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Title: Modeling field scale unsaturated flow and transport processes

Abstract

The scales of concern in subsurface transport of contaminants from low-level radioactive waste disposal facilities are in the range of 1 to 1,000 m. Natural geologic materials generally show very substantial spatial variability in hydraulic properties over this range of scales. Such heterogeneity can significantly influence the migration of contaminants. It is also envisioned that complex earth structures will be constructed to isolate the waste and minimize infiltration of water into the facility. The flow of water and gases through such facilities must also be a concern. A stochastic theory describing unsaturated flow and contamination transport in naturally heterogeneous soils has been enhanced by adopting a more realistic characterization of soil variability. The enhanced theory is used to predict field-scale effective properties and variances of tension and moisture content. Applications illustrate the important effects of small-scale heterogeneity on large-scale anisotropy and hysteresis and demonstrate the feasibility of simulating two-dimensional flow systems at time and space scales of interest in radioactive waste disposal investigations. Numerical algorithms for predicting field scale unsaturated flow and contaminant transport have been improved by requiring them to respect fundamental physical principles such as mass conservation. These algorithms are able to provide realistic simulations of systems withmore » very dry initial conditions and high degrees of heterogeneity. Numerical simulation of the movement of water and air in unsaturated soils has demonstrated the importance of air pathways for contaminant transport. The stochastic flow and transport theory has been used to develop a systematic approach to performance assessment and site characterization. Hypothesis-testing techniques have been used to determine whether model predictions are consistent with observed data.« less

Authors:
; ;  [1]
  1. Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Civil and Environmental Engineering
Publication Date:
Research Org.:
Nuclear Regulatory Commission, Washington, DC (United States). Div. of Regulatory Applications; Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Civil and Environmental Engineering
Sponsoring Org.:
Nuclear Regulatory Commission, Washington, DC (United States)
OSTI Identifier:
10182911
Report Number(s):
NUREG/CR-5965
ON: TI94018975; TRN: 94:018997
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: Aug 1994
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; 54 ENVIRONMENTAL SCIENCES; LOW-LEVEL RADIOACTIVE WASTES; ENVIRONMENTAL TRANSPORT; FLOW MODELS; SOILS; LIQUID FLOW; GAS FLOW; RADIOACTIVE WASTE FACILITIES; SITE CHARACTERIZATION; TWO-DIMENSIONAL CALCULATIONS; GROUND WATER; 052002; 540250; WASTE DISPOSAL AND STORAGE; SITE RESOURCE AND USE STUDIES

Citation Formats

Gelhar, L.W., Celia, M.A., and McLaughlin, D. Modeling field scale unsaturated flow and transport processes. United States: N. p., 1994. Web. doi:10.2172/10182911.
Gelhar, L.W., Celia, M.A., & McLaughlin, D. Modeling field scale unsaturated flow and transport processes. United States. doi:10.2172/10182911.
Gelhar, L.W., Celia, M.A., and McLaughlin, D. 1994. "Modeling field scale unsaturated flow and transport processes". United States. doi:10.2172/10182911. https://www.osti.gov/servlets/purl/10182911.
@article{osti_10182911,
title = {Modeling field scale unsaturated flow and transport processes},
author = {Gelhar, L.W. and Celia, M.A. and McLaughlin, D.},
abstractNote = {The scales of concern in subsurface transport of contaminants from low-level radioactive waste disposal facilities are in the range of 1 to 1,000 m. Natural geologic materials generally show very substantial spatial variability in hydraulic properties over this range of scales. Such heterogeneity can significantly influence the migration of contaminants. It is also envisioned that complex earth structures will be constructed to isolate the waste and minimize infiltration of water into the facility. The flow of water and gases through such facilities must also be a concern. A stochastic theory describing unsaturated flow and contamination transport in naturally heterogeneous soils has been enhanced by adopting a more realistic characterization of soil variability. The enhanced theory is used to predict field-scale effective properties and variances of tension and moisture content. Applications illustrate the important effects of small-scale heterogeneity on large-scale anisotropy and hysteresis and demonstrate the feasibility of simulating two-dimensional flow systems at time and space scales of interest in radioactive waste disposal investigations. Numerical algorithms for predicting field scale unsaturated flow and contaminant transport have been improved by requiring them to respect fundamental physical principles such as mass conservation. These algorithms are able to provide realistic simulations of systems with very dry initial conditions and high degrees of heterogeneity. Numerical simulation of the movement of water and air in unsaturated soils has demonstrated the importance of air pathways for contaminant transport. The stochastic flow and transport theory has been used to develop a systematic approach to performance assessment and site characterization. Hypothesis-testing techniques have been used to determine whether model predictions are consistent with observed data.},
doi = {10.2172/10182911},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1994,
month = 8
}

