Delaware Department of Natural Resources and Environmental Control MURDERKILL RIVER WATERSHED REVISED NUTRIENT AND DISSOLVED OXYGEN TMDLs Prepared by: HDR|HydroQual KCDW – 178287 March 2014 i PREFACE As required by the Federal Clean Water Act, the Delaware Department of Natural Resources and Environmental Control (DNREC) is responsible for implementing water quality monitoring and assessment activities in the State and also for establishing Total Maximum Daily Loads (TMDLs) on impaired State surface waters as indicated on the State’s 303(d) List. On May 12, 2005, the Cabinet Secretary of DNREC issued Order No. 2005-W-0025 adopting amended Total Maximum Daily Load (TMDL) Regulations for nutrients and oxygen consuming compounds for the entire Murderkill River Watershed. Since promulgation of the 2005 amended TMDL, a multi-year monitoring, research and modeling study of the Murderkill River Watershed by DNREC and other cooperating agencies and institutions resulted in proposing scientifically-based, site-specific dissolved oxygen (DO) criteria and nutrient targets for the tidal Murderkill River. This multi-year effort necessitated revisions to the 2005 TMDL that will comply with the proposed site-specific DO criteria and nutrient targets for the tidal Murderkill River. The proposed revisions to the Murderkill River Watershed nutrient and DO TMDL will be presented during a Public Hearing to be held on April 29, 2014 at the DNREC main office in Dover. All comments received before and during the Public Hearing process will be considered by DNREC. Based on the comments received, the report may be modified accordingly. i CONTENTS Section Page 1 INTRODUCTION ......................................................................................................................... 1-1 1.1 303(D) LISTED WATERBODIES ................................................................................ 1-4 1.2 DESIGNATED USES ...................................................................................................... 1-6 1.3 EXISTING WATER QUALITY STANDARDS AND NUTRIENT TARGETS ........................................................................................................................... 1-6 1.4 SITE-SPECIFIC DISSOLVED OXYGEN CRITERIA AND NUTRIENT TARGETS ........................................................................................................................... 1-6 2 WATERSHED CHARACTERISTICS........................................................................................ 2-1 2.1 LAND USE ......................................................................................................................... 2-1 2.2 POINT SOURCES ............................................................................................................ 2-1 3 WATERSHED MONITORING ................................................................................................. 3-1 3.1 OVERALL WATER QUALITY ASSESSMENT ....................................................... 3-2 3.2 SOURCES OF POLLUTION ......................................................................................... 3-3 4 MODELING FRAMEWORKS ................................................................................................... 4-1 4.1 MODEL SEGMENTATION/DELINEATION ....................................................... 4-2 4.2 MODEL SETUP & CALIBRATION/VALIDATION ............................................. 4-2 4.2.1 WATERSHED MODEL .................................................................................... 4-4 4.2.2 TIDAL HYDRODYNAMIC MODEL ......................................................... 4-11 4.2.3 TIDAL WATER QUALITY MODEL .......................................................... 4-11 4.3 MODEL DEVELOPMENT SUMMARY .................................................................. 4-25 5 LOADING SCENARIO ASSESSMENT .................................................................................. 5-1 5.1 NO KCRWTF DISCHARGE SCENARIO ................................................................. 5-3 5.2 TIDAL MARSH LOAD IMPACT SCENARIO ....................................................... 5-11 5.3 NATURAL BACKGROUND SCENARIO ............................................................... 5-21 6 SITE-SPECIFIC NUTRIENT TARGETS ................................................................................. 6-1 6.1 KCRWTF TREATMENT LEVEL SCENARIOS ...................................................... 6-1 6.2 PROPOSED SITE-SPECIFIC NUTRIENT TARGETS .......................................... 6-3 7 REVISED TMDL ANALYSIS ..................................................................................................... 7-1 7.1 TOTAL MAXIMUM DAILY LOADS AND THEIR ALLOCATIONS ............... 