Environmental Risk Assessment Reports - Chapter 17

CHAPTER 17 Groundwater Modeling in Health Risk Assessment Jeanette H. Leete CONTENTS I. Introduction.................................................................................................357 II. Groundwater Modeling Reports.................................................................358 III. Technical Aspects of Groundwater Modeling............................................358 A. Definition of “Model” ..................................................................358 IV. Technical Aspects of Contaminant Transport............................................365 A. Physical and Chemical Forces Influencing Movement................365 B. Model Misuse, Limitations, and Sources of Error ......................366 C. Groundwater Quality Monitoring.................................................367 V. Conclusion..................................................................................................367 References...................................................................................................367 I. INTRODUCTION Groundwater modeling refers to the construction and operation of a model that can mimic the actual behavior of groundwater in an aquifer system. There are several kinds of groundwater models: electrical analog, physical (most physical models look like ant farms packed with layers of sand and clay), and mathematical. For this primer, we use “groundwater model” to mean a mathematical model. A mathematical model is a set of equations and assumptions chosen to represent a groundwater system. Computer programs then solve these sets of equations. Groundwater modeling is extremely useful for developing credible risk assess-ments where groundwater is a potential exposure pathway. Groundwater modeling is employed during the risk assessment process in the hazard evaluation and exposure assessment steps. Modeling is used to evaluate the possible contaminant transport 357 © 2001 by CRC Press LLC 358 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS pathways (so that exposure potential can be evaluated). Groundwater modeling can provide information about changes in concentration from source to discharge or withdrawal point. Assessing the risk to humans or to the environment of a constituent of ground-water requires the ability to predict exposure to this groundwater. This requires knowledge of the direction and amount of groundwater movement; the chemical nature of the water; concentration of the undesirable constituent at the point or area of entry into groundwater; possible interactions with the aquifer material and natural groundwater; interconnections to other water sources (discharge to springs, pumping from wells, hydraulic connections between aquifers); and potential for transforma-tions during transport, such as adsorption, dilution, and dispersion. Direct measurement of this information is generally impossible because of lim-ited access to subsurface information. Movement of water or contaminated water in the subsurface cannot be directly observed, nor can continuous measurements over an area be taken. Available information is always limited to point information at a limited number of locations. If done efficiently and well, groundwater modeling can combine sparse data into a coherent representation of the workings of a hydrogeo-logic system. That information can then be used to predict the current and future extent of contamination and pathways of exposure. II. GROUNDWATER MODELING REPORTS Groundwater modeling reports are typically produced as one large deliverable. This format is acceptable for simple physical and geochemical settings, and for situations where previous work has created a credible understanding of the geology of the area, and has defined the existing hydrogeochemistry and extent of contamination. A groundwater modeling report should be broken into several interim deliver-ables for complex or poorly understood settings. Examples of complex settings include multiaquifer problems, flow in fractured formations, and situations where groundwater withdrawals are variable in amount, timing, and location. Possible logical subreports include Site Geology and Conceptual Hydrogeologic Setting; Ground Water Flow Model Calibration and Verification; and Ground Water Transport Modeling. A series of smaller reports allows the project manager to review inter-mediate results and ensure that the project is on “solid ground” before authorizing subsequent work. If necessary, the project manager may arrange for peer review by a second consultant, selected to review the specific report segment. III.TECHNICAL ASPECTS OF GROUNDWATER MODELING A. Definition of “Model” A model is a characterization of a real system. In hydrogeology, as mentioned in the introduction, there are several classes of models. These classes are discussed below. © 2001 by CRC Press LLC GROUNDWATER MODELING IN HEALTH RISK ASSESSMENT 359 1. Conceptual Models Conceptual models describe and offer an explanation of “how groundwater works” in a given system. Conceptual models should always precede data collection. For example, the regional geology would be described in a conceptual model along with the locations and nature of the bounding conditions on the aquifer (which might be rivers, discharge areas, recharge areas, faults, and areas where the aquifer is not present). An example of a conceptual model could read: The groundwater system in the study area consists of a stack of three regional aquifers, within a vaguely bowl-shaped basin of horizontally layered Paleozoic sedimentary rock, over a crystalline bedrock surface. The uppermost unit consists of varying thicknesses of glacial materials. Where these materials are sandy and of sufficient thickness, they too can serve as local aquifers. Preglacial drainage systems have cut through all but the deepest of the aquifers. Recharge to the system is focused where aquifers subcrop beneath sandy glacial deposits, and where aquifers appear at the surface. A major river system bisects the study area. The valley is incised from 100 to 300 feet below the general surface elevations, and forms the major discharge zone for the regional aquifer system, and thus a major boundary to the system. From such a model (i.e., the description and accompanying geologic cross-sections and maps), the risk assessment professional can form a mental picture of regional groundwater flow directions and groundwater/surface water interactions. General opinions of cause and effect are given in a conceptual model, but for predictions and analysis of local conditions, dynamic models are necessary. 