Analysis of nitrogen differentiation characteristics and control mechanism in the groundwater of Naozhou Island

The Naozhou Island, located in the Leizhou Peninsula region, is a typical basaltic volcanic island. Due to the lack of surface rivers on the island, groundwater serves as the primary freshwater source for both production and daily life. In recent years, with the increase in human activities, nitrogen pollution from agricultural practices and residential activities has gradually become a major threat to the quality of groundwater on Naozhou Island. Therefore, understanding the sources, distribution characteristics, and control mechanisms of nitrogen on Naozhou Island is crucial for the protection of the island’s groundwater environment. Based on data integration and field investigations, this study explores the sources, distribution characteristics, and controlling factors of nitrogen in the groundwater system of Naozhou Island from a hydrogeochemical perspective. The results show that the nitrogen in the groundwater of Naozhou Island mainly originates from agricultural fertilization and domestic wastewater discharge. Ammonium nitrogen and nitrate nitrogen in the groundwater exhibit significantly different spatial distribution patterns: ammonium nitrogen is primarily found in the groundwater discharge zones along the island’s periphery, while nitrate nitrogen is mainly concentrated in the groundwater recharge zones at the island’s center, with nitrate nitrogen pollution being more severe. Additionally, there is a spatial gradient and conversion between the two. This spatial distribution and mutual transformation of nitrogen are determined by the island’s unique redox zoning pattern.