A method for identifying and preventing the diving plumes in alluvial aquifers

The phenomenon of “diving plumes” frequently occurs in some particular hydrogeological settings where the groundwater recharge flows into the top of a shallow water table aquifer and, once inside the aquifer, this water begins to move downwards. Due to the preferred horizontal flow path, it can push contaminant plumes downwards up to the deepest aquifers. Therefore, especially in urban areas, where the water table depression is more pronounced, the pollutants are conveyed to the most important groundwater resources for human consumption, often causing a severe decay of the drinking water. This paper thus deals with the most reliable criteria to delimit the areas in which the hydrogeological settings expose the aquifers to the deepening of the contaminants. To this end, this paper considers several contaminations of alluvial deposits in the Functional Urban Area of Milan. It analyses the hydrogeological setting and uses a mathematical model to highlight the deformations of the flow net produced by each of them. In this way, it was possible to identify the structures that facilitate the deepening of pollution and to set up an appropriate monitoring network that highlights the location of the plumes and allows their attenuation.