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Title: Characterization of contaminant transport by gravity, capillarity and barometric pumping in heterogeneous. 1998 annual progress report

Abstract

'The intent of this research program is to obtain an improved understanding of vadose zone transport processes and to develop field and modeling techniques required to characterize contaminant transport in the unsaturated zone at DOE sites. For surface spills and near-surface leaks of chemicals, the vadose zone may well become a long-term source of contamination for the underlying water table. Transport of contaminants can occur in both the liquid and gas phases of the unsaturated zone. This transport occurs naturally as a result of diffusion, buoyancy forces (gravity), capillarity and barometric pressure variations. In some cases transport can be enhanced by anisotropies present in hydrologic regimes. This is particularly true for gas-phase transport which may be subject to vertical pumping resulting from atmospheric pressure changes. For liquid-phase flows, heterogeneity may enhance the downward transport of contaminants to the water table depending on soil properties and the scale of the surface spill or near-surface leak. Characterization techniques based upon the dynamics of transport processes are likely to yield a better understanding of the potential for contaminant transport at a specific site than methods depending solely on hydrologic properties derived from a borehole. Such dynamic-characterization techniques can be useful for evaluating sitesmore » where contamination presently exists as well as for providing an objective basis to evaluate the efficacy of proposed as well as implemented clean-up technologies. The real-time monitoring of processes that may occur during clean-up of tank waste and the mobility of contaminants beneath the Hanford storage tanks during sluicing operations is one example of how techniques developed in this effort can be applied to current remediation problems. In the future, such dynamic-characterization methods might also be used as part of the site-characterization process for determining suitable locations of new DOE facilities that have the potential of introducing contamination into the vadose zone. This report summarizes work and accomplishments at the midpoint of the 3-year project. The authors have pursued the concept of a vadose-zone observatory (VZO) to provide the field laboratory necessary for carrying out the experiments required to achieve the goals of this research. The approach has been: (1) to carry out plume release experiments at a VZO allowing the acquisition of several different kinds of raw data that, (2) are analyzed and evaluated with the aid of highly detailed, diagnostic numerical models. Because the soil properties of a single VZO are unlikely to cover the full range of conditions encountered at all DOE facilities, the authors anticipate studying at least two and possibly three sites spanning a wide range of hydrologic and geologic properties.'« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Livermore National Lab., CA (US)
Sponsoring Org.:
USDOE Office of Environmental Management (EM), Office of Science and Risk Policy
OSTI Identifier:
13611
Report Number(s):
EMSP-54950-98
ON: DE00013611
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
44; 58; 54; Progress Report; Measuring Instruments; Flow Models; Remedial Action; PROGRESS REPORT; MEASURING INSTRUMENTS; FLOW MODELS; REMEDIAL ACTION

Citation Formats

Carrigan, C.R., and Hudson, G.B. Characterization of contaminant transport by gravity, capillarity and barometric pumping in heterogeneous. 1998 annual progress report. United States: N. p., 1998. Web. doi:10.2172/13611.
Carrigan, C.R., & Hudson, G.B. Characterization of contaminant transport by gravity, capillarity and barometric pumping in heterogeneous. 1998 annual progress report. United States. doi:10.2172/13611.
Carrigan, C.R., and Hudson, G.B. 1998. "Characterization of contaminant transport by gravity, capillarity and barometric pumping in heterogeneous. 1998 annual progress report". United States. doi:10.2172/13611. https://www.osti.gov/servlets/purl/13611.
@article{osti_13611,
title = {Characterization of contaminant transport by gravity, capillarity and barometric pumping in heterogeneous. 1998 annual progress report},
author = {Carrigan, C.R. and Hudson, G.B.},
abstractNote = {'The intent of this research program is to obtain an improved understanding of vadose zone transport processes and to develop field and modeling techniques required to characterize contaminant transport in the unsaturated zone at DOE sites. For surface spills and near-surface leaks of chemicals, the vadose zone may well become a long-term source of contamination for the underlying water table. Transport of contaminants can occur in both the liquid and gas phases of the unsaturated zone. This transport occurs naturally as a result of diffusion, buoyancy forces (gravity), capillarity and barometric pressure variations. In some cases transport can be enhanced by anisotropies present in hydrologic regimes. This is particularly true for gas-phase transport which may be subject to vertical pumping resulting from atmospheric pressure changes. For liquid-phase flows, heterogeneity may enhance the downward transport of contaminants to the water table depending on soil properties and the scale of the surface spill or near-surface leak. Characterization techniques based upon the dynamics of transport processes are likely to yield a better understanding of the potential for contaminant transport at a specific site than methods depending solely on hydrologic properties derived from a borehole. Such dynamic-characterization techniques can be useful for evaluating sites where contamination presently exists as well as for providing an objective basis to evaluate the efficacy of proposed as well as implemented clean-up technologies. The real-time monitoring of processes that may occur during clean-up of tank waste and the mobility of contaminants beneath the Hanford storage tanks during sluicing operations is one example of how techniques developed in this effort can be applied to current remediation problems. In the future, such dynamic-characterization methods might also be used as part of the site-characterization process for determining suitable locations of new DOE facilities that have the potential of introducing contamination into the vadose zone. This report summarizes work and accomplishments at the midpoint of the 3-year project. The authors have pursued the concept of a vadose-zone observatory (VZO) to provide the field laboratory necessary for carrying out the experiments required to achieve the goals of this research. The approach has been: (1) to carry out plume release experiments at a VZO allowing the acquisition of several different kinds of raw data that, (2) are analyzed and evaluated with the aid of highly detailed, diagnostic numerical models. Because the soil properties of a single VZO are unlikely to cover the full range of conditions encountered at all DOE facilities, the authors anticipate studying at least two and possibly three sites spanning a wide range of hydrologic and geologic properties.'},
doi = {10.2172/13611},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1998,
month = 6
}

