Impacts of N2O Oversaturated Sewage Effluents on the Spatial Distribution of Riverine N2O: Insights from Sanya Estuaries, Hainan Province

Rivers and estuaries are recognized as significant sources of atmospheric greenhouse gas nitrous oxide (N2O), primarily through diffusive pathways. Anthropogenic nitrogen contributions to surface water bodies can alter the baseline emissions from natural sources; however, due to high spatial variability and limited datasets, the specific sources and sinks contributing to N2O remain poorly understood. This study investigates the sources and sinks of nitrous oxide (N2O) in river systems located in Sanya, Hainan Province, China. In April 2023, we collected 48 samples of river water and seawater, measuring hydrochemical parameters in situ while analyzing N2O concentrations in the laboratory. The results indicate that N2O concentrations vary significantly across different river systems within Sanya. Specifically, N2O concentrations ranged from 0.33 to 307.18 nmol/L for samples from the Sanya River, 2.28 to 3113.46 nmol/L for samples from Damaoshui River, 5.72 to 122.75 nmol/L for Tengqiao River waters, and between 11.08 and 18.07 nmol/L for Ningyuan River waters; coastal seawater exhibited concentrations ranging from 2.42 to 21.96 nmol/L. Notably, we observed that riverine N2O levels near sewage discharge points were oversaturated—indicating a peak concentration—which subsequently declined towards levels more consistent with those found in natural river systems as one approaches the mouths of estuaries. Both Sanya River and Damaoshui River appear to be significant sources of N2O; conversely, coastal seawater is not considered a substantial source. Our data suggest that wastewater discharges may play a critical role in influencing N2O levels within river waters by directly introducing oversaturated effluents into these ecosystems.