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Massachusetts Estuary Project(MEP) <br /> `Linked Watershed EmbaymentModel Peer Review <br /> atmospheric loads cannot be controlled at the local level. Cape Cod residences,and commercial, <br /> institutional and industrial parcels donunantly use on-site disposal, although some areas have installed <br /> sanitary sewers and central treatment. The geology,sanitary infrastructure,parcel density and topography <br /> indicate on-site septic systems are a major source of nitrogen load to the watershed. Dominance of septic <br /> systems as nitrogen sources in receiving water bodies is by no means unique to the Cape,and occurs in <br /> other areas of the country that are septic served,even with lower population densities and less conductive <br /> soils and groundwater(e.g.Kaushal et at.2011). An excellent review of on-site septic'systems risk in <br /> terms of nitrogen contamination is given by Gold and Sims(2000),including systems on glacial outwash <br /> systems similar to Cape Cod. Specific research on seasonally used septic systems in similar geologic <br /> settings in Rhode Island has shown specific vulnerability to nitrogen leaching(Postma et al. 1992)due to <br /> incomplete formation of biological clogging mats. <br /> Estimating loading from individual and institutional activity at the parcel level is necessarily imprecise as <br /> details of every residential and non-residential unit cannot be measured,and can only be inferred using <br /> limited measurements or surveys and statistical methods. However,the methods used by the SMAST <br /> Team are reasonable and consistent with approaches used by others working locally,including the MDEP <br /> (Horsley&Witten, 1996),the Cape Cod Commission'(Eichner and Cambareri, 1992),and the Woods <br /> Hole Marine Biological Laboratory(Valiela et al. 1997a and Latimer and Charpentier,2010)as well as <br /> other'areas of the country(e.g.Law et al.2004, Osmond et al.2004,Groffinan et al.2004). Importantly, <br /> rather than relying on literature values for key information,such as the nitrogen concentration of septic <br /> effluent,the volume of wastewater per parcel or per capita, and lawn fertilization rates,care has been <br /> taken to use local measurements and inference using data collected in the area: <br /> 1. Septic effluent nitrogen sources: Detailed Geographic Information System(GIS) <br /> analysis of parcel-scale water use was used to estimate total septic and stormwater <br /> input,and mapped into detailed groundwater flowpaths derived from USGS <br /> MODFLOW simulations. Septic wastewater volume is typically difficult to reliably <br /> estimate as water supply for septic-served parcels often comes from individual <br /> groundwater wells. `However,the Cape is somewhat unique with water supply <br /> provided by local utilities, allowing parcel-scale billing to be used to estimate septic <br /> recharge,after allowing for outdoor water and other consumptive use. This obviates <br /> some of the problems of estimating occupancy and per capita water use to estimate <br /> effluent volumes as water use is directly known, Nitrogen concentrations of <br /> wastewater varies with water use patterns, and local measurements have been used to <br /> constrain these estimates. <br /> 2. Lawn fertilizer sources: Parcel scale surveys(households,golf courses,etc.)of <br /> fertilizer use estimated fertilization rates similar to those estimated elsewhere in the <br /> eastern US (e.g.Law et al. 2004, Osmond et al. 2004). While these estimates do not <br /> have the spatial precision that septic volume estimates have(which have parcel-scale <br /> billing information as discussed above),it provides reasonable estimates of total lawn <br /> fertilizer nitrogen applied within groundwater catchment areas. Estimates of lawn <br /> nitrogen attenuation and leaching rates are inferred from measurements in similar <br /> soils(Cape'Cod and Long Island) and remain an additional source of uncertainty. <br /> 3. - Stonnwater sources: Given the high infiltration capacities of Cape Cod glacial <br /> moraine and outwash derived soils,the flat topography with numerous small <br /> wetlands and depressions, and road construction patterns,stormwater largely enters <br /> groundwater systems through run-on infiltration(unpervious runoff routing to <br /> pervious areas)or into local wetlands. Local high density commercial development <br /> may provide limited areas of higher stormwater input,but these comprise small areas <br /> of the Cape. Detailed GIS analysis of groundwater catchment area land covers, <br /> December 30,2011 2 <br />