Regional characteristics of nitrate sources and distributions in the shallow groundwater of the Lake Baiyangdian watershed

Lake Baiyangdian is located in Xiong'an New Area, China, where groundwater is the primary water supply. Groundwater nitrate (NO₃^-) contamination is common in the Baiyangdian Lake watershed because of industrial and domestic wastewater discharge and over-application of agricultural fertilizer. However, the source characteristics and NO₃^- distribution across the entire watershed are still unclear. In this study, NO₃^- samples collected from rivers and shallow groundwater over the past decade were analyzed. Samples were also collected in December 2016 from the Lake Baiyangdian watershed area, and the spatio-temporal NO₃^- distributions of groundwater and the effects of various sources on groundwater NO₃^- were analyzed using water chemical ions and stable nitrate nitrogen isotopes (δ¹⁵N-NO₃^-). The results showed that the NO₃^- concentration in shallow groundwater differed, and the nitrogen sources had variable effects, particularly from the hills to the plains. In the hilly area, high NO₃^- concentrations measured in the alluvial valley groundwater were attributed to local rural sewage, with the highest NO₃^- concentration of 313 mg·L^-1; while the regional farmland manure application over several decades was the main cause of commonly high groundwater NO₃^- concentration in recent years. Rainy season leaching led to NO₃^- concentrations two times greater than that during the dry season, which exceeded the World Health Organization's (WHO) standard (50 mg·L^-1) and threatened downstream water quality safety. Of the shallow groundwater samples collected in the plains in December 2016, 21.5% had NO₃^- concentrations exceeding the WHO standard. The median groundwater nitrate concentrations trended downward from upstream to downstream in geomorphological type (proluvial fan: 42.4 mg·L^-1 > alluvial-proluvial fan: 24.1 mg·L^-1 > alluvial-proluvial plain: 6.0 mg·L^-1 and river zone: 6.2 mg·L^-1), but the median δ¹⁵N-NO₃ isotopes trended upward (proluvial fan: 12.8‰ and alluvial-proluvial fan: 11.3‰ < alluvial-proluvial plain: 16.7‰ < river zone: 20.9‰), indicating that denitrification increased from upstream to downstream. High aquifer sediment permeability in the proluvial fan and alluvial-proluvial fan regions increase the risk of nitrate leaching into the aquifer. Sewage (33.3%) and manure (34.0%) were primary sources of groundwater nitrate and caused the deviation from the WHO standard rate. In regions with lakes and depressions, groundwater nitrate was affected by industrial and domestic sources and fertilization, and, compared to other regions, groundwater nitrate was higher near the domestic and industrial wastewater river (but also had drastically different surface pollution control measures). However, the reduced conditions in other lake and depression regions lowered the groundwater nitrate concentration (< 10 mg·L^-1). This study provides suggestions for managing nonpoint source pollution in the Lake Baiyangdian watershed shallow groundwater based on the regional source characteristics and nitrate distribution, particularly for vulnerable places, such as hilly areas, the proluvial/alluvial-proluvial fan region of the piedmont plain, and wastewater influence areas.