351 CHAPTER 16 Surface Water Modeling Bruce T. Rodgers CONTENTS I. Introduction 351 II. Model Objective 352 III. Model Classification 352 A. Degree of Detail 353 B. Principal Model Components 353 IV. Model Selection 355 A. Budget 355 B. Function 355 C. Options 355 D. Resources 355 V. Sources of Models 355 VI. Conclusion 356 References 356 I. INTRODUCTION Surface water models serve as an integral part of the risk assessment process when the contaminant pathway or exposure media includes streams, rivers, estuaries, coastal regions, or open ocean. These models quantify the physical and chemical characteristics of the environmental compartments through which a contaminant moves. They also quantify the spatial and temporal extent of contaminant concen- trations in surface water environments. These models are analytical tools, used to support ERA planing and subsequent assessment of remedial options, or pollution prevention strategies. LA4111/ch16 Page 351 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC 352 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS Surface water models are used when the required information cannot be mea- sured directly. As an example, consider the case of a river containing contaminated sediment. Field measurement alone can identify the quality of the sediment and size of the contaminated area. However, a surface water model together with field mea- surement is necessary to determine the cause of contamination and the effectiveness of remediation. In general, surface water models are comprised of a series of mathematical equations representing various processes occurring in an aquatic environment. Each equation, or group of equations, mathematically reproduces the abiotic or biotic interactions of a chemical with its environment. This produces a simulation of what would happen if a chemical were to enter a surface water environment and be transported to some point away from its point of entry. A wide variety of models are available to surface water modelers. Each model provides a different degree of detail about each environmental component. The art of model usage lies in the selection of an appropriate model to reflect the needed level of detail for use in a specific application. A project manager and project team must generally understand the various types of surface water models, and the advantages and disadvantages of each. This under- standing aids in determining the appropriate level of effort to invest in modeling activities. II. MODEL OBJECTIVE Surface water models explain and predict the interaction of a chemical with its environment. Explanation refers to the relationships between source loading, water and contaminant movement, water and sediment chemistry, and biological response. Using models to understand these relationships helps in planning an ERA by iden- tifying critical elements affecting contaminant pathways and exposure. Prediction involves the simulation of risk. Unlike field measurements where risk assessors must deal with existing environmental conditions, models can use any specified physical or chemical input to quantify change in risk. For example, the model can quantify the change in risk attributed to a reduction in contaminant loading, or a change in the physical or chemical characteristic of the aquatic envi- ronment. This information is useful to assess the effectiveness of remedial options or pollution prevention strategies. III. MODEL CLASSIFICATION An array of models exist for the prediction and assessment of surface waters. Each model is classified based on the degree of detail provided and the environmental component simulated. LA4111/ch16 Page 352 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC SURFACE WATER MODELING 353 A. Degree of Detail The degree of detail provided by a surface water model refers to the complexity of the mathematical equations used to simulate the environmental processes, such as the movement of water or contaminants. At one end of the spectrum are screening level models which are simple to use and fairly generic. At the other end of the spectrum are complex models which are difficult to use and highly specific to a particular application. A wide variety of models reside between these extremes. Screening level models describe environmental processes in general, “coarse” terms based on simple linear relationships. Such simple models require very little site specific information and can be applied by contractors with only a general understanding of modeling and the environment being simulated. Accordingly, screening level models are appropriate in applications which focus on general or relative impacts without regard for high accuracy, as is the case during the initial planning of the ERA. However, these models may oversimplify the critical environ- mental component required for a sufficiently accurate assessment of imposed risk. Complex models simulate the physical, chemical, and biological environment using complicated mathematical equations. The degree of resolution and precision of these models depends on the quality of site specific field data, and by the understanding and experience of the contractor. Such models are appropriate where a high degree of confidence is required, but are generally cumbersome for planning level analysis. B. Principal Model Components The principal components of the surface water model are source loading, water movements, contaminant movement, water and sediment chemistry, and biological response. Biological response is the most significant component for an ERA because it specifically addresses risk to exposed organisms. The validity of the model com- ponent used to determine biological response is limited by the accuracy of each individual component comprising the overall model. 1. Source Loading Source loading refers to the amount of contaminant released to the aquatic environ- ment by an identified source. Examples of sources include treated waste waters from industries and municipalities, urban stormwater runoff, combined sewer overflows, agricultural runoff, groundwater inflow, and contaminated sediments. In each of these cases, and where possible, source loading is best determined through direct field monitoring. The model is useful to estimate source loading when field infor- mation is not available. The model is also useful to establish a relationship between the amount of contaminant and factors contributing to its release. This relationship allows assessment of the probability of source loading, and the effectiveness of source control. LA4111/ch16 Page 353 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC 354 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS 2. Water Movement Water movement refers to the physical characteristics of the water body which affect the movement of contaminants. Specific physical characteristics of concern include the quantity of water available for mixing with the contaminant, and the pathway and speed of contaminant transport. The appropriate degree of detail depends on the spatial and temporal resolution required for the risk assessment. At one extreme are models which provide no space or time resolution. These models are appropriate for the most simplistic application involving uniform exposure over prolonged dura- tions, such as a small lake with contaminated sediments throughout. At the other extreme are three dimensional dynamic models which resolve the variations in water movement throughout the water body and through time. Such a complex model may be appropriate for assessment of a chemical spill or waste water discharge within a tidal estuary. 