Coupled hydrochemical-microbial approach to groundwater pollution in covered and exposed karst systems

Karst aquifers are highly vulnerable to anthropogenic contamination due to their rapid flow dynamics and limited self-purification capacity. However, the mechanisms controlling pollutant transport in different karst settings remain insufficiently understood. This study conducts a comparative investigation of groundwater pollution transport in a covered karst system (Wuming Karst Basin, WKB) and an exposed karst system (northwestern Guangxi Karst Area, NGKA) in southern China. By integrating hydrochemical analysis with microbial 16S rRNA sequencing, and applying a dual-source tracking framework combining Positive Matrix Factorization (PMF) and the Bayesian-based Sourcetracker model, this work systematically characterizes pollution sources and their co-migration behavior in distinct geomorphic contexts. Results show that groundwater in both systems is dominated by Ca-HCO₃ hydrochemistry controlled by carbonate weathering, but exhibits distinct anthropogenic signatures. In WKB, pollution is primarily attributed to domestic sewage, with PMF identifying Na⁺ and K⁺ enrichment and Sourcetracker revealing a dominant microbial contribution of 60.44 %. In contrast, NGKA displays multi-source pollution dominated by agriculture (41.34 %), with rapid pollutant infiltration facilitated by exposed karst conduits. Notably, both systems exhibit significant contributions from uncharacterized sources (WKB: 37.36 %; NGKA: 14.37 %), underscoring the limitations of current source apportionment methods. This study is the first to integrate chemical and microbial tracers in a comparative karst setting, providing new insights into the coupled transport mechanisms of ions and microbes. The proposed framework offers a robust foundation for targeted pollution mitigation and groundwater protection strategies in vulnerable karst environments.