Chemical origin and water quality assessment of groundwater in the forest-cropland transition zone of northern Xiaoxing’anling Mountains

As a crucial part of the ecological security barrier in northern China, the Xiaoxing'anling Mountains serve as an important water conservation area in Northeast China. This study focuses on the groundwater in Aihui District, Heihe City, which lies in the forest-cropland transition zone in the northern Xiaoxing'anling Mountains. By comprehensively applying spatial analysis and data statistical methods, we adopted the Inverse Distance Weighting (IDW) to analyze spatial interpolation based on key indicators of groundwater, including pH, sulfate, fluoride and nitrate. Subsequently, by overlaying factors such as topography, geomorphology, land use types and hydrogeological conditions, this research probed into the chemical genesis and spatial distribution patterns of groundwater in the study area. The results indicate that the overall water quality of groundwater in the study area is moderately alkaline. Its cations and anions are dominated by Ca²⁺ and \mathrmNO_3^- respectively. The variation coefficient of \mathrmSO_4^2- was the highest, suggesting its highly heterogeneous spatial distribution. Additionally, a strong positive correlation was observed between \mathrmSO_4^2- and Mg²⁺, while a negative correlation between F⁻ and Ca²⁺. The indicators of chloride, sodium ions (Na⁺), and total dissolved solids (TDS) demonstrated excellent performance. The indicators of fluoride, pH value and sulfide showed good performance, with most of them meeting the Class III water quality standard. However, the nitrate indicator showed relatively poor performance, specifically, 46.83% of the samples did not meet the Class III water quality standard. Spatially, there was a gradually improving trend from the cultivated area to the forest area. Comprehensively considered, nitrate is the primary factor affecting the groundwater environmental grade in the study area. Specifically, the water quality in the southwestern part is relatively better, while that in the southeastern part is comparatively worse. Through comprehensive analysis, it is considered that the immoderate application of pesticides and fertilizers is the driver to cause the differences in water quality.