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TECHNICAL REPORTS

Landscape and Watershed Processes

Nitrogen Loads through Baseflow, Stormflow, and Underflow to Rehoboth Bay, Delaware

^a Watershed Assessment Section, Delaware Department of Natural Resources and Environmental Control, 820 Silver Lake Boulevard, Suite 220, Dover, DE 19904
^b College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958-1298
^c Delaware Geological Survey, University of Delaware, Newark, DE 19716-7501

^* Corresponding author (ullman{at}udel.edu)

Received for publication September 26, 2005. A detailed study of water and nitrogen (N) discharge from a small, representative subwatershed of Rehoboth Bay, Delaware, was conducted to determine total N loads to the bay. The concentrations of ammonium (NH₄⁺), nitrate + nitrite (NO₃^– + NO₂^–), and dissolved and particulate organic N were determined in baseflow and storm waters discharging from Bundicks Branch from October 1998 to April 2002. A novel hydrographic separation model that accounts for significant decreases in baseflow during storm events was developed to estimate N loads during unsampled storms. Nitrogen loads based on gauged flows alone (7100–19 100 kg/yr) significantly underestimated those based on land use–land cover (LULC) and estimated N export factors from different classes of LULC (32 000–40 600 kg/yr). However, when ungauged underflow and associated N loads were included in the total loads (25500–33800 kg/yr), there was much better agreement with LULC export models. This suggests that in permeable coastal plain sediments, underflow contributes significantly to N fluxes to estuarine receiving waters, particularly in drier years. Based on the similarity in LULC, N loads from the Bundicks Branch subwatershed were used to estimate upland loads to the entire Rehoboth Bay Watershed (259 000–316000 kg/yr). These N loads from the watershed were much greater than those from direct atmospheric deposition (49000–64 500 kg/yr) and from a local wastewater treatment plant (9700–13700 kg/yr). While the watershed was the principal source of N at all times during the year, the relative contributions from the watershed, wastewater, and direct atmospheric deposition varied predictably with season.

Abbreviations: LULC, land use–land cover • PON, particulate organic nitrogen • RBWTP, Rehoboth Beach Wastewater Treatment Plant • TDN, total dissolved nitrogen • TN, total nitrogen

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