Watershed scale controls on urea transport in a coastal plain river network
Open Access
- Author:
- Tzilkowski, Sarah Suzanne
- Graduate Program:
- Forest Resources
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 19, 2012
- Committee Members:
- Elizabeth Weeks Boyer, Thesis Advisor/Co-Advisor
Anthony Buda, Thesis Advisor/Co-Advisor
Michael Gooseff, Thesis Advisor/Co-Advisor
Matt Richard Marshall, Thesis Advisor/Co-Advisor - Keywords:
- urea
hydrology
nitrogen
stormflow
water quality
nutrient loading
dissolved organic nitrogen
Chesapeake Bay - Abstract:
- Increasing nutrient loads delivered from the landscape to coastal ecosystems has widely been recognized as a major contributor to coastal eutrophication and as a driver of the escalation of harmful algal blooms (HABs). Urea, a form of organic nitrogen, is a common nutrient found in fertilizers, manures, and human waste, and is gaining recognition as a preferred nutrient for the development of toxic HABs. While several studies have documented elevated urea, concentrations in tributaries draining the Delmarva Peninsula and within the Chesapeake Bay, little is known about the key controls that influence urea delivery from the landscape to surface waters. Here, in attempt to address the need to better understand urea fate and transport, land management and hydrologic controls on in-stream urea, specifically urea-nitrogen (urea-N), concentrations were investigated in the Manokin River. The Manokin River is a Coastal Plain watershed (244 km2) on the Delmarva Peninsula that drains directly to the Chesapeake Bay and is characterized by rural development coupled with intensive agriculture, particularly poultry production. Monthly synoptic sampling during baseflow conditions from March 2010 to June 2011 was conducted throughout the watershed in order to represent the variety of potential point and non-point sources of urea-N. Sampling was also conducted during stormflow conditions from August 2010 to August 2011 using time-weighted automated (SIGMA) samplers at select sites within the watershed. Temporal baseflow trends illustrate higher average urea-N concentrations through the summer months (0.043 mg L-1) and generally lower (0.020 mg L-1) concentrations during the winter months. Spatially, mean urea-N concentrations in the agricultural ditches were significantly higher than all other sampling locations, including groundwater, surface water, and wastewater treatment plant effluent, suggesting ditch networks may be ‘hotspots’ for urea-N within the watershed. Mean urea-N concentrations were significantly (p < 0.05) lower (R2 = 65.4%) in agriculturally dominated watersheds and significantly (R2 = 35.8%) higher in watersheds draining greater proportions of wetlands. Urea-N concentrations in wetland dominated headwaters were significantly correlated with water temperature (R2 = 58.9%), dissolved organic matter (R2 = 75.2%), and dissolved organic carbon to dissolved organic nitrogen (DOC:DON) ratios (R2 = 36.6%). Based on these correlations and stream channel and riparian zone characteristics, in-situ production of urea-N through bacterial decomposition of organic matter and phytoplankton and photochemical breakdown of organic matter was a significant source of urea-N at baseflow. Although baseflow urea-N was not influenced by the presence of poultry agriculture, excess nutrient inputs via litter application may indirectly support in-situ urea-N production in ditches. Stormflow was found to be the predominant urea-N delivery mechanism to downstream coastal waters, as urea-N concentrations typically increased 3 to 9 times above baseflow concentrations during storms at most sites. Spatially, drainage ditches tended to have the highest mean stormflow urea-N concentrations than all other sites monitored during events. Dry antecedent conditions and low water tables led to greater changes in urea-N concentrations and higher event mean urea-N concentrations. Storms with high rainfall intensity also produced higher event mean urea-N concentrations, possibly due to rapid shallow lateral flow. Flow pathways of urea-N were estimated using hysteresis trends in urea-N concentrations and hydrograph separation. Trends suggest that urea-N typically moved through quicker flow pathways, including subsurface lateral flow rather than slower flow pathways (e.g., groundwater). Based on the similar responses between urea-N and nitrate during events following dry antecedent conditions, agricultural sources of urea-N may be important. Additionally, in situ production of urea-N in stagnant ditch environments during baseflow conditions could contribute to observed peaks in urea-N during stormflow conditions. Annual urea-N loads for a sub-basin in the Manokin River watershed were estimated using USGS Load Estimator software (LOADEST). The cumulative estimated load of urea-N for January 1, 2010 to December 31, 2011 was 109 kg of urea-N. Seasonally, over the two year time period, spring urea-N loads were significantly higher than any other season, representing 51.2% of the total delivered load. During this same two year time period, stormflow represented approximately 18% of the time and delivered 55% of the urea-N load. Although most of the annual delivery of urea-N to the coastal zone occurred during the spring, one intense event in August of 2011 delivered 13.6% of the two-year total load. Following this late summer storm, urea-N concentrations in tidal rivers near the Chesapeake Bay increased above levels known to stimulate harmful algal blooms, highlighting the importance of stormflow in urea-N delivery through the surface water network.