Volume I Stressor Analysis Report for the Benthic Macroinvertebrate Impairments in the Accotink Creek Watershed, Fairfax County, Virginia Prepared for Virginia Department of Environmental Quality Prepared by Interstate Commission on the Potomac River Basin Revised June 21, 2017 Cover Photo Accotink Creek near Hooes Road, Virginia. 2008. Photo by Virginia Department of Environmental Quality Disclaimer The opinions expressed in this report are those of the authors and should not be construed as representing the opinions or policies of the United States government or the signatories or Commissioners to ICPRB. Revised: 06/21/2017 Table of Contents List of Tables........................................................................................................................................................................ iv List of Figures....................................................................................................................................................................viii Acronyms ............................................................................................................................................................................xiii Units of Measure ............................................................................................................................................................... xv Executive Summary..........................................................................................................................................................ES-1 1 Introduction................................................................................................................................................................. 1-1 1.1 Applicable Water Quality Standards........................................................................................................ 1-3 1.1.1 Designated Uses ...................................................................................................................................... 1-3 1.1.2 Water Quality Criteria .......................................................................................................................... 1-3 1.1.3 Aquatic Life Use....................................................................................................................................... 1-4 1.2 Impairment Listings........................................................................................................................................ 1-4 1.3 Goals of Stressor Identification Analysis................................................................................................ 1-8 2 Watershed Description............................................................................................................................................ 2-1 2.1 Watershed Description and Identification ............................................................................................ 2-1 2.1.1 Topography............................................................................................................................................... 2-3 2.1.2 Hydrogeomorphic Regions................................................................................................................. 2-4 2.1.3 Soils .............................................................................................................................................................. 2-5 2.1.4 Land Use..................................................................................................................................................... 2-9 2.1.5 Population and Households .............................................................................................................2-16 2.2 Permitted Facilities .......................................................................................................................................2-16 2.2.1 Facilities with Individual Permits..................................................................................................2-17 2.2.2 Facilities with General Permits.......................................................................................................2-20 2.2.3 Municipal Separate Storm Sewer Systems (MS4s).................................................................2-23 2.2.5 Sewers.......................................................................................................................................................2-26 3 Analysis of Monitoring Data .................................................................................................................................. 3-1 3.1 Analysis of Biological Monitoring Data................................................................................................... 3-2 3.1.1 DEQ Benthic Monitoring...................................................................................................................... 3-2 3.1.2 EPA Biological Monitoring................................................................................................................3-11 3.1.3 Fairfax County Biological Monitoring ..........................................................................................3-14 3.1.4 Volunteer Monitoring .........................................................................................................................3-23 Stressor Identification Analysis for Accotink Creek Watershed i Revised: 06/21/2017 3.1.5 Summary of Biological Monitoring in the Accotink Creek Watershed...........................3-27 3.2 Habitat Assessment.......................................................................................................................................3-27 3.2.1 DEQ Habitat Assessment...................................................................................................................3-27 3.2.2 FCDPWES Habitat Assessment and Infrastructure Inventory...........................................3-31 3.3 Geomorphic Assessment.............................................................................................................................3-37 3.3.1 DEQ Geomorphic Assessment.........................................................................................................3-37 3.3.2 Fairfax County SPA Geomorphic Assessment...........................................................................3-41 3.3.3 EPA Particle Size Analysis.................................................................................................................3-43 3.4 Flow .....................................................................................................................................................................3-44 3.5 Analysis of Conventional Water Quality Monitoring Data ............................................................3-48 3.5.1 Temperature...........................................................................................................................................3-55 3.5.2 pH................................................................................................................................................................3-61 3.5.3 Dissolved Oxygen..................................................................................................................................3-64 3.5.4 Specific Conductance...........................................................................................................................3-69 3.5.5 Total Dissolved Solids.........................................................................................................................3-75 3.5.6 Chloride ....................................................................................................................................................3-79 3.5.7 Turbidity ..................................................................................................................................................3-84 3.5.8 Total Suspended Solids and Suspended Sediment .................................................................3-90 3.5.9 Ammonia..................................................................................................................................................3-96 3.5.10 Nitrate .......................................................................................................................................................3-98 3.5.11 Total Kjeldahl Nitrogen...................................................................................................................3-100 3.5.12 Total Nitrogen.....................................................................................................................................3-103 3.5.13 Total Orthophosphate......................................................................................................................3-105 3.5.14 Total Phosphorus...............................................................................................................................3-107 3.5.15 Summary of Conventional Water Quality Data .....................................................................3-110 3.5.16 FCDPWES Water Quality Monitoring ........................................................................................3-113 3.5.17 EPA Water Quality Monitoring ....................................................................................................3-114 3.6 Analysis of Metals and Toxics Monitoring Data..............................................................................3-116