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Suitability of Parametric Models to Describe the Hydraulic Properties of an Unsaturated Coarse Sand and Gravel (1998)

Mace, Andy ; Rudolph, David L. ; Kachanoski, R. Gary

Oxford, UK : Blackwell Publishing Ltd

Ground water 36 (1998), S. 0

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ISSN: 1745-6584

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Energy, Environment Protection, Nuclear Power Engineering , Geosciences

Notes: The performance of parametric models used to describe soil water retention (SWR) properties and predict unsaturated hydraulic conductivity (K) as a function of volumetric water content (θ) is examined using SWR and K(θ) data for coarse sand and gravel sediments. Six 70 cm long, 10 cm diameter cores of glacial outwash were instrumented at eight depths with porous cup ten-siometers and time domain reflectometry probes to measure soil water pressure head (h) and θ, respectively, for seven unsaturated and one saturated steady-state flow conditions. Forty-two θ(h) and K(θ) relationships were measured from the infiltration tests on the cores. Of the four SWR models compared in the analysis, the van Genuchten (1980) equation with parameters m and n restricted according to the Mualem (m = 1 - 1/n) criterion is best suited to describe the θ(h) relationships. The accuracy of two models that predict K(θ) using parameter values derived from the SWR models was also evaluated. The model developed by van Genuchten (1980) based on the theoretical expression of Mualem (1976) predicted K(θ) more accurately than the van Genuchten (1980) model based on the theory of Burdine (1953). A sensitivity analysis shows that more accurate predictions of K(θ) are achieved using SWR model parameters derived with residual water content (θr) specified according to independent measurements of θ at values of h where θ/h ∼ 0 rather than model-fit θr values. The accuracy of the model K(θ) function improves markedly when at least one value of unsaturated K is used to scale the K(θ) function predicted using the saturated K. The results of this investigation indicate that the hydraulic properties of coarse-grained sediments can be accurately described using the parametric models. In addition, data collection efforts should focus on measuring at least one value of unsaturated hydraulic conductivity and as complete a set of SWR data as possible, particularly in the dry range.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1745-6584.1998.tb02818.x

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Ground Penetrating Radar Imaging of an Aquifer During a Pumping Test (2000)

Endres, Anthony L. ; Clement, William P. ; Rudolph, David L.

Oxford, UK : Blackwell Publishing Ltd

Ground water 38 (2000), S. 0

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ISSN: 1745-6584

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Energy, Environment Protection, Nuclear Power Engineering , Geosciences

Notes: We have imaged the temporal and spatial response of an unconfined aquifer during a pumping test using ground penetrating radar (GPR) profiling. In particular, we have observed the transient behavior of the reflection generated by the water content variation occurring in the transition zone between the overlying residually saturated material and the water saturated capillary fringe below. The progressive arrival time delay of this reflection was measured and used to infer its drawdown that occurred during the pumping test. We also observed several other phenomena on the GPR profiles that are related to drainage: (1) the development of a series of diffractions indicating localized irregularities in water saturation and (2) the velocity pull-up of stratigraphic reflections due to increased electromagnetic (EM) wave velocity in the overlying section.Comparing the GPR profiling data and piezometer measurements, we have observed that the drawdown of the transition zone reflection is smaller and delayed relative to the measured hydraulic head drawdown. From the combined GPR and piezometer data, we have inferred that a 0.20 m increase in the combined thickness of the transition zone and capillary fringe occurred before the drawdown of the GPR transition zone reflection commenced. Once achieved, this 0.20 m increased thickness remained for the duration of the pumping test. Using the distance-drawdown relationship obtained from GPR profiling, we have estimated the drained water volume due to the downward movement of the transition zone. The results of our analysis account for only a fraction of the pumping well production, approximately 45% on the first day and about 25% on the second day. Of the various reasons examined to explain this result, we have concluded that it is likely that the behavior of the GPR reflection originating from the transition zone is representative of the shallower, less saturated portion of the transition zone and undetected drainage is occurring in the deeper, more saturated portion of the transition zone.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1745-6584.2000.tb00249.x

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Estimating Ground Water Flux into Large Lakes: Application in the Hamilton Harbor, Western Lake Ontario (2000)

Harvey, F. Edwin ; Rudolph, David L. ; Frape, Shaun K.