3. Contaminant Movement Contaminant movement refers to transport of chemicals from the identified source to a receptor. Since chemicals are often transported with water, contaminant move- ment relates to water movement. However, water and contaminant movement are distinguished as separate modeling components since the effort required to provide the same spatial and temporal detail differ greatly. This distinction is important when the project manager compares bids, since the estimated cost depends more on the degree of detail specified for water movement than contaminant movement. 4. Water and Sediment Chemistry Water and sediment chemistry refers to the physical, chemical, or biochemical change of a contaminant while in transport from source to receptor, and at rest in sediment. Sediment is included since many of the contaminants of concern occur in the organic fraction of suspended or bottom sediment. The proper degree of detail is dictated by the contaminant of concern and by the nature of the issue under consideration. First order kinetics are the simplest application but may over simplify the transformation of complex chemicals. More complex models account for parent- compound/daughter-product sequences, ionic speciation, and sorption to dissolved organic carbon and solids. 5. Biological Response Biological response refers to the uptake and accumulation of contaminants in the aquatic receptor. This component is often addressed independently of the surface water model, however, it is included to stress the importance of overall integration. The biological response is the focal point of a surface water model. It defines the contaminants of concern, means of exposure, and nature of imposed risk. It is the basis from which the model function is defined. LA4111/ch16 Page 354 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC SURFACE WATER MODELING 355 IV. MODEL SELECTION In many ways, model selection is similar to purchasing a new automobile — the selection seems infinite, the price seems irrational, and clear answers are difficult to find. As with selecting an automobile, it is difficult to outline a series of steps to ensure the appropriate selection. Personal choice always plays a factor, yet several basic criteria must be recognized. These criteria include budget, function, options, and resources. A. Budget A realistic budget for the surface water model must be identified at the beginning of the ERA. If cost is not a factor then this should be stated. The degree of detail provided by the model is limited by effort spent. Budgetary constraints will focus selection towards the appropriate class of model. B. Function Function relates to the principal model components and degree of detail required of the model. It is subtle, and requires a strong understanding of modeling, the envi- ronment being simulated, and issues to be resolved. The contaminants of concern, means of exposure, and nature of imposed risk must be clearly rationalized prior to defining function. For a complex ERA, a screening level model is often necessary to determine the components and detail required to accurately and realistically resolve the application. C. Options Options refer to the “bells-and-whistles” of the model. Surface water models are merely a tool to support the risk manager. As such, they must test ideas and com- municate results. Options, such as graphical user interfaces, graphical display, and linkages to databases and information systems, open the model to a much wider audience than otherwise possible. D. Resources Resources refer to the quality and quantity of site-specific field data. Although it is easy to be tempted, one cannot use a sophisticated surface water model without supporting data. If data are limited, then the degree of model detail must also be limited. V. SOURCES OF MODELS Surface water models can be obtained through various sources. The U.S. EPA and the U.S. Army Corp of Engineers are excellent sources of well-tested and docu- LA4111/ch16 Page 355 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC 356 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS mented models. Universities are also excellent sources of models. Regulatory agen- cies are generally familiar with models from these sources, and are usually ready to accept them as legitimate. Contractors may also offer models that they have either developed or modified. These models demonstrate the expertise and experience of the contractor; however, they are generally less familiar to regulatory agencies than those developed in the public sector. Therefore, they should be thoroughly reviewed and tested prior to use. VI. CONCLUSION Surface water models are a tool to support the risk assessment. They are used to explain and predict the interaction of a chemical with its environment. The com- ponents of the surface water model include: (1) source loading, to estimate the amount of chemical entering the water; (2) water movement, to estimate the trans- port pathway; (3) contaminant movement, to estimate the region of exposure; (4) water and sediment chemistry, to estimate the change in chemical composition; and (5) biological response, to estimate the risk to exposed organisms. A wide variety of models are available to address each component of the surface water model. Each model provides a different degree of detail ranging from screening level to highly complex. The art of model usage lies in the selection of the appropriate model to reflect the needed level of detail for use in a specific application. REFERENCES Ambrose, R.B., WASP4, A Hydrodynamic and Water Quality Model Theory, User’s Manual and Programmer’s Guide, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA, 1987. Brown, L.C., and Barnwell, T.O., Enhanced Stream Water Quality Models QUAL2E and QUAL2E-UNCAS: Documentation and User Manual, Office of Research and Develop- ment, U.S. Environmental Protection Agency, Athens, GA, 1987. Burns, L.A., Cline, D.M., and Lassiter, R.R., Exposure Analysis Modeling System (EXAMS): User Manual and System Documentation, Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, GA, 1982. Thomann, R.V., and Mueller, J.A., Principles of Surface Water Quality Modeling and Control, Harper & Rowe, New York, 1987. U.S. Army Corp of Engineers, Cequal-w2: A Numerical Two-Dimensional, Laterally-Aver- aged Model of Hydrodynamics and Water Quality, U.S. Army Corp of Engineers, Wash- ington, 1986. LA4111/ch16 Page 356 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC . excellent sources of well-tested and docu- LA4111/ch16 Page 355 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC 356 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS mented. Engineers, Cequal-w2: A Numerical Two-Dimensional, Laterally-Aver- aged Model of Hydrodynamics and Water Quality, U.S. Army Corp of Engineers, Wash- ington, 1986. LA4111/ch16 Page 356 Wednesday,. of source control. LA4111/ch16 Page 353 Wednesday, December 27, 2000 2:55 PM © 2001 by CRC Press LLC 354 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS 2. Water Movement Water