2/16/2012 SEWER COMMISSION Minutes

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Massachusetts Estuary Project(MEP) Linked Watershed Embayment Model PeerRevieiv_ including storinwater generation from transportation(roads,parking lots),roof and other runoff-generating areas was completed to estimate stormwater volumes,with concentrations estimated from standard sources. While individual parcel,road and other land cover estimates of nitrogen input to the watershed and recharge to groundwater will be uncertain,averaging over the number of parcels and total impervious areas in each groundwater catchment area provides the total and mean application and recharge rates required for this analysis. These means and totals are more stable and have less uncertainty than individual parcels(based on the law of large numbers) and are the required information for loading magnitude and delivery patterns to the estuaries. Moreover,independent measurements of groundwater loads by Kroeger et al. (1999,2006)and Valiela et al. (2000)for Green Pond approximately match the MEP estimates,building confidence that the MEP results are reasonable. Key Issue 3—Uncertainty and Sensitivity Analysis Sensitivity analysis evaluates the effects of changes in input values,parameter estimates,assumptions and algorithms on a model's results. Uncertainty analysis investigates the effects of lack of knowledge and other potential sources of error in the model,and when conducted in combination with sensitivity analysis, allows a model user to be adequately informed about the confidence that can be placed in the model results. U.S.EPA(2009)recormnends sensitivity analysis as the principal evaluation tool for characterizing the most and least important sources of uncertainty in environmental models. There are numerous and varied inputs to the load estimates as discussed above(e.g. septic nitrogen concentrations,fertilizer leaching rates, groundwater recharge rates, etc.). Further, some important contributing processes are captured with only one-or two-digit accuracy—for example,50%o of the nitrogen transported through each of the major freshwater ponds is assumed to be attenuated. These low- accuracy inputs imply that nitrogen load estimates with five-digit accuracy,as included in the MEP reports,are impossible. The uncertainty in these inputs also implies that there are margins of uncertainty around the resulting load estimates. While the Panel does not recommend any wholesale changes in the approach to estimating nitrogen loads to the bays and estuaries at this stage,we recommend changes in the presentation of MEP results so as to acknowledge explicitly that there is uncertainty in the load estimates and to provide some estimate of the degree of uncertainty. The Panel further recornmends that model sensitivity analyses be conducted for nitrogen mass loads for each specific estuary. A healthy recognition that there is uncertainty would encourage planning bodies to pursue an adaptive monitoring and management strategy as they move forward to understand and remedy the impacts of nitrogen on bays and estuaries. Such an adaptive strategy is wise in light of the uncertainties in predicting the response of bays and estuaries to future load reductions. Key Issue 4—Nitrogen Attenuation in Groundwater The SMAST Team assumes there is no attenuation of nitrogen in groundwater,which they appropriately indicate to be a conservative approach(Howes et al. 2002). While this is a reasonable and conservative assumption,there is ample information in the technical literature to show that nitrate(NO3)from septic systems is almost certainly converted to nitrogen gas(N2)by bacterially-mediated denitrification in the subsurface,but the extent varies widely. Denitrification occurs by a microbially mediated stepwise chemical reaction as follows: NO3-—NO2-—NO—>N20-->N2 Anaerobic conditions and a substrate to support microbial populations(typically carbon)are needed for denitrification and most denitrification occurs in the vadose zone in the near vicinity of septic system leaching fields and subsurface wastewater discharges. In saturated groundwater, the extent of denitrification is highly variable and depends primarily on the availability of dissolved organic carbon (DOC)(Pabich et al. 2001). Kinetic models for denitrification indicate that the reaction proceeds quickly _.�� December 30,2011