Constraining Subsurface Connectivity by Water-Trace Experiment in Deep, Semi-Arid Karst Terrain

This pilot water-trace experiment evaluates subsurface hydraulic connectivity between an ephemeral channel and a deep fractured-carbonate aquifer (vadose zone thickness >300 m) to assess managed aquifer recharge (MAR) feasibility in semi-arid settings. The study site comprises an ephemeral channel incised into fractured limestone, selected for its documented infiltration capacity and proximity to structural lineaments expected to enhance vertical permeability through the deep unsaturated zone. The primary objective was to constrain groundwater flowpaths and transit times under controlled low-flow recharge conditions relevant to MAR operations. Rhodamine WT tracer was introduced via continuous reclaimed water outflow at approximately 0.95 m³/min (250 gpm) over 48 h. Monitoring was conducted at two downgradient municipal wells and one upgradient site using passive charcoal receptors, water grab samples, and continuous in-situ fluorometry. While measured concentrations were below standard quantitative detection thresholds, contemporaneous anomalous fluorescence signals were observed at a municipal production well (2.7 km downgradient) by two independent methods 11-13 d after dye introduction, yielding an apparent groundwater velocity of approximately 200-250 m/d. No detection occurred at the further downgradient municipal well or upgradient site. The results provide preliminary evidence of rapid subsurface connectivity in this fractured-carbonate system. The transit time and flowpath geometry have important implications for MAR design, particularly regarding residence time and water quality. This study demonstrates that controlled water-tracer introduction coupled with redundant detection methods can constrain connectivity in semi-arid karst-matrix aquifers and provides a transferable methodological framework for MAR feasibility evaluation.