11/15/2012 SEWER COMMISSION Minutes

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and findings from MEP analyses completed in other estuaries such as Wareham River and Centerville River, the MEP Technical Team concluded that an upper limit of<0.50 mg TN L"' tidally averaged TN at the threshold station (ER01) would result in healthy infaunal habitat throughout the western branch of Eel Pond. • Within the Quashnet River, Hamblin Pond/Little River and Jehu Pond/Great River sub- embayments within the Waquoit Bay System, it was necessary to select 3 sentinel locations. The Quashnet River Estuary operates independent from the Hamblin Pond and Jehu Pond Estuaries, except as they share common source waters from Waquoit Bay. Their interaction is primarily through their effect on the nitrogen level with Waquoit Bay. The sentinel system within the Quashnet River Estuary was set within the upper/mid basin (region above the bridge). Achieving the nitrogen threshold at this station will also improve benthic habitat in the lower basin. Since there is no historical evidence that the Quashnet River Estuary supported eelgrass, the threshold nitrogen concentration was based upon restoring benthic habitat at the sentinel station. The target nitrogen concentration to restore infaunal habitat is based upon the high quality infaunal sites in lower Hamblin Pond and in Little River (Stations 176 and 170, Figure VII-9). The tidally averaged nitrogen levels at these sites are 0.498 and 0.524 mg TN L" respectively. These values are consistent with the infaunal guidance levels within the Popponesset Bay sub-embayments of 0.5 to 0.4 mg TN L"' (0.5 mg TN L"') being the upper threshold value). Based upon these data a conservative estimate for the infaunal threshold for the Quashnet River Estuary is 0.50 mg TN L"', with 0.52 likely to represent a slight stress, but still high quality habitat. • Within the Hamblin Pond/Little River and Jehu Pond/Great River Estuaries the sentinel locations were placed within the pond basins. The target nitrogen threshold focuses on eelgrass restoration of these systems. Setting the threshold for these ponds was not straight-forward given that eelgrass was almost completely lost from the main basin of Waquoit Bay prior to significant loss from the Hamblin Pond and Jehu Pond Estuaries in the 1980's. As such, the approach taken by the MEP Technical Team was to develop the threshold nitrogen level for these Ponds in relation to the nitrogen level in the main bay, which serves as the source water(boundary condition) to the ponds. Based upon a main bay boundary condition of 0.38 mg TN L-' (upper eelgrass threshold) the nitrogen levels in the Ponds would necessarily have been >0.38 mg TN L"', given the gradients established by the interplay of loading and hydrodynamics. This is consistent with the existence of a few diminishing small patches of eelgrass at nitrogen levels on the order of 0.5 mg TN L"' in these ponds in 2001-2003. Based upon the modeling it appears that Jehu Pond could support eelgrass at a nitrogen threshold of 0.446 mg TN L"'. Although Hamblin Pond is similar to Jehu Pond in gross structure, it has very different loading and attenuation characteristics. The result is that the structure of the system produces much lower nitrogen levels so a threshold of 0.38 mg TN L-' was selected to allow for Uncertainties. It is important to note that the analysis of future nitrogen loading to the Waquoit Bay estuarine system focuses upon additional shifts in land-use from forest/grasslands to residential and commercial development. However, the MEP analysis indicates that significant increases in nitrogen loading can occur under present land-uses, due to shifts in occupancy, shifts from seasonal to year-round usage and increasing use of fertilizers. Therefore, watershed-estuarine nitrogen management must include management approaches to. prevent increased nitrogen loading from both shifts in land-uses (new sources) and from loading increases of current land-uses. The overarching conclusion of Executive Summary 10