Integrating MODPATH modelling and tritium data to assess groundwater residence time in the Campo de Montiel Aquifer (Spain).

This research outlines work conducted by the Geological Survey of Spain (IGME-CSIC) within the SIGLO-PRO project (PID2021-128021OB-I00/AEI/ https://doi.org/10.13039/501100011033/FEDER, UE) to characterise groundwater residence times in the Campo de Montiel aquifer. The study combines numerical particle-tracking simulations using MODPATH with existing empirical groundwater dating based on tritium measurements undertaken by CEDEX (Centro de Estudios y Experimentación de Obras Públicas, Spanish Ministry of Transport and Sustainable Mobility) across multiple sampling campaigns between 1971 and 2007.
A total of 61 observation points were incorporated, including wells and springs, each with one to four measurements collected over four multi-decadal sampling periods. Backward particle-tracking simulations were initiated from these sampling locations, using columns of particles in wells and radial distributions around springs to approximate natural recharge capture zones and flow pathways across the carbonated aquifer system.
Model outputs indicate that the mean groundwater residence time across the network ranges between approximately 13 and 36 years, in broad agreement with the tritium-derived ages. Although both approaches contain inherent uncertainties, their convergence supports the robustness of the residence-time estimates and suggests the aquifer behaves as a moderately slow-turnover groundwater reservoir under current recharge conditions.
Travel time across the unsaturated zone has been also considered. Although limited field evidence is available for this parameter in the Campo de Montiel system, estimates informed by previous carbonated aquifer studies suggest lag times of several months, and alternative methods—such as those proposed by Fenton et al.—are being evaluated to refine these values further. These preliminary results will be incorporated into future modelling iterations to improve understanding of recharge-to-discharge transit times, particularly for springs where shallow pathways may dominate.
Overall, the integration of particle tracking and isotopic dating provides a coherent first-order estimate of groundwater age structure for the Campo de Montiel aquifer. These findings form a basis for assessing vulnerability, understanding contaminant transport potential, and evaluating future scenarios of groundwater abstraction and recharge variability under climate and land-use change.