Benthic TMDLs for the Goose Creek Watershed Submitted by Virginia Department of Environmental Quality Virginia Department of Conservation and Recreation Prepared by Interstate Commission on the Potomac River Basin March 1, 2004 Revised April 27, 2004 ICPRB Report 04-5 This report has been prepared by the staff of the Interstate Commission on the Potomac River Basin. Support was provided by the Virginia Department of Environment Quality and the U.S. Environmental Protection Agency. The opinions expressed are those of the authors and should not be construed as representing the opinions or policies of the United States or any of its agencies, the several states, or the Commissioners of the Interstate Commission on the Potomac River Basin. Benthic TMDLs for the Goose Creek Watershed Submitted by Virginia Department of Environmental Quality Virginia Department of Conservation and Recreation Prepared by Interstate Commission on the Potomac River Basin March 1, 2004 Revised April 27, 2004 __________________________________________ Benthic TMDLs For the Goose Creek Watershed TABLE OF CONTENTS TABLE OF CONTENTS ........................................................................................................I LIST OF TABLES ................................................................................................................IV LIST OF FIGURES ..............................................................................................................VI EXECUTIVE SUMMARY ................................................................................................ VII ACKNOWLEDGEMENTS ...........................................................................................XXIII CHAPTER ONE: INTRODUCTION.................................................................................. 1 1.1 TMDL Definition and Regulatory Information ............................................................. 1 1.2 Impairment Listing......................................................................................................... 1 1.3 Applicable Water Quality Standards ............................................................................. 4 CHAPTER TWO: WATERSHED CHARACTERIZATION........................................... 5 2.1 General Description of the Goose Creek Watershed ..................................................... 5 2.2 Climate........................................................................................................................... 5 2.3 Geology and Soils.......................................................................................................... 8 2.4 Ecoregion ....................................................................................................................... 8 2.5 Land Use and Population............................................................................................... 9 2.6 Streamflow................................................................................................................... 11 2.7 VADEQ’s Biological Assessment ............................................................................... 12 2.8 Biological Assessment By Other Organizations.......................................................... 16 2.8.1 Loudoun County Baseline Biological Monitoring Survey ................................... 16 2.8.2 Northern Virginia Regional Index of Biotic Integrity .......................................... 16 2.8.3 Citizen Monitoring................................................................................................ 17 2.8.4 Summary of Benthic Monitoring Data from Other Sources................................. 18 CHAPTER THREE: STRESSOR IDENTIFICATION OF THE IMPAIRMENTS TO THE BENTHIC COMMUNITYIN LITTLE RIVER AND LOWER GOOSE CREEK ................................................................................................................................................. 19 3.1 Introduction.................................................................................................................. 19 3.1.1 Potential Stressors................................................................................................. 19 3.1.2 Sources of Data..................................................................................................... 19 3.2 Description of Biological Monitoring Sites................................................................. 19 3.2.1 Description of Significant Features in Vicinity of the Lower Goose Creek Impairment...................................................................................................................... 19 3.2.2 Description of Significant Features in Vicinity of the Little River Impairment... 20 3.3 Toxics and Heavy Metals............................................................................................. 22 3.3.1 Chronic Toxicity Study of Little River and Goose Creek .................................... 22 3.3.2 Monitoring of Toxic Chemicals and Metals in the Water Column ...................... 22 3.3.3 Monitoring Toxic Chemicals and Metals in Sediment ......................................... 23 3.3.4 Monitoring Toxic Chemicals and Metals in Fish Tissue...................................... 24 _______________________________________________________________________i __________________________________________ Benthic TMDLs For the Goose Creek Watershed 3.3.5 Pesticide Use at the Goose Creek Golf Course..................................................... 27 3.3.6 Summary of the Evidence for the Identification of Toxics and Heavy Metals as Stressors .......................................................................................................................... 27 3.4 Nutrients....................................................................................................................... 27 3.4.1 Benthic Macroinvertebrate Monitoring ................................................................ 27 3.4.2 Physical Characterization and Habitat Assessment .............................................. 27 3.4.3 Water Quality Monitoring Data............................................................................ 28 3.4.3.1 Trophic State Classification........................................................................... 31 3.4.4 2002 Nonpoint Source Assessment ...................................................................... 32 3.4.5 Diurnal DO Measurements ................................................................................... 32 3.4.6 Summary of Analysis of Available Evidence Concerning Nutrients ................... 33 3.5 Hydromodification....................................................................................................... 34 3.5.1 Physical Characterization and Habitat Assessment .............................................. 34 3.5.2 Water Quality Data ............................................................................................... 34 3.5.3 Impact of Hydromodification ............................................................................... 35 3.6 Sediment ...................................................................................................................... 37 3.6.1 Benthic Macroinvertebrate Monitoring ................................................................ 37 3.6.2 Physical Characterization and Habitat Assessment .............................................. 38 3.6.3 Water Quality Monitoring Data............................................................................ 38 3.6.4 2002 Nonpoint Source Assessment ...................................................................... 40 3.6.5 Summary of Available Evidence Concerning Sediment ...................................... 40 3.7 Conclusion of the Stressor Identification For Goose Creek and Little River.............. 40 CHAPTER FOUR: REFERENCE WATERSHED SELECTION ................................. 41 4.1 Reference Watershed Approach .................................................................................. 41 4.2 Screening Procedure for Reference Watersheds.......................................................... 41 4.3 Comparison of Goose Creek and Little River With Their Reference Watersheds ...... 43 CHAPTER FIVE: DETERMINATION OF SEDIMENT LOADS FOR THE IMPAIRED AND REFERENCE WATERSHEDS ........................................................... 46 5.1 The GWLF Model........................................................................................................ 46 5.1.1 Simulation of Watershed Hydrology .................................................................... 47 5.1.2 Simulation of Erosion and Sediment Transport.................................................... 47 5.1.3 Seasonal Variability and Critical Conditions........................................................ 48 5.2 Hydrology Simulation and Calibration of the Impaired and Reference Watersheds... 48 5.2.1 Meteorological Data.............................................................................................. 50 5.2.2 Initial Parameter Values........................................................................................ 50 5.2.3 Hydrology Calibration .......................................................................................... 52 5.3 Sediment Source Assessment ...................................................................................... 60 5.3.1 Erosion From the Land Surface ...........................................................................