7-1 7.2 TMDL ENDPOINTS ....................................................................................................... 7-2 7.3 TMDL MODEL OUTPUT PRESENTATION .......................................................... 7-3 7.4 INTERPRETATION OF RESULTS .......................................................................... 7-11 8 THE REVISED MURDERKILL TMDL ................................................................................... 8-1 ii 8.1 CONSIDERATION OF THE IMPACT OF BACKGROUND POLLUTANTS 8-4 8.2 CONSIDERATION OF CRITICAL ENVIRONMENTAL CONDITIONS ...... 8-4 8.3 CONSIDERATION OF SEASONAL VARIATIONS ............................................. 8-5 8.4 CONSIDERATION OF MARGIN OF SAFETY ...................................................... 8-5 8.5 CONSIDERATION OF MODEL CAPABILITIES AND LIMITATIONS ........ 8-6 8.6 REASONABLE ASSURANCE ...................................................................................... 8-7 8.7 TMDL IMPLEMENTATION / PUBLIC PARTICIPATION ................................ 8-8 9 REFERENCES ................................................................................................................................ 9-1 iii FIGURES Figure Page Figure 1. Murderkill River Watershed Study Area .......................................................................... 1-2 Figure 2. Murderkill River Tidal Reaches where Site-Specific DO Criteria Apply .................... 1-8 Figure 3. Murderkill River Watershed HSPF Segmentation and Land Uses .............................. 2-2 Figure 4. Murderkill River Watershed 2007 Land Use for Major Sub-watersheds.................... 2-3 Figure 5. Murderkill River Watershed Septic System Distribution .............................................. 2-5 Figure 6. Murderkill River ECOMSED/RCA Model Grid .......................................................... 4-3 Figure 7. Murderkill River HSPF Flow Calibration/Validation (2007-2008) ............................. 4-5 Figure 8. Water Quality Model Calibration at Station 206561, Double Run at Barratts Chapel Rd. (Rd. 371) .......................................................................................................... 4-6 Figure 9. Water Quality Model Calibration at Station 206461, McGinnis Pond at McGinnis Pond Rd. (Rd. 378) .......................................................................................... 4-7 Figure 10. Water Quality Model Calibration at Station 206071, Andrews Lake at Rd. 380 Brdg. .............................................................................................................................. 4-8 Figure 11. Water Quality Model Calibration at Station 206451, Coursey Pond at Canterbury Rd. (Rt. 15) at Rd. 388 Bridge ...................................................................... 4-9 Figure 12. Water Quality Model Calibration at Station 206361, McColley Pond at Canterbury Rd. (Rt. 15) near Spillway ........................................................................... 4-10 Figure 13. Hydrodynamic Model Flux Calibration for August 2007 ........................................... 4-12 Figure 14. Hydrodynamic Model Flux Validation for August 2008 ............................................ 4-13 Figure 15. Hydrodynamic Model Salinity Calibration for August 2007 ...................................... 4-14 Figure 16. Hydrodynamic Model Salinity Calibration for August 2007 ...................................... 4-15 Figure 17. Hydrodynamic Model Salinity Calibration for August 2007 ...................................... 4-16 Figure 18. Hydrodynamic Model Salinity Validation for August 2008 ....................................... 4-17 Figure 19. Hydrodynamic Model Salinity Validation for August 2008 ....................................... 4-18 Figure 20. Hydrodynamic Model Salinity Validation for August 2008 ....................................... 4-19 Figure 21. Water Quality Model Calibration/Validation (2007-2008) ......................................... 4-21 Figure 22. Water Quality Model Calibration/Validation (2007-2008) ......................................... 4-22 Figure 23. Water Quality Model Calibration/Validation (2007-2008) ......................................... 4-23 Figure 24. Water Quality Model Calibration/Validation (2007-2008) ......................................... 4-24 Figure 25. Tidal Murderkill River Zones............................................................................................ 5-2 Figure 26. Model Sensitivity Run for No KCRWTF Loads (Summer Average Results) ........... 5-4 Figure 27. Loading Scenario Results at Bowers Beach (206101).................................................... 5-5 Figure 28. Loading Scenario Results at Milford Neck Wildlife Levee (206141) .........................