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Massachusetts Estuary Project(MEP) <br /> Linked Watershed Emba�ment Model Peer Review <br /> 5. KEY ISSUES IDENTIFIED BY PANEL <br /> 5.1. Introduction <br /> The Panel identified key issues with the MEP modeling approach,and its application to the Pleasant Bay <br /> and Bourns Pond case studies, and organized these issues by topic area. Presentation of each key issue <br /> includes discussion,conclusions and recommendations. <br /> Identification of these key issues by the Panel does not imply that the MEP modeling approach is flawed <br /> and in need of repairs. Rather,the opinion of the Panel is that the MEP modeling approach is <br /> scientifically defensible and functionally adequate, and can be improved going forward Within the <br /> recommended adaptive management fi-amework. This is a normal process for management of complex <br /> water resource systems like the estuaries of Cape Cod. The towns should proceed within this framework <br /> to develop and finpletnent wastewater and nutrient management-plans, and make improvements along the <br /> way to increase scientific understanding and reduce management uncertainties. <br /> 5.2. Model-Based Issues <br /> 5.2.1. Groundwater and Watershed Nitrogen Loads <br /> Key Issue I-Groundwater Modeling <br /> Cape Cod differs from most other watersheds in that drainage of the watershed to receiving water bodies <br /> occurs largely via groundwater rather than via surface water flow over the land surface. Groundwater <br /> discharge to the estuaries occurs either indirectly through base flow in small streams and rivers,or <br /> directly through seepage at the shoreline, in the intertidal zone and more limited submarine discharge. In <br /> most surface-water-drained watersheds, delineation of the watershed boundary is simply a matter of <br /> identifying the ridge lines that separate watersheds from topographic maps,and delineating the watershed <br /> by connecting ridge,lines appropriately,or automatically from digital elevation models. This is not <br /> possible for Cape Cod. Thus,in order to delineate watersheds,the SMAST Team depended upon maps of <br /> aquifer areas contributing groundwater flow to the various bays and estuaries. These maps were <br /> developed using groundwater flow models developed by the USGS based on the widely used USGS <br /> MODFLOW computer code. The USGS has been working With and refining their groundwater models of <br /> Cape Cod for decades,'the work is very well done,and the personnel highly qualified. The Panel is <br /> therefore confident that this aspect of the MEP studies has been done using appropriate tools and with <br /> sufficient accuracy, <br /> Key Issue 2—Nitrogen Load Estimation <br /> A key aspect of the MEP modeling approach is the estimation of nitrogen loads generated within the <br /> watershed and the subsequent partial attenuation of those loads during transport to the receiving bays and <br /> estuaries. This requires consideration of all significant sources of nitrogen inputs to the contributing <br /> watershed and directly to the estuaries. For all sources delivered to groundwater,and surface water,the <br /> nitrogen concentrations and the volumes(effluent volumes,effective precipitation)need to be estimated. <br /> Major sources are currently atmospheric deposition,stormwater,-fertilizer for agriculture,urban lawns <br /> and recreational areas; and,wastewater loading from on-site disposal(septic systems) and outfalls from <br /> sewage treatment plants. Controllable loads include wastewater,fertilizer and stormwater,while <br /> December 30,2011 <br />