Planning Assistance to States Study for MADEP Brief of Assabet River Sediment and Dam Removal (for Sediment P-flux Reduction) Feasibility Study DECEMBER 2, 2009
EPA (Clean Water Act) Requires States to: EPA (Clean Water Act) Requires States to: Set Limits on Pollutants Set Limits on Pollutants Total Maximum Daily Load (TMDL) Total Maximum Daily Load (TMDL) Maximum amount of pollutant that can enter a Maximum amount of pollutant that can enter a water body … …and still meet water quality and still meet water quality water body standards standards
PHOSPHORUS TMDL FOR PHOSPHORUS TMDL FOR ASSABET RIVER ASSABET RIVER � Developed in 2004 � Developed in 2004 � Requires reductions of P loadings from four treatment plants to � Requires reductions of P loadings from four treatment plants to 0.1 mg/L during the growing season 0.1 mg/L during the growing season � 90% reduction in sediment phosphorus load � 90% reduction in sediment phosphorus load
Wastewater Treatment Plants Wastewater Treatment Plants will meet 0.1 seasonal limit will meet 0.1 seasonal limit with new facilities with new facilities � Westborough � Westborough – Includes Shrewsbury & Hopkinton – Includes Shrewsbury & Hopkinton – Construction underway Construction underway – � � Hudson Hudson – Construction almost complete Construction almost complete – � � Marlborough West Marlborough West – Includes Northborough Includes Northborough – – Plans approved Plans approved – – – Awaiting permit modification from Awaiting permit modification from EPA EPA � � Maynard Maynard – Construction contract signed Construction contract signed –
Corps Planning Assistance to States Study MADEP requested the Corps provide information on sediment and dam removal: – relative to the TMDL goal of 90 % reduction in phosphorus release from the sediments NOTE: The Corps is not involved in permitting the WWTFs discharges to the river - EPA and MADEP permit the WWTFs on the river
CDM assisted Corps in Study Technical Reports prepared by CDM for this study are: � Modeling Report – June 2008 � Sediment Management Plan – Dec 2008 Available at: http://www.nae.usace.army.mil/projects/ma/assab etriver/assabetriver.htm
There several Communities located along the ~32 mile river and the Communities share this resource
Dams create manmade impoundments along the Assabet River
Dams Sites Date from 1700 and 1800s Dam Name Town Year Built Dam site dates from Aluminum City Northborough 1925 pre-1900 Allen Street Northborough 1900 1720 Hudson Hudson Repaired 1987 1860 Gleasondale Stow 1924/1883 1750 Ben Smith Maynard 1850 1850 Powdermill Acton 1924 (being pre-1835 rebuilt)
The extent of influence of a Dam on water levels The extent of influence of a Dam on water levels extends beyond the visual pool extends beyond the visual pool Dam Structural Dam Height (ft) Estimated Extent of Dam influence Aluminum City 8.5 About 0.1 mile in Northborough Allen Street 12 About 0.6 miles to River St. in Northborough Hudson 15 About 1.2 miles to Chapin Rd. in Hudson Gleasondale 12 About 1.5 miles to Cox Rd. in Hudson Ben Smith 9 About 5 miles to Route 62 in Stow Powdermill 13 About 1 mile to Crane Ave in Maynard
CDM Modeling Methodology CDM Modeling Methodology -> River bed profile and Sediment sediment movement transport model Revised XS -> Water surface Water surface / profiles and river hydraulic model hydraulics Revised hydraulics Watershed and -> Water quality river water benefits quality model Sediment P fluxes Phosphorus flux -> Sediment P fluxes model
Scenarios Analyzed Scenarios Analyzed Wastewater Treatment Plant River Sediment Scenario Improvements Removal Dam Removal Existing conditions (2000) √ Planned WWTF improvements √ Dredging 3 ft from each impoundment √ Full dam removal all 6 dams √ Partial dam Hudson, removal Gleasondale, Ben Smith √ Partial dam Ben Smith removal
Findings – – WWTF Improvements WWTF Improvements Findings ( 2005 NPDES Permits of 1.0 TP winter and 0.1 TP growing season) ( 2005 NPDES Permits of 1.0 TP winter and 0.1 TP growing season) � Reducing phosphorus discharges from the � Reducing phosphorus discharges from the WWTFs has several impacts on water quality: WWTFs has several impacts on water quality: � Lower instream P concentrations in river Lower instream P concentrations in river � � Lower algal counts Lower algal counts � � Improved DO Improved DO � � Less algal settling Less algal settling � � Reduced P flux from sediment due to lower algal Reduced P flux from sediment due to lower algal � settling settling 60% reduction in P flux 60% reduction in P flux Note: high summer P fluxes are due not only to algal settling high summer P fluxes are due not only to algal settling Note: and cycling through sediment, but also the high P in the and cycling through sediment, but also the high P in the sediment during winter winter time time sediment during