Technical Report:

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  • This final report summarizes the work and accomplishments of our three-year project. We have pursued the concept of a Vadose-Zone Observatory (VZO) to provide the field laboratory necessary for carrying out the experiments required to achieve the goals of this research. Our approach has been (1) to carry out plume release experiments at a VZO allowing the acquisition of several different kinds of raw data that (2) are analyzed and evaluated with the aid of highly detailed, diagnostic numerical models. The key feature of the VZO constructed at Lawrence Livermore National Laboratory (LLNL) is the variety of plume-tracking techniques thatmore » can be used at a single location. Electric resistance tomography (ERT) uses vertical arrays of electrodes across the vadose zone that can monitor electrical resistance changes in the soil as a plume moves downward to the water table. These resistance changes can be used to provide ''snapshots'' of the progress of the plume. Additionally, monitoring wells have been completed at multiple levels in the vicinity of a central infiltration site. Sensors emplaced at different levels include electrically conducting gypsum blocks for detecting saturation changes, thermistors for monitoring temperature changes and pressure transducers for observing barometric changes at different levels in the vadose regime. The data from these sensors are providing important information about the state of the gas- and liquid-phase dynamics of the infiltration process. Similarly, access ports at different levels have been used to supply gas-phase samples while lysimeters yield liquid-phase samples. Studies involving gas-phase tracers were carried out at LLNL and at an Orange County Water District site in southern California to evaluate the time-dependent chemical signature of a plume that was spiked with an array of dissolved noble-gas tracers. Our work also correlate chemical signatures with those of the above-mentioned sensors that track the physical changes in the vadose zone. From the VZO at the LLNL site and from 3-D diagnostic simulations of our very first tracer-spiked plume infiltration event, we produced a much better understanding of the implications of soil heterogeneity for unsaturated zone contaminant transport at DOE sites. Even though the LLNL VZO site is considered to be hydrologically ''tight'' owing to the low permeability of the clays and silts that dominate the soil formations there, we find that saturation increases resulting from a near-surface ''leak'' reach the water table across the 20-meter-thick vadose zone in only tens of hours. This rapid transport at the site cannot be accurately simulated by layered models that derive their hydrologic properties from borehole-soil samples. In the LLNL vadose zone, layered infiltration models clearly underpredict of the rate of contaminant infiltration to the water table. Chemical transport simulations based on layered models of the Hanford vadose zone also appear to drastically underpredict contaminant migration. Furthermore, only simulations assuming a heterogeneous regime ''threaded'' by extremely high-permeability pathways can explain the rapid increase in saturation observed with ERT near the water table. Three-dimensional predictive models of a hypothetical tritiated water leak that are based on the above mentioned VZO infiltration-experiment diagnostic models were run. Tritiated water is an excellent conservative tracer and the infiltration simulations predict, in very good agreement with VZO experiments, that a continuous hypothetical tritium release (2-3 liters/rein) would be expected to reach the water table at significant concentrations within days. The numerical model suggests that this arrival time is determined by the amount of time required, infiltrating liquid at a given rate, to flush one pore volume in the soil between the infiltration point and the water table. Another infiltration event monitored by ERT demonstrated that infiltration could occur even more rapidly (within hours) as a result of apparent ''fastpaths'' in the inhomogeneous soil regime. Because heterogeneity and ''fast paths'' are so important for understanding the transport of contaminants to the water table and such pathways are inherently three-dimensional, one- and even two-dimensional models of layered soils, as have sometimes been used at Hanford, are likely to be inadequate for evaluating vadose zone transport processes.« less
  • 'Vadose regimes can be the sites of complex interactions between the atmosphere and groundwater. When a volatile contaminant exists as free product or in dissolved form in the vadose environment, upward transport can occur with the contaminant ultimately being vented as a vapor into the atmosphere. This transport happens naturally and can be enhanced by anisotropy resulting from heterogenities in the vadose regime. Several stages in the transport process are involved in going from a volatile, liquid state contaminant to a contaminant vapor vented at the surface. In a three-year effort, called the Vadose Zone Transport Study, the authors aremore » investigating, with the aid of existing data, new field studies involving dissolved tracer gases and 3-D diagnostic computer simulations that provide a framework to interpret the observations, the detailed nature of each of these stages of transport in several different kinds of vadose regimes. They are emphasizing the impact of features specific to a site, that is, the local geology and hydrology, on each stage of the transport process. In particular they want to better understand how the time scales for (1) partitioning contaminants from the liquid to the vapor states and then (2) transporting the vapor out of the vadose regime are dependent on the specific character of a site. Such time-scale information will be important for evaluating the potential of contaminant sources as well as remediation strategies including natural remediation approaches.'« less
