Solute‐Specific and Spatially Variable Concentration–Discharge Relationships in Volcanic Hillslope Groundwater

Abstract

Understanding how groundwater concentration–discharge (C–Q) relationships are controlled within hillslopes requires resolving internal hydrochemical dynamics, a mechanistic challenge rarely addressed because groundwater chemistry and discharge are rarely monitored simultaneously within the critical zone. Here, an extensively instrumented drainage tunnel system in a steep volcanic hillslope served as an internal groundwater observation platform, enabling the analysis of a 2‐year record of hydrochemical and discharge dynamics from four sub‐vertical drains and two integrating weirs. Despite relatively uniform lithology and vegetation cover, groundwater C–Q relationships varied markedly between locations and among solutes. Weathering products, including Na ⁺ , Ca ²⁺ , and dissolved Si, generally showed dilution to near‐chemostatic behavior, with spatial differences reflecting contrasting reaction limitation and subsurface flow path routing. In contrast, K ⁺ , Mg ²⁺ , and SO ₄ ²⁻ more commonly exhibited enrichment, indicating wet season mobilization of near‐surface, chemically enriched source zones. The radioactive tracer ²²² Rn responded positively to large rainfall events, suggesting enhanced radon emanation under wet conditions and the activation and reconnection of subsurface pathways. Comparisons between individual drains and integrated weirs further showed that aggregation effects can reshape local groundwater C–Q signals, including damping some local dilution patterns and modifying hysteresis patterns. These results demonstrate that groundwater within a single volcanic hillslope is internally heterogeneous and highlight the importance of internal groundwater observations for disentangling the coupled roles of weathering reactions, flow path activation, and fluid residence time in shaping groundwater C–Q dynamics.