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Title: Geostatistical analysis of field hydraulic conductivity in compacted clay

Abstract

Hydraulic conductivity (K) of fractured or porous materials is associated intimately with water flow and chemical transport. Basic concepts imply uniform flux through a homogeneous cross-sectional area. If flow were to occur only through part of the area, actual rates could be considerably different. Because laboratory values of K in compacted clays seldom agree with field estimates, questions arise as to what the true values of K are and how they should be estimated. Hydraulic conductivity values were measured on a 10 x 25 m elevated bridge-like platform. A constant water level was maintained for 1 yr over a 0.3-m thick layer of compacted clay, and inflow and outflow rates were monitored using 10 x 25 grids of 0.3-m diameter infiltration rings and outflow drains subtending approximately 1 x 1 m blocks of compacted clay. Variography of inflow and outflow data established relationships between cores and blocks of clay, respectively. Because distributions of outflow rates were much less and bore little resemblance to the distributions of break-through rates based on tracer studies, presence of macropores and preferential flow through the macropores was suspected. Subsequently, probability kriging was applied to reevaluate distribution of flux rates and possible location of macropores. Sitesmore » exceeding a threshold outflow of 100 x 10/sup -9/ m/s were classified as outliers and were assumed to probably contain a significant population of macropores. Different sampling schemes were examined. Variogram analysis of outflows with and without outliers suggested adequacy of sampling the site at 50 randomly chosen locations. Because of the potential contribution of macropores to pollutant transport and the practical necessity of extrapolating small plot values to larger areas, conditional simulations with and without outliers were carried out.« less

Authors:
;
Publication Date:
Research Org.:
Dept. of Agriculture, University Park, PA (USA)
OSTI Identifier:
6413799
Alternate Identifier(s):
OSTI ID: 6413799
Report Number(s):
CONF-8704137-
Journal ID: CODEN: IMGJB
Resource Type:
Conference
Resource Relation:
Journal Name: J. Int. Assoc. Math. Geol.; (United States); Journal Volume: 20:4; Conference: MGUS '87, Redwood City, CA, USA, 13 Apr 1987
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; CLAYS; HYDRAULIC CONDUCTIVITY; GEOLOGIC DEPOSITS; STATISTICAL MODELS; POLLUTANTS; ENVIRONMENTAL TRANSPORT; TOXIC MATERIALS; COMPACTING; COMPUTERIZED SIMULATION; FILTRATION; FRACTURED RESERVOIRS; GROUND WATER; HYDROLOGY; KRIGING; LAND POLLUTION CONTROL; MAPPING; PERMEABILITY; POROSITY; PROBABILITY; SAMPLING; TRACER TECHNIQUES; WATER INFLUX; WATER POLLUTION CONTROL; CONTROL; HYDROGEN COMPOUNDS; ISOTOPE APPLICATIONS; MASS TRANSFER; MATERIALS; MATHEMATICAL MODELS; MATHEMATICS; OXYGEN COMPOUNDS; POLLUTION CONTROL; SEPARATION PROCESSES; SIMULATION; STATISTICS; WATER 510200* -- Environment, Terrestrial-- Chemicals Monitoring & Transport-- (-1989); 520200 -- Environment, Aquatic-- Chemicals Monitoring & Transport-- (-1989)

Citation Formats

Rogowski, A.S., and Simmons, D.E. Geostatistical analysis of field hydraulic conductivity in compacted clay. United States: N. p., 1988. Web.
Rogowski, A.S., & Simmons, D.E. Geostatistical analysis of field hydraulic conductivity in compacted clay. United States.
Rogowski, A.S., and Simmons, D.E. Sun . "Geostatistical analysis of field hydraulic conductivity in compacted clay". United States. doi:.
@article{osti_6413799,
title = {Geostatistical analysis of field hydraulic conductivity in compacted clay},
author = {Rogowski, A.S. and Simmons, D.E.},
abstractNote = {Hydraulic conductivity (K) of fractured or porous materials is associated intimately with water flow and chemical transport. Basic concepts imply uniform flux through a homogeneous cross-sectional area. If flow were to occur only through part of the area, actual rates could be considerably different. Because laboratory values of K in compacted clays seldom agree with field estimates, questions arise as to what the true values of K are and how they should be estimated. Hydraulic conductivity values were measured on a 10 x 25 m elevated bridge-like platform. A constant water level was maintained for 1 yr over a 0.3-m thick layer of compacted clay, and inflow and outflow rates were monitored using 10 x 25 grids of 0.3-m diameter infiltration rings and outflow drains subtending approximately 1 x 1 m blocks of compacted clay. Variography of inflow and outflow data established relationships between cores and blocks of clay, respectively. Because distributions of outflow rates were much less and bore little resemblance to the distributions of break-through rates based on tracer studies, presence of macropores and preferential flow through the macropores was suspected. Subsequently, probability kriging was applied to reevaluate distribution of flux rates and possible location of macropores. Sites exceeding a threshold outflow of 100 x 10/sup -9/ m/s were classified as outliers and were assumed to probably contain a significant population of macropores. Different sampling schemes were examined. Variogram analysis of outflows with and without outliers suggested adequacy of sampling the site at 50 randomly chosen locations. Because of the potential contribution of macropores to pollutant transport and the practical necessity of extrapolating small plot values to larger areas, conditional simulations with and without outliers were carried out.},
doi = {},
journal = {J. Int. Assoc. Math. Geol.; (United States)},
number = ,
volume = 20:4,
place = {United States},
year = {Sun May 01 00:00:00 EDT 1988},
month = {Sun May 01 00:00:00 EDT 1988}
}

