Application of NBL-REF bridge method to determine groundwater quality threshold in the Changbai Mountain area

Groundwater pollution poses a serious threat to both the ecological environment and human health. Defining groundwater quality thresholds has become a fundamental prerequisite for effective groundwater protection. The Changbai Mountain area, as a crucial ecological barrier and water conservation zone in China, relies heavily on groundwater to sustain regional ecological security and meet domestic water demands. Therefore, scientifically establishing groundwater quality thresholds is of great significance for this region. Based on observed groundwater quality data, this study integrates probability plots, the 2−σ iterative method, and distribution function analysis to determine the natural background values of groundwater. Subsequently, the NBL-REF (Natural Background Level-Reference Value) bridging method is employed to define threshold values for 20 groundwater quality indicators in the Changbai Mountain region. Results show that although groundwater in the area is subject to some anthropogenic disturbance, the natural background values of most indicators remain below their respective reference values, indicating that the overall water quality is still relatively good. However, the background values of NO3-N and Al exceed the reference values, while those of DO, NH3-N, and Fe are close to the upper limits, suggesting the need for enhanced management and control. The calculated thresholds not only reflect the baseline status of groundwater quality in the region but also provide a basis for regulatory management. The findings offer scientific support for groundwater resource protection in the Changbai Mountain area and demonstrate potential applicability to other mountainous and plain regions.