Groundwater flow and microbial contaminant dynamics in low-permeability heterogeneous aquifers: a multidisciplinary study from the Northern Apennines, Italy

Abstract

Low-permeability aquifers hosted in marls, sandstones, and pelites represent strategic but often complex water storage bodies in regions where extensive and highly productive groundwater reservoirs are absent. Understanding the hydrogeological functioning of these systems and their vulnerability to microbial contamination is a key scientific challenge, relevant to many areas worldwide where similar lithological and structural settings occur. In this framework, the area of Varano de’ Melegari (Northern Apennines, Italy) was selected as a representative test site to investigate groundwater flow and fecal contamination dynamics within low-permeability aquifers. The study area, covering approximately 15 km2, includes two formations of the sedimentary Epiligure Succession, the Monte Piano and Ranzano Formations, and hosts numerous wells and springs used for irrigation, domestic supply, and livestock farming. The main objective of this preliminary work was toidentify the processes governing the occurrence and spatial distribution of fecal indicators in such aquifer systems, and to develop a conceptual framework applicable to comparable hydrogeological contexts. A multidisciplinary approach was employed, combining (i) detailed geological surveys, (ii) long-term monitoring of discharge, hydraulic head, and (iii) sampling for bacteriological (fecal indicators), and isotopic (Tritium) analyses. The integrated results provided a preliminary interpretation of the hydraulic behavior of this heterogeneous system. Despite the low permeability of the aquifer, both piezometric levels and spring discharge responded rapidly to rainfall events, with discharge ranging from 0,0 to 0,3 L/s. The hydraulic gradient was consistently high and locally reached values of approximately 50% (i ≈ 0,5), in agreement with the topographic gradient, as expected for low-permeability media. In addition, the low tritium content (0,46–4,51 TU) suggests the coexistence of both faster and slower flow components, as well as mixing between waters associated with groundwater flow paths of different lengths and/or depths within the aquifer. From a microbiological perspective, the rapid response of the aquifer to effective infiltration was further confirmed. However, the spatial and temporal distribution of fecal indicators (Fecal Enterococci) was found to be primarily controlled by local meteorological and microclimatic conditions, particularly by the influence of temperature on bacterial colony growth in the surrounding soils. The results also provide new insights for further investigation in both qualitative and quantitative fields for these kinds of aquifers.