Solute-specific and spatially variable concentration–discharge relationships in hillslope critical zone groundwater

Hillslope groundwater in the critical zone (CZ) links subsurface water storage, hydrologic connectivity, and chemical export, yet its concentration–discharge (C–Q) relationships remain poorly constrained because internal discharge and chemistry are rarely observed simultaneously. The Poshan drainage tunnel system in Hong Kong consists of two sub-tunnels intersected by dense sub-vertical drains (SVDs), providing a valuable groundwater observation platform for investigating how hydrologic processes control solute concentrations within the volcanic rock hillslope. From April 2023 to April 2025, groundwater was sampled biweekly to monthly at the two tunnel outlets, the high tunnel weir (HTW) and low tunnel weir (LTW), and at four representative SVDs distributed along the tunnels across position and elevation. Concentrations of major ions, dissolved Si, and ²²²Rn were measured, and discharge at each location was monitored or calculated. Power law fitting, hysteresis index, and reactive transport model inversion were used to analyze C–Q relationships and to identify controls on solute export and transport. Hydrologic deconvolution was applied to time series of rainfall, groundwater levels from 9 piezometers, and discharge at the two weirs (HTW and LTW) to obtain residence time distributions, thereby constraining groundwater discharge and hydrologic connectivity. Across the hillslope, C–Q relationships showed strong solute-specific and spatial variability. At the weir scale, upslope groundwater more commonly exhibited enriching or diluting C–Q behaviors, whereas downslope groundwater more often showed chemostatic C–Q relationships. At the SVD scale, an upslope SVD showed significant dilution of Cl⁻ but strong enrichment of ²²²Rn with discharge, whereas a downslope SVD showed dilution of Na⁺, Cl⁻, and NO₃⁻ together with enrichment of K⁺, SO₄²⁻, and Si with discharge. The C–Q relationships of different solutes showed distinct behaviors: Na⁺, Cl⁻, and NO₃⁻ generally showed dilution or near chemostasis. In contrast, K⁺, Mg²⁺, and SO₄²⁻ more frequently enriched at high discharge, consistent with seasonal accumulation in shallow reservoirs followed by storm-driven flushing. Si displayed intermediate behavior, tending toward dilution or weak enrichment depending on location. ²²²Rn showed strong enrichment at specific sites, which may indicate rapid activation of short flow paths and groundwater mobilization. Hysteresis loop direction also varied by solute and location, with more counter-clockwise loops in the upslope area where solute signals lagged behind discharge and followed a seasonal cycle. More clockwise loops were observed for K⁺, Mg²⁺, and SO₄²⁻ in the downslope area, suggesting relatively abundant source reservoirs. Hydrologic deconvolution further indicated shorter mean residence times downslope than upslope, while groundwater near the tunnel could be discharged rapidly. Overall, the spatial and solute-specific C–Q variability within the volcanic CZ reflects the spatial distribution of solute storage and the interplay of thermodynamic limits, reaction kinetics, and residence time, and apparent chemostasis at larger scales may be due to mixing-driven signal averaging along heterogeneous flow paths.