2/16/2012 SEWER COMMISSION Minutes

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Massachusetts Estuary Project(MEP) -------- —. Linked Watershed Emboyment Model Peer Review----- ---------- ---- ----------� �--_be reduced to 0.15 in a depth-averaged model.The Panel has recommended to the SMAST Team that this approach be explored for determination of dispersion coefficients in the RMA4 model. Key Issue 3 -RMA4 Application to Bioactive Nitrogen in Pleasant Bay Total nitrogen concentration in the water column is the state variable in the RMA4 model applications for all of the MEP estuaries to date, with the exception of Pleasant Bay. Bioactive nitrogen concentration is the state variable for the Pleasant Bay application. Total nitrogen is the sum of DIN,PON and DON,and bioactive nitrogen is the sum of only DIN and PON. The SMAST Team considered DON to be a large non-active pool generally separate from the nitrogen fractions active in eutrophication(Howes et al. 2006). In discussions with the SMAST Team,they stated that the reason for applying the RMA4 model to bioactive nitrogen in Pleasant Bay was a large,unexplained difference in observed total nitrogen concentrations between the ocean boundary monitoring station(PBA-17a) and the monitoring stations in the lower portion of Pleasant Bay(PBA-18,PBA-19 and PBA-01). The SMAST Team judged that this difference was inconsistent with the highly dispersive mixing in lower Pleasant Bay, and that bioactive nitrogen was a more appropriate state variable because it appeared to be relatively constant within and outside of Pleasant Bay. The results for observed total nitrogen and bioactive nitrogen concentrations in Table VI-I of Howes et al. (2006) appear to support this reasoning. Mean total nitrogen concentrations at PBA-01,the station in the lower-most portion of Pleasant Bay, and PBA-17a,the station used to specify the ocean boundary concentration in the RMA4 model,are 0.433 and 0.232 mg/L,respectively. Mean bioactive nitrogen concentrations at these same stations are 0.105 and 0.094 mg/L,respectively. There appears to be a large spatial gradient in total nitrogen concentration,but a small or no gradient in bioactive nitrogen concentration. However,the observations for the ocean boundary station(PBA-17a)represent only summer 2005 conditions because data were unavailable for other years,while the observations for all of the stations within Pleasant Bay,including PBA-01,represent summer average conditions for 2000-2005. The SMAST Team provided the Panel with mean values for observed total and bioactive nitrogen concentrations for individual summers for selected stations in the lower portion of Pleasant Bay. Comparisons using data for only 2005 appear to show small or no spatial gradients for either total nitrogen or bioactive nitrogen concentrations between stations PBA-01 and PBA-17a. Total nitrogen concentrations at PBA-01 (lower bay)and PBA-17a(ocean boundary)for 2005 are 0.290 and 0.232 mg/L,respectively. Bioactive nitrogen concentrations for 2005 at these same stations are 0.086 and 0.094 mg/L,respectively. The Panel recommmends that the SMAST Team conduct a detailed review of the primary data for total and bioactive nitrogen concentrations for monitoring stations in lower Pleasant Bay and the vicinity of the ocean boundary. Depending on the results of such a review,the SMAST Team might re-consider their decision to apply the RMA4 model to bioactive nitrogen instead of total nitrogen. If the RMA4 model could be applied to total nitrogen in Pleasant Bay,this would result in consistency among the model applications across'all of the MEP estuaries,and would eliminate potential ambiguity in the use of bioactive nitrogen for only Pleasant Bay. Key Issue 4-Sensitivity Analysis As discussed above,in Section 5.2.1,U.S.EPA(2009)recommends sensitivity analysis as the principal evaluation tool for characterizing the most and least important sources of uncertainty in environmental models. The SMAST Team conducted sensitivity analyses for the Great Pond Case Study(Howes et al. 2001)that included watershed nitrogen.loads,atmospheric deposition,benthic fluxes and dispersion coefficients. These analyses did not include nitrogen concentration at the ocean boundary. Furthermore, December 30,2011