Building a Model:Contaminant Transport

The final step in a complete watershed model is contaminant transport. If the model properly simulates both flow and sediments, it will likely be able to correctly simulate reactive transport as well, since the proper flow conditions are being simulated in the model. As discussed in Section 11, contaminants may be simulated as simple first order reactants, or nutrients may be simulated using the NSM. Use of first order constituents requires that the user have explicit information about the contaminants being simulated, and appropriate reaction rates, as the user must supply all the reaction rates for the model. Since the user has complete control to specify the rates, the user has great latitude in calibrating the constituent transport model. When using NSM, many different reactions occur. Most of the reaction rates are hidden from the user, and are calculated by NSM. Calibration of parameters is limited.

The user is therefore required to determine which method is best for thier individual case. If nutrients are the contaminants of concern, then the NSM should obviously be considered first. If other contaminants need to be simulated, then they will need to be simulated as first order reactants. It should be noted that static kinetic rates, or even first order rates, may not be appropriate for the reactants being considered. In addition, while the simple constituents gives the user great latitude in specifying and adjusting rates to match observed data, the user must assure that the rates are reasonable for both the constituents and conditions being simulated. While conceptually simple, the application of the simple constituents actually requires more knowledge about the constituents (i.e. chemisty) than the use of NSM, where the expert knowledge of the contaminants (phosphorous and nitrogen) has been programmed into the method.