A global synthesis of dual nitrate isotope values in rivers and groundwaters

Exponential human population growth and the rapid co-development of agricultural and industrial sectors have caused a sharp increase of nitrogen loading to rivers and groundwaters worldwide since the 1950s. Reactive nitrogen species (e.g., nitrate, ammonium) are widely distributed compounds in rivers and groundwaters primarily as a result of diverse agricultural activities utilizing N-containing fertilizers and anthropogenic non-point sources, such as the disposal of sewage by centralized and individual systems, animal feeding operations, and elevated atmospheric N deposition. Systematic efforts to identify global patterns in nitrogen loss processes using nitrogen isotopes have mostly targeted soil and plant systems but remain rather limited for surface and/or groundwater systems. Here, synthesized published (4,492) and new data (425) for nitrogen and oxygen isotopes of nitrate in rivers and groundwater generated under an IAEA Coordinated Research Project, which aimed to utilize the application of nitrogen isotope techniques to assess nitrogen pollution in rivers and groundwaters, are presented. Among the two water types, we found that groundwater had higher average nitrate concentrations (~5.0 mg L^-1 NO₃-N) versus rivers (~2.0 mg L^-1 NO₃-N), slightly higher δ¹⁵N and much higher δ¹⁸O (+7.6 ‰ and +4.3 ‰, respectively) compared to rivers (+7.0 ‰ and +1.8 ‰, respectively). Seasonal variations in the concentrations and the isotopic compositions of N-species were found to be temperature related, given that biological activity increases with water temperature. Across a range of Köppen climate types, we found the δ¹⁵N and δ¹⁸O of NO₃ in rivers systematically increased when moving from temperate to tropical climates, following the increase of the average air temperature.