Cabin John Creek Watershed Assessment Summary Document October 2018 Prepared for: Montgomery County Department of Environmental Protection 255 Rockville Pike, Suite 120 Rockville, MD 20850 Prepared by: Cabin John Creek Watershed Assessment Summary Document Table of Contents Acronym List ................................................................................................................................................. ii 1 Introduction .......................................................................................................................................... 1 Implementation Plans ........................................................................................................................... 1 Runoff Management and Impervious Cover Treatment ....................................................................... 1 Total Maximum Daily Loads (TMDLs) ................................................................................................... 1 Trash and Litter Management .............................................................................................................. 1 2 Goals and Existing Conditions ............................................................................................................... 3 2.1 Existing Conditions in the Cabin John Creek Watershed .............................................................. 3 Introduction to Cabin John Creek Watershed ...................................................................................... 3 Subwatershed Summaries .................................................................................................................... 5 Watershed Land Use ........................................................................................................................... 13 Existing Stormwater Management Facilities ...................................................................................... 13 2.2 Problems Facing the Cabin John Creek Subwatersheds ............................................................. 15 Biological and Habitat Conditions ....................................................................................................... 15 Water Quality Issues ........................................................................................................................... 15 2.3 Existing Pollutant Loads and Impervious Surfaces ...................................................................... 17 Sediment Loads per the TMDL for Cabin John Creek Watershed ....................................................... 17 Bacteria Loads per the TMDL for Cabin John Creek Watershed ......................................................... 17 Nutrient Loads .................................................................................................................................... 18 Impervious Surfaces ............................................................................................................................ 18 Existing Trash Loads ............................................................................................................................ 18 3 Inventory of Stormwater Management Opportunities ...................................................................... 20 3.1 Types of Stormwater Management ............................................................................................ 20 Stream Restoration ............................................................................................................................. 20 Regenerative Stormwater Conveyance (RSC) and Outfall Stabilization ............................................. 20 Stormwater Management Facility ...................................................................................................... 20 Stormwater Management .................................................................................................................. 20 Potential Green Streets Corridor ........................................................................................................ 21 3.2 Inventory of Stormwater Management Opportunities .............................................................. 21 3.3 Inventory of Stormwater Management Opportunities .............................................................. 25 4 References .......................................................................................................................................... 26 Appendix A Catchment Assessments Appendix B Methods Memorandum Page i Cabin John Creek Watershed Assessment Summary Document Acronym List DA – drainage area DEP – Department of Environmental Protection DU – dwelling unit EPA – US Environmental Protection Agency ESD – environmental site design HUC – hydrologic unit code HOA – homeowners association IA – impervious area IP – Implementation Plan MDE – Maryland Department of the Environment M‐NCPPC – Maryland‐National Capital Parks and Planning Commission MS4 – Municipal Separate Storm Sewer System NPDES – National Pollutant Discharge Elimination System ROW – right‐of‐way RSC – Regenerative Stormwater Conveyance TMDLs – Total Maximum Daily Loads TN – total nitrogen TP – total phosphorus TSS – total suspended solids WIP – Watershed Implementation Plan WLAs – Waste Load Allocations WSSC – Washington Suburban Sanitary Commission Page ii Cabin John Creek Watershed Assessment Summary Document 1 Introduction In 2016 the Montgomery County Department of Environmental Protection (DEP) initiated a watershed assessment for the Cabin John Creek watershed. The last time a full watershed assessment had been completed for Cabin John Creek was in 2004. Desktop and field methodologies (see Appendix B) were employed in 2017 through early 2018 to update DEP’s understanding of the current conditions within the watershed. These assessments were used to document future restoration opportunities within the watershed as an update to opportunities identified in the Cabin John Creek Implementation Plan submitted to the Maryland Department of the Environment (MDE) in 2012. This Summary Document provides an overview of the restoration opportunities identified and will assist DEP in complying with future National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer System (MS4) Permit requirements and in developing an updated Total Maximum Daily Load Implementation Plan for the Cabin John Creek watershed. Relevant MS4 Permit requirements are described in further detail below. Implementation Plans As required by the County’s current MS4 Permit, DEP develops watershed assessments by evaluating current water quality conditions and then identifying and ranking structural, non‐structural and programmatic watershed restoration opportunities for each County watershed. Full watershed assessments include field investigations, prioritized restoration project inventories, and cost estimates. Implementation plans to identify restoration opportunities, estimate treatment to be provided by those opportunities, determine watershed restoration potential, evaluate the ability of the watersheds to meet applicable Total Daily Maximum Loads (TMDLs), and provide schedules and cost estimates. This Summary Document is not intended to serve as a full implementation plan. It identifies restoration opportunities and provides estimated treatment provided by restoration opportunities but does not include evaluation of compliance with applicable TMDLs. Runoff Management and Impervious Cover Treatment The County’s current MS4 permit requires that the County restore an additional 20% of untreated impervious cover within the County’s MS4 Permit area which translates to stormwater control for an additional 3,778 impervious acres. The County has made significant progress toward meeting this additional requirement for watershed restoration through multiple programs and anticipates that it will meet this goal by December 2020. The County utilizes watershed assessments, such as the one documented within this Summary Document, to continue to identify future restoration opportunities. Total Maximum Daily Loads (TMDLs) The permit requires the County to develop implementation plans to achieve progress toward the County’s Waste Load Allocations (WLAs) associated with the TMDLs that existed when the permit was issued in 2010. These plans were developed and submitted within 1 year of the start of the permit, as required. Additional TMDLs were added after the permit was issued and TMDL implementation plans either have been completed or are included in a Countywide Coordinated Implementation Strategy (2012). TMDLs specific to the Cabin John Creek watershed are addressed in Section 2.3. Trash and Litter Management The County actively participates in multiple programs and partnerships designed to meet the goals of the Potomac River Watershed Trash Treaty. Initiatives directly related to regional campaigns include 1 Cabin John Creek Watershed Assessment Summary Document ongoing education and outreach for recycling and litter reduction, mass media outreach campaigns, and litter removal from streets, stormwater ponds, and transit stops. 2 Cabin John Creek Watershed Assessment Summary Document 2 Goals and Existing Conditions 2.1 Existing Conditions in the Cabin John Creek Watershed Introduction to Cabin John Creek Watershed The Cabin John Creek watershed is a Maryland 8 and 12‐digit code watershed located in southeastern Montgomery County. As shown in Figure 2.1, nine subwatersheds, comprise the Cabin