Nitrogen loads through baseflow, stormflow, and underflow to Rehoboth Bay, Delaware

Area of Science:

• Environmental Science
• Hydrology
• Water Quality

Background:

• Nitrogen (N) loading into estuarine systems is a critical environmental concern.
• Understanding N sources and fluxes is essential for managing coastal water quality.

Purpose of the Study:

• To determine total nitrogen loads to Rehoboth Bay from a representative subwatershed.
• To compare N loads estimated by different methods, including gauged flows, land use-land cover (LULC), and underflow contributions.

Main Methods:

• Collected water samples for ammonium, nitrate + nitrite, and organic N concentrations in baseflow and storm waters.
• Developed a novel hydrographic separation model to estimate N loads during unsampled storms.
• Utilized land use-land cover (LULC) data and export factors to estimate N loads.

Main Results:

• Nitrogen loads based solely on gauged flows (7,100–19,100 kg/yr) significantly underestimated LULC-based estimates (32,000–40,600 kg/yr).
• Incorporating ungauged underflow and associated N loads (25,500–33,800 kg/yr) improved agreement with LULC models.
• Watershed N loads (259,000–316,000 kg/yr) were substantially higher than atmospheric deposition (49,000–64,500 kg/yr) and wastewater treatment plant inputs (9,700–13,700 kg/yr).

Conclusions:

• Underflow significantly contributes to nitrogen fluxes in permeable coastal plain sediments, especially in drier years.
• The watershed is the primary source of nitrogen to Rehoboth Bay, with seasonal variations in relative contributions from watershed, wastewater, and atmospheric sources.