Sources and cycling processes of nitrogen revealed by stable isotopes and hydrochemistry in a typical agricultural lake basin

Increased exposure to nitrate has been reported in many arid and semi-arid areas where groundwater is used as the drinking water source, presenting a major public health threat. Groundwater nitrate pollution is becoming serious due to intensive agricultural activities in the Nansi Lake Basin, Shandong Province, Northern China, through which the trunk line of the South-to-North Water Diversion Project passes. This study characterized potential nitrogen pollution sources by combined analysis of hydrochemical components and stable nitrate isotopes (δ15N–NO3-, δ18O–NO3-); the contribution of different sources was estimated using the SIAR model. The results show groundwater nitrate contamination in the Nansi Lake Basin is widespread both west (NSW) and east (NSE) of the lake. Manure and sewage (M&S), soil nitrogen (SN), and chemical fertilizer (CF) are potential NO3−-N sources in the study area, with the order of contribution being M&S (78.38%) > SN (15.02%) > CF (6.60%) in areas west of the lake and M&S (59.40%) > SN (30.38%) > CF (10.22%) in areas east of the lake, indicating the significant impact of human activities. The relationship between d-excess and δ15N–NO3- suggests irrigation practices increase soil moisture, resulting in more N from 15N-depleted soils leaching into the groundwater, increasing the contribution of SN. The predominant processes of the nitrogen cycle in the basin are different in the NSW and NSE areas. In the upper-middle reaches of the NSW area, nitrification dominates the nitrogen cycle due to aerobic conditions and elevated salinity; however, denitrification occurs in local downstream aquifers at depths ranging from 10 to 50 m. In contrast, SN plays a bigger role in the NSE, where the mineralization of soil organic nitrogen is the predominant process, followed by nitrification of M&S and CF nitrogen sources due to rapid groundwater flow in the carbonate and metamorphic rock aquifers.