1、Identification of Key Assumptions and Models for the Development of Total Maximum Daily LoadsRegulatory Analysis and Scientific Affairs Publication Number 4736 November 2006Identification of Key Assumptions and Models for the Development of Total Maximum Daily LoadsRegulatory and Scientific AffairsA
2、PI PUBLICATION 4736FIRST EDITION, NOVEMBER 2006PREPARED BY:Tischler/KocurekRound Rock, TexasACKNOWLEDGMENTS THE FOLLOWING PEOPLE ARE RECOGNIZED FOR THEIR CONTRIBUTIONS OF TIME AND EXPERTISE DURING THIS STUDY AND IN THE PREPARATION OF THIS REPORT: API STAFF CONTACT Roger Claff, Regulatory Analysis an
3、d Scientific Affairs MEMBERS OF THE CLEAN WATER ISSUES TASK FORCE Rees Madsen, Task Force Chairman, BP P.L.C. John Cruze, Task Force Vice Chairman, ConocoPhillips Jeffrey Adams, BP America Incorporated Gregory Biddinger, ExxonMobil Refining and Supply Company Mickey Carter, ConocoPhillips Robert Goo
4、drich, ExxonMobil Research and Engineering John King, Marathon Ashland Petroleum Susie King, ConocoPhillips Jonnie Martin, Shell Oil Products US Pat Netsch, ChevronTexaco Corporation Pepsi Nunes, Marathon Ashland Petroleum LLC David Pierce, ChevronTexaco Corporation Jeff Richardson, BP P.L.C. George
5、 Stalter, BP P.L.C. Kim Wiseman, ChevronTexaco Corporation Jenny Yang, Marathon Oil Company David Zabcik, Shell Oil Products US Identification of Key Assumptions and Models for the Development of Total Maximum Daily Loads Abstract This study identifies and reviews the most widely used, publicly avai
6、lable watershed and receiving water models used in total maximum daily load (TMDL) analysis. These models are the primary tool states and EPA use to establish TMDLs, the pollutant loading budgets required when a state determines that a surface water body does not achieve applicable surface water qua
7、lity standards. Applicable models range from simple mass balances to highly sophisticated computer models that simulate dynamic water quality variations. Watershed models are used to predict point and nonpoint source pollutant loadings in runoff from different types of land use. Receiving water mode
8、ls are used to predict receiving water quality as a function of pollutant loadings and hydrologic conditions. The applicability of these models and their complexity, input data requirements, and prediction capabilities are described. The most important model input requirements for developing scienti
9、fically supported water quality simulations are identified and prioritized. In the case of watershed models, the most important variables are: (1) the physical characteristics of the watershed; (2) the land uses; and (3) the loading functions that relate pollutant loadings to land use. The key data
10、requirements for receiving water models are: (1) the adequate characterization of hydraulics, which governs the transport of pollutants; (2) the pollutant transformation rates; and (3) the pollutant sources. The review of available TMDL models emphasizes that site-specific data must be available to
11、calibrate and validate whichever model is selected to meet the TMDL objectives. An essential element of any TMDL is validation of water quality model predictive capability, using a field data set that is independent of the data used for model calibration. Also, a component of every TMDL should be se
12、nsitivity analyses of model predictions to allow probability analysis of uncertainty. Table of Contents Page 1 Introduction . 1 Objective . 2 Scope . 2 Organization . 3 2 Summary of TMDL Modeling 4 TMDL fundamentals 4 Watershed models 5 Receiving water models 8 3 TMDL Fundamentals. 10 Modeling funda
13、mentals . 11 Scoping the TMDL study. 13 Selecting a model 16 Boundary conditions (pollutant loadings) 18 4 Watershed Models. 21 Loading equations. 22 Comprehensive watershed modeling 23 Selecting a watershed modeling approach . 26 Data sources for watershed models 29 5 Receiving Water Models. 31 Ste
14、ady state models 32 Dynamic models 36 Boundary conditions 41 Other models. 43 Websites 47 References 48List of Tables Page 2-1 Fundamental Development of TMDL Development . 5 2-2 Watershed Model Selection Considerations 6 2-3 Key Watershed Model Variables 7 2-4 Receiving Water Model Selection Consid
15、erations . 8 2-5 Key Receiving Water Model Variables . 9 3-1 Watershed Model Simulation Capabilities 17 3-2 Receiving Water Model Simulation Capabilities. 19 4-1 Example Watershed Equations . 22 4-2 Input Data for Watershed Equations 23 4-3 Examples of Comprehensive Watershed Models 25 4-4 Selecting
16、 a Comprehensive Watershed Model 27 4-5 Data Sources for Watershed Models . 29 5-1 Examples of Steady-state Water Quality Models . 33 5-2 Data Requirements for Steady-state Water Quality Models. 35 5-3 Examples of Dynamic Water Quality Models . 37 5-4 Data Requirements for Dynamic Water Quality Mode
17、ls. 40 5-5 Steady-state Model Boundary Conditions 42 5-6 Dynamic Model Boundary Conditions 43 5-7 Examples of Mixing Zone Models 45 5-8 Examples of Ecological Models 46 List of Figures Page 3-1 Selecting the Geographic Area for the TMDL 15 4-1 Selection and application of watershed models . 30 Execu
