"Modeling Phosphorus Loading for Total Maximum Daily Loads (TMDL)"

Major Points:

1. Federal Clean Water Act requires states to list waters not meeting water quality standards on the 303d list and to write a Total Maximum Daily Load Report (TMDL) on each waterbody on the list.
2. The TMDL establishes the allowable pollutant loading from all contributing sources at a level necessary to achieve the applicable water quality standards. The TMDLs must account for seasonal variability and include a margin of safety (MOS) to account for uncertainty of how pollutant loadings may impact the receiving water’s quality.
3. After public comment and final approval by the EPA, the TMDL will be incorporated into the watershed action plan to be developed by the Executive Office of Environmental Affairs Basin Team (see below) and serve as a guide for future implementation activities. Where permits for wastewater and other discharges are required, TMDLs will be used by DEP to set appropriate limits.
4. Massachusetts has more than 560 lakes on the 303d list. Most of these are listed for nutrient issues and TMDLs reports are needed on phosphorus. EPA requires these reports to be completed within 12 years, but DEP has limited staff.
5. We decided to use MassGIS landuse coverages of each lake watershed with phosphorus export coefficients to estimate phosphorus loads to the lakes.
6. As described in Mattson and Isaac (1999) "Calibration of Phosphorus Export Coefficients for Total Maximum Daily Loads of Massachusetts Lakes" in the Journal of Lake and Reservoir Management 15(3):209-219, we used existing data on measured exports for a group of lakes to statistically calibrate export coefficients for landuse types in Massachusetts. We used a "stagewise" regression technique to estimate reasonable coefficients to fit the model and used another set of lakes to verify the model.
7. Further details on the 303d list, the TMDL strategy, and examples of TMDLs is available on the DEP web site at www.state.ma.us/dep/brp/wm/wmpubs.htm

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