Co-occurrence and health risk assessment of geogenic contaminants and nitrate in bedrock groundwater from a mountainous area, South Korea

The risk of groundwater contamination by geogenic and anthropogenic contaminants is increasing, particularly in mountainous regions where diverse bedrock types and agricultural practices can create conditions for co-occurrence of contaminants. We investigated 122 groundwater samples from fractured bedrock aquifers with diverse lithology in a mountainous area in South Korea to characterize the co-occurrence of arsenic, uranium, fluoride, boron, and nitrate, and to assess potential human health risks. Stable isotopes of water distinguished regional flow systems with depleted δ 18 O and high pH, distinct from local flow systems influenced by evaporation and fertilizer-derived nitrate. Uranium contamination was predominant at granite–metamorphic contact zones, and elevated nitrate in granitic bedrock regions indicated enhanced weathering under acidified conditions. Sedimentary areas showed boron enrichment from dissolution of carbonate and clay minerals. Northern highland zones exhibited spatially coherent arsenic–fluoride co-occurrence due to prolonged water–bedrock interactions, with approximately 25% of samples exceeding both noncarcinogenic and carcinogenic risk thresholds. Because simultaneous arsenic and fluoride exposure elevates human health risks beyond individual contaminant effects, our findings highlight the need for integrated management strategies addressing contaminant co-occurrence and combined human health risks through flow system-based monitoring tailored to lithological controls. • We assessed geogenic and agricultural contamination in mountainous aquifers. • Stable isotopes revealed distinct regional and local groundwater flow systems. • Lithology and flow systems jointly control geogenic contaminant mobilization. • Nitrate in granitic aquifers bedrock indicated accelerated weathering under acidic conditions. • As and F - in groundwater exceeded noncarcinogenic and carcinogenic risk thresholds.