Harmful Metal Export from Abandoned Mines Controlled by Hydrodynamic and Biogeochemical Drivers

Abandoned mines are significant contaminant sources to rivers and lakes, with metal(loid) leaching and acid drainage impacting water quality and ecosystem function. While mine drainage is conventionally assumed to contribute contaminants continuously over decadal time scales, recent findings indicate episodic pulses of metal(loid) export with uncertain drivers. Here, we investigate a former lead–silver-zinc mine (Reiche Zeche, Germany) that drains directly into Elbe River tributaries, identifying spatial and temporal peaks in contaminant release, i.e., “hotspots” of reactivity. Through biogeochemical monitoring and molecular profiling of dissolved organic matter (DOM) and microbial communities, we reveal dynamic contaminant sources. One hotspot exhibited a release pattern controlled by density stratification, in which deeper, isolated layers with iron-oxidizing bacteria intensively accumulated metals and DOM. We demonstrate that specific molecular formulas within DOM are tightly coupled to metals, such that DOM can act as a persistent tracer of hotspot activity while also potentially enhancing metal(loid) complexation, stabilization, and release processes. A broader survey of abandoned mines (n = 28) revealed similar DOM signatures downstream of leaching sites, suggesting similarities in the contaminant release dynamics. These findings provide a framework for novel contaminant monitoring approaches and highlight the need for improved water management strategies at subsurface–surface interfaces in postmining landscapes.