Nitrate-nitrogen transport in streamwater and groundwater in a loess covered region: Sources, drivers, and spatiotemporal variation.

Water quality is an increasing concern in the dry regions of the world as it affects and reduces the quantity of available water. Our objective was to investigate the sources, drivers, spatiotemporal patterns of nitrate‑nitrogen (NO-N) transport in the streamwater and groundwater in a dry and a wet season in seven large rivers located in the Loess Plateau of China (640,000 km, 100 million population), which is a region with marked influence of human activities on streamflow and groundwater. We collected 510 streamwater and groundwater samples and found that NO-N was significantly lower in the dry season (< 5.0 mg L) than the wet season (> 5.0 mg L). In the wet season, NO-N was lower in the streamwater than groundwater; however, the spatial variation in the NO-N was greater in streamwater, with higher concentrations in two rivers (Wei and Fen). The source characterization using stable isotopes of NO from the Wei River showed that chemical N fertilizers and soil organic N contributed ~ 75% of NO to streamwater and that soil organic N was the greatest contributor of NO to groundwater (~ 60%) than streamwater (< 40%). The spatial pattern of NO-N was dominated by fertilizer application and varied seasonally with rainfall-runoff and streamflow-groundwater connectivity. Our results showed the complicated patterns and sources of NO pollution in streamwater and groundwater and highlight that more emphasis should be placed to prevent and restore the degraded water quality in the dry regions.