Plum Creek Headwaters Subwatershed Sediment TMDL Blair County, Pennsylvania Prepared by: Final Draft Submitted for EPA approval, May 2021 TABLE OF CONTENTS EXECUTIVE SUMMARY......................................................................................................................................................................................... 1 Table 1. Summary of Annual Average TMDL (TMDLAvg) Variables for the Plum Creek Headwaters Subwatershed 1 th Table 2. Summary of 99 Percentile Daily Loading TMDL (TMDLMax) Variables for the Plum Creek Headwaters Subwatershed ........................................................................................................................................................... 1 INTRODUCTION ...................................................................................................................................................................................................... 2 Table 3. Aquatic-Life Impaired Stream Segments in the Plum Creek Headwaters Subwatershed per the 2018 Final Pennsylvania Integrated Report ................................................................................................................................ 3 Figure 1. Plum Creek Headwaters Subwatershed. .................................................................................................... 4 Table 4. Existing NPDES Permitted Discharges in the Plum Creek Headwaters Subwatershed and their Potential Contribution to Sediment and Phosphorus Loading. ................................................................................................. 5 TMDL APPROACH ................................................................................................................................................................................................... 6 SELECTION OF THE REFERENCE WATERSHED ....................................................................................................................................... 6 Table 5. Comparison of the Impaired (Plum Creek Headwaters) and Reference (Black Hole Creek) Subwatersheds. ......................................................................................................................................................... 7 Table 6. Existing NPDES-Permitted Discharges in the Black Hole Creek Subwatershed and their Potential Contribution to Sediment Loading. ........................................................................................................................... 9 Figure 2. Black Hole Creek Subwatershed. .............................................................................................................. 13 Figure 3. Stream conditions within the Plum Creek Headwaters Subwatershed. ................................................... 14 Figure 5. Landscapes within the Plum Creek Headwaters Subwatershed. .............................................................. 16 Figure 6. Landscapes within the Plum Creek Headwaters Subwatershed. .............................................................. 17 Figure 7. Agricultural practices in the Plum Creek Headwaters Subwatershed that may exacerbate sediment loading .................................................................................................................................................................... 18 Figure 8. Practices within the Plum Creek Headwaters Subwatershed that may be protective against agricultural pollution. ................................................................................................................................................................. 19 Figure 9. Example substrate conditions for stream segments originating in the mountainous/hilly areas of the Black Hole Creek Subwatershed. ............................................................................................................................. 20 Figure 10. Example substrate conditions within the White Deer Golf Course region of the Black Hole Creek Subwatershed. ........................................................................................................................................................ 21 Figure 11. Examples of beaver activity in the Black Hole Creek Subwatershed. ..................................................... 22 Figure 12. Examples of stream substrate conditions within the lower mainstem of the Black Hole Creek. ........... 23 Figure 13. Example landscapes within in the Black Hole Creek Subwatershed. ...................................................... 24 ii Figure 14. Examples of conditions within the Black Hole Creek Subwatershed that may help prevent sediment pollution. ................................................................................................................................................................. 25 Figure 15. Examples of conditions within the Black Hole Creek Subwatershed that may exacerbate sediment loading. ................................................................................................................................................................... 26 Figure 16. Approximate area of the Susquehanna Ordnance Depot within the Black Hole Creek Subwatershed. 27 Figure 17. Photograph looking into the Susquehanna Ordnance Depot from the fenced periphery along Maple Hill Drive. ....................................................................................................................................................................... 28 HYDROLOGIC / WATER QUALITY MODELING ....................................................................................................................................... 29 Figure 18. Riparian buffer analysis in the Plum Creek Headwaters Subwatershed ................................................ 32 Figure 19. Riparian buffer analysis in the Black Hole Creek Subwatershed ............................................................ 33 CALCULATION OF THE TMDLAVG .................................................................................................................................................................. 34 Table 7. Existing Annual Average Loading Values for the Black Hole Creek Subwatershed, Reference ................. 34 Table 8. Existing Annual Average Loading Values for the Plum Creek Headwaters Subwatershed, Impaired ....... 35 Table 9. Calculation of an Annual Average TMDL Value for the Plum Creek Headwaters Subwatershed ............. 36 CALCULATION OF LOAD ALLOCATIONS ................................................................................................................................................... 36 MARGIN OF SAFETY .......................................................................................................................................................... 36 WASTELOAD ALLOCATION .................................................................................................................................................. 36 LOAD ALLOCATION ............................................................................................................................................................ 37 LOADS NOT REDUCED AND ADJUSTED LOAD ALLOCATION ........................................................................................................ 37 Table 10. Average Annual Load Allocation, Loads Not Reduced and Adjusted Load Allocation ............................ 37 CALCULATION OF SEDIMENT LOAD REDUCTIONS ............................................................................................................................. 38 Table 11. Average Annual Sediment Load Allocations for Source Sectors in the Plum Creek Headwaters Subwatershed ......................................................................................................................................................... 38 CALCULATION OF A DAILY MAXIMUM “TMDLMAX” VALUE .............................................................................................................. 39 Table 12. Calculation of TMDLMax for the Plum Creek Subwatershed ..................................................................... 39 th Table 13. 99 Percentile of Daily Loading TMDL (TMDLMax) Variables for the Plum Creek Headwaters Subwatershed ......................................................................................................................................................... 40 th Table 14. Allocation of the 99 Percentile Daily Load Allocation (LAMax) for the Plum Creek Headwaters Subwatershed ......................................................................................................................................................... 40 CONSIDERATION OF CRITICAL CONDITIONS AND SEASONAL VARIATIONS ......................................................................... 41 RECOMMENDATIONS ........................................................................................................................................................................................ 41 PUBLIC PARTICIPATION .................................................................................................................................................................................. 42 CITATIONS .............................................................................................................................................................................................................. 42 APPENDIX A: BACKGROUND ON STREAM ASSESSMENT