18、tive Summary The American Petroleum Institute (API) commissioned this evaluation of models for developing total maximum daily loads (TMDL) as required by Section 303(d) of the Clean Water Act. TMDLs are required when a state determines that a surface water body does not achieve applicable surface wa
19、ter quality standards. The TMDL is designed to identify the pollutant sources causing and/or contributing to the impaired water quality, and to determine allowable point and nonpoint source pollutant loadings that will assure the water quality standard is achieved. Water quality models are the prima
20、ry tool states and EPA use to establish TMDLs. Applicable models range from simple mass balances to highly sophisticated computer models that simulate dynamic water quality variations. There are two basic categories of models used for TMDL studies: (1) watershed models, and (2) receiving water model
21、s. Watershed models are used to predict point and nonpoint source pollutant loadings in runoff from different types of land use. Receiving water models are used to predict receiving water quality as a function of pollutant loadings and hydrologic conditions. Some comprehensive models link watershed
22、pollutant load prediction and receiving water quality effects analysis. Dynamic water quality models are available to simulate temporal variations in water quality. Ecological models are receiving water models that simulate chemical transport and transformation in aquatic food webs. Publicly availab
23、le watershed and receiving water models are available for virtually every category of TMDL. Generally, a publicly available model that is well documented and has been demonstrated to generate scientifically acceptable predictions should be used for TMDLs. The data requirements for watershed and rece
24、iving water models become increasingly demanding as the models become more complex and sophisticated. Model selection must consider the nature of the water quality impairment and the complexity of the model needed to adequately simulate the source-effect relationship. As a general rule of thumb, the
25、 entity performing the TMDL should select the least complex model that meets the TMDL objectives. This study identifies and reviews the most widely used, publicly available watershed and receiving water models. The applicability of these models and their complexity, input data requirements, and pred
26、iction capabilities are described. The most important model input requirements for developing scientifically supported water quality simulations are identified and prioritized. In the case of watershed models, the most important variables are: (1) the physical characteristics of the watershed; (2) t
27、he land uses; and (3) the loading functions that relate pollutant loadings to land use. The key data requirements for receiving water models are: (1) the adequate characterization of hydraulics, which governs the transport of pollutants; (2) the pollutant transformation rates; and (3) the pollutant
28、sources. Section 2 of the report summarizes the most important data for each type of watershed and water quality model described in this report. The review of available TMDL models emphasizes that site-specific data must be available to calibrate and validate whichever model is selected to meet the
29、TMDL objectives. An essential element of any TMDL is validation of water quality model predictive capability, using a field data set that is independent of the data used for model calibration. Also, a component of every TMDL should be sensitivity analyses of model predictions to allow probability an
30、alysis of uncertainty.American Petroleum Institute Section 1 TMDL Model Development 1 Introduction Section 1 This report is intended to provide the reader with an understanding of the use of models in the development and implementation of total maximum daily loading (TMDL) studies. The Clean Water A
31、ct (CWA, Section 303(d) requires a TMDL when a surface water body does not achieve a surface water quality standard or designated use.1Regulations promulgated by the U.S. Environmental Protection Agency (EPA) require states and tribes to perform TMDLs for all water bodies under their jurisdiction th
32、at they have identified as impaired waters pursuant to the provisions of Section 303(d)(1) of the CWA. Section 303(d)(2) requires EPA to approve or disapprove state lists of impaired waters and TMDLs developed by states to eliminate the impairments. If a TMDL is approved, the TMDL is to be incorpora
33、ted into the states continuing planning process required by Section 303(e). If EPA disapproves a states TMDL, then EPA is required to develop the TMDL and the state must include the EPA-derived TMDL into its continuing planning process. Water quality models, which simulate the fate and transport of
