Many drainage systems in the Neponset watershed outfall to the wetland that surround streams, rivers, ponds, and lakes. Using the MassDEP wetlands dataset, we assigned AU IDs to outfalls based on which impaired waters were associated with those outfalls. Figure 5. Using the MassDEP wetlands dataset to assign more outfalls to AU IDs. In some cases, an outfall may be situated on a rivulet that is too small to be depicted in any of the three hydrological datasets described above. In those cases, the topographic contours can help assign these outfalls AU IDs (Figure 6). The contours show areas where higher hills and ridges separate riverbeds into distinct basins. Finally, the last few ambiguous outfalls can be assigned the MassGIS drainage sub-basins layer. Figure 6. Using topographic contours to associate outfalls with Impaired Waters Figure 7. Topographic Contours in Sharon. Figure 8. Drainage Sub-basins. Ranking Catchments Once we delineated outfall catchments for Milton and linked outfalls with Impaired Waters in Milton and all Neponset Watershed Municipalities, we ranked Milton’s outfall catchments according to how likely they were to contribute illicit discharges to the receiving waters. We used CommunityViz, a GIS-based assessment tool for planning, to prioritize outfalls and catchments using the program’s suitability wizard. This suitability analysis evaluates sites based on a set of specified quantitative criteria. In this case, the presence of older homes, industrial uses, septic systems, and other physical features were summarized into a composite score from 0 to 100. Catchments with a median score or below were considered low priority, catchments within the third quartile were considered medium, and those within the top quartile were considered high priority, or most likely to contribute illicit discharges to impaired waters. Land use were designated as medium or high potential for illicit discharges using information found in the 2004 manual from Center for Watershed Protection, titled “Illicit Discharge Detection and Elimination: a guidance manual for program development and technical assessments”. The guidance manual provided illicit discharge pollution potential for a variety of land uses and included their SIC codes in Attachment A, which were compared to and converted to NAICS codes during this process. We used the following criteria for the analysis:
Figure 8. Catchment Ranking, low risk (hatched), medium (cross hatched), and high risk (solid). in dark gray represent high priority catchments, or catchments with the highest chance of contributing pollution to the stormwater system. Cross hatching represents catchments with a moderate chance, and hatching is the lowest chance. Much of the hatched area, for example, is in the Blue Hills conservation area. Table 4. Milton Outfall Catchment Ranking Results. 3 = High, 2 = Medium. AU_ID 104 Town of Milton MA73-26 0.8 94.32 3 248 Town of Milton MA73-29 1.9 89.51 3 253 Town of Milton MA73-29 4.9 83.09 3 160 Town of Milton MA73-03 0.9 76.50 3 188 Town of Milton MA73-03 0.7 67.32 3 23 Town of Milton MA73-02 2.2 67.32 3 217 Town of Milton MA73-29 0.8 66.40 3 7 Town of Milton MA73-29 4.0 64.76 3 178 Town of Milton MA73-26 0.6 62.40 3 178 Town of Milton MA73-26 0.5 62.22 3 103 Town of Milton MA73-26 1.5 58.42 3 25 Town of Milton MA73-02 4.1 57.72 3 134 Town of Milton MA73-26 0.4 56.28 3 231 Town of Milton MA73-29 0.6 56.27 3 76 Town of Milton MA73-29 2.7 54.32 3 207 Town of Milton MA73-03 1.5 52.80 3
154 Town of Milton MA73-26 0.6 52.65 3 170 Town of Milton MA73-26 0.4 52.23 3 82 Town of Milton MA73-26 1.5 52.14 3 158 Town of Milton MA73-03 4.4 51.33 3 0 Town of Milton MA74-09 1.0 51.23 3 186 Town of Milton MA73-04 2.1 51.10 3 193 Town of Milton MA73-04 2.0 50.58 3 110 Town of Milton MA73-26 0.4 50.46 3 205 Town of Milton MA73-03 0.4 50.39 3 0 Town of Milton MA73-26 4.8 49.96 3 146 Town of Milton MA73-29 1.5 49.78 3 190 Town of Milton MA73-03 0.7 49.55 3 226 Town of Milton MA73-04 2.1 49.41 3 207 Town of Milton MA73-03 2.8 49.25 3 129 Town of Milton MA74-09 2.3 48.50 3 198 Town of Milton MA73-03 2.9 48.43 3 27 Town of Milton MA73-29 19.1 47.41 3 183 Town of Milton MA73-03 2.9 47.06 3 83 Town of Milton MA73-26 0.9 46.73 3 209 Town of Milton MA73-03 4.5 46.54 3 15 Town of Milton MA73-29 8.7 45.71 3 27 Town of Milton MA73-29 10.4 45.13 3 230 Town of Milton MA73-29 3.3 44.38 3 199 Town of Milton MA73-04 2.0 44.35 3
