Unprocessed Atmospheric Nitrate in Waters of the Northern Forest Region in the U.S. and Canada

Little is known about the regional extent and variability of nitratefrom atmosphericdeposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers ( δ18Oand δ15N) of nitratesources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitratewas transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrateduring baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrateduring stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric...