Spatial pattern of groundwater chemistry in a typical piedmont plain of Northern China driven by natural and anthropogenic forces

Groundwater is crucial for human society's development in piedmont plains, yet its hydrogeochemistry often exhibits complex spatial distributions due to the interplay of nature and human factors. Ninety-two phreatic groundwater samples were collected from a typical piedmont plain in northern China and analyzed using self-organizing map combined with hydrogeochemical simulation, diagrams, and the entropy-weighted water quality index. Groundwater samples were categorized into four clusters, demonstrating a gradual hydrogeochemical facies evolution from HCO₃-Ca to Cl-Mg·Ca and Cl-Na, along with an increase in NO₃^- content in the order of clusters IV, II, III, and I. Natural processes, including silicates weathering and reverse cation-exchange, establish the natural fundamental framework of groundwater chemistry, which is furtherly sculptured by agricultural substances input. Groundwater quality was predominantly excellent or good, with entropy-weighted water quality index (EWQI) values below 100 at over 92% of the sampling sites. Groundwater quality is relatively poorer in the upstream areas near the mountains and along the Hutuo River, where the stratum permeability is high, but improves in the downstream areas where permeability is lower. Agricultural land use and spatial variation in aquifer permeability are responsible for the observed spatial variations in groundwater chemistry. Agricultural contaminants warrant attention for the protection of groundwater quality in piedmont plains that with long-term agricultural activities, especially in the upstream areas near the mountains. This research improves the understanding of the spatial distribution and variation of groundwater chemistry in piedmont plains, and provides scientific guidance for related groundwater development and management.