Submitted to: American Geophysical Union
Publication Type: Abstract only
Publication Acceptance Date: 9/14/2012
Publication Date: 12/3/2012
Citation: Tsilkowski, S., Buda, A.R., Boyer, E., Bryant, R.B., May, E. 2012. Toward understanding mechanisms controlling urea delivery in a coastal plain watershed[abstract]. American Geophysical Union. Poster No. H13C-1353. Interpretive Summary: An interpretive summary is not required.
Technical Abstract: Improved understanding of nutrient mobilization and delivery to surface waters is critical to protecting water quality in agricultural watersheds. Urea, a form of organic nitrogen, is a common nutrient found in fertilizers, manures, and human waste, and is gaining recognition as an important driver of coastal eutrophication, particularly through the development of harmful algal blooms. While several studies have documented elevated urea concentrations in tributaries draining to the Chesapeake Bay, little is known about the potential sources and flow pathways responsible for urea delivery from the landscape to surface waters, as well as how these sources and pathways might vary with changing seasons, antecedent conditions, and storm types. In this study, we investigated hydrologic controls on urea delivery in the Manokin River watershed through the analysis of urea concentration dynamics and hysteresis patterns during seven storm events that occurred in 2010 and 2011. The Manokin River is a Coastal Plain watershed (11.1 km**2) on the Delmarva Peninsula that drains directly to the Chesapeake Bay and is characterized by extensive rural development coupled with intensive agriculture, particularly poultry production. Sampling was conducted through monthly grab sampling at baseflow conditions and time-weighted, automated (Sigma) samplers during stormflow events. Monitored storms were chosen to represent a spectrum of antecedent conditions based on precipitation and groundwater levels in the area. Flushing from the landscape during events was found to be the predominant urea delivery mechanism, as urea concentrations increased 3-9 times above baseflow concentrations during storms. The timing and number of flushes, as well as the degree of increased concentrations were dependent on antecedent conditions and the nature of the storm event. For instance, during an intense (13.7 mm hr**-1), short-duration (4 hrs) storm in August of 2010 when antecedent conditions were dry (5-day antecedent precipitation index = 0 mm), we observed and an anticlockwise hysteresis pattern and a delayed, yet high peak in urea concentrations (0.17 mg L**-1) on the falling limb of the hydrograph. These trends suggest that urea was delivered via slow, diffuse flow pathways and from sources distal from the sampling point. In contrast, during a less intense (3.2 mm hr**-1), longer duration (22 hrs) storm in October of 2010 when antecedent conditions were wetter (5-day antecedent precipitation index = 67.31 mm), we observed a clockwise hysteresis pattern and a smaller peak in urea concentrations (0.06 mg L**-1) timed with the hydrograph peak. Here, the trends suggest that urea delivery occurred through faster flow pathways (e.g., shallow lateral flow) and from proximal (e.g., near-stream, in-stream) sources of urea. Collectively, these trends demonstrate that urea is flushed to streams during storm events, but that the mechanism for delivery depends on antecedent conditions, as well as the nature of the storm event.