Hydrochemical fingerprints and health risk assessment of groundwater contamination in the Bohai Sea region, China

Seawater intrusion and human activities have significantly impacted coastal groundwater quality in many regions worldwide. This study systematically assessed groundwater chemistry, its suitability for drinking and irrigation (sample size, n = 3034), and exposure risks (n = 2863) across three key sub-regions of the Bohai Sea area: Bohai Bay, Liaodong Bay, and Laizhou Bay. Significant seasonal variations observed in groundwater chemistry at different depths in Bohai Bay region, with severe contamination from salinity-alkalinity and nitrogen-fluoride. SO₄-Cl-Na type groundwater predominates in Bohai Bay and Laizhou Bay areas, primarily influenced by evaporation dissolution and seawater mixing, while carbonate weathering and reverse ion exchange play secondary roles. In the Liaodong Bay region, SO₄-Cl-Ca-Mg water is dominant, driven primarily by carbonate weathering and reverse ion exchange. Groundwater evolution over time follows a complex sequence: Bohai Bay (mainly freshening) < Liaodong Bay (freshening > intrusion) < Laizhou Bay (freshening ≈ intrusion). Groundwater in the Bohai Sea region exhibits high sodium percentage, sodium adsorption ratio, total hardness, corrosion ratio, and residual sodium carbonate, with exceedance levels ranked as Bohai Bay > Laizhou Bay > Liaodong Bay, while irrigation efficiency follows the opposite trend. Fluoride, nitrogen, and bromine contamination pose significant health risks, particularly to children, and are likely linked to both anthropogenic interventions (such as industrial layout) and natural (geological) factors. Diseases types and incidence rates among residents closely correlate with groundwater pollutant levels, emphasizing the need for targeted pollution reduction and dynamic management strategies.