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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

View larger version (60K): [in a new window]   Fig. 1. The Inland Bays watershed showing the locations of Rehoboth Bay, the Bundicks Branch and Millsboro Pond subwatersheds and gauging stations, the Rehoboth Beach Wastewater Treatment Plant (RBWTP), the urban and sewered areas of the watershed, and the Georgetown and Cape Henlopen rain collection sites.

View larger version (32K): [in a new window]   Fig. 2. Rainfall (at both Georgetown and Cape Henlopen) and gauged discharge at Bundicks Branch during the study period. Monthly rainfall did not vary greatly across the watershed. The baseflow contributions to the gauged discharge, QBF, determined by the traditional and alternative hydrographic separation models are shown. There are substantial differences between these models only during the wettest months.

View larger version (29K): [in a new window]   Fig. 3. Comparison of the traditional hydrographic separation model with the alternative model used in this study. The traditional method yields a maximum value of gauged baseflow (QBF), a minimum gauged stormflow (QSF), and therefore apparently negative values of stormflow nitrogen concentration (CSF) and storm loads (LSF) during some periods during storms. The alternative procedure yields a minimum QBF, maximum value of QSF, and only rarely negative values of CSF and LSF.

View larger version (18K): [in a new window]   Fig. 4. Dissolved N concentrations (NO3– and total dissolved nitrogen, TDN) at Bundicks Branch decreased with increasing baseflow discharge (QBF) during the study period. The concentrations at low flow appear to be a good estimate of median ground water concentrations in the Bundicks Branch subwatershed.

View larger version (48K): [in a new window]   Fig. 5. Relative magnitude and seasonal distribution of total nitrogen (TN) loads (LT = LBF + LSF + LUF, where BF is baseflow, SF is stormflow, and UF is underflow) from the Bundicks Branch watershed to the Rehoboth Bay ecosystem during the study period. Baseflow N loads [= LBF(TDN) + LBF(PON), where TDN is total dissolved nitrogen and PON is particulate organic nitrogen] were highest during high flow periods as TN in baseflow was less variable during the experimental period. Total dissolved N predominated at all times, but PON loads had a seasonal pattern and were largest during the wettest periods.

View larger version (28K): [in a new window]   Fig. 6. Discharge components, N concentrations and speciation, and N loads during the storm of 10 Apr. 2002, the smallest of the sampled storms. During this storm N concentrations and speciation varied little with discharge and therefore the gauged loads were proportional to discharge.

View larger version (40K): [in a new window]   Fig. 7. Discharge components, N concentrations and speciation, and N loads during the storm of 10–12 Aug. 2001. The storm had two peaks and showed clear evidence of a "first-flush" effect during the initial rise in the hydrograph, with particulate organic nitrogen (PON) becoming a dominant component of the N load.

View larger version (20K): [in a new window]   Fig. 8. Storm loads determined by the alternative hydrographic separation method for 17 sampled storms at Bundicks Branch. Although there was substantial variation in the storm loads (a), there was substantially less variation in the average storm concentration (SF, b), which can therefore be used to estimate storm loads for unsampled storms from discharge alone.

View larger version (40K): [in a new window]   Fig. 9. Monthly unit underflow (QUF*) at Bundicks Branch based on the difference between unit gauged flows (QG*) at Millsboro Pond and Bundicks Branch. This is a minimum estimate of QUF*, based on the assumption that there is no underflow at Millsboro Pond. Underflow was a significant fraction of total discharge at Bundicks Branch in all but the wettest periods and dominated in the driest periods.

View larger version (38K): [in a new window]   Fig. 10. Comparison of monthly N loads from the watershed to Rehoboth Bay with loads due to atmospheric deposition and municipal wastewater (Rehoboth Bay Wastewater Treatment Plant, RBWTP). Nitrogen loads from the watershed predominated at all times of the year, but the relative importance of N sources varied predictably through the year.