Environmental isotopes and multi-tracer evidence to elucidate deep groundwater circulation and fault-driven connectivity in complex karst systems: Case of the Middle Atlas, Morocco

Study region The Middle Atlas Mountains in Morocco is a structurally complex karst where carbonate plateaus and major faults cut Liassic carbonates and Triassic evaporites, sustaining groundwater resources for adjacent semi-arid plains. Study focus A multi-tracer approach to constrain regional flow architecture, recharge elevation, groundwater age, and the processes controlling mineralization across unconfined and confined karst compartments. We combined major-ion hydrochemistry with environmental isotopes (δ²H, δ¹⁸O, ³H, and 14 C) from 60 springs and 8 surface water sites, the study quantifies recharge elevations, groundwater residence times, and the geochemical drivers of mineralization in both unconfined and confined aquifer compartments. New hydrological insights for the region The study establishes the first massif-scale isotopic baseline, defining a new Local Meteoric Water Line (δ²H = 7.99 δ¹⁸O + 14.56) and a vertical δ¹ ⁸O gradient of −0.23‰ per 100 m, placing recharge altitudes between 700 m and 2490 m. Findings reveal a dual-flow system: fast-flowing conduit networks with modern water and confined compartments containing significantly older groundwater. Mineralization is primarily driven by subsurface Triassic evaporite dissolution. Furthermore, the Tizi N’Tretten fault zone is identified as a critical structural control facilitating vertical circulation and inter-basin flow, providing a vital framework for managing Mediterranean karst resources.