Ground Water Models Scientific and Regulatory Applications (1990) / Chapter Skim
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7 RESEARCH NEEDS
7 RESEARCH NEEDS
Pages 249-284
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From page 249... ...
Two comments are in order before the recommendations are presented. First, the focus of this study has been the status of ground water models; and therefore associated areas of expertise (e.g., climatic scenarios and exposure assessment models)
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From page 250... ...
Identifying key or cornerstone issues relevant to a host of policy goals is essential so that limited resources can be devoted to the development of technology necessary to achieve national goals on the environment and economy. Certainly, as a nation we should maintain a leadership role in hydrogeologic studies for a variety of reasons; the application of ground water models in regulation and litigation is only one.
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From page 251... ...
The committee members, primarily ground water modelers, recognize that evaluation of modeling accuracy is a broad topic influenced heavily by subjective decisions made when climate scenarios are developed, site characterization plans are made and data are analyzed, and subsurface conceptual models are formalized.
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From page 252... ...
USE OF MODELS There is no doubt that increasingly greater scientific emphasis is being placed on the use of predictive computer models in ground water hydrology and geochemistry. Early applications of ground water models emphasized qualitative or relative evaluation of several alternatives.
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The general subject of contamination of ground water is discussed in some detail in a report by the National Research Council (1984) entitled Groundwater Contamination.
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From page 254... ...
With regard to the use of deep geologic deposits for the disposal of nuclear wastes, Niederer (1988) believes that certainty is as important as safety.
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From page 255... ...
Finally, interdisciplinary efforts that bring together site geologists, hydrologists, geochemists, geostatisticians, and health physicists are essential if ground water models and allied technologies are to be routinely applied to study and solve contamination problems with confidence. Basic Understanding and Process Models Two paths have been taken toward improving our basic understanding and developing more predictive ground water models: (1)
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From page 256... ...
Multiphase Fluid Flow and Transport Models An obvious trend in research is to extend modeling capabilities to new classes of problems. A case in point is the commonly encountered problem of multiphase fluid flow and transport accompanied by dissolved component transport in water.
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From page 257... ...
Solute transport modeling requires that the accumulation of sorbed material be accounted for and that the rate of desorption be described. In addition, realistic
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From page 258... ...
Modeling of solute transport in such a situation must involve mass balances in the gas phase and description of the transfer rates between the gas phase and other phases. Not only do these requirements add to the computational demands, but they are not easily described with our current knowledge.
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From page 259... ...
The truth, at this point in time, lies somewhere between the extremes, but perhaps closer to the pessimist's point of view. The basis for this somewhat negative evaluation is the fact that researchers in geochemistry have yet to demonstrate that any of the popular geochemical models can be fully validated against field or laboratory data.
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From page 260... ...
Therefore the available geochemical models with comprehensive reactions and databases do not seem to be appropriate for solute transport modeling. Instead, simpler versions of these comprehensive codes have been developed
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From page 261... ...
Of course, great flexibility is needed in such a computational shell because there are so many types of chemical reactions exhibiting kinetic behavior. Biological reactions present a major modeling challenge.
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From page 262... ...
Probabilistic Methods Methods available in ground water hydrology for obtaining estimates of uncertainty involve two general approaches: deterministic porous-media models, where the probability component enters primarily through parameter variations, and stochastic porous-media models, where the probability component enters through the treatment of the medium itself as well as through parameter variations (Gutjahr, 1988~. These two approaches differ in how probabilities are assigned and incorporated and how the process is modeled, i.e., as deterministic or stochastic.
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From page 263... ...
Despite recent advancements, neither the stochastic nor the deterministic method has satisfactorily resolved the role of scale-dependent dispersion. With one or both of these probabilistic approaches, a broadly based technology is required to quantify uncertainty throughout the modeling process for subsurface systems, and a significant effort is needed to demonstrate the relevance of conceptual models, mathematical approaches, and characterization techniques.
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From page 264... ...
Because of the level of current activity in field-scale modeling and the strength with which opposing views are held5 it is not clear that a single state of the art can be agreed upon for conservative solute transport. Codes that embody state-of-th~art process models are relatively few in number.
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From page 265... ...
Calibration and validation are areas of research in ground water modeling. Site-specific data on initial and boundary conditions, along with material and fluid properties, when combined with computer software or codes, form a mode} of the site.
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From page 266... ...
