BENNETT CREEK WATERSHED ASSESSMENT March 20, 2008 Bennett Creek Watershed Assessment ABSTRACT The Bennett Creek Watershed, which is a subwatershed of the Lower Monocacy Basin, was the fourth watershed to be selected for a retrofit assessment by the Frederick County National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer System (MS4) permit program. The first part of the assessment involved gathering all available information on the condition of the watershed. The next step involved using this data to rank the fifteen Bennett Creek subwatersheds in order of priority for restoration. Stressors were then identified at random and targeted site locations throughout the watershed by using a series of logical steps based on the US EPA Stressor Identification Guidance Document. Impairments were evaluated, candidate causes of impairment were described, relationships between causes, stressors and biotic conditions were assessed, and probable stressors were identified based on strength of evidence. Stressors varied among subwatersheds, but nutrient enrichment and habitat degradation were the most commonly cited candidate causes of impairment, followed by excessive sediment and turbidity. The next phase of this analysis will involve identifying priority restoration sites and recommending projects to reverse, prevent, or slow stream and watershed degradation. Effectiveness of stressor reduction projects and best management practices will be evaluated by the county based on monitoring the stressors they are designed to control (sediment, water temperature, nutrients). Effectiveness of the overall watershed management will be evaluated based on the overall biological condition of the watershed as monitored through continued probabilistic sampling. Tetra Tech, Inc. ii Bennett Creek Watershed Assessment Table of Contents ABSTRACT 1. WATERSHED CHARACTERIZATION .................................................................1 1.1 Background ......................................................................................................1 1.2 Bennett Creek Watershed Overview ..............................................................1 2. STRESSOR SOURCE INVENTORY......................................................................14 2.1 Overview .......................................................................................................14 2.1.1 Response indicators (biological) ................................................................14 2.1.2 Stressor indicators (physical, chemical, hydrologic, biological) .............15 2.1. Source indicators (land use/land cover, NPDES permits, other) .............17 2.2. Spatial and temporal distribution of data ..................................................18 2.3. Data sufficiency .............................................................................................19 3. IDENTIFICATION OF STRESSORS .....................................................................38 3.1. Methods ..........................................................................................................38 3.1.1. Stressor Identification Process....................................................38 3.1.2. Data Analysis ................................................................................39 3.2. Results and Discussion .................................................................................41 3.2.1. Description of Impairment ..........................................................41 3.2.2. Conceptual Model/Candidate Causes ........................................42 3.2.3. Correlation Analyses ...................................................................44 3.2.3.1. Bennett Creek ..................................................................44 3.2.3.2. Northern Piedmont .........................................................47 4. PLEASANT BRANCH – STRESSOR IDENTIFICATION .................................49 4.1. Description of Impairment ...........................................................................49 4.2. Conceptual Model/Candidate Causes .........................................................49 4.3. Analysis of Evidence–Associating candidate causes ..................................49 4.4. Characterization of Causes ..........................................................................50 4.4.1. Elimination of candidate causes .................................................50 4.4.2. Strength of evidence .....................................................................50 4.4.3. Identification of probable causes ................................................50 4.5. Summary of Results ......................................................................................52 5. BENNETT CK – UPPER MAINSTEM – STRESSOR IDENTIFICATION .....79 5.1. Description of Impairment ...........................................................................79 5.2. Conceptual Model/Candidate Causes .........................................................79 5.3. Analysis of Evidence–Associating candidate causes ..................................79 5.4. Characterization of Causes ..........................................................................80 5.4.1. Elimination of candidate causes .................................................80 5.4.2. Strength of evidence .....................................................................80 5.4.3. Identification of probable causes ................................................80 5.5. Summary of Results .....................................................................................81 6. FAHRNEY – STRESSOR IDENTIFICATION .....................................................99 6.1. Description of Impairment ...........................................................................99 6.2. Conceptual Model/Candidate Causes .........................................................99 6.3. Analysis of Evidence – Associating candidate causes: Fahrney (east) .....99 6.4. Characterization of Causes: Fahrney (east) .............................................100 6.4.1. Elimination of candidate causes ...............................................100 Tetra Tech, Inc. iii Bennett Creek Watershed Assessment 6.4.2. Strength of evidence ...................................................................100 6.4.3. Identification of probable causes ..............................................100 6.5. Analysis of Evidence – Associating candidate causes: Fahrney (west) ..103 6.6. Characterization of Causes: Fahrney (west) ............................................103 6.6.1. Elimination of candidate causes ...............................................103 6.6.2. Strength of evidence ...................................................................104 6.6.3. Identification of probable causes ..............................................104 6.7. Summary of Results ....................................................................................105 7. MONOCACY DIRECT – NORTH - STRESSOR IDENTIFICATION ............138 7.1. Description of Impairment .........................................................................138 7.2. Conceptual Model/Candidate Causes .......................................................138 7.3. Analysis of Evidence – Associating candidate causes ..............................138 7.4. Characterization of Causes ........................................................................139 7.4.1. Elimination of candidate causes ...............................................139 7.4.2. Strength of evidence ...................................................................139 7.4.3. Identification of probable causes ..............................................139 7.5. Summary of Results ...................................................................................141 8. LITTLE BENNETT – STRESSOR IDENTIFICATION ....................................160 8.1. Description of Impairment .........................................................................160 8.2. Conceptual Model/Candidate Causes .......................................................160 8.3. Analysis of Evidence – Associating candidate causes ..............................160 8.4. Characterization of Causes ........................................................................161 8.4.1. Elimination of candidate causes ...............................................161 8.4.2. Strength of evidence ...................................................................161 8.4.3. Identification of probable causes ..............................................161 8.5. Summary of Results ...................................................................................162 9. BENNETT MIDDLE MAINSTEM– STRESSOR IDENTIFICATION ............177 9.1. Description of Impairment .........................................................................177 9.2. Conceptual Model/Candidate Causes .......................................................177 9.3. Analysis of Evidence – Associating candidate causes ..............................177 9.4. Characterization of Causes ........................................................................177 9.4.1. Elimination of candidate causes ...............................................177 9.4.2. Strength of evidence ...................................................................178 9.4.3. Identification of probable causes ..............................................178 9.5. Summary of Results ...................................................................................178