Distribution characteristics and source identification of nitrate in the underground water of coastal areas of South China: A case study of Zhuhai City

Nitrate pollution could present in the underground water due to the human activities like agriculture, industry, residents and mariculture in coastal areas. In order to find out the distribution characteristics and sources of nitrate in the underground water of the coastal areas of South China, the authors in this paper took a typical coastal city-Zhuhai as an example. The mathematical statistics, ion ratio and principal component analysis were adopted to study the distribution characteristics of water chemical, nitrate, nitrite and ammonia nitrogen, and the sources of nitrate in the underground water. The results show that the underground water is mainly weak acid, and conductivity of deep confined underground water is obviously higher than that of shallow underground water. The chemical type of underground water is mainly Ca²⁺·Mg²⁺-HCO₃^- and Na⁺-Cl^-·SO₄^2- type. The dissolved inorganic nitrogen (DIN) of pore confined groundwater in loose rocks is mainly composed of NH₄⁺, and the DIN of other types of underground water is mainly NO₃^-. Nitrate and nitrite exceed the standard in sporadic water samples, and the concentration of nitrate is relatively high in the water samples of hilly areas and intermountain valleys. The ammonia nitrogen of 18.01% underground water samples in loost rocks exceeds the standard, and the high concentration ammonia nitrogen water samples are mainly distributed in the coastal plain area. The chemical characteristics of underground water in the study area is mainly controlled by the weathering, evaporation and seawater mixing of silicate and sulfate minerals. Nitrate in the underground water is mainly from atmospheric precipitation and human activities. Nitrate in clastic pore fissure groundwater, granite metamorphic rook fissure groundwater and pore unconfined groundwater mainly comes from atmospheric precipitation, and agricultural activities. Nitrate in pore confined groundwater in loose rocks and carbonate fissure karst water comes from domestic sewage and mariculture wastewater. This research results could provide scientific basis for the development, utilization and protection of the underground water in the coastal areas of South China.