Modelling the Density Contrast Effect on a Chlorinated Hydrocarbon Plume Reaching the Shore Line

Diffused petroleum and chlorinated hydrocarbon contamination was detected in a sandy aquifer below a chemical plant in Southern Italy. The contamination was due to underground leaking tanks and pipes. The site is located near the shore line and is bordered by canals which, in combination with pumping wells, control the groundwater flow direction toward the sea. In this study, a comprehensive three-dimensional flow model was developed and calibrated to simulate the general groundwater flow system and to individuate a flow line. On this latter, a detailed field investigation was performed in order to determine the fate of dissolved hydrocarbons. Depth profiles obtained from multi-level samplers located along the modelled flow line, including measurements of hydrocarbons, all major ions and dissolved gasses, were used to constrain the conceptual model. These data were then included into a two-dimensional transport model in order to verify the efficacy of the hydraulic barrier (HB) in preventing the hydrocarbon plume to reach the shore line. Two different approaches were used in the transport simulation, one accounting for density-dependent flow and the other not. The calibrated models show that the plume length and consequently, the submarine groundwater discharge of contaminants is slightly different for the two approaches. For the simulation not accounting for the density contrast between freshwater and saltwater, the mass of contaminant discharged downstream to the HB was underestimated and also the reconstructed plume geometry was different than the observed. Moreover, the reconstruction of the saltwater intrusion interface (SWII) with the two different approaches was substantially different. This study demonstrates that at field site, variable density processes should be carefully taken into account not only when the modelling is devoted to the reconstruction of the SWII but also when the modelling is targeting the fate of hydrocarbons at sites affected by SWII, in order to provide accurate data on which soundly environmental management of the coastal zone can be based.