2. Dynamic Models A dynamic model can be changed to reflect changing conditions, that is, it can be manipulated. Physical models, scale models of the groundwater system, can be built in aquariums or narrow plexiglass “ant farms” of sand, gravel, and clay or other porous materials. The surface topography, complete with lakes and/or streams, can be represented — wells can be built of acrylic or other clear tubing (so that water levels can be observed); leaky underground storage tanks can be made from empty film canisters with pinholes and an access pipe made from a straw. With some imagination, patience, and visits to the hardware store, most types of groundwater problems can be built into a physical model. A physical model can show groundwater movement in response to regional flow, and can show response to pumping of the model’s wells. Food coloring can be added to the recharge water or to water at a contaminant source in order to reveal the flow paths of the water. Because of the difficulty in deriving quantifiable results from such models, and the amount of time needed to rebuild it every time a change is needed, these models are rarely used today to solve groundwater problems. They have, however, proven to be very useful in the public meeting forum where they can be used to demystify the concepts of groundwater flow and contaminant transport. The flow of electricity through a conductor is analogous to the flow of ground-water through an aquifer, the realization of which was the breakthrough which © 2001 by CRC Press LLC 360 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS allowed the development of mathematical solutions to groundwater flow problems. Accordingly, some of the first groundwater models were built as networks of resistors and capacitors. The aquifer characteristics were scaled into the model by using different resistors to represent the transmission of water and different capacitors to represent the storage of water. When such a model was finished, current represented the flow of water and voltage represented the hydraulic head (which can be under-stood as the water level in wells which penetrate the aquifer). These models are called electrical analog models. Electrical analog models can take months to build and adjust so that they represent the aquifer system under study, and they tend to take up quite a bit of lab space. Three-dimensional flow can be modeled by con-necting two or more horizontal models to each other with the appropriate electronics to represent leakage between the layers. Electrical analog models are rarely built today because other models are easier to work with. Today’s uses still include permanent museum and public education displays. Stochastic models are statistical models. Much recent research, and possibly hundreds of recent papers, have explored the use of stochastics in the modeling of groundwater flow and contaminant transport, but the method has not gained wide acceptance among practitioners. This is almost certainly due to the complexity of the concepts employed and the fact that none of the many modeling approaches presented has become a standard. It is possible that rapid progress toward an accepted standard could be made in the next several years. Mathematical models are derived from the physical laws that govern the situation (e.g., conservation of mass, conservation of momentum, and Darcy’s equation) with simplifying assumptions about the aquifer and about the edges of the modeled area. Analytical models can be used to solve very simple problems (e.g., the aquifer can be assumed to be the same in every direction and only one value for each parameter is needed). Equations are set up which represent the system variables (e.g., hydraulic head) over the domain of the model. The resulting analytical model of groundwater flow will be a set of partial differential equations that can be solved directly using calculus. Analytical models for solute transport can be created in a similar fashion. The results of more than one analytical model run can be combined to produce a solution to a more complicated situation. One could, for example, set up an analytical model which produced a solution for the hydraulic gradient over the area of concern, then use a different analytical model to predict the movement of con-taminants in response to those gradients. The data requirements for an analytical model are not extensive, because after all, only one number can be used for each system parameter. Analytical models can be solved quickly with an inexpensive programmable calculator or personal com-puter. Graphical solution of some of the less complicated flow equations is possible. For example, flow nets combine lines which describe flow paths and lines which represent equal hydraulic head to provide a visualization of the groundwater flow field. Once constructed, a flow net can be used for prediction of flow directions and amounts. It is clear that many real world problems are not simple enough to be accurately assessed with simple analytical or graphical models. Where enough is known about © 2001 by CRC Press LLC GROUNDWATER MODELING IN HEALTH RISK ASSESSMENT 361 a hydrogeologic or contaminant transport problem to be able to characterize the system with variable aquifer parameters and detailed boundary conditions, the groundwater flow equations cannot be directly solved with calculus; rather, they must be approximated by systems of algebraic equations. Groundwater models using this technique are termed numerical models. Calculations must be carried out repeat-edly over the entire system of equations until a solution is reached. The process is repeated every time a change in any of the model data is made. Mathematical models have replaced other types of models as the speed of computers has increased and the cost of computers has decreased. As few as 15 years ago, the best high speed computers the major universities had to offer often took hours to complete one run of a numerical model. Because of computing costs, these model runs were often done overnight at lower rates. The results were picked up in the morning (if indeed the program had run without fatal errors), and during the day necessary changes were made in model input for the next night’s run. Today’s personal computers provide the speed and flexibility needed to handle many model runs, of even very complex models, in one day, and advanced workstations allow the calculation of detailed three-dimensional models, and provide graphical color output of the results in seconds. 3. Model Selection The particular problem at hand will determine which of the methods is appropriate. Each of the modeling approaches discussed above has its limitations, advantages, and disadvantages. The essential question is: Can this method answer my question most efficiently? There is a tendency in the groundwater profession to turn to the numerical models without consideration of the less elegant methods. To counteract this bias, the following questions should be posed as part of the model selection process: · What is the model’s purpose? The scope of the study may be such that answers can be obtained from analytical models or from graphical solutions. · What data are available to characterize the aquifer system? If the aquifer system can only be described in general terms, what is the justification for the use of a complex model? · Is the collection of additional data to be part of the study? If so, then a preliminary model can be constructed to guide data acquisition and eventual construction of a full model. If the decision is made that a numerical model is indeed necessary, an appropriate numerical method should be selected. There are three basic approaches to numerical modeling: finite difference, finite element, and analytic element. As mentioned above, the continuous equations that describe conditions in the aquifer at every point can only be directly solved for very simple situations. To accommodate more complex aquifer characteristics, the study area is divided up into smaller pieces. In both the finite difference and finite element approaches, each aquifer segment is described by an algebraic equation or set of equations, all of © 2001 by CRC Press LLC ... - tailieumienphi.vn