Bacteria, Nutrient, and Dissolved Oxygen TMDL Development for Mispillion River and Cedar Creek, Delaware August 2006 Prepared for: Delaware Department of Natural Resources and Environmental Control Prepared by: Water Resources and TMDL Center Tetra Tech, Inc. Bacteria, Nutrient, and Dissolved Oxygen TMDL Development for Mispillion River and Cedar Creek, Delaware August 2006 Prepared for: Delaware Department of Natural Resources and Environmental Control Prepared by: Tetra Tech, Inc. TMDL Development for Mispillion River and Cedar Creek TABLE OF CONTENTS Preface ..........................................................................................................................................................1 1.0 Introduction............................................................................................................................................1 1.1 Background Information.....................................................................................................................1 1.2 Impairment Listing..............................................................................................................................4 1.3 Applicable Water Quality Standards..................................................................................................8 1.3.1 Dissolved Oxygen.......................................................................................................................9 1.3.2 Nutrients......................................................................................................................................9 1.3.3 Bacteria .....................................................................................................................................10 2.0 Data Review and Source Assessment ..................................................................................................11 2.1 Data Sources ....................................................................................................................................11 2.2 Data Review.....................................................................................................................................11 2.2.1 Dissolved Oxygen......................................................................................................................14 2.2.2 Nutrients.....................................................................................................................................15 2.2.3 Bacteria ......................................................................................................................................19 2.3 Source Identification.........................................................................................................................21 2.3.1 Point Sources .............................................................................................................................21 2.3.2 Nonpoint Sources.......................................................................................................................25 3.0 TMDL Endpoint Determination...........................................................................................................30 4.0 TMDL Methodology............................................................................................................................32 4.1 Watershed Model.............................................................................................................................33 4.1.1 Configuration of Key Model Components................................................................................33 4.1.1.1 Watershed Segmentation....................................................................................................33 4.1.1.2 Land Use Representation ...................................................................................................36 4.1.1.3 Hydrology Representation .................................................................................................37 4.1.1.4 Pollutant Representation ....................................................................................................38 4.1.1.5 Meteorological Representation ..........................................................................................41 4.1.1.6 Waterbody Representation.................................................................................................43 4.1.2 Watershed Model Calibration and Validation...........................................................................44 4.2 Receiving Water Model Configuration............................................................................................51 4.2.1 Grid Generation ........................................................................................................................52 4.2.2 Water Quality Model Structure.................................................................................................54 4.2.3 Boundary Conditions ................................................................................................................54 4.2.4 Initial Conditions ......................................................................................................................57 4.2.5 Model Calibration and Validation.............................................................................................57 4.3 Assumptions and Limitations...........................................................................................................63 4.3.1 Assumptions..............................................................................................................................63 4.3.2 Limitations ................................................................................................................................64 5.0 TMDL Results and Calculations..........................................................................................................65 5.1 Load Allocations (LAs) ...................................................................................................................67 5.2 Waste Load Allocations (WLAs).....................................................................................................71 5.3 Margin of Safety (MOS)..................................................................................................................72 5.4 Consideration of Critical Conditions ...............................................................................................72 5.5 Consideration of Seasonal Variation................................................................................................73 6.0 Reasonable Assurance and Implementation ........................................................................................74 7.0 Public Participation..............................................................................................................................76 References...................................................................................................................................................77 TOC-1 TMDL Development for Mispillion River and Cedar Creek Appendix A: Nutrients and DO TMDL Modeling Results Appendix B: Bacteria TMDL Modeling Results Appendix C: Baseline Accumulation Rates for Nutrients, BOD, and Enterococcus Appendix D: TMDL Loads and Reductions by Land Use TOC-2 TMDL Development for Mispillion River and Cedar Creek FIGURES Figure 1-1. Location of the Mispillion River and Cedar Creek watersheds. ...............................................3 Figure 1-2. Location of the impaired segments in the Mispillion River watershed.....................................6 Figure 1-3. Location of the impaired segments in the Cedar Creek watershed. ..........................................7 Figure 2-1. Sampling sites in the Mispillion River watershed...................................................................12 Figure 2-2. Sampling sites in the Cedar Creek watershed. ........................................................................13 Figure 2-3. Average, maximum, and minimum DO concentrations along the Mispillion River mainstem, from upstream to downstream.............................................................................................................14 Figure 2-4. Average, maximum, and minimum DO concentrations along the Cedar Creek mainstem, from upstream to downstream. ....................................................................................................................15 Figure 2-5. Average, maximum, and minimum TN concentrations along the Mispillion River mainstem, from upstream to downstream.............................................................................................................16 Figure 2-6. Average, maximum, and minimum TP concentrations along the Mispillion River mainstem, from upstream to downstream.............................................................................................................17 Figure 2-7. Average, maximum, and minimum TN concentrations along the Cedar Creek mainstem, from upstream to downstream. ....................................................................................................................18 Figure 2-8. Average, maximum, and minimum TP concentrations along the Cedar Creek mainstem, from upstream to downstream. ....................................................................................................................18 Figure 2-9. Average, maximum, and minimum enterococcus concentrations