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

Abstract

Abstract:Understanding the influence of storm events on nitrate (NO 3 ) dynamics is important for efficiently managing NO 3 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 NO 3 (υ 15 N-NO 3 and υ 18 O-NO 3 ) and water (υ 18 O-H 2 O) from baseflow to peakflow. Peakflow fractions of event NO 3 and event water were then computed using two-component mixing models to elucidate NO 3 flow pathway differences among the five sites. For the forested upland site, storm size appeared to affect NO 3 sources and flow pathways. During small storms (<35 mm rainfall), greater event NO 3 fractions than event water fractions indicated the prevalence of atmospheric NO 3 source contributions at peakflow. During larger storms (>35 mm rainfall), event NO 3 fractions were less than event water fractions at peakflow suggesting that NO 3 was flushed from stored sources via shallow subsurface flow pathways. For the urbanized site, wash-off of atmospheric NO 3 was an important NO 3 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 NO 3 at peakflow suggested the dominance of ground water flow pathways during storms. These ground water flow pathways likely flushed stored NO 3 sources into the stream, while deep soils in the karst lowlands also may have promoted NO 3 assimilation. The results of this study illustrated how NO 3 isotopes and υ 18 O-H 2 O could be combined to show key differences in water and NO 3 delivery between forested uplands, karst valleys and fully urbanized watersheds.

Sources and Transport of Nitrogen in Arid Urban Watersheds

“…Assuming that 50% of all pervious areas are fertilized, 5 this would be the equivalent of 69 to 130 kg N ha −1 y −1 input via fertilization, an order of magnitude larger than estimates of total annual N deposition of <6 to 18 kg/ha. 45,46 As in previous studies, 4,20,47,48 we found that f atm in urban stormwater was high, averaging 34%. The contribution of NO 3 − to total NO 3 − atm export from watersheds ranged from 4 to 53% across all observed events ( Figure 5), and observations of f atm for individual samples within events ranged much more widely (0 to 80% over all samples).…”

“…At the two surface-drained watersheds there were significant negative correlations between δ 15 N and δ 18 O (ρ = −0.87, p < 0.05 and ρ = −0.63, p < 0.05) during the 13 December 2011 event. These results support findings from other studies suggesting that variation in NO 3 − isotopes in urban 4,20,47,48,50,53 and nonurban 48,49 waters is primarily a result of the mixing of sources rather than biogeochemical processing along flowpaths during runoff events. Across a range of land uses, isotopic evidence of denitrification at the watershed scale has only been documented during baseflow conditions and is largely limited to agricultural watersheds.…”

Nitrogen pollution and source identification of urban ecosystem surface water in Beijing

“…When compared with the TN concentrations of rivers and lakes in the Beijing metropolitan area in 2003, at which point they were all below 2 mg $L -1 [17], TN concentrations showed an increasing trend. The TN concentrations reported in this study are higher than those in the Taihu lake region [26], in the Haicheng river basin [27]and in the Beijiang river in south China [28], but close to the TN level of the Thames river in the UK [29], and of stream water in Pennsylvania, USA [30]. These increasing TN concentrations may be due to intensification of human activities caused by urbanization, and agrees with other studies which have shown that urbanization results in elevated TN and NO -3 -N concentrations in watersheds in developing areas [31,32], and that urbanization soon takes over as the major source of nitrogen in urban freshwater ecosystems [27].…”

“…Atmospheric nitrogen deposition rates for Beijing have been estimated at 75.13 mg$m -2 , and, from June to September 2009, showed a significantly increasing trend [51]. Other authors have reported that direct runoff of atmospheric NO -3 -N, mobilized from impervious surfaces, is an important source of NO -3 -N in urban watersheds during storm events [1,30,52]. In Beijing, atmospheric nitrogen deposition may account for a larger proportion of nitrogen pollution in the upper stream region relative to the downstream reaches, where the contribution from sewage was higher.…”

Nitrogen Fluxes Into, Out Of, and Within a Virginia Permaculture Livestock Farm

“…The hydrologic cycle is a key driver of Nr loss to the environment and surface waters in particular are sites of significant Nr transport and transformation (Carpenter et al 1998, Seitzinger et al 2002. Streams with watersheds that are dominated by agricultural land uses often transport significant amounts of N because N application rates are so high (Howden et al 2011, Buda andDeWalle 2009). During base flows, N transport via the hydrologic cycle is typically in the form of NO 3 -, which is highly soluble (Howden et al 2011).…”