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Title: Flow and transport at the Las Cruces trench site: Experiment IIb

Abstract

The US Nuclear Regulatory Commission (NRC) has been directed by Congress in the Low Level Waste Policy Act of 1980 to develop regulatory guidance and assist the individual states and compacts in siting and assessing future low level radioactive waste (LLW) disposal facilities. Three water flow and solute transport experiments were performed as part of a comprehensive field trench study near Las Cruces, New Mexico to test deterministic and stochastic models of vadose zone flow and transport. This report presents partial results from the third experiment (experiment IIb). Experiments IIa and b were conducted on the North side of the trench, on a plot 1.22 m wide by 12 m long, perpendicular to the trench. The area was drip irrigated during two time periods with water containing a variety of tracers. The advance of the water front during the two irrigation episodes was measured with tensiometers and neutron probes. Solute front positions were determined from soil solution sampling through suction samplers and from disturbed sampling. The results from experiment IIb show predominantly downward water movement through the layered unsaturated soil, as evidenced from neutron probe data and gravimetric sampling. Tritium plumes were only half as deep and half as widemore » as the water plumes at 310 days after the beginning of experiment IIb. Chromium, applied as Cr(VI), moved a readily as, and similar to tritium, but there was a loss of mass due to reduction of Cr(VI) to Cr(III). Chloride and nitrate, initially present at high concentrations in the soil solution, were displaced by the low concentration irrigation water, resulting in chloride and nitrate concentration distributions that looked like negative images of the tritium distributions. The extensive data presented should serve well as a data base for model testing.« less

Authors:
;  [1]; ;  [2]
  1. New Mexico State Univ., Las Cruces, NM (United States)
  2. Arizona Univ., Tucson, AZ (United States). Dept. of Soil and Water Science
Publication Date:
Research Org.:
Nuclear Regulatory Commission, Washington, DC (United States). Div. of Regulatory Applications; New Mexico State Univ., Las Cruces, NM (United States); Arizona Univ., Tucson, AZ (United States). Dept. of Soil and Water Science
Sponsoring Org.:
Nuclear Regulatory Commission, Washington, DC (United States)
OSTI Identifier:
515518
Report Number(s):
NUREG/CR-6437
ON: TI97008405; TRN: 97:016600
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: Jul 1997
Country of Publication:
United States
Language:
English
Subject:
05 NUCLEAR FUELS; RADIOACTIVE WASTE FACILITIES; HYDROLOGY; NEW MEXICO; SITE CHARACTERIZATION; ENVIRONMENTAL TRANSPORT; TRACER TECHNIQUES; IRRIGATION; EXPERIMENTAL DATA; HYDRAULIC CONDUCTIVITY; SOILS

Citation Formats

Vinson, J., Hills, R.G., Wierenga, P.J., and Young, M.H. Flow and transport at the Las Cruces trench site: Experiment IIb. United States: N. p., 1997. Web. doi:10.2172/515518.
Vinson, J., Hills, R.G., Wierenga, P.J., & Young, M.H. Flow and transport at the Las Cruces trench site: Experiment IIb. United States. doi:10.2172/515518.
Vinson, J., Hills, R.G., Wierenga, P.J., and Young, M.H. Tue . "Flow and transport at the Las Cruces trench site: Experiment IIb". United States. doi:10.2172/515518. https://www.osti.gov/servlets/purl/515518.
@article{osti_515518,
title = {Flow and transport at the Las Cruces trench site: Experiment IIb},
author = {Vinson, J. and Hills, R.G. and Wierenga, P.J. and Young, M.H.},
abstractNote = {The US Nuclear Regulatory Commission (NRC) has been directed by Congress in the Low Level Waste Policy Act of 1980 to develop regulatory guidance and assist the individual states and compacts in siting and assessing future low level radioactive waste (LLW) disposal facilities. Three water flow and solute transport experiments were performed as part of a comprehensive field trench study near Las Cruces, New Mexico to test deterministic and stochastic models of vadose zone flow and transport. This report presents partial results from the third experiment (experiment IIb). Experiments IIa and b were conducted on the North side of the trench, on a plot 1.22 m wide by 12 m long, perpendicular to the trench. The area was drip irrigated during two time periods with water containing a variety of tracers. The advance of the water front during the two irrigation episodes was measured with tensiometers and neutron probes. Solute front positions were determined from soil solution sampling through suction samplers and from disturbed sampling. The results from experiment IIb show predominantly downward water movement through the layered unsaturated soil, as evidenced from neutron probe data and gravimetric sampling. Tritium plumes were only half as deep and half as wide as the water plumes at 310 days after the beginning of experiment IIb. Chromium, applied as Cr(VI), moved a readily as, and similar to tritium, but there was a loss of mass due to reduction of Cr(VI) to Cr(III). Chloride and nitrate, initially present at high concentrations in the soil solution, were displaced by the low concentration irrigation water, resulting in chloride and nitrate concentration distributions that looked like negative images of the tritium distributions. The extensive data presented should serve well as a data base for model testing.},
doi = {10.2172/515518},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Jul 01 00:00:00 EDT 1997},
month = {Tue Jul 01 00:00:00 EDT 1997}
}

Technical Report:

