2/16/2012 SEWER COMMISSION Minutes

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Massachusetts Estuary Project(MEP) _ Linked Watershed Embayment Model Peer Review In both the Pleasant Bay and Bourns Pond applications,Acoustic Doppler Current Profiler(ADCP) velocity measurements were available across the inlets and for several other transects. These measurements were used to compute the fluxes of water at these locations,thus providing data for an independent corroboration of the RMA2 model. The measured water fluxes closely match those computed by RMA2. Thus, the Panel concludes that the RMA2 models for Pleasant Bay and Bournes Pond are well corroborated. Key Issue 5-Will the RMA2 and RMA4 Models be Updated to Include the 2007 Breach in Pleasant Bay? After this issue was raised,Applied Coastal Research and Engineering provided a report describing a study for the New England District of the Corps of Engineers in which the 2007 breach was included in the Pleasant.Bay model. The impact of the breach is to increase tidal flushing by about 15%. Given that implementation of TMDLs for the Cape Cod estuaries and bays is projected to be an ongoing process for the next 20-30 years, and the uncertainty of future conditions of the breach,the Panel anticipates that the numerical models will continue to evolve. As part of this process,the Panel recommends that the bathymetry and geometry in the RMA2 and RMA4 models continue to be updated as new data become available. 5.2.2.2.Estuarine Water Quality Model Key Issue 1-RMA4 Model Conceptual Framework Total nitrogen(TN)is the sum of dissolved inorganic nitrogen(DIN), dissolved organic nitrogen(DON) and particulate organic nitrogen(PON). The RMA4 water quality model is calibrated to TN in Bournes Pond(Howes et al. 2005)and bioactive nitrogen(DIN+PON)in Pleasant Bay(Howes et al. 2006). In the actual estuaries,PON is a component of TN and bioactive nitrogen that settles from the water column into the sediment bed. To model TN or bioactive nitrogen in the water column,this PON settling flux must be taken into account. One way to do this is to include a net apparent settling velocity in the model, and another way is to adjust a nitrogen input load to compensate for lack of inclusion of an explicit apparent net settling velocity. In the SMAST RMA4 water quality model,PON settling flux is represented implicitly as an adjustment to observed benthic regeneration fluxes. The observed benthic fluxes are discounted by an assumed PON settling flux so that when they are specified as a sediment-water input load to the model,the PON which has remained in the water column does not get double-counted. Basically,this approach"pre-calibrates" the water quality model with a fixed value for PON apparent net settling velocity. A strength of this approach is that it is based on observed data for benthic fluxes,but a concern is that it is conducted outside the mass balance equation in the model. Consequently,there is no direct constraint to ensure that the correct adjustment has been made. Furthermore,because this approach implicitly"pre- calibrates"PON net settling velocities,it limits the flexibility to adjust PON net settling fluxes as part of the model calibration. As the models continue to evolve and improve, the Panel recommends that the SMAST Team consider representing PON settling flux using a net apparent settling velocity in the RMA4 water quality model. A benefit of this approach would be closer alignment between the conceptual framework of the model and net settling fluxes for PON in the estuaries._Additional benefits are that observed benthic fluxes could be directly specified as model input,thus enhancing transparency, and that the model calibration,using apparent net settling velocities,would be conducted within the mass balance equation of the model. These improvements to the RMA4 water quality model would strengthen the calibrations to the estuaries and reduce uncertainties in computed nitrogen concentrations. _ December 30,2011 