Dynamics of stream nitrate sources and flow pathways during stormflows on urban, forest and agricultural watersheds in central Pennsylvania, USA

Understanding the influence of storm events on nitrate (NO₃^-) dynamics is important for efficiently managing NO₃^- pollution. In this study, five sites representing a downstream progression of forested uplands underlain by resistant sandstone to karst lowlands with agricultural, urban and mixed land-use were established in Spring Creek, a 201 km² mixed land-use watershed in central Pennsylvania, USA. At each site, stream water was monitored during six storm events in 2005 to assess changes in stable isotopes of NO₃^- (υ¹⁵N-NO₃^- and υ¹⁸O-NO₃^-) and water (υ¹⁸O-H₂O) from baseflow to peakflow. Peakflow fractions of event NO₃^- and event water were then computed using two-component mixing models to elucidate NO₃^- flow pathway differences among the five sites. For the forested upland site, storm size appeared to affect NO₃^- sources and flow pathways. During small storms (<35 mm rainfall), greater event NO₃^- fractions than event water fractions indicated the prevalence of atmospheric NO₃^- source contributions at peakflow. During larger storms (>35 mm rainfall), event NO₃^- fractions were less than event water fractions at peakflow suggesting that NO₃^- was flushed from stored sources via shallow subsurface flow pathways. For the urbanized site, wash-off of atmospheric NO₃^- was an important NO₃^- source at peakflow, especially during short-duration storms where event water contributions indicated the prevalence of overland flow. In the karst lowlands, very low fractions of event water and even lower fractions of event NO₃^- at peakflow suggested the dominance of ground water flow pathways during storms. These ground water flow pathways likely flushed stored NO₃^- sources into the stream, while deep soils in the karst lowlands also may have promoted NO₃^- assimilation. The results of this study illustrated how NO₃^- isotopes and υ¹⁸O-H₂O could be combined to show key differences in water and NO₃^- delivery between forested uplands, karst valleys and fully urbanized watersheds.