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 km(2) 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- (delta N-15-NO3- and delta O-18-NO3-) and water (delta O-18-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 delta O-18-H2O could be combined to show key differences in water and NO3- delivery between forested uplands, karst valleys and fully urbanized watersheds. Copyright (C) 2009 John Wiley & Sons, Ltd.