Oxford, UK : Blackwell Publishing Ltd

Ground water 38 (2000), S. 0

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ISSN: 1745-6584

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Energy, Environment Protection, Nuclear Power Engineering , Geosciences

Notes: Over the last two decades, there has been a growing recognition of the importance of evaluating ground water's contribution to lakes. As a result, a number of techniques have been developed for measuring hydraulic properties across lake bottoms, primarily in the littoral zone. However, for larger, deeper lakes such as the Great Lakes these techniques are impractical in the profundal zone. As a result, many water balance calculations in these settings omit the ground water component altogether owing to the difficulties encountered in making the necessary hydraulic observations across the deeper lake bottom sediments.In this study, a methodology is developed for determining the flux into large, deep lakes using a combination of existing and recently developed techniques. The methodology is applied to the Hamilton Harbor, a natural bay at the western end of Lake Ontario, to estimate the ground water contribution to the harbor's water budget. Hydraulic gradients were monitored in 37 piezometers within the harbor during 1993 and 1994. Calculated hydraulic gradients, along with sediment hydraulic conductivities measured or estimated using a number of techniques, are used to estimate ground water flux to the harbor through Thiessen polygon weighting.Measured hydraulic gradients ranged from −0.333 to 0.430, the majority being upward indicating ground water discharge conditions. Gradients were varied across the harbor and increased in magnitude closer to shore. The total ground water contribution to the harbor was estimated to be 2.1 × 107 m3/yr. Compared with other hydrological components, ground water was slightly larger than the yearly precipitation input, and approximately 8 % of the total surface inflows to the harbor, and 2 % of the total surface outflow through the Burlington ship canal, which connects the harbor to Lake Ontario.The computed ground water flux suggests that despite the fact that ground water flux to large lakes may typically be a smaller input than surface water inputs, it is still a significant component of the overall water budget and should not be automatically omitted from water balance calculations.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1745-6584.2000.tb00248.x

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Significance of Unsaturated Flow and Seepage Faces in the Simulation of Steady-State Subsurface Flow (1999)

Romanoa, Connie G. ; Frind, Emil O. ; Rudolph, David L.

Oxford, UK : Blackwell Publishing Ltd

Ground water 37 (1999), S. 0

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ISSN: 1745-6584

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Energy, Environment Protection, Nuclear Power Engineering , Geosciences

Notes: A fully saturated, free-surface ground water flow model and a variably saturated model are applied to the solution of the free-surface ground water flow problem in order to determine the influence of the unsaturated flow component on the accuracy of the saturated flow solution and the water balance for homogeneous systems. The significance of the seepage face that must exist to provide a transition between a phreatic surface and a downstream surface water body is also evaluated. A series of simple generic cross-sectional test cases are used as the basis for the numerical investigations. The fully saturated and variably saturated models used in this study are two-dimensional Galerkin finite-element models that incorporate exit seepage face options. The fully saturated solution is found to be almost identical to the variably saturated solution below the water table, the only significant difference being that the variably saturated model predicts a slightly lower water table due to horizontal flow components in the capillary fringe. This difference is negligible for large-scale systems, snowing that for homogeneous systems, unsaturated flow can be neglected and steady-state flow including seepage face discharge can be realistically simulated by a fully saturated free-surface model. For aquifer-scale systems where discretization limitations may prevent the explicit representation of seepage faces, the seepage face can be omitted because of its negligible influence on the overall water balance at the larger scale.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1745-6584.1999.tb01151.x

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5

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Large-Scale Aquitard Consolidation Near Mexico City (1993)

Ortega-Guerrero, Adrian ; Cherry, John A. ; Rudolph, David L.

Oxford, UK : Blackwell Publishing Ltd

Ground water 31 (1993), S. 0

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ISSN: 1745-6584

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Energy, Environment Protection, Nuclear Power Engineering , Geosciences

Notes: The extensive lacustrine Chalco Plain in the southeastern part of Mexico City is underlain by an aquitard up to 300 m thick composed of a layered sequence of very porous (80-90%) fine-grained, organic-rich Quaternary deposits, with thin horizontal interbeds of volcanic sand (“Capas Duras”). The aquitard overlies a thick sequence of alluvial-pyroclastic sediments which form a highly productive regional aquifer. The Chalco Plain was a shallow lake until the 1940s when it was drained for agricultural use and human habitation. Historic information indicates that the Chalco Plain was an area of ground-water discharge prior to the onset of heavy ground-water extraction from the semiconfined aquifer. Due to aquifer pumping, however, the hydraulic gradient has reversed throughout the full thickness of the aquitard in areas where the aquitard is thin (〈 100m), and recharging conditions now prevail. Where the aquitard is thick, the hydraulic head data show a progressive decline with time even though the hydraulic gradient still indicates upward flow in at least the upper part of the lacustrine sequence. Between the early 1960s, when major ground-water extraction began beyond the periphery of the aquitard, and the onset in 1982 of heavy pumping from aquifers beneath the aquitard, the land surface subsided approximately 3 m. An additional subsidence of 2 m occurred between 1984 and 1989, causing a shallow lake to form and gradually expand. If the present rate of ground-water withdrawal from the Chalco Basin continues, total land subsidence in the middle of the plain will