Field Investigation of Drainage System Clogging in Karst Tunnels: Characterizing Groundwater Chemistry and Mineral-Scale Features

Clogging in underground drainage systems caused by mineral scaling poses a critical threat to the operational safety of karst tunnels. This study conducted a comprehensive field investigation across 11 operating karst tunnels in Southern China, combining macro-visual inspections, internal pipeline detections, hydrochemical evaluations, and micro-characterizations (XRD/SEM) to reveal scaling distribution and crystallization mechanisms. Statistical analyses across 1559 defect points indicate that secondary structural defects—dominated by lining seepage and construction joint leakage—dominate the defect inventory, with normalized occurrence rates broadly similar across the four surveyed lithologies; the observational nature of the survey and the limited number of tunnels per lithology, however, preclude a definitive causal attribution. Hydrochemical profiling revealed that groundwater leaching through cementitious linings creates strongly alkaline (pH up to 13.65) and highly hard mother liquors, and saturation-index calculations show that all waters with measurable carbonate were supersaturated with respect to both calcite and aragonite. Mineralogical characterizations identified five mineral phases: stable rhombohedral calcite, acicular aragonite, spherical vaterite, and minor impurities. A systematic correlation analysis of the 25 paired water-scale samples, however, did not confirm that any single hydrochemical variable robustly controls polymorph selection: only total hardness showed a suggestive negative association with aragonite content (ρ = −0.45, p = 0.024) that did not survive correction for multiple testing. The proposed single-ion controls on crystal polymorphs should therefore be regarded as working hypotheses, and kinetic and hydraulic factors likely modulate the final phase assemblage. These findings provide realistic boundary parameters for crystallization kinetic simulations and offer crucial theoretical guidance for developing targeted scale mitigation and drainage maintenance strategies.