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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE Executive Summary Fluid flow and solute transport within the vadose zone, the unsaturated zone between the land surface and the water table, is the cause of expanded plumes arising from localized contaminant sources, and an understanding of vadose zone processes is an essential prerequisite for cost-effective contaminant remediation efforts. Contamination of the vadose zone can result from many causes, including chemical spills, leaky underground storage tanks, leachate from waste disposal sites and mine tailings, and application of agricultural chemicals. Another major environmental concern is the potential for long-term migration of radionuclides from low- and high-level nuclear waste disposal facilities. Development of flow and transport models for the vadose zone is a key requirement for designing remediation and long-term stewardship strategies. The presence of fractures and other channel-like openings in the vadose zone poses a particularly significant problem, because such features are potential avenues for rapid transport of chemicals from contamination sources to the water table. The underpinning of any vadose zone fluid transport model is the conceptualization of (1) the relevant processes, (2) the structure of the subsurface, and (3) the potential events or scenarios that impact the behavior of the modeled system. These conceptualizations together form a “conceptual model,” and it is such conceptual models that are the focus of this Panel's study. In cases where multiple competing conceptual models could lead to drastically different conclusions, strategies for evaluating these models must be based on sound technical criteria. The need to develop such strategies and criteria was a key reason for the appointment of this Panel (see Box 1).
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE BOX 1 Statement of Task A panel under the auspices of the U.S. National Committee for Rock Mechanics will organize and conduct a workshop on conceptual models of fluid infiltration in fractured media. The study will focus on the scale, complexity, and site-specific conditions and processes that need to be determined in order to develop an appropriate conceptual infiltration model. Examples of questions that may be addressed are: (1) Does the conceptual model provide an adequate characterization of the system? (2) How well does the model perform in comparison with competing models? (3) Is the data base adequate to estimate model parameters with sufficient reliability that the associated prediction uncertainties are acceptable in light of the intended application of the model? (4) What are the opportunities for field testing and verification of the model? The Panel will produce a consensus report of its findings and conclusions. A series of individually authored papers that were presented at the workshop will be appended to the report. When carefully developed and supported by field data, models can be effective tools for understanding complex phenomena and for making informed predictions. However, model results are always subject to some degree of uncertainty due to limitations in field data and incomplete knowledge of natural processes. Thus, when models form the basis for decision-making, uncertainty will be an inescapable component of environmental management and regulation. A key consideration in any modeling process is whether the model has undergone sufficient development and testing to address the problem being analyzed in a sufficiently meaningful manner. After reviewing the process through which conceptual models of flow and transport in the fractured vadose zone are developed, tested, refined, and reviewed, the Panel produced the following conclusions grouped according to the two major topics addressed in this report: (1) general considerations during the development and testing of conceptual models, and (2) flow and transport in the fractured vadose zone. These conclusions are followed by the Panel's recommendations for research activities that will contribute to the conceptual modeling process. CONCLUSIONS ON DEVELOPMENT AND TESTING OF CONCEPTUAL MODELS Development of the conceptual model is the most important part of the modeling process. The conceptual model is the foundation of the quantitative, mathematical representation of the field site (i.e., the mathematical model), which in turn is the basis for the computer code used for simulation.