34、pollutants, are the principal tools that are used to develop and implement a TMDL. These models are designed to be predictive tools that will allow the regulatory agency to determine the amount of reductions in pollutant loading that will be required of each contributing source in order to assure th
35、at the surface water achieves the relevant water quality criterion and/or designated use. Because models are central to the development and implementation of TMDLs, the American Petroleum Institute (API) Clean Water Issues Task Force commissioned this review of the types of models used for TMDL stud
36、ies and the assumptions underlying their development and use. 1Section 303(d)(1)(A) identifies these as surface waters where technology-based effluent standards promulgated pursuant to Sections 301(b)(1)(A) and (B) are not stringent enough to implement any applicable water quality standard. Such sur
37、face waters are referred to as “impaired.” American Petroleum Institute Section 1 TMDL Model Development 2 Objective This report focuses on the types of models used for TMDLs, the key assumptions underlying the models, how models are selected for specific surface waters and impairments, the data req
38、uired to apply the models to a specific surface water and impairment, and how the predictive capability of the models is assessed. EPA has published a report entitled Compendium of Tools for Watershed Assessment and TMDL Development.2EPAs Compendium provides detailed descriptions of most of the mode
39、ls that are included in this review. The Water Environment Research Foundation (WERF) has also published a survey and assessment of water quality models that is available as a CD-ROM.3This API review is not intended to be a substitute for EPAs Compendium, other EPA guidance, the WERF assessment, and
40、 published technical references on water quality modeling. The reader should refer to the EPA compendium and WERF report for more detailed descriptions of watershed and water quality models. A list of published references on modeling is included in this report. Scope This review covers different typ
41、es of water quality models applied to TMDLs performed to date. It also includes models recommended for, but not necessarily applied to, TMDLs. Because there are literally dozens of water quality models, both public and proprietary, this review focuses on well-documented models that are in the public
42、 domain and most commonly recommended for TMDL use. A range of model complexity is represented in this review. Models that are applicable to a wide range of surface water constituents were evaluated for this study. This report is designed to inform users about the application of models to specific t
43、ypes of TMDL problems, with emphasis on assuring that predictions of water quality are as reliable and accurate as practical, given the time and resources available to conduct the TMDL. This review does not recommend any specific model for any particular application. 2EPA, 1997, Compendium of Tools
44、for Watershed Assessment and TMDL Development, EPA 841-B-97-006, Office of Water, Washington, D.C. 3WERF, 2001, Water Quality Models: A Survey and Assessment, Order No. D13209TC (CD-ROM), Washington, D.C. American Petroleum Institute Section 1 TMDL Model Development 3 Organization This report is org
45、anized into the following sections: 1. Introduction; 2. Summary of TMDL modeling; 3. TMDL Fundamentals; 4. Watershed Models; and 5. Receiving Water Models. A list of websites for downloading the publicly available models described in this report is presented at the end of the report. A list of refer
46、ences on TMDLs and models is also provided following the website list. American Petroleum Institute Section 2 TMDL Model Development 5 Summary of TMDL Modeling Section 2 The objective of this study was to review federal, state, and regional TMDL methodologies and guidance to identify key assumptions
47、, variables, and input data required to develop waste load allocations (WLA) for point sources and load allocations (LA) for non-point sources that, when implemented, will restore water quality in an impaired water body so that it meets applicable water quality criteria and designated uses. The revi
48、ew focused on the models available to federal, state, regional, and local regulatory authorities to perform TMDL studies. The term “models” is used in its broadest sense, ranging from simple desktop calculations to complex mathematical models that must be run on powerful computers. The review is bro
49、ken into 3 categories: (1) an overview of TMDL modeling including how models are selected and how the boundaries of the water body to be modeled are specified; (2) watershed models used to predict point and non-point source pollutant loadings from sub-watersheds and watershed; and (3) receiving water quality models that are used to predict the transport and fate of specific water quality constituents in a surface water body. The conclusions drawn from this review are presented in this summary section. TMDL Fundamentals Fundamental steps in the TMDL process are: identifying water quality