111 Town of Milton MA73-29 2.9 44.16 3 242 Town of Milton MA73-04 0.9 44.02 3 236 Town of Milton MA73-29 4.0 43.92 3 228 Town of Milton MA73-04 2.5 43.59 3 7 Town of Milton MA73-29 12.9 43.47 3 39 Town of Milton MA73003 2.0 43.38 3 84 Town of Milton MA73-26 1.1 43.24 3 231 Town of Milton MA73-29 3.7 43.16 3 93 Town of Milton MA73-29 0.5 42.97 3 134 Town of Milton MA73-26 0.8 42.59 3 100 Town of Milton MA73-29 2.4 42.01 3 113 Town of Milton MA73-29 8.1 41.79 3 191 Town of Milton MA73-04 4.9 41.58 3 174 Town of Milton MA73-03 2.8 41.29 3 239 Town of Milton MA73-26 1.2 41.23 3 230 Town of Milton MA73-29 10.6 41.17 3 251 Town of Milton MA73-29 0.4 41.11 3 133 Town of Milton MA73-29 15.0 41.09 3 121 Town of Milton MA73-29 4.1 41.06 3 177 Town of Milton MA73-26 0.4 40.95 3 99 Town of Milton MA73-29 0.9 40.70 3 193 Town of Milton MA73-04 2.9 40.64 3 147 Town of Milton MA73-26 3.7 40.01 3 79 Town of Milton MA73-29 0.8 39.63 3
94 Town of Milton MA73-29 2.7 39.61 3 241 Town of Milton MA73-26 2.2 39.60 3 173 Town of Milton MA73-04 2.7 39.46 3 129 Town of Milton MA74-09 1.6 39.16 2 243 Town of Milton MA73-02 4.0 38.92 2 106 Town of Milton MA73-29 2.4 38.78 2 215 Town of Milton MA73-03 2.9 38.71 2 188 Town of Milton MA73-03 20.6 38.57 2 178 Town of Milton MA73-26 5.5 38.57 2 163 Town of Milton MA73-03 9.6 38.54 2 148 Town of Milton MA73-29 33.0 38.46 2 234 Town of Milton MA73-29 40.9 37.83 2 117 Town of Milton MA73-26 1.9 37.53 2 122 Town of Milton MA73-29 12.8 37.23 2 92 Town of Milton MA73-29 2.0 37.02 2 125 Town of Milton MA73-26 13.6 36.95 2 87 Town of Milton MA73-26 2.9 36.91 2 123 Town of Milton MA73-26 2.5 36.91 2 81 Town of Milton MA73-29 3.4 36.83 2 0 Town of Milton MA74-10 2.4 36.78 2 152 Town of Milton MA73-29 2.4 36.76 2 200 Town of Milton MA73-03 3.6 36.64 2 86 Town of Milton MA73-29 2.3 36.48 2 0 Town of Milton MA73-03 2.1 36.31 2
173 Town of Milton MA73-04 2.1 36.29 2 214 Town of Milton MA73-29 4.2 36.22 2 41 Town of Milton MA73003 2.2 36.04 2 159 Town of Milton MA73-26 42.2 35.78 2 183 Town of Milton MA73-03 13.9 35.43 2 169 Town of Milton MA73-26 12.4 35.41 2 155 Town of Milton MA73-03 8.9 35.22 2 85 Town of Milton MA73-26 13.3 35.01 2 179 Town of Milton MA73-04 3.4 34.98 2 203 Town of Milton MA73-29 1.4 34.96 2 43 Town of Milton MA73-02 10.0 34.81 2 218 Town of Milton MA73-29 7.1 33.89 2 0 Town of Milton MA74-10 0.4 33.77 2 219 Town of Milton MA73-04 6.9 33.56 2 93 Town of Milton MA73-29 8.5 33.53 2 181 Town of Milton MA73-04 3.2 33.52 2 150 Town of Milton MA73-26 1.4 33.50 2 131 Town of Milton MA73-29 22.4 33.46 2 187 Town of Milton MA73-04 2.8 33.45 2 104 Town of Milton MA73-26 2.0 33.35 2 119 Town of Milton MA73-26 6.9 33.33 2 138 Town of Milton MA73-29 15.1 33.11 2 202 Town of Milton MA73-03 5.4 32.99 2 87 Town of Milton MA73-26 4.1 32.88 2
123 Town of Milton MA73-26 2.5 32.85 2 178 Town of Milton MA73-26 120.5 32.83 2 165 Town of Milton MA73-26 16.9 32.30 2 45 Town of Milton MA73-29 27.7 32.19 2 188 Town of Milton MA73-03 2.0 32.03 2 64 Town of Milton MA73-29 0.7 32.03 2 259 Town of Milton MA73-04 13.3 31.69 2 104 Town of Milton MA73-26 7.4 31.64 2 84 Town of Milton MA73-26 19.7 31.16 2 52 Town of Milton MA73-02 8.5 31.11 2 89 Town of Milton MA73-26 6.4 31.02 2 159 Town of Milton MA73-26 5.8 30.87 2 185 Town of Milton MA73-03 35.1 30.85 2 195 Town of Milton MA73-03 17.0 30.72 2 39 Town of Milton MA73003 0.6 30.70 2 120 Town of Milton MA73-29 13.8 30.64 2 88 Town of Milton MA73-29 24.0 30.33 2 210 Town of Milton MA73-29 11.4 30.31 2 104 Town of Milton MA73-26 90.3 30.29 2 151 Town of Milton MA73-03 10.8 30.14 2 136 Town of Milton MA73-29 44.8 30.05 2 42 Town of Milton MA73003 6.0 30.01 2 208 Town of Milton MA73-29 14.3 29.93 2 31 Town of Milton MA73-29 68.4 29.78 2