Key factors affecting groundwater nitrate levels in the Yinchuan Region, Northwest China: Research using the eXtreme Gradient Boosting (XGBoost) model with the SHapley Additive exPlanations (SHAP) method

Groundwater is a vital natural resource that has been extensively used but, unfortunately, polluted by human activities, posing a potential threat to human health. Groundwater in the Yinchuan Region is contaminated with NO₃^-, which is harmful to the local population. This study utilized the eXtreme Gradient Boosting (XGBoost) model with the SHapley Additive exPlanations (SHAP) method to identify the key factors influencing groundwater nitrate pollution in the Yinchuan Region. The SHAP feature dependence plots revealed the intricate relationship between NO₃^- levels and TDS, Mn²⁺, TFe, and pH in complex groundwater systems. The results indicate that the high levels of groundwater NO₃^- are primarily caused by the combined effect of irrigation water from the Yellow River, shallow groundwater depth, unfavorable drainage, water recharge, overuse of fertilizers, and geological factors such as weathering nitrogen-bearing rocks. Hydrochemical parameters such as Mn²⁺, Fe²⁺, and pH create a strong reducing groundwater environment, resulting in lower NO₃^- concentrations in this region. Well depth and soil organic carbon at a depth of 80-100 cm have a negative impact on NO₃^- concentrations; conversely, sand in soil depths 0-20 cm and 100-150 cm and climatic factors such as precipitation have a weak but positive effect on the level of NO₃^- in groundwater in the region. The recommendation is to quickly and extensively implement a farming water-conservancy transformation project, reducing water-intensive crops, promoting groundwater use for irrigation in areas where soil salinization is a concern are proposed. This research could provide local agencies with a scientific foundation for sustainable management of farming and groundwater in the Yinchuan Region, ultimately benefiting the entire Yinchuan Plain.