  • 'Dynamic microbial attachment/detachment occurs in subsurface systems in response to changing environmental conditions caused by contaminant movement and degradation. Understanding the environmental conditions and mechanisms by which anaerobic bacteria partition between aqueous and solid phases is a critical requirement for designing and evaluating in-situ bioremediation efforts. This interdisciplinary research project will provide fundamental information on the attachment/detachment dynamics of anaerobic bacteria in heterogeneous porous media under growth and growth-limiting conditions. Experiments will provide information on passive and active attachment/detachment mechanisms used by growing anaerobes capable of reductive dechlorination. Theoretical representations of these attachment/detachment mechanisms will be incorporated into existing flowmore » and transport models that incorporate heterogeneity effects and can be used to predict behavior at field scales. These mechanistic-based models will be tested against experimental data provided through controlled laboratory experiments in heterogeneous porous media in large (meter-scale) 2-D flow cells. In addition to a mechanistic-based predictive model, this research will lead to new theories for the transient spatial distribution of microbial populations and contaminant plumes in heterogeneous porous media, improving the capability for designing staged remediation strategies for dealing with mixed contaminants.'« less
  • 'The goal of the research is to study the migratory behavior of contaminants in subsurface fractured systems using naturally occurring uranium- and thorium-series radionuclides as tracers under in-situ physico-chemical and hydrogeologic conditions. Naturally occurring U- and Th-series disequilibria can provide information on the rates of adsorption-desorption and transport of contaminants as well as on fluid transport and rock dissolution in a natural setting. The authors are developing a realistic model of contaminant migration in the Snake River Plain Aquifer beneath the INEEL by evaluating the retardation processes involved in the rock/water interaction. The major tasks are to: (1) determine themore » natural distribution of U, Th, Pa and Ra isotopes in the groundwater as well as in rock minerals and sorbed phases, and (2) study rock/water interaction processes using U/Th series disequilibria and a statistical analysis-based model code for the calculation of in-situ retardation factors of radionuclides and rock/water interaction time scales. This study will also provide an improved understanding of the hydrogeologic features of the site and their impact on the migration of contaminants. This report summarizes results after 20 months of a 36-month project. Studies performed at LANL include analysis of the long-lived nuclides {sup 238}U, {sup 235}U, {sup 234}U, {sup 230}Th, {sup 226}Ra, {sup 232}Th, and {sup 231}Pa by thermal ionization mass spectrometry (TIMS). Studies performed at the Univ. of Southern California include the measurement of short-lived naturally occurring radionuclides by decay-counting techniques and the development of models to predict the migration behavior of these radionuclides. Initial efforts began with analysis of 31, 0.5L water samples obtained through routine sampling by USGS and INEEL personnel. One significant observation from these data is that {sup 234}U/{sup 238}U activity ratios are highest in waters that emanate from local recharge areas to the northwest of the INEEL and they decrease in the direction of groundwater flow. Contours of high {sup 234}U/{sup 238}U ratios delineate preferential flow paths extending southward from the local recharge areas. The uranium data have allowed delineation of these preferential flow pathways in greater detail than obtained by other methods. The contour map of {sup 234}U/{sup 238}U ratios also identifies isolated pockets with low {sup 234}U/{sup 238} U ratios. These are suspected to represent groundwater which has undergone more extensive water/rock interaction as a result of either longer residence times of faster reaction rates. These results are important for understanding the hydrologic context of the site and crucial for interpreting the complete data sets for the U and Th decay chains.'« less
  • This final technical report summarizes the goals, objectives, experimental results, and continuum and stochastic modeling results from an University Research Initiative project focused on multiphase fluid flow and contaminant transport processes in heterogeneous multiphase systems. This report also annotates the many journal articles, book chapters, reports, newsletter articles, and professional meeting presentations and abstracts produced from this project, and lists the post doctoral associates, doctoral students, and masters students supported by this project. The vast contributions to the scientific literature produced by this project demonstrate the significant impact that this project has on advancing basic science in this important area.