2/16/2012 SEWER COMMISSION Minutes

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Massachusetts Estuary Project(MEP) Linked Watershed Embayment Model Peer Review l sources were assumed to enter the groundwater system,with negligible direct surface water delivery to ponds, streams or estuaries. The inputs to these watershed models are the following: • Septic o Parcel-level water use o Applied nitrogen concentrations • Fertilizer o Application rates and lawn size • Stormwater and nonpoint sources o Nitrogen load factors based on event mean concentrations The hydrodynamic models compute flow velocities and tidal heights in the estuaries. The inputs to these models are inflows from the watershed,bathymetry and ocean boundary conditions for tidal heights. The salinity models compute dispersion coefficients in the estuaries. The inputs to these models are the flows and tidal heights from the hydrodynamic models,and ocean boundary concentrations for salinity. The nitrogen water quality models compute nitrogen concentrations in the estuaries. The inputs to these models are the flows and tidal heights from the hydrodynamic models,dispersion coefficients from the salinity models,nitrogen mass loads from the watershed, atmosphere and benthic regeneration,and ocean boundary conditions for nitrogen concentration. Threshold nitrogen concentration targets are developed by MEP for each specific estuary solely on the basis of observed monitoring data. These data are used to link nitrogen concentrations in the estuaries to water quality(chlorophyll and dissolved oxygen)and ecological(benthic communities and eelgrass) responses. This is considered a"reference"approach for developing site-specific numeric nutrient criteria (Howes et al.2003,U.S.EPA 2001). Finally,to conduct nitrogen load planning scenarios and develop TMDLs,these nitrogen concentration targets are linked to nitrogen concentrations computed by the nitrogen water quality model,and then back to nitrogen mass loads computed by the watershed model(Figure 2). However,because of uncertainties incumbent in management and response of nitrogen-enriched estuaries,and consistent with the recommended adaptive management framework,the DSS should continue to inform and reinforce the process beyond TMDL development. Continued monitoring,evaluation and adjustment of the modeling tools(Figure 1),as well as the management actions,are essential to cost effective outcomes. 4.3. Scientific Credibility and Functional Adequacy The Panel finds that the MEP modeling approach is scientifically credible. It is consistent with current understanding of existing conditions for Cape Cod estuaries, based on available data. The components in the approach are well-known and documented. Computation of watershed nitrogen loads is strongly data- driven and quantitatively linked to estuarine nitrogen concentrations. A fundamental principle in the development and application of environmental models to inform management decisions is that there should be compatibility among the study questions and objectives,available data and resources,and level of model complexity. The Panel finds that the level of complexity in the components and linkages of the MEP modeling approach is simple,parsimonious and well balanced within this context. The Panel also finds that the MEP modeling approach is functionally adequate. This approach is - specifically designed for groundwater dominated systems and explicitly considers nitrogen loads from septic systems,the dominant controllable watershed source of nitrogen for Cape Cod estuaries. The MEP modeling approach is appropriate and useful,within the overall DSS,for evaluating alternative scenarios and informing nutrient management plans, and is consistent with existing nationwide TMDL practices. December 30,2011 0