different types of LNAPL and the lack of some site-specific data among which the porosity whose sensitivity analysis has revealed that its reduction provides a reduction of about 20% of estimated volume.