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE The context in which a conceptual model is developed constrains the range of its applicability. A conceptual model is by necessity a simplification of the real system, but the degree of simplification must be commensurate with the problem being addressed. It is important to recognize that model predictions require assumptions about future events or scenarios, and are subject to uncertainty. Quantitative assessment of prediction uncertainty should be an essential part of model prediction. A suite of predictions for a range of different assumptions and future scenarios is more useful than a single prediction. Testing and refinement of the conceptual model are critical parts of the modeling process. Reasonable alternative conceptualizations and hypotheses should be developed and evaluated. In some cases, the early part of a study might involve multiple conceptual models until alternatives are eliminated by field results. Although model calibration does provide a certain level of model testing, a good fit to the calibration data does not necessarily prove that the model is adequate to address the issues in question. A model that matches different types of calibration data collected under different field conditions is likely to be more robust than a model that matches a limited range of calibration data. However, if the model cannot be calibrated to match the calibration data, this is an indication that the conceptualization should be re-examined. Checking model simulation results against field data (that were not used for calibration) is one, but not the exclusive, approach to model testing. Where all field data are needed for calibration, and none are left for further testing, this does not mean that the model cannot be used for prediction. From an operational perspective, the goal of model testing is to establish the credibility of the model. In addition to testing and evaluation, the credibility of a model can be enhanced by independent peer review and by maintaining an open flow of information so that the model is available for scrutiny by concerned parties. CONCLUSIONS ON FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE There exists a body of field evidence indicating that infiltration through fractured rocks and structured soils does not always occur as a wetting front advancing at a uniform rate; accordingly, any model simulations that assume a uniform wetting front within a homogeneous medium may provide erroneous estimates of flux and travel times through the vadose zone. The current state of knowledge is not adequate to determine which processes are likely to control unsaturated flow and transport at a given field site. Although laboratory and theoretical analyses demonstrate that film flow in frac-
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE tures can transport fluid and solute at rates substantially higher than transport by capillary flow, the significance of film flow at the field scale is controversial. Although not identical, structured soils and fractured rocks exhibit many similarities in flow and transport processes. Macropores and aggregates in structured soils are respectively analogous to fractures and matrix blocks in rock, and therefore communication between workers in both soil science and fractured rock fields will be mutually beneficial. Models of varying complexity have been developed for preferential flow, but their adequacy for field-scale application requires further testing. In order to avoid explicitly simulating the mechanisms that cause preferential flow, current models simulate fast and slow flow by use of a composite hydraulic conductivity curve or by dual permeability domains. Further testing is required to determine whether such models are adequate for field-scale application over a broad range of field conditions. The interaction between fracture and matrix exerts a strong control on fluid and solute movement. However, the strength of this interaction in the field is not well known. The simplified representation of this interaction in current models also requires further evaluation. Solute transport in the fractured vadose zone can exhibit complex behavior due to the large variations in fluid velocity and the interplay of advective and diffusive transport between fractures and matrix. Solute transport models are more complex than flow models, and can involve multiple regions to represent the diversity of macropore and micropore sizes. Environmental tracers should be included in field investigation strategies from the very beginning of a site characterization program. Use of geochemical and environmental tracer data in several studies has led to substantial revisions of conceptual models initially based upon hydrodynamic analysis. RECOMMENDED RESEARCH Flow and transport in the fractured vadose zone have been, and will continue to be, an active area of research in both the soil science and subsurface hydrology disciplines. The research recommended in this report is not meant to be inclusive. Instead, the list below reflects topics that address the issues identified in this report so that conceptual models of flow and transport can be improved. Fundamental research to understand flow and transport processes in unsaturated fractures should continue. Particular emphasis should be placed on understanding mechanisms that cause non-uniform (preferential) flow, film flow, and intermittent behavior. Research is needed to understand the spatial variability in vadose zone properties, and to develop upscaling methods. Spatial variability is a key cause of model uncertainty, because the subsurface cannot be exhaustively sampled. Up-
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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE scaling methods are needed to derive field-scale flow and transport properties from small-scale laboratory measurements. There is a need for comprehensive field experiments in several fractured vadose zone geologic environments. These experiments should be designed to understand the controlling processes for a broad range of field conditions, to evaluate methods of parameter upscaling, and to test alternative conceptual models. Current models should be evaluated for their adequacy for simulating flow and transport in the presence of fingering, flow instability, and funneling. Of particular importance is the evaluation of transfer coefficients to represent fluid and solute exchange between fracture and matrix. There is a need to develop quantitative assessments of prediction uncertainty for models of flow and transport in the fractured vadose zone. Meaningful quantification of uncertainty should be considered an integral part of any modeling endeavor, as it establishes confidence bands on predictions given the current state of knowledge about the system. Research should be undertaken to develop improved techniques for geochemical sampling of the fractured vadose zone. Improved sampling techniques will facilitate the use of environmental tracers and geochemical data for conceptual model building.
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