Spatial Distribution and Source Apportionment of Nitrate in Groundwater in a Representative Agricultural Region of the Southern Piedmont of the Qinling Mountains, China

Abstract In order to study the source of nitrate pollution in shallow groundwater in the agricultural area, surface water, groundwater and soil samples were collected, and the conventional chemical indicators and nitrogen and oxygen isotopes were analyzed. The method of combining nitrate δ15N and δ18O was used to identify the source of nitrate pollution. The results show that the concentration of NO3−(N) in the groundwater is between 3.18mg/L and 10.32mg/L, and the water quality of BC203, TCQ3020 and TCQ3029 wells belongs to Class III(5mg/L). From the perspective of spatial distribution, the pollution degree of water quality in the upper reaches is stronger than that in the lower reaches, that in the tributaries is stronger than that in the main stream, and it gradually decreases along the flow direction; vertically, the soil nitrate content decreases with the increase of depth, and suddenly increases at -1 meter, indicating that the groundwater nitrate pollution depth in this area has risen to -1 meter. The δ15N and δ18O values of nitrate ranged from 7.2‰ to 14.6‰ and from-8.31‰ to-7.39‰, respectively. Combined with the field survey, it was determined that the main sources of nitrate were animal manure and domestic sewage(MS), soil nitrogen(SN).