Deciphering pollution sources and mechanisms controlling groundwater chemistry in a typical dense agricultural plain on Yungui Plateau.

Groundwater is a critical resource for economic growth and livelihoods in the dense agricultural plains of plateaus. However, contaminations from various sources pose significant threats to groundwater quality. Understanding the sources of groundwater contamination and the mechanisms of hydrochemical control is essential for the sustainable development of agriculturally intensive plains. This research utilizes 23 datasets of groundwater chemical measurements to apply hierarchical clustering analysis, positive matrix factorization, and hydrochemical analysis techniques. Through these methods, the study identifies the sources of groundwater contamination and deciphers the hydrochemical control mechanisms within a representative intensive agricultural plain region of Yungui Plateau. The finds indicate that groundwater in the plain primarily derives from the rainfall occurred in the surrounding mountains. During the long underground flow process, groundwater undergoes water-rock interactions and ion exchanges with various lithological strata, resulting in the formation of distinct hydrochemical types. As it traverses regions influenced by human activities, groundwater encounters varying levels and types of contamination. Consequently, there is a notable variation in groundwater quality across different areas of the plain. Groundwater is dominated by the hydrochemical faces of HCO₃-Ca type in the southern part of the plain. Groundwater in the piedmont region of this part exhibits the highest quality, acting as the baseline for the overall groundwater quality of the plain. Groundwater in agricultural areas of this part is severely polluted by nitrate-rich agricultural wastewater. In the central urban area, under the control of municipal wastewater discharge and denitrification, groundwater is to some extent polluted by NH₄ ⁺. In the northern sector of the plain, groundwater chemistry exhibits greater diversity due to variations in geological strata and exposure to a range of pollution sources. The majority of the regions are contaminated with SO₄ ^2- and Cl^- and present a predominance of Cl-Na type for groundwater hydrochemical facies. Groundwater at the northernmost end is polluted by NO₂ ^-, NH₄ ⁺, and P. In addition, there is also a small amount of groundwater near the lake that is heavily polluted by fertilizers. This study provides valuable insights for the development of sound groundwater management strategies, applicable not only to the current agricultural plain but also to analogous regions worldwide. PRACTITIONER POINTS: This study probed the impact of agricultural pollution on the groundwater hydrochemistry in a cultivated plain. The research pinpointed the origins and contributions of groundwater chemicals in the cultivated agricultural plain. A conceptual model was established to illustrate groundwater chemistry formation in an intensive agricultural irrigation plain on Yungui Plateau.