Technical Report:

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  • The development of accurate mathematical models to predict field-scale solute transport in the saturated and unsaturated zones is hampered by the lack of reliable data on field-scale transport parameters. A critical review of the available literature on studies conducted at 55 saturated zone and 28 unsaturated zone sites produced 99 and 8 longitudinal dispersivity values, respectively. In the saturated zone, the scale of observation for all the data ranged from 0.75 m to 100 km with longitudinal dispersivities from 0.01 to 5500 m. However, only five sites produced highly reliable dispersivity data, based on an evaluation of the test configuration,more » the tracer monitoring, and the data analysis method for each site. The largest scale of high reliability dispersivities was only 115 m. The high reliability data subset indicates that the dispersivity initially increases with the scale of observation. But it is not clear whether the dispersivity increases indefinitely with scale or reaches an asymptotic value as is assumed in classical modelling and predicted by recent stochastic theories. In the unsaturated zone the dispersivity ranged from 1 mm to 0.7 m and appeared to increase with the scale of observation from 1 m to 20 m; however, most experiments were at scales of about 2 m. The transport process is dominated by the lateral movement of solutes in dry, high tension soils whereas in nearly saturated soils the solutes and water can move rapidly downward through the macrostructures. There is a clear need to conduct controlled large-scale field experiments in both the saturated and unsaturated zones to obtain reliable dispersivities at increasing scales and to identify the controlling transport mechanisms.« less
  • A field-scale, unsaturated flow and solute transport experiment at the Las Cruces trench site in New Mexico was simulated as part of a blind'' modeling exercise to demonstrate the ability or inability of uncalibrated models to predict unsaturated flow and solute transport in spatially variable porous media. Simulations were conducted using a recently developed multiphase flow and transport simulator. Uniform and heterogeneous soil models were tested, and data from a previous experiment at the site were used with an inverse procedure to estimate water retention parameters. A spatial moment analysis was used to provide a quantitative basis for comparing themore » mean observed and simulated flow and transport behavior. The results of this study suggest that defensible predictions of waste migration and fate at low-level waste sites will ultimately require site-specific data for model calibration.« less
  • Over the past several years, the authors have performed experimental studies focused on understanding small-scale flow processes within discrete fractures and individual matrix blocks; much of the understanding gained in that time differs from that underlying the basic assumptions used in effective media representations. Here they synthesize the process level understanding gained from their laboratory studies to explore how such small-scale processes may influence the behavior of fluid flow in fracture networks and ensembles of matrix blocks at levels sufficient to impact the formulation of intermediate-scale effective media properties. They also explore, by means of a thought experiment, how thesemore » same small-scale processes could couple to produce a large-scale system response inconsistent with current conceptual models based on continuum representations of flow through unsaturated, fractured rock. Based on their findings, a number of modifications to existing dual permeability models are suggested that should allow them improved applicability; however, even with these modifications, it is likely that continuum representations of flow through unsaturated fractured rock will have limited validity and must therefore be applied with caution.« less
  • Unsaturated flow in heterogeneous fractured porous rock was simulated using a stochastic continuum model (SCM). In this model, both the more conductive fractures and the less permeable matrix are generated within the framework of a single continuum stochastic approach, based on non-parametric indicator statistics. High-permeable fracture zones are distinguished from low-permeable matrix zones in that they have assigned a long range correlation structure in prescribed directions. The SCM was applied to study small-scale flow in the vicinity of an access tunnel, which is currently being drilled in the unsaturated fractured tuff formations at Yucca Mountain, Nevada. Extensive underground testing ismore » underway in this tunnel to investigate the suitability of Yucca Mountain as an underground nuclear waste repository. Different flow scenarios were studied in the present paper, considering the flow conditions before and after the tunnel emplacement, and assuming steady-state net infiltration as well as episodic pulse infiltration. Although the capability of the stochastic continuum model has not yet been fully explored, it has been demonstrated that the SCM is a good alternative model feasible of describing heterogeneous flow processes in unsaturated fractured tuff at Yucca Mountain.« less