Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2009
Publication Date: 9/11/2009
Publication URL: http://www3.interscience.wiley.com/cgi-bin/fulltext/122591186/PDFSTART
Citation: Buda, A.R., Dewalle, D.R. 2009. Dynamics of stream nitrate sources and flow pathways during stormflows on urban, forest and agricultural watersheds in central Pennsylvania, USA. Hydrological Processes. 23(23):3292-3305. Interpretive Summary: Identification of nitrate pollution sources in surface and ground waters is an important management priority within mixed land-use basins in the Chesapeake Bay watershed. To address this need, we utilized stable isotopes in nitrate and water to investigate potential sources and flow pathways of nitrate in stream water within Spring Creek, a 201 square-kilometer mixed land-use watershed located in central Pennsylvania. Six storm events were sampled at five sites representing a downstream progression of forested uplands underlain by sandstone to karst lowlands with agricultural, urban, and mixed land-uses. The results of the study illustrated major differences in water and nitrate delivery mechanisms between forested uplands and karst valleys, and confirmed the dominance of overland flow pathways in urbanizing basins.
Technical Abstract: Potential sources and flow paths of stream water nitrate were assessed using stable isotopes in nitrate (delta 15N-NO3 and delta 18O-NO3) and water (delta 18O-H2O) in Spring Creek, a 201 square-kilometer mixed land-use watershed located in central Pennsylvania. Six storm events were sampled during 2005 using a nested and spatially-distributed sampling design (5 sites). The sites represented a downstream progression of forested uplands underlain by resistant sandstone to karst lowlands with agricultural, urban, and mixed land-uses. Stream samples were collected during antecedent baseflow conditions and at or near peakflow for each event. Contributions of event nitrate and event water during each storm were computed using two-component mixing models. Comparisons among sites of the fractions of event nitrate and event water at peakflow showed three distinct types of flowpath and nitrate source variations: urban, karst lowlands, and forested uplands. At peakflow for the urbanized site, high proportions of event water at peakflow were matched closely by high fractions of event nitrate (mean ratio of event nitrate to event water was 1.03) showing the dominant influence of overland flow pathways and atmospheric nitrate sources. In the karst lowlands, very low fractions of event water and even lower fractions of event nitrate at peakflow (mean ratios of event nitrate to event water ranged from 0.11 to 0.25) showed the dominance of piston-flow flow generation processes and flushing of stored nitrate. The forested upland site with steeper terrain, shallow soils, and sandstone bedrock showed an intermediate response (mean ratio of event nitrate to event water was 0.65) that was dependent on storm size. During small storms (less than 1.3 cm of precipitation), small fractions of event water were accompanied by large fractions of event nitrate from atmospheric sources, which suggested that channel precipitation flow pathways and/or wash-off of dry deposition were important. A threshold for flushing of soil nitrate appeared to be surpassed during larger storms (greater than 1.3 cm of precipitation) where large fractions of event water were accompanied by large fractions of pre-event nitrate that most likely was flushed from stored soil sources via interflow flow pathways. Overall, this study illustrated for the first time how nitrate isotopes and delta 18O-H2O could be combined to show major differences in water and nitrate delivery mechanisms between forested uplands and karst valleys, and confirm the dominance of overland flow pathways in urbanizing basins.