Cases requiring a million or more nodes are within reach for both two- and three-dimensional problems; however, only physical processes such as convection and dispersion have been simulated. Hydrogeologists have always been quick to take new ideas and put them to work in models, and this trend is expected to continue as new theoretical ideas develop and new computer hardware becomes available.
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From page 267... ...
, in which they develop a methodology for optimizing the numerical solution of sets of simultaneous equations developed to describe chemical reactions in moving fluids; at present, the method is limited to small sets of equations, but additional mathematical research might make it useful for larger systems. As an aside, mathematical research might be more profitably conducted by individuals and by organizations that are not as richly endowed in computing resources as some of the national centers.
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From page 268... ...
, and advice in estimating parameters for contaminant transport models (McClymont and Schwartz, 1987~. Different approaches can be used to solve problems.
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From page 269... ...
The main components of this system are a contaminant transport model, two expert systems (EXPAR and EXINS) , and a plotting package.
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From page 270... ...
The entire package was designed for users with minimal expertise in the use of a computer and modeling. As such, it represents one of the important potential uses of expert systems in relation to contaminant transport modeling.
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From page 271... ...
A tendency exists to describe ground water models, the application of those models, and the necessary research as a logical progression and, thereby, leave the impression that modelers believe that another generation of models is the answer. In reality, this would perpetuate a myth.
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From page 272... ...
is only one component of this broader topic. There is a need to provide through integrated efforts the interdisciplinary technologies necessary to discover, characterize, and solve ground water contamination problems.
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From page 273... ...
The identification of necessary data, suitable instrumentation, appropriate sample network design, and data interpretation methods is an interdisciplinary effort. Research should proceed toward highly integrated interdisciplinary methods in order to sample the subsurface environment.
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From page 274... ...
Often, many species must be followed, including products, and these species may be affected in very different manners by other mechanisms, such as sorption or volatilization. Another area of interdisciplinary research involves the disposal of liquid hazardous wastes by subsurface injection through wells into deep aquifers.
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From page 275... ...
The extreme difficulty and cost involved in obtaining adequate field and laboratory data prior to construction of deep-well injection facilities contribute to the increasing use of predictive computer modeling. Predictive modeling potentially offers a means to minimize, or at least to optimize, the drilling of numerous test and monitoring wells and possibly to fill existing gaps in knowledge.
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From page 276... ...
POLICY TRENDS AND SUPPORT FOR RESEARCH An EPA study found that existing ground water models do not account for all processes affecting the fate and impact of contaminants. For example, the flow and transport of organic solvents are influenced by the hysteresis in multiphase soil-fluid characteristics and by biotic and abiotic fate processes; neither is accounted for in existing and available codes.
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From page 277... ...
Thus the need to regulate LLW sites does not directly justify research on complex hydrogeologic systems. This regulation provides no guidance on measures of confidence; however, all subsurface environments are uncertain or unknown to some degree.
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From page 278... ...
Current research seeks, in part, methods to quantify certainty by relating uncertainty in knowledge of the subsurface to uncertainty in predictions of future events. The "truth" of the subsurface environment is not known; therefore research toward methods of quantifying uncertainty must treat the
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From page 279... ...
Methods of uncertainty analysis that include the influence of subjective decisions on mode} results will help to ensure the proper use of models by revealing cases where ignorance outweighs knowledge. A number of governmental agencies are active in subsurface environmental studies; however, it is not clear if this contributes to the problem or to the solution of developing theoretically sound and computationally correct ground water models.
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From page 280... ...
It is true that ground water models that consider spatial and temporal changes appear to be advanced technology when compared to our understanding of geochemical and microbiological phenomena. However, more advanced methods of ground water characterization and modeling are needed in order to understand with confidence where a contaminant ~ in the subsurface so that the effectiveness of bioremediation methods for in situ treatment of contaminants can be estimated.
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From page 281... ...
Pp. 185-189 in Proceedings of the Eighth International Joint Conference on Artificial Intelligence, Karlsrnhe, West Germany.
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1986. A demonstration expert system to aid in assessing ground water contamination potential by organic chemicals.
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From page 283... ...
Pp. 122-129 in Proceedings of the Eighth International Joint Conference on Artificial Intelligence, Karlsrnhe, West Germany.
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From page 284... ...
1989. ^ crklcs1 ev~lustlon of recent deveL opponents in ~drogeoche=~1 transport models of re~ctlve multlcbem~1 components.
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