Deterioration of groundwater quality along an increasing intensive land use pattern in a small catchment

Land use change has greatly influenced groundwater quality worldwide. Identifying the effects of different intensive land uses on the groundwater quality is the first step in taking proper action to solve the problem. In this study, we compared the effects of different intensive land uses (region A, natural vegetation; region B, cereal fields; region C, kiwifruit orchards) in the Yujiahe catchment between 2015 and 2017 in Shaanxi, China, on the major ions and stable isotopes of nitrate (δ15N–NO3– and δ18O–NO3–). The NO3- groundwater concentrations increased from region A to region B and region C; NO3- concentrations in shallow groundwater were higher than those of deep groundwater in region C (55.3 vs. 28.9 mg/L, respectively). The NO3- concentrations in region A and region B did not exceed the WHO standard of 50 mg/L. However, 56.3% and 22.2% of the shallow and deep groundwater samples have NO3- concentrations exceeding the standard in region C, respectively. The average electrical conductivity (EC) values of springs in region A and shallow groundwater in regions B and C were 438, 525, and 753 µs/cm, respectively. Concentrations of Ca2+, Mg2+, Na+, Cl-, and HCO3- ions and nitrogen isotope values increased from region A to region C, indicating that intensive land use change has modified groundwater hydrochemical composition, and deteriorated groundwater quality. This study has highlighted the significant effect of intensive land use of orchards at the small catchment scale on the groundwater quality.