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

Abstract

AbstractUnderstanding 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− (δ15N‐NO3− and δ18O‐NO3−) and water (δ18O‐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 δ18O‐H2O could be combined to show key differences in water and NO3− delivery between forested uplands, karst valleys and fully urbanized watersheds. Copyright © 2009 John Wiley & Sons, Ltd.