Geochemical characteristics and driving factors of high-ammonium groundwater in the rapid urbanization of the Pearl River Delta

High concentration of ammonium nitrogen in groundwater is a potential threat to drinking water safety and ecological environment. Compared with nitrate nitrogen, the high concentration of ammonium nitrogen not only has a variety of man-made sources, but also the natural sedimentary environment is the main cause of high ammonium groundwater. The rapid urbanization of the Pearl River Delta was taken as a case study to investigate the environmental characteristics and driving factors of high ammonium groundwater by means of mathematical statistics and principal component analysis methods. The results show that the concentration of ammonium in the groundwater in the study area is as high as-180 mg/L. Among 1539 groups of groundwater samples, 69 groups of high-ammonium groundwater with ammonium ion concentration greater than 10 mg/L, 23 groups of high-ammonium fertilizer water yield ammonium ion concentration greater than 30 mg/L. Compared with the historical hydrochemical data from 2005 to 2008, the proportion of high-ammonium groundwater sample sites in the pore aquifers of newly-added construction land from 2009 to 2018 is 6.5%, which is 1.25 times that of ten years ago. The high ammonium groundwater is distributed in the silty and silty aquifer developed in the low lying basement and depression at the bottom of Quaternary in the delta plain area. The sediments rich in organic matter and total organic carbon, such as silt layer, are the "ammonium producing layer" in the Pearl River Delta region, and the mineralization of organic nitrogen is the main driving force of the high ammonium groundwater in the urbanized pore aquifer in the delta plain region. The leakage and infiltration of domestic sewage from urbanization expansion and ammonium-rich industrial wastewater are the important sources of ammonium-nitrogen in high ammonium-rich groundwater in the urban-rural junction. The reduction environment rich in organic matter is the main cause of the high ammonium groundwater in the delta plain. Lixiviation, cation exchange adsorption and sea-land interaction are the main hydrogeochemical processes of the evolution of high-ammonium groundwater water quality in the Pearl River Delta.