Pore network model to predict flow processes in unsaturated calcarenites

Abstract

The knowledge of infiltration mechanisms in vadose zone is the key to forecast the components of the hydrologic cycle such as run-off generation and aquifer recharge. Besides, slope stability, settlements and bearing capacity of foundations, and rock weathering are issues in which infiltration processes play an important role. In Apulia and Basilicata (Southern Italy) representative calcarenites outcrops are exposed along both the coastline and internal areas. These calcarenites belong to the Calcarenite di Gravina Fm. (Middle Pliocene-Lower Pleistocene) and are mainly constituted by fine- medium- and coarse-grained packstones and grainstones. The whole geological formation represents an important hydrogeologic unit which controls groundwater recharge and transport of contaminants within a complex, multilayered system comprising a wide and deep aquifer hosted into the Mesozoic basement. A smart analytical and numerical tool based on the pore bundle model conceptualization and the Richards’ equation was developed to predict infiltration and retention mechanism of calcarenites. This work investigated the impact of bimodal poresize distribution on the unsaturated flow from dry to wet conditions obtained through conventional and unconventional laboratory tests and petrophysical characterization, also completed with mercury intrusion porosimetry and image analysis. Laboratory experiments were carried out on medium-grained grainstones sampled at Canosa di Puglia (Tufarelle locality), by means of infiltration tests conducted starting from a different degree of saturation and varying the inlet flow rate. The experimental data were compared with the pore network model prediction. For the rock samples used, the study disclosed that macroporosity mainly affects the propagation of the wetting front and infiltration rate. Thus, the wetting front develops principally during the infiltration of water through the interconnected macropores following the pathways having minimum flow resistance with a gravity driven flow velocity higher than the diffusive flow though micropores.