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  • Results of hydraulic-conductivity tests conducted in the field and laboratory are presented for soils collected from 11 compacted-clay test pads or liners. The field tests were conducted with sealed double-ring infiltrometers to define the field-scale hydraulic conductivity (K{sub F}). The laboratory tests were conducted using large undisturbed block specimens (diameter {>=} 0.3 m) and small specimens collected in thin-wall sampling tubes (diameter = 71 mm). Results of tests at low effective stress showed that the hydraulic conductivity of the block specimens was similar to K{sub F} at each site. The hydraulic conductivities of block specimens from sites where K{sub F}more » exceeded 10{sup {minus}9} m/s decreased by a factor of 4 when the effective stress was increased from 10 to 44 kPa. In contrast, the hydraulic conductivities of specimens from the other sites were reduced only by a factor of 1.5 under the same increment of effective stress.« less
  • A regional groundwater flow model has been developed to be used as a management tool for the Albuquerque Basin. It is crucial to recognize the impact of the inherent uncertainty in aquifer hydrogeology when applying the model, and an understanding of the effects of uncertainty can be accomplished using a probabilistic approach to address the role of natural variability of aquifer properties on management strategies. Statistical analysis shows that the hydraulic conductivity data is moderately skewed to the right, but is not lognormal. Geostatistical analysis revealed zonal anisotropy oriented due north-south, which is directly related to the flow direction ofmore » the ancestral Rio Grande which laid down the Upper Santa Fe Group deposits. The presence of multiple depositional environments within the Upper Santa Fe Group violates the assumption of stationarity. This can be circumvented by choosing the simulation search radius so that local stationarity holds, or by separating the basin into two portions to be simulated separately and then combined for flow model analysis.« less
  • Observations of the spatial variability of hydraulic conductivity at a tracer test site, located at Columbus Air Force Base in Mississippi, are presented. Direct measurements of hydraulic conductivity of the heterogeneous alluvial aquifer at the site were made using borehole flowmeter logging, slug tests, and a laboratory permeameter to test undisturbed soil cores. Indirect methods estimating hydraulic conductivity were also evaluated, including soil grain size analyses, surface geophysical surveys, and mapping of sediment facies. The spatial covariance of the 2,187 hydraulic conductivity values obtained with the borehole flowmeter method was examined. The log hydraulic conductivity variance ([sigma][sub 1n K][sup 2])more » and the horizontal and vertical correlation scales ([lambda][sub h] and [lambda][sub [upsilon]]) of 4.5, 12.8 m, and 1.6 m, respectively, were estimated assuming second-order stationarity of the conductivity field. The covariance parameters are uncertain with bounding values that are 24-76% above or below the estimate. Covariance parameters estimated with more limited nonflowmeter data were within the same magnitude as those obtained using the extensive flowmeter data, suggesting that data from a variety of methods may be used to provide approximate values of the autocovariance parameters. Nonstationarity of the 1n K field was examined by removing three-dimensional polynomial trend surfaces and calculating variograms of the residuals. Significantly lower estimates for [sigma][sub 1n K][sup 2], [lambda][sub h] and [lambda][sub [upsilon]] of 2.7, 4.8 m, and 0.8 m, respectively, were obtained from the third-order log conductivity residuals. After trend removal, the bounding parameter values differ 15-44% from the estimated values. 40 refs., 16 figs., 4 tabs.« less
  • A field test was conducted to determine if freeze-thaw cycles cause increases in hydraulic conductivity in the field, as has been observed in laboratory tests. A test pad of compacted clay was instrumented for monitoring temperatures and climatic conditions and partially insulated. The measurements indicated that up to 10 cycles of freeze-thaw occurred in the uninsulated portion of the test pad whereas no freezing occurred in the insulated portion. Hydraulic-conductivity tests were conducted before and after winter to determine if changes in hydraulic conductivity occurred as a result of winter exposure. In situ hydraulic-conductivity tests showed that the overall hydraulicmore » conductivity was similar in the insulated and uninsulated portions of the test pad before and after winter. No change in overall hydraulic conductivity occurred in the uninsulated region because the depth of frost penetration was only 30% of the thickness of the test pad. However, tests on block specimens collected from near-surface soil that underwent freezing and thawing increased in hydraulic conductivity by a factor of 50--300. Similar increases in hydraulic conductivity were measured using laboratory freeze-thaw tests.« less
  • Infiltration tests were conducted in the laboratory to study water movement during infiltration in compacted clay and to determine the suction head at the wetting front as defined in the Green-Ampt infiltration equation. Tests were conducted on two clays, having low and high plasticity indices. Results of the tests show that the wetting front is diffuse and becomes increasingly so as it penetrates deeper into the soil. The wetting front is also more diffuse for molding water contents near or wet of optimum water content and for clay with higher plasticity index. Suction head at the wetting front has beenmore » found to decrease as the wetting front penetrates into the soil, with the rate of dissipation depending on the molding water content and soil type. For the clay with low plasticity index, the initial suction head at the wetting front was largest and decreased the fastest when the molding water content was near optimum. For the clay with high plasticity index, similar initial suction head and rate of dissipation were measured at various molding water contents. Soil-specific equations are presented to estimate the suction head at the wetting front using the air-entry pressure.« less