Nitrate and Fluoride as Complementary Indicators of Groundwater Evolution Along a Flow Path: A SOM-PMF Coupled Approach.

Groundwater contamination by nitrate (NO3 -) and fluoride (F-) is pervasive, yet the coupled mechanisms governing their evolution along flow paths remain poorly understood. Most studies examine them separately, neglecting their potential to serve as complementary tracers of anthropogenic and natural processes. Here, we propose a coupled SOM-PMF framework to identify the sources and co-evolution of NO3 - and F- in the Ganjiang River Basin. Self-organizing maps (SOM) applied to 67 field samples resolved a five-stage hydrochemical sequence from the recharge zone (C1) to the confined zone (C5), capturing both systematic trends and anomalous sites. NO3 - peaked at 14.42 mg/L in the transition zone (C3), reflecting intense local anthropogenic inputs, whereas F- reached a maximum of 6 mg/L in the confined zone (C5), associated with strongly alkaline conditions at sites such as XF043. Positive matrix factorization (PMF) then quantified the factors controlling this evolution. NO3 - was dominated by anthropogenic factor F4 (78.3%), tracking agricultural and domestic inputs and their attenuation under reducing conditions. F- enrichment was driven primarily by the dissolution of fluoride-bearing minerals under alkaline conditions (F3, 76.1%) and further amplified by evaporative concentration in the confined zone (F5, 14.5%). Although NO3 - and F- show no direct statistical correlation, together they form a complementary tracer system that distinguishes the combined influence of anthropogenic and natural processes. The coupled SOM-PMF approach provides a transferable, data-driven tool for source apportionment and risk zoning of multi-contaminant groundwater system.