Hotspots and hot moments of metal mobilization: dynamic connectivity in legacy mine waters

Abstract. Monitoring and treatment of contaminated mine water conventionally focuses on end-of-pipe assessment and remediation techniques, at the downstream outlet of mining sites after closure. Conversely, the initial stages of pollutant release and their pathways within abandoned mines have been largely overlooked. This study examines subsurface mining-affected anthropogenic structures and the dynamic hydrogeochemical loadings and drainage pathways within them, revealing how variable subsurface flow activation impacts metal(loid) mobilization and opens novel direct mitigation options. We identified complex hydrological patterns through the mine (Reiche Zeche, Ore Mountains, Germany) in which percolation paths were dynamically connected to the drainage based on flow conditions. Using in-situ sensors, hydrogeochemical monitoring and stable water isotopes, we reveal a hydrodynamic regime in which episodic shifts in subsurface connectivity govern metal(loid) mobilization from localized storage zones, ultimately controlling solute export to surface waters. We use concentration–discharge (C–Q) relationships, hysteresis indices, and the Pollution Load Index (PLI) to evaluate metal transport during the annual pattern of flow regimes. Our analyses of event-scale C–Q hysteresis patterns reveal site- and element-specific shifts in flow path activation in a very short time. Despite low flow periods traditionally considered low risk for contaminant mobilization, contaminant hotspots within poorly connected hydrological zones can emerge during these times, with high pollution potential and solute accumulation governed by the sequence and timing of crossing or exceeding a connectivity or flow threshold, as described by the hydrological fill-and-spill and geochemical lotic-lentic cycle concepts. Notably, Zn loads during low flow, pre-flush periods reached levels up to six times higher than median values. Preceding the flushing events, geochemical and microbial-mediated metal leaching create the spatially distributed contaminant stock, remobilized during reconnection events. With a large proportion of heavy metal loads occur during low flow and especially just before the high flow (flush) period, source-related mitigation with decentralized water treatment structures becomes much more feasible than end-of-pipe solutions that require higher throughput volumes and multi-element filtering. This work also highlights the need for event-sensitive monitoring and treatment strategy options that prioritize internal system behavior to mitigate pollution risk in abandoned mines and other caverned hydrological systems.