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  • Two water flow and solute transport experiments were performed as part of a comprehensive field trench study near Las Cruces, New Mexico. These experiments were designed to provide data to test deterministic and stochastic models of vadose zone flow and transport. In Experiment 1, a 4 m by 9 m area was irrigated for 10 days with water containing tritium. Thereafter, water was applied without tritium for an additional 76 days. Simple one-dimensional uniform and layered soil deterministic models for infiltration adequately predicted the overall movement of the wetting front during infiltration, but poorly predicted point values for water contentmore » due to spatial variability. Use of the layered soil model, rather than the uniform soil model, did not consistently improve prediction accuracy, for this particular field application. In Experiment 2, a 1.22 m by 12 m area was irrigated for 11.5 days with water containing tritium and bromide. Thereafter, water was applied without tracers for an additional 64 days. Water and bromide moved fairly uniformly during infiltration, whereas high concentrations of tritium developed on one side of the irrigated area. During redistribution, tritium moved little, whereas bromide displayed significant movement both downward and to one side. A two-dimensional deterministic model for water flow showed qualitative, but not quantitative, agreement with observations. A two-dimensional deterministic model for solute transport poorly described tritium and bromide movement during redistribution. 24 refs., 25 figs., 4 tabs.« less
  • The objective of this work was to test the ability of the PORFLO-3 computer code to simulate water infiltration and solute transport in dry soils. Data from a field-scale unsaturated zone flow and transport experiment, conducted near Las Cruces, New Mexico, were used for model validation. A spatial moment analysis was used to provide a quantitative basis for comparing the mean simulated and observed flow behavior. The scope of this work was limited to two-dimensional simulations of the second experiment at the Las Cruces trench site. Three simulation cases are presented. The first case represents a uniform soil profile, withmore » homogeneous, isotropic hydraulic and transport properties. The second and third cases represent single stochastic realizations of randomly heterogeneous hydraulic conductivity fields, generated from the cumulative probability distribution of the measured data. Two-dimensional simulations produced water content changes that matched the observed data reasonably well. Models that explicitly incorporated heterogeneous hydraulic conductivity fields reproduced the characteristics of the observed data somewhat better than a uniform, homogeneous model. Improved predictions of water content changes at specific spatial locations were obtained by adjusting the soil hydraulic properties. The results of this study should only be considered a qualitative validation of the PORFLO-3 code. However, the results of this study demonstrate the importance of site-specific data for model calibration. Applications of the code for waste management and remediation activities will require site-specific data for model calibration before defensible predictions of unsaturated flow and containment transport can be made. 23 refs., 16 figs., 3 tabs.« less
  • The LAS CRUCES TRENCH SITE DATABASE includes raw and processed experimental data obtained from the vadose zone model validation experiments performed on the New Mexico State Ranch northeast of Las Cruces, New Mexico. Included are data characterizing the unsaturated hydraulic properties for the experimental site and dynamic data, such as water contents, tensions, and solute concentrations, obtained during two infiltration redistribution experiments. At the site, a 26.4m long, 4.5m wide, and 5 mile deep trench was constructed in undisturbed soil. During construction a total of 594 disturbed soil samples and 594 associated soil cores were taken from 9 distinct soilmore » layers identified on the north wall. Samples were also taken from three vertical transects on this wall. Data included in the database are sample locations, a description of the 9 soil layers, bore hole saturated hydraulic conductivities and outflow data, laboratory saturated hydraulic conductivities, soil water retention data, particle size distribution data, bulk density data, parameters used to fit the van Genuchten equation to the water retention data, and estimated van Genuchten parameters for each layer and the entire site. Data obtained from two experiments designed to provide data for testing deterministic and stochastic models of vadose zone flow and transport are also included.« less
  • Computer modelling has become an essential part of the licensing process of low level waste disposal sites. Validation of models requires high quality data from carefully controlled field experiments. This report provides the soil physical data obtained at a field site near Las Cruces, New Mexico. A large trench was constructed in undisturbed soil to provide horizontal access to irrigated plots on the sides of the trench and to provide soil samples. Details are provided on the construction of the trench and on the physical properties of the samples collected during construction of the trench. A total of 594 disturbedmore » samples and 594 core samples were collected. Morphological characteristics of the soil at the site are presented, as well as data on the saturated hydraulic conductivity determined in the field with the bore hole permeameter and in the laboratory with a constant head method. Soil water retention data, particle size distribution data, and bulk density data are also provided. The van Genuchten equation was fitted to the soil water retention data and the parameters fitted in this equation are presented in the report. All results are presented in tabular form, ASCII format on a floppy disk available at the Nuclear Energy Software Center (NESC) Argonne National Laboratory. 3 figs., 51 tabs.« less
  • A series of dynamic field experiments have been performed at the Las Cruces Trench Site to provide data to test deterministic and stochastic models for water flow and solute transport in spatially variable unsaturated soils. Two experiments were performed to provide support for model validation efforts during Phase 1 of INTRAVAL and a third experiment is currently underway to support the INTRAVAL Phase 2 efforts. The third experiment utilizes different boundary and initial conditions and additional chemical tracers. The data from the third experiment along with model predictions from several modeling groups will be used to test models for watermore » flow and solute transport during infiltration and redistribution. This report summarizes the Las Cruces Trench Site model validation efforts and presents the INTRAVAL Phase 2 validation plans. The Phase 2 validation strategy is discussed in detail. 29 refs., 11 figs., 4 tabs.« less