Ground Water Models Scientific and Regulatory Applications (1990) / Chapter Skim
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OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS
OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS
Pages 1-21
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From page 1... ...
. In the case of contaminant transport, a much larger number of diverse and complicated processes are involved.
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As the following discussion explains, the most satisfactory results to date have come with models involving the flow of water or the transport of a single nonreactive contaminant in a saturated porous medium. As systems become more complicated through partial saturation, the presence of several mobile fluids, fracturing, or the existence of reacting contaminants, many more questions arise about the adequacy or validity of the underlying process models.
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From page 3... ...
The numerical demands of all but the simplest unsaturated zone simulation models are formidable, and accurate three-dimensional unsaturated flow modeling capabilities are not available to most consultants. Yet many important contamination problems, such as leaking underground storage tallks, infiltrating pesticides, and leaching mining wastes, afl.ect the unsaturated zone.
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The status of contaminant transport modeling depends greatly on the chemical species and phase of interest. In general, the pros cesses that influence the transport of dilute, nonreactive aqueous phase solutes are wed understood, at least in saturated media.
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Most ground water contaminant modelers would probably agree that multiphase contaminant transport Is the area where the basic physical mechanisms that control contaminant movement and degradation are least well understood and most difficult to model. Yet a wide range of important contaminants probably travel as separate liquid or gaseous phases when they move through the subsurface environment.
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It seems apparent, however, that it would be unwise to rely solely on any single source of information when deciding how to formulate regulations, carry out a cleanup, or protect public health. Models should be supplemented by carefully conceived field work, which not only provides data for estimating model inputs but also provides an independent confirmation of conditions in the subsurface environment.
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From page 7... ...
At the present time, conceptual issues and/or problems in obtaining data on parameter values limit the reliability and therefore the applicability of flow models Involving unsaturated media, fractured media, or two or more liquids. As a group, flow processes are among the most widely characterized hydrogeologic processes.
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From page 8... ...
Contaminant transport is the outcome of mass transport processes, such as advection, diffusion, and mechanical dispersion, that move the mass and a multitude of mass transfer processes that redistribute mass within or between phases through chemical and biological reactions. Present-day understanding of mass transport developed from early studies on laboratory columns and more recent well-documented tracer studies in the field.
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From page 9... ...
As was the experience with flow, fracturing adds considerable complexity to mass transport. The issue of whether fractures are open or highly channel~zed, the importance of diffusion into the matrix, and how mixing occurs at fracture intersections make conceptualization of even mass transport processes uncertain.
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From page 10... ...
Examples of some simplifications commonly used in ground water models include the assumption that an aquifer consists of a perfectly homogeneous, elastic material, or that the aquifer is made up of a small number of alternating homogeneous layers. The differences between the geologic reality of heterogeneity and the simplifications that may be used in ground water models make it scientifically dangerous and potentially misleading to blindly apply generic ground water models to any specific hydrogeological situation.
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In such situations, there is no alternative but the use of contaminant transport models. A regulation that requires contaminant transport modeling reflects an implicit decision to require a given level of detail and allow a given level of uncertainty.
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It is impossible to specify by a generally applicable regulation a contaminant transport mode} that would be scientifically valid in all applications and over the typical life of a regulation. In some circumstances, it may be appropriate to specify the use of a particular contaminant transport model.
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The regulatory agencies should not develop a list of sanctioned ground water models. Models are used to evaluate a wide range of subsurface conditions for a variety of purposes.
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Appropriate and successful models of ground water flow and transport can range from simple analytical solutions for one-dimensional flow in a homogeneous aquifer to highly complicated numerical codes designed to simulate multiphase transport of reactive species in heterogeneous, three-dimensional porous media. A useful mode!
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From page 15... ...
One of the key requirements in successfully applying flow or contaminant transport models is good-quality, site-specific data. Such data provide feasible bounds on the potable range of controlling parametere or boundary conditione, thereby minimizing the impact of data uncertainty as a major source of uncertainty associated with mode]
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Good documentation of ground water models throughout the model~ng process is necessary because of the complexities ~n~rol~red. A hydrogeologic computer mode!
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As ground water mode] mage has increased, a shortage of qualified staff capable of appropriately applying modem has been identifidl.
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To adroit the misuse of ground water flow and transport models, agencies and companies should employ qualified and well-tra~ned personnel. Ground water flow and transport models are complex computer codes.
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Research should be undertaken to provide the field and laboratory data necessary to validate How and transport models. Given that some types of models cannot be validated with existing, rather limited knowledge about some types of flow and mass transport processes, it is recommended that research be undertaken to fill in information gaps.
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From page 20... ...
Recommendations for the Future Governments, academic institutions, and private Industry need to provide financial resources and substantially Increase the pool of qualified personnel In the spectrum of fields essential to ground water modeling. A severe shortage of qualified personnel exists in the areas of hydrogeology, ground water hydrology, and organic and aqueous geochemistry.
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From page 21... ...
The third area the role of modeling in decisionmaking, including legal and social interactions-must be understood if the courts, enforcement agencies, industries, and the affected public are to obtain the benefits of modeling. The last two areas characterization through welI-controlled field and laboratory studies and development of new approaches for parameter estimation and new measurement techniques are essential if fate, transport, and remediation are to be measured in the subsurface, which is otherwise not easily accessible to human observation.
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