The TMDL Module is the process used to calculate total maximum daily load (TMDL) anywhere within a watershed. The idea behind TMDL is that there is some amount of pollutant which can be assimilated by a water body without impairing that water body for its intended uses. Point and/or nonpoint source loading can be scaled up or down by a fixed multiplier for any number of pollutants. The point source or nonpoint source loading for a single pollutant can then be calculated to meet a criterion for a designated use. Following is the six-step process:
The first step is to choose the scenario for the TMDL. Next to Specify scenario:, choose a scenario from the list. If the desired scenario is not listed, go to Scenario / Manager in the menu and copy an existing scenario to a new one and open it or open an existing scenario.
The second step is to Specify Water Quality Limited Section: to indicate the impaired stream or reservoir segment for which a TMDL is to be calculated.
A designated use must be chosen from the list next to Designated Use:. To create a new designated use not in the list, click on New. Then choose which criterion to evaluate from the list next to Criterion: or create a new criterion by clicking on New. Next to Specify TMDL Pollutant:, select the pollutant for which a TMDL is to be calculated. This may or may not be the same as the parameter in the criterion. For a dissolved oxygen criterion, as shown above, the pollutant for which a TMDL is needed could be BOD or ammonia. Check whether point source loading or nonpoint source loading should be calculated so that the criterion is met. Whichever is not selected can be reduced (or even increased) by a fixed amount. In the example above, point source loading of BOD is to be calculated given a 30% reduction in ammonia from point sources and a 20% reduction in ammonia from nonpoint sources.
Next, run a simulation. A TMDL simulation is actually several (usually 3) iterative simulations. The level of whatever loading is selected to be calculated for the specified pollutant is adjusted so that the water quality criterion is met by less than 1%. In the above example, the phosphorus loading from point sources is adjusted up and down until the resulting minimum weekly average dissolved oxygen concentration is between 5.00 and 5.05 mg/l. The TMDL calculated has no built-in safety factor. It is left to the user and stakeholders to apply an appropriate safety factor to the resulting TMDL.
After running the TMDL simulation, view the results in terms of loading and water quality. Click on Loading to see how much point and nonpoint loading is in the calculated TMDL. Click on Water Quality to determine if, with the TMDL loading, water bodies are no longer impaired. Sometimes, reducing point or nonpoint source loading to zero might not be enough. In that case, the parameter for which the TMDL was calculated might not be what is causing the water quality impairment.
If the TMDL is acceptable, click on Save TMDL. That will add the TMDL to the list of saved TMDLs shown in the dialog which pops up. To remove or view saved TMDLs at any time, click on View TMDLs.
To run TMDLs for multiple unrelated parameters, make a TMDL simulation for each parameter. After the first TMDL is calculated, get the point and nonpoint load multipliers from the View TMDLs spreadsheet and enter them in the main spreadsheet in the TMDL module. Then proceed with the TMDL for the next parameter. The loading from the initial TMDL will be used while calculating the next TMDL.