Spatiotemporal Variation of Groundwater Nitrate Concentration Controlled by Groundwater Flow in a Large Basin: Evidence From Multi-Isotopes (15N, 11B, 18O, and 2H)

Elevated and increasing ${{\text{NO}}_3}^{-}$ concentration in groundwater affect groundwater supplies in China and elsewhere. However, how groundwater flow affects ${{\text{NO}}_3}^{-}$ concentration in groundwater has yet to be fully understood. Herein, multi-isotopes (¹⁵N, ¹¹B, ¹⁸O, and ²H) and local indicators of spatial association (LISA) were used to elucidate the spatiotemporal variation, sources, and patterns of ${{\text{NO}}_3}^{-}$ and its response to groundwater flow in Poyang Lake Basin where agriculture, industry and urban coexist. The location of ${{\text{NO}}_3}^{-}$ hotspots identified by LISA tended to move from the middle to lower reaches of Ganfu Plain with groundwater flow, and hotspots area expanded in the upper reaches of Xin River Basin and northwest of the study area during the transition from dry season to wet season. Our results revealed that variations of regional ${{\text{NO}}_3}^{-}$ concentration were controlled by groundwater recharge or flow mode (vertical or lateral), biogeochemical processes and sources (sewage and manure). In some areas with the single stratigraphic structure (unconfined aquifer), spatiotemporal variation of ${{\text{NO}}_3}^{-}$ concentration was influenced by local pollution sources and vertical recharge of current precipitation (vertical flow). In some areas with binary structures (confined aquifer), groundwater was mainly recharged by lateral flow and ${{\text{NO}}_3}^{-}$ concentration was mainly affected by mixing effect of upstream groundwater, reflecting human activities in the upper reaches rather than local human activities. In lakeside floodplain, groundwater ${{\text{NO}}_3}^{-}$ was attenuated by the dissimilatory ${{\text{NO}}_3}^{-}$ reduction to ${{\text{NH}}_4}^{+}$. This study provides a novel insight into groundwater flow controlling on spatiotemporal distribution of ${{\text{NO}}_3}^{-}$ concentration in the regional scale.