11/15/2012 SEWER COMMISSION Minutes

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Benthic animal indicators were consistent with the levels of oxygen depletion, chlorophyll- a and organic enrichment, including macroalgal accumulation, within all of the sub-embayments of the Waquoit Bay System. The System is presently supporting benthic habitat ranging from minimally/moderately impaired to significantly impaired. It should be noted that, given the loss of eelgrass beds, throughout the main basin of Waquoit Bay, eastern and lower Eel Pond (fringing beds in Childs River), as well as the Quashnet River, Hamblin Pond/Little River and Jehu Pond/Great River, it is clear that the Waquoit Bay Embayment System is clearly impaired by nutrient overloading throughout its tidal reaches. Based upon the infaunal community survey it appears that most of the Waquoit Bay Embayment System is presently supporting impaired benthic animal habitat, primarily resulting from nitrogen and organic enrichment, periodic oxygen stress and in some areas, accumulations of drift macroalgae that "smother" benthic animals. At present, high quality benthic habitat is only found within the lower basin of Eel Pond and the Seapit River. These areas do not have significant accumulations of macroalgae or oxygen depletion and have relatively oxidized sediments comprised of medium to fine sands with low organic enrichment or consolidated muds. Overall, the pattern of infaunal habitat quality throughout the Waquoit Bay Embayment System is consistent with measured dissolved oxygen concentrations, chlorophyll, nutrients and organic matter enrichment in this system. Classification of habitat quality necessarily includes the structure of the specific estuarine basin, specifically as to whether a basin area is wetland influenced or an open water tidal embayment. Based upon this analysis it is clear that most of the benthic animal habitat within the Waquoit Bay Embayment System is moderately to significantly impaired (Quashnet River, severely degraded) by nitrogen and organic matter enrichment, while the moderate to high quality benthic animal habitat is primarily found in the region of the Seapit River down to the Eel Pond inlet. The proximate cause of impairment is organic matter enrichment and oxygen depletion, stemming ultimately from nitrogen enrichment. Total nitrogen levels within the significantly impaired basins presently range from 0,65 to 1.20 mg TN L-', levels typical of other estuarine basins with significant impairment of benthic animal habitat throughout southeastern Massachusetts estuaries. 3. Conclusions of the Analysis The threshold nitrogen level for an embayment represents the average water column concentration of nitrogen that will support the habitat quality being sought. The water column nitrogen level is ultimately controlled by the integration of the watershed nitrogen load, the nitrogen concentration in the inflowing tidal waters (boundary condition) and dilution and flushing via tidal flows. The water column nitrogen concentration is modified by the extent of sediment regeneration and by direct atmospheric deposition. Threshold nitrogen levels for this embayment system were developed to restore or maintain SA waters or high habitat quality. In this system, high habitat quality was defined as supportive of eelgrass and supportive of diverse benthic animal communities. Dissolved oxygen and chlorophyll-a were also considered in the assessment. Watershed nitrogen loads (Tables ES-1 and ES-2) for the Town of Falmouth and Mashpee Waquoit Bay embayment system was comprised primarily of wastewater nitrogen. Land-use and wastewater analysis found that generally about 60% - 85% of the controllable watershed nitrogen load to the embayment was from wastewater. A major finding of the MEP clearly indicates that a single total nitrogen threshold can not be applied to Massachusetts' estuaries, based upon the results of the Great, Green and Executive Summary 8