Nitrate Fate and Decadal Shift Impacted by Land Use Change and Enhanced Rural Tourism Activities in a Typical Karst Aquifer System as Revealed by Dual Nitrate Isotopes and Hydrochemistry

Nitrate pollution in oxygenated karst aquifers is common due to nitrification and anthropogenic inputs. However, the shift of nitrogen sources influenced by enhanced rural tourism activities and land use changes are not well understood. In this study, hydrochemistry and dual nitrate isotopes of water samples from a karst basin in Chongqing, southwestern China were employed to investigate the nitrate fate and its decadal change during a period of 2007–2008 and 2017–2019. The results showed that δ15N nitrate and δ18O nitrate values at the groundwater basin resurgence averaged 9 ± 3.4‰ and 2.5 ± 3.4‰, respectively, with a mean NO3 – concentration of 19.7 ± 5.4 mg/L in 2017–2019, clearly exceeding natural background levels. The dual isotope results suggested that nitrification occurred at the sampled sites. From 2007–2008 to 2017–2019, the mean δ15N nitrate values from the primary sink point and the resurgence of the underground river water samples increased by 11.4‰ and 4.8‰, respectively. A Bayesian isotopic mixing model based on the nitrate isotopes revealed that soil organic nitrogen (SON) and manure and sewage (M&S) proportions for the groundwater increased by 34% and 23%, respectively, while chemical fertilizer (CF) and atmospheric precipitation (AP) proportions decreased by 32% and 25%, respectively. These decadal changes resulted from reforestation practices and enhanced rural tourism activities in the basin, which were evidenced by the land use change. The elevated nitrogen load from the rapid development of rural tourism is likely to increase this contamination in the near future if the infrastructure cannot meet the demands. The results from this study could contribute to minimizing environmental health risks in drinking water when rural tourism activities are increasing.