TRACING STREAM NITRATE IN A CENTRAL PENNSYLVANIA MIXED LAND-USE BASIN USING STABLE ISOTOPES, BACTERIA, AND INORGANIC CHEMICALS

Open Access
Author:
Buda, Anthony Robert
Graduate Program:
Forest Resources
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 08, 2007
Committee Members:
  • David Russell Dewalle, Committee Chair
  • Michael Allan Arthur, Committee Member
  • Hunter J Carrick, Committee Member
  • William Edward Sharpe, Committee Member
  • Chitrita Debroy, Committee Member
Keywords:
  • bacteria source tracking
  • storm events
  • mixing
  • tracers
  • nitrogen
  • discriminant analysis
  • ridge and valley
  • karst
Abstract:
Excess nitrate in terrestrial and aquatic ecosystems has resulted in numerous water quality problems throughout the U.S., which emphasizes the need to develop effective management plans to deal with nitrate pollution, especially in mixed land-use watersheds. Stable nitrate isotopes have been promoted as a useful tool for studying nitrate sources and cycling, but the application of these tracers in mixed land-use watersheds remains limited. To address this need, five studies were undertaken to improve the ability to trace nitrate sources and movements in Spring Creek, a small (201 km2) mixed land-use karst watershed located in the Ridge and Valley Province of central Pennsylvania. The first study (Chapter 2) was intended to improve our understanding of nitrate stable isotope variations in precipitation, which is essential for tracing sources of nitrate in streams during storm runoff periods. Results of the study showed that information about oxidant levels in the atmosphere and knowledge of storm tracks can be very useful for explaining seasonal and within-storm variations of nitrate stable isotopes in precipitation. The second study (Chapter 3) assessed nitrate isotope transformations, especially denitrification, in losing and gaining stream reaches along the main-stem of Spring Creek within the karst valley to test the assumption that nitrate isotopes in stream water conserve their original source signatures. Results of this study showed that nitrate stable isotopes were relatively unaffected by in-stream processes in losing reaches, and only changed in the gaining reaches when new and isotopically distinct nitrate sources were introduced into the stream. The third study (Chapter 4) was undertaken to better understand nitrate sources and flow pathways during storm events in a mixed land-use watershed. The study illustrated for the first time how stable isotopes in nitrate and water could be combined to show major differences in nitrate delivery mechanisms between forested uplands and karst valleys, and confirmed the dominance of overland flow pathways in urbanizing basins. The fourth study (Chapter 5) tested whether microbial source tracking techniques with E. coli bacteria could be used to develop indicators of human and animal pollution in stream water to address difficulties in distinguishing sewage (human) and manure (animal) sources of nitrate using stable isotopes. Applications of a phenotypic (serotyping) and a molecular (ERIC-PCR) source tracking technique indicated that animal sources of E. coli were predominant throughout the Spring Creek watershed and that human E. coli sources were much less important. The results suggested that differentiation of human and animal pollution sources may be difficult in watersheds that are primarily serviced by sewage treatment (e.g. Spring Creek), and that future studies of this sort should consider identifying watersheds where human and animal pollution sources are more widely distributed. The final study (Chapter 6) was designed to determine how well land-use and nitrate pollutant sources could be discriminated using nitrate stable isotopes, inorganic chemistry, and bacterial indicator variables in a mixed land-use setting. The results of this study showed that nitrate isotopes were sufficient for capturing basic differences in principal nitrate sources from precipitation, forested land-uses, and sewage; however, inorganic chemical tracers enabled discrimination among land-uses and geology that could not necessarily be captured using nitrate isotopes alone. Future studies applying combined tracer approaches in mixed land-use watersheds should therefore consider the value of using nitrate isotopes alone to discriminate basic nitrate sources as well as the added benefit of using inorganic chemistry to discriminate on other watershed characteristics such as land-use and geology. Overall, the five studies in this dissertation demonstrated valuable new techniques and highlighted new directions of research that will help to further our understanding of nitrate sources, transformations, and flow pathways in mixed land-use watersheds.