11/15/2012 SEWER COMMISSION Minutes

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Threshold nitrogen levels for the embayment systems in this study were developed to restore or maintain SA waters or high habitat quality. High habitat quality was defined as supportive of eelgrass and infaunal communities. Dissolved oxygen and chlorophyll-a were also considered in the assessment. The nitrogen thresholds developed in Section VIII-2 were used to determine the amount of total nitrogen mass loading reduction required for restoration of eelgrass and infaunal habitats in the Waquoit Bay embayment system. Tidally averaged total nitrogen thresholds derived in Section VIIIA were used to adjust the calibrated constituent transport model developed in Section VI. Watershed nitrogen loads were sequentially lowered, using reductions in septic effluent discharges only, until the nitrogen levels reached the threshold level at the sentinel stations chosen for the Waquoit Bay system. It is important to note that load reductions can be produced by reduction of any or all sources, increasing embayment flushing or by increasing the natural attenuation of nitrogen within the freshwater systems to the embayment. The load reductions developed in Section VIII represent only one of a suite of potential reduction approaches that need to be evaluated by the community. The presentation is to establish the general degree and spatial pattern of reduction that will be required for restoration of this nitrogen impaired embayment. The Massachusetts Estuaries Project's thresholds analysis, as presented in this technical report, provides the site-specific nitrogen reduction guidelines for nitrogen management of the Waquoit Bay embayment system shared between Towns of Falmouth and Mashpee. Future water quality modeling scenarios should be run which incorporate the spectrum of strategies that result in nitrogen loading reduction to the embayment. For Illustrative purposes, the MEP analysis has initially focused upon nitrogen loads from on-site septic systems as. a test of the potential for achieving the level of total nitrogen reduction for restoration of the embayment system. The concept was that since nitrogen loads associated with wastewater generally represent 60% - 85% of the controllable watershed load to the Waquoit Bay embayment system and are more manageable than other of the nitrogen sources, the ability to achieve needed reductions through this source is a good gauge of the feasibility for restoration of these systems. 2. Problem Assessment(Current Conditions) A habitat assessment was conducted throughout the Waquoit Bay embayment system based upon available water quality monitoring data, historical changes in eelgrass distribution, time-series water column oxygen measurements of dissolved oxygen and chlorophyll, and benthic community structure. At present the overall Waquoit Bay Embayment system (comprised of: Waquoit Bay, Eel Pond, Childs River, Quashnet River, Sage Lot Pond, Hamblin Pond/Little River and Jehu Pond/Great River) is generally showing impaired habitat quality resulting from nitrogen enrichment (Section VII, Table VIII-1). This indicates that nitrogen management of this system will be for restoration rather than for protection or maintenance of an unimpaired system. In general, the habitat quality within the basins of this System is manifested by the temporal changes in eelgrass coverage and benthic community characteristics, which are consistent with the observed levels of nitrogen and organic matter enrichment and magnitude of oxygen depletion, as well as the sediment characteristics and general absence to only sparse macroalgal accumulations. The distribution and levels of habitat impairment within the Waquoit Bay Embayment System is consistent with the moderate to significant level of nitrogen enrichment. The near complete loss of the extensive eelgrass beds within the Waquoit Bay Embayment System makes restoration of this resource the primary focus for nitrogen management, with the associated goal of restoring impaired benthic habitat in areas of the system (all subembayments) showing clear signs of impaired benthic communities. Executive Summary 5