The importance of 3D geological modelling for MAR planning: An example from the Po valley margin (northern Italy)

Managed Aquifer Recharge (MAR), defined as the intentional and controlled infiltration of water into aquifer, has been successfully applied to enhance groundwater quantity and quality, restore overexploited system, prevent saltwater intrusion, limit land subsidence, mitigate flooding and support other environmental benefits.  

The planning phase and the correct site selection are essential for creating a fully functional MAR system. During this phase, several parameters are usually considered, involving geomorphology, water management aspects, hydrology, groundwater quantity and quality, and aquifer characteristics. However, the latter is often limited to a few hydrogeological features of the aquifer, such as storage capacity, hydraulic conductivity, and lithology.

This study aims to highlight the importance of detailed stratigraphic knowledge of the aquifer system for MAR site selection, through a 3D geological modelling approach focused on understanding the geometries, volumes and the degree of interconnection between aquifer bodies. These data are crucial for identifying areas where subsurface conditions are most favorable for MAR development.

The 3D geological modelling approach is proposed for the Middle Pleistocene–Holocene gravelly fluvial deposits located at the margin between the uplifting Apennine chain and the subsiding Po Plain, in northern Italy. Stratigraphy of alluvial deposits of the Secchia River, one of the main tributaries of the Po River, was reconstructed through a grid of seven stratigraphic cross-sections covering an area of about 650 km², subsequently implemented in a 3D model using the software Leapfrog Geo.

The 3D geological modelling was based on stratigraphic correlations between outcrops, boreholes and water-well data down to 350 m depth, most of which are available in a regional geological database.

Results highlight a cyclic alternation of gravel bodies and mud layers. Mud layers increase in thickness away from the river-valley outlet, whereas gravel bodies show a parallel decrease in thickness and in their degree of interconnection. Moreover, an overall upward increase in thickness and degree of interconnection is observed. Gravels are poorly sorted and clast-supported, with clasts ranging from a few centimetres to half a metre, often showing imbrication. The mud layers, which locally contain thin pebble layers and carbonate concretions, may laterally transition to lens-shaped gravel or sand bodies with fining-upward trends.

Overall, the gravel bodies represent efficient groundwater reservoirs, while the muddy horizons may act as hydraulic barriers to subsurface flow. By integrating stratigraphic results with information on water table fluctuation, it is possible to identify areas where hydrogeological conditions, lithology, and aquifer-body connectivity are most favorable for MAR development.