Sanchez et al. (2026) Hotspots and hot moments of metal mobilization: dynamic connectivity in legacy mine waters

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-02-17
• Authors: Anita Alexandra Sanchez, Maximilian P. Lau, Sean Adam, Sabrina Hedrich, Conrad Jackisch
• DOI: 10.5194/hess-30-945-2026

Research Groups

• Interdisciplinary Environmental Research Centre, Technische Universität Bergakademie Freiberg, Freiberg, 09599, Germany
• Institute of Mineralogy, Technische Universität Bergakademie Freiberg, Freiberg, 09599, Germany
• Institute of Drilling Technology and Fluid Mining, Technische Universität Bergakademie Freiberg, Freiberg, 09599, Germany
• Institute of Biosciences, Technische Universität Bergakademie Freiberg, Freiberg, 09599, Germany

Short Summary

This study reveals that metal(loid) mobilization in abandoned underground mines is governed by episodic shifts in subsurface hydrological connectivity, particularly during low-flow and pre-flush periods, leading to disproportionate contaminant release from localized storage zones that are often overlooked by conventional monitoring.

Objective

• To understand how dynamic contaminant mobilization within underground mine systems can be effectively monitored and translated into targeted, in-situ treatment strategies that move beyond conventional end-of-pipe approaches.
• To characterize the temporal evolution of flow regimes and their influence on metal(loid) concentrations and loads.
• To determine the geochemical signatures associated with localized storage and release zones (hotspots) and episodic release events (hot moments).
• To evaluate how phase-dependent flow and concentration–discharge (C–Q) relationships can inform adaptive, near-source mine water treatment strategies.

Study Configuration

• Spatial Scale: Reiche Zeche mine, Ore Mountains, Germany. The study focused on a single slanted vertical extraction structure spanning three levels (103 m, 149 m, and 191 m below surface) with 26 sampling sites, specifically analyzing four distinct flow paths (sites 1, 2, 3A, and 3B).
• Temporal Scale: Over two years (3 February 2022 to 31 May 2024) with 42 sampling campaigns (one-to-three-week intervals). High-resolution monitoring (daily autosampler, hourly UV-Vis spectrometer) was conducted for 10 months (16 May 2022 to 14 February 2023 for autosampler, 16 May 2022 to 23 May 2023 for spectrolyzer) at site 2.

Methodology and Data

• Models used:
  • Concentration–Discharge (C–Q) relationships (log10-log10 space, slope 'b', ratio of coefficients of variation of concentration and discharge (CVc/CVq))
  • Pollution Load Index (PLI)
  • Hysteresis Index (HI) methods (Lloyd et al., 2016; Zuecco et al., 2016; HARP method by Roberts et al., 2023)
  • Fill-and-spill concept
  • Lotic-lentic cycle concept
  • Self-Calibrating Palmer Drought Severity Index (PDSI)
  • Quinlan’s Cubist modeling (for UV-Vis spectral data interpretation)
  • Hierarchical rule-based classification algorithm for geochemical phases
• Data sources:
  • In-situ sensors: Pressure sensors (Levelogger5, Solinst Georgetown) for continuous water level and flow monitoring.
  • Hydrogeochemical monitoring: Manual water sampling (42 campaigns), pH, conductivity, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and metal(loid) concentrations (iron (Fe), zinc (Zn), arsenic (As), copper (Cu), cadmium (Cd), lead (Pb), aluminum (Al), nickel (Ni), and manganese (Mn)) via inductively coupled plasma optical emission spectroscopy (ICP-OES).
  • Stable water isotopes (δ2H, δ18O) via cavity ring-down spectroscopy (L-2130i, Picarro Santa Clara).
  • Automated sampling: ISCO autosampler (daily samples).
  • High-frequency monitoring: Online UV-Vis spectrometer probe (spectro::lyser V3, s::can GmbH Vienna) for hourly absorbance measurements.
  • Meteorological data: Automated station at the surface of Reiche Zeche mine.
  • Reference data: Average values from the Elbe river at Magdeburg station for PLI background concentrations.
  • Data Repository: B2SHARE Data Repository (Sanchez et al., 2026) and Zenodo (Jackisch and Sanchez, 2026) for code.

Main Results

• Contaminant mobilization is controlled by episodic shifts in internal hydrological connectivity and threshold exceedance, rather than continuous seepage or solely high-flow events.
• Low-flow and pre-flush periods are high-risk intervals for solute accumulation in poorly connected storage zones, forming "hotspots" of high contaminant potential.
• Subsequent hydrological reconnection triggers "hot moments" of sharp but short-lived contaminant releases, accounting for a disproportionate share of annual metal loads (e.g., 50–56% of total annual Zn load at site 3A occurred in 2–3 months preceding flushing).
• Site 3A consistently exhibited the highest Pollution Load Index (PLI) values (often exceeding 500), which declined sharply early in flush events, indicating solute buildup during low connectivity and rapid export upon flow path reactivation.
• C–Q relationships revealed strong dilution patterns (negative slopes) at deeper sites (3A, 3B) for Zn and Cd, suggesting depletion of accumulated pools upon flushing, while site 2 showed less pronounced dilution.
• Isotopic differences between deeper and shallower sites narrowed considerably during flush phases, indicating reactivation of previously disconnected zones and increased hydrological connectivity.
• High-frequency monitoring at site 2 demonstrated that transitions from solute accumulation to flushing occurred within hours, with density stratification creating temporary solute traps that are rapidly flushed.
• Zn loads during low-flow, pre-flush periods reached values up to six times higher than median values.

Contributions

• Challenges conventional end-of-pipe monitoring and remediation by demonstrating that significant contaminant mobilization occurs during hydrologically quiet (low-flow and pre-flush) intervals due to internal hydrological thresholds and episodic connectivity.
• Establishes a transferable framework for diagnosing contaminant risks in legacy mine settings by integrating event-scale C–Q relationships, hysteresis analysis, and geochemical phase classification with fill-and-spill and lotic-lentic concepts.
• Highlights the critical need for event-sensitive, near-source monitoring and adaptive remediation strategies that prioritize internal system dynamics to mitigate pollution risk more efficiently and scalably.
• Extends the application of stable water isotopes and C–Q analysis tools to subsurface mine drainage systems, providing a promising means for tracing complex contaminant sources and transport processes.

Funding

• Dr. Erich-Krüger Foundation (Project: "Source Related Control and Treatment of Saxon Mining Water")
• TU Bergakademie Freiberg (for open-access publication)

Citation

@article{Sanchez2026Hotspots,
  author = {Sanchez, Anita Alexandra and Lau, Maximilian P. and Adam, Sean and Hedrich, Sabrina and Jackisch, Conrad},
  title = {Hotspots and hot moments of metal mobilization: dynamic connectivity in legacy mine waters},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-945-2026},
  url = {https://doi.org/10.5194/hess-30-945-2026}
}