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Massachusetts Estuary Project CMEP) <br /> _ Linked Watershed EmbaymentModel Peer Revievv <br /> while DOC is present but essentially stops once DOC concentrations drop to low levels. Thus,whatever <br /> DOC is introduced at wastewater sources is rapidly consumed near the source. Further,frorn the source <br /> there is little remaining DOC and denitrification is typically minimal. Recent work suggests iron fi•om <br /> pyrite weathering or other materials may substitute in the absence of organic carbon,but this is not <br /> thought to be significant in this area. In some localities,more DOC may be added fioin natural sources, <br /> particularly where a thin vadose zone(shallow water table)allows more transport of DOC from shallow <br /> soils to the ground water or potentially from buried organic horizons(e.g.Hill,2011). The Panel's rough <br /> estimate from the literature is that if nitrogen attenuation were incorporated into the analysis it would <br /> reduce the groundwater loads on the order of only a few percent. Given the other uncertainties in the <br /> nitrogen loads generally, andthe variability in denitrification,this potential error is relatively minor and <br /> would probably be best addressed in sensitivity analyses recommended above. <br /> The fact that DOC limits denitrification suggests some possibilities for local in-situ removal of nitrate via <br /> permeable reactive barriers or DOC injection of some sort. Several researchers have tested these <br /> approaches and they may constitute a cost-effective alternative to sewering. For example,trenches filled <br /> with sawdust have been found to provide DOC and enhance denitrification(e.g. Schipper et al. 2010). <br /> Finally, one fact is clear:natural groundwater denitrification is not the solution to the Cape's nitrate <br /> problem. Although inclusion or exclusion of denitrification from the mass balance adds some uncertainty <br /> to the exact loads,discharge of nitrogen to the estuaries via groundwater is a significant load whether or <br /> not there is an accounting for denitrification in groundwater. <br /> As mentioned above,significant attenuation(50%) of nitrogen load is assumed to take place in flowpaths <br /> passing through freshwater ponds, streams and wetlands, as mapped by the MODFLOW simulation. <br /> Estimates of attenuation were based on limited measurements of stream nitrogen concentrations and flows <br /> to estimate annual loads,compared to the linked watershed nitrogen loading and groundwater delivery <br /> models, as well as surveys of pond physical and hydrodynamic conditions. These losses can <br /> considered to occur at the interface between groundwater and surface water systems, and represent an <br /> important transition where water is passing through organic material,which can act as substrate for <br /> microbial denitrification or immobilization, Plant uptake and burial, or incorporation into long tern <br /> storage, are additional attenuation processes that may be active in these regions. However,no details of <br /> specific processes are incorporated into these estimates of attenuation,which are determined by simple <br /> mass balance estimates. This is consistenfiwith the level of process specification in other model <br /> components and should be treated as an additional source of uncertainty. <br /> 5.2.2. Estuarine Hydrodynamics and Water Quality <br /> As noted above in Section 4.2,the MEP modeling approach includes the application of a numerical <br /> groundwater model, a numerical hydrodynamic model and a numerical water quality model. Once these <br /> models are corroborated for a particular water body(e.g. Pleasant Bay),they offer highly effective tools <br /> for supporting the determination of TMDLs. With this approach,many scenarios can be modeled to <br /> assess the impact of different levels of nitrogen loading and/or physical changes on water column <br /> concentrations of nitrogen. <br /> The particular numerical models used by the SMAST Teain for the computation of hydrodynamics and <br /> water quality of the Cape Cod estuaries are known as RMA2(Donnell et al. 2011)and RMA4(Letter.et <br /> al. 2011),respectively. The RMA2 hydrodynamic model serves as the hydraulic"chassis"for the RMA4 <br /> water quality model. These two models share the same spatial segmentation grid and numerical solution <br /> method.-Various issues raised by the Panel concerning the numerical hydrodynamic and water quality <br /> models are discussed below. <br /> p December 30,2011 4 <br />