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API Publ 4736:2006 pdf download

API Publ 4736:2006 pdf download.ldentification of Key Assumptions and Models for the Development of Total Maximum Daily Loads.
The objective of the TMDL is to alI(kate allowable pollutant loadings to point and non-point sources, thereby assuring that a surface water body complies with a waler quality criterion and its designated uses. To satisfy this objective, a method is required to predict waler quality resulting from pollutant loadings and the physical, chemical, and biological charactcristics of the surface water body. The methodology for making such predictions is a cause-effect “model” of the surface water body. The term “model” can be interpreted broadly — a model may be as simple as a mass balance performed by hand or as complex as a multi-dimensional fate and transport model that simulates multiple interrelated water quality constituents. The simplest definition of a model is that it is a mathematical formulation of a physical, chemical, and/or biological surface water system that can be used to predict the responses (effects) of the system to an actual, assumed or predicted set of inputs (causes).
The TM DL. approach assumes that specific water quality constituents with numeric values determine if a designated use is impaired. either directly as a water quality criterion (e.g.. a toxicity-derived criterion for a nietal) or indirectly as a causal variable (e.g.. phosphorus for nutrient enrichment). States also have narrative water quality criteria such as “no toxics in toxic amounts” or “nutrients that cause nuisance growth of aquatic plants.” A narrative criterion cannot be addressed by a TMDL unless and until an appropriate numerical translator for the criterion is developed by the state.4 Most states have not developed the necessary numeric translators for their narrative criteria. This fact means that hefbrc a TMDL can he developed for a receiving water that is identilied as having an impairment of a narrative criterion, causal water quality constituents that can be defined by numeric values must he identified as the first step of a TMDL
There are two general categories of water quality model that are typically used for conducting TMDLs6:
I. Watershed models, which typically are used to predict non- point source pollutant loadings: and
2. Receiving water models that simulate the transport and fate of water quality constituents.
Ecosystem or ecological models are a form of receiving water model that simulates the relationship between the biology of aquatic organisms and their physical and chemical environment. They can be linked to or are a functional component of a receiving water quality model, but may also he a completely separate analytical procedure that uses water quality model Output data and physical system descriptions as inputs.
Both calegones of mtIels require simulation of the surface water system hydrodynamics. which is the fundamental transpoil mechanism for pollutants in surface water bodies. Hydrodynamics may be incorporated directly into the models or modeled separately and then used as inputs to the loading or receiving water model.
Simulation models may also be “steady-state” or “dynamic” with respect to surface water hydrology. A steady-stale model simulates water quality or pollutant loadings under a set of specified. invariant hydrologic (stream flow, tides, andlor precipitation) input conditions. Historically, most receiving water modeling lbr constituents such as dissolved oxygen has been done with steady- state hydrologic models, with the input assumptions typically representing some form of critical hydrologic condition (e.g., low stream flow). Dynamic models simulate the varying response of water quality to variable input conditions including physical factors such as rainfall, sunlight and temperature.


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