HOP BROOK SEDIMENT AND DAM REMOVAL STUDY MARLBOROUGH AND SUDBURY MASSACHUSETTS DRAFT REPORT Department of the Army New England District U.S. Army Corps of Engineers Concord, MA July 2008 TABLE OF CONTENTS INTRODUCTION .............................................................................................................. 1 Study Authority............................................................................................................... 1 Study Background........................................................................................................... 1 Goals and Objectives ...................................................................................................... 2 Study Area ...................................................................................................................... 2 Prior Studies and Reports................................................................................................ 6 EXISTING CONDITIONS................................................................................................. 6 Dam Conditions .............................................................................................................. 6 Dam Safety...................................................................................................................... 8 Existing Uses .................................................................................................................. 9 Sediment Quantity and Quality..................................................................................... 10 Natural Resources ......................................................................................................... 12 Introduction............................................................................................................... 12 Fish............................................................................................................................ 13 Aquatic Invertebrates................................................................................................ 14 Wetlands and Riparian Vegetation ........................................................................... 15 Invasive Species........................................................................................................ 17 Wildlife ..................................................................................................................... 18 Rare Species Habitat................................................................................................. 19 Restoration of Anadromous Fish .............................................................................. 20 American Eel ............................................................................................................ 21 Cultural Resources Identification ................................................................................. 21 Real Estate Identification.............................................................................................. 22 ALTERNATIVES............................................................................................................. 22 Alternative 1: Removal of Hager Pond Dam............................................................... 23 Alternative 2: Removal of Carding Millpond, and Stearns Millpond Dams ................ 23 Alternative 3: Removal of Carding Millpond Dam ...................................................... 23 Alternative 4: Partial Breach of Hager Pond Dam........................................................ 24 Alternative 5: Partial Breach of Carding Millpond Dam.............................................. 24 Alternative 6: Partial Breach of Carding Millpond and Stearns Millpond Dams......... 24 Alternative 7: Partial Breach of Hager Pond, Carding Millpond and Stearns Millpond Dams ............................................................................................................................. 24 Alternative 8: Dredging of Hager Pond, Grist Millpond, Carding Millpond and Stearns Millpond........................................................................................................................ 24 MODELING RESULTS AND COMPARISONS OF ALTERNATIVES....................... 25 i PRELIMINARY COSTS.................................................................................................. 26 ADDITIONAL ALTERNATIVES................................................................................... 29 SUMMARY...................................................................................................................... 30 REFERENCES ................................................................................................................. 31 LIST OF TABLES Table 1. Dam Information, Year Built............................................................................... 7 Table 2. Impoundment Characteristics .............................................................................. 7 Table 3. General Characteristics of Dams ......................................................................... 8 Table 4. Dam Safety Hazard Rating .................................................................................. 9 Table 5. Existing Uses ....................................................................................................... 9 Table 6. Sediment Volumes............................................................................................. 11 Table 7. Chemical groups where one or more samples exceeded guidelines.................. 11 Table 8. List of Warm-water Fish Species....................................................................... 14 Table 9. Estimated Wetland Areas................................................................................... 17 Table 10: Rare Species List .............................................................................................. 20 Table 11. Dam Owners .................................................................................................... 22 LIST OF FIGURES Figure 1. Basin Map........................................................................................................... 4 Figure 2. River Schematic.................................................................................................. 5 APPENDICES Appendix A – Dam Site Visit Reports and Sediment Management Plan Appendix B – Cost Estimates Appendix C - Sediment Quality Appendix D - Natural Resources Appendix E - Cultural Resources Appendix F - Real Estate Information Appendix G - Hydrologic and Hydraulic Analysis Appendix H – Additional Alternatives ii INTRODUCTION Study Authority This investigation is being conducted by the Corps of Engineers, New England District under the Work for Others authority pursuant to 31 U.S.C Sec 6505 (Intergovernmental Cooperation Act). The study was performed using 100% non-Federal funding from the Massachusetts Department of Environmental Protection (MADEP). Study Background Hop Brook and its impoundments currently do not meet state requirements for water quality. Areas behind dams experience low dissolved oxygen and excessive growth of aquatic vegetation. Both factors result in degraded aquatic habitat. The primary issue is too much phosphorus input to the waterway. Phosphorus, a nutrient, when elevated above normal background levels causes excessive production of floating and rooted aquatic plants. This vegetation growth and decomposition negatively impacts the water column’s dissolved oxygen levels. Adequate dissolved oxygen is required to support aquatic life. Phosphorus loadings originate from both point and non-point sources. Point sources include the Marlborough East Wastewater Treatment Plant (MWWTP), while non-point sources include internal recycling of phosphorus from sediments and storm water runoff. Current wastewater discharge permit (September 2004) jointly issue by the U.S. Environmental Protection Agency (EPA) and MADEP sets long term limits for phosphorus to 0.1 mg/l from April through November and 0.75 mg/l from December through March. The permit requires compliance with the phosphorus limit within four years. The earlier NPDES permit limits allowed for phosphorus levels to be 0.75 mg/l in the effluent (EPA press release # 04-09-10). Hop Brook has been the subject of several water quality studies during recent years due to the excessive nutrients (phosphorus and nitrogen) that have entered the brook from the watershed. The MWWTP discharges into Hop Brook near its headwaters, and it is believed that excess nutrients (phosphorus and nitrogen) from this discharge are primarily responsible for eutrophication at four downstream impoundments, resulting in excessive aquatic macrophytes and algal growth. Current upgrades to the MWWTP include 1 improvements to its tertiary treatment to limit concentrations of phosphorus and nitrogen. The impact of this wastewater treatment plant is significant in that during periods of low flow (July-September) it is believed that at least 75% of the total stream volume in Hop Brook is comprised of effluent from this facility (ENSR Report, 2001). The downstream impoundments affected by these excess nutrients lie within six miles of the headwaters of Hop Brook, in the City of Marlborough and town of Sudbury. These include (in order from upstream to downstream) Hager Pond, Grist Millpond, Carding Millpond and Stearns Millpond. The Study Area is shown in Figure 1. In addition to the nutrients entering these impoundments from the