Groundwater Age Modelling to Refine Knowledge on the Hydrogeological Functioning of the Shallow Alluvial Aquifer in the Po River Basin

Over the past 30 years, groundwater (GW) age derivation has attracted attention in researchers and professionals to overcome quality-related challenges in the management of aquifer systems. GW ages can be determined following either hydraulic-based techniques or isotope-based approaches. By the term age modelling, instead, reference is made to developed methods that qualitatively couple hydrodynamics with isotope-based ages in order to reduce the uncertainties involved. Here we present an age modelling application to the shallow alluvial aquifer in the Po River Basin. We used the 3H/3Hetri method to assess the aquifer renewability in a portion of the plain where the extremely variable recharge contributions and the intrinsic variability in aquifer geometry, make sometimes water management arduous. Results from 44 wells showed 3H activities ranging from 0.52 to 5.02 T.U.. To use 3He for age computations, the fraction produced by 3H decay (3Hetri) must be separated by other sources: dissolved He in solubility equilibrium with the atmosphere (Heeq), excess air trapped at the vadose zone (Heea) or 3He produced from 6Li (3Herad). The 3He/4He ratios normalized to that of the atmosphere range from 0.30 to 3.25 revealing the dominant presence of 3Hetri with a minor contribution of Herad. The slight Herad contribution to the Hetot identifies older GWs, reaching values of 60±20yr, detected near the piedmont and in the higher plain. The downstream middle plain, instead, revealed younger ages of about 24±2yr. Chronometers also unraveled mixing processes, detailing older GWs in the higher plain as the result of a young component containing 3H and another older than 70 years mostly 3H-free. In the middle plain, oppositely, mixing counts at least two components where the youngest portraits shorter flowpaths able to lower background aquifer age values, particularly where return flow due to intensive irrigation occurs, thus acting as a young recharge component to the aquifer. The age modelling approach turned to be effective in deepening the knowledge related to the hydrogeological functioning of the alluvial aquifer in the Po River Basin, by elucidating the relations between residence time and flow patterns. GW ages interpretation along with information from wells’ lithologs, screens locations and intervals, vadose zone behaviors and recharge patterns allows the identification of reliable conceptual models and sustains efficient management strategies of GW resources.