Findings - - Sediment Removal/Dredging Sediment Removal/Dredging Findings � Based on USGS study, total P sediment � Based on USGS study, total P sediment concentration in the impoundments was highest in concentration in the impoundments was highest in top 3 ft of sediment top 3 ft of sediment � Removing the top 3 ft of sediment from each � Removing the top 3 ft of sediment from each impoundment results in the following water quality impoundment results in the following water quality impacts: impacts: � Reduced Reduced sediment phosphorus flux lasts only for a sediment phosphorus flux lasts only for a � few years; as dredged areas fill back in, P flux will ; as dredged areas fill back in, P flux will few years increase back to previous levels increase back to previous levels � Dredging increases residence time in impoundments Dredging increases residence time in impoundments � � Reduces reaeration and dissolved oxygen Reduces reaeration and dissolved oxygen � � Increases algal growth from deeper impoundments Increases algal growth from deeper impoundments � Dredging does not improve water quality Dredging does not improve water quality
Findings – – Dam Removal Dam Removal Findings � Dam removal results in the following water � Dam removal results in the following water quality impacts: quality impacts: � Reduced residence time Reduced residence time � � Reduced biomass production Reduced biomass production � � Improved DO Improved DO � � Less algal settling Less algal settling � � Reduced P flux from sediment due to lower algal Reduced P flux from sediment due to lower algal � settling settling Additional 20% reduction in P flux from dam Additional 20% reduction in P flux from dam removal for a total 0f 80%. removal for a total 0f 80%. Note: Removing the dams for the larger Removing the dams for the larger Note: impoundments has the greatest benefit impoundments has the greatest benefit
Estimated P-Flux Reduction P-Flux Scenario P Flux Change (mg P/m 2 -day) D/S: 21.6 Base Condition (2000) No Change U/S: 12.0 Planned Improvements D/S: 8.6 (WWTP TP @ 0.1 mg/l – ~ 60 % reduction summer U/S: 4.8 @ 1.0 mg/l - winter) D/S: 4.3 Planned Improvements plus ~ 80 % reduction Dam Removal – 6 dams U/S: 2.4 Planned Improvements plus ~ 80% reduction (Hudson and D/S: 4.3 Dam Removal – 3 dams d/s impoundments only) U/S: 4.8 (Hudson, Gleasondale, Ben ~ 60% reduction (u/s Smith) impoundments) ~ 70% reduction (Ben Smith Planned Improvements plus D/S: 6.5 and Powdermill Dam Removal – 1 dam impoundments only U/S: 4.8 (Ben Smith) ~ 60% reduction (Gleasondale and u/s impoundments)
Summary of P-Flux Seasonal Analysis (using P-flux model only) Although WWTF permitting is not part of this study - The P-flux model based on limited laboratory data indicated that winter instream P concentration may have an effect on summer sediment flux rates – reductions in phosphorus levels in WWTF discharges during the non-growing season may make a significant contribution toward achieving water quality standards – this contribution could not be quantified in this study MADEP is gathering additional data to help them assess this contribution as part of their Assabet River monitoring program
Considerations for Dam Removal Cost, Impacts, Permits
Dam removal - - sediment behind dams will sediment behind dams will Dam removal need to be managed to prevent downstream need to be managed to prevent downstream sediment movement sediment movement Sediment volume to be managed/dredged to prevent downstream movement of sediment with dam removal * Dam (CY) Aluminum City 1,300 Allen Street 2,230 Hudson 71,560 Gleasondale 27,860 Ben Smith 67,600 Powdermill 65,830 * Modeling used to determine volumes that would move downstream * Modeling used to determine volumes that would move downstream
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