API Publ 4736-2006 pdf download.Identification of Key Assumptions and Models for the Development of Total Maximum Daily Loads.
There is also a distinction within the dynamic modeling approach between those model formulations that assume equilibrium conditions between phases (i.e., mass transfer resistance between solids, water, and atmosphere is negligible) and those that assume that resistance to mass transfer between phases is significant. Dynamic models that simulate changing point and non-point source loads, hydrology, and hydrodynamics and equilibrium between solid, aqueous, and atmospheric phases are acceptable for pollutants with physical and chemical properties that are consistent with this assumption. If mass transfer resistance is important for a pollutant, then a model incorporating non-equilibrium conditions should be used for water quality simulations. Models may also be characterized as deterministic or empirical. A deterministic model uses theoretical mathematical constructions of the physical, chemical, and biological processes in a surface water to develop the cause-effect relationships that it simulates. An empirical model is a mathematical formulation that does not attempt to directly describe the underlying physical, chemical, or biological processes, but instead uses statistical methods or observed relationships to develop cause-effect relations between system inputs and outputs. It is common for deterministic models to incorporate empirical relationships for some of the internal functional relationships required to simulate receiving water quality and ecosystems. Similarly, most empirical models are based on an evaluation of the theoretical cause-effect relationships among the variables that are used as inputs and the predicted outputs. Thus, most of the models described in this report are based on a mix of deterministic and empirical relationships.
Each state, tribe, commonwealth and territory 7 has its own unique method for designating surface waters as impaired and listing them on its CWA Section 303(d) list. EPA’s 2002 and 2004 listing guidance documents 8 are designed to make the listing procedures more consistent, but the individual state listing procedures will still remain distinct. The most important distinction between state listing programs is the amount of quality-assured data upon which a state bases its listing decisions. States that require an extensive database consisting of quality- assured field data for 303(d) listing may have sufficient data available to define the scope (geographic extent and types of sources and conditions contributing to the impairment) of the required TMDL modeling to remedy the impairment. In other states, the first step in the TMDL process may be developing and implementing a sampling program to collect the data to adequately characterize the extent of the water quality impairment (including verifying that an impairment actually exists and that the water quality criteria and designated uses are appropriate).
The geographic scope of a TMDL should encompass all portions of the upstream drainage area that contribute significant amounts of the constituent(s) that cause or contribute to the impaired water quality in the listed surface water segment. It may be practical and justified to limit the upstream drainage area included in the analysis. For example, if the constituent causing the impairment is absent, or is present in de minimis quantities in the water entering the impaired segment from upstream, and projections indicate that it is unlikely that future activities will contribute significant amounts of the constituent, it is acceptable to model only the impaired segment. Another way to state this condition is that if the “background” or “ambient” concentration of a constituent at a location upstream of the impaired surface water is a small fraction of the amount of the constituent that would cause or contribute to the impairment, and that background or ambient concentration is not projected to increase significantly in the future, it is acceptable to use that location as the upstream model boundary. Unless those conditions are satisfied, the upper boundary of the modeled watershed must be moved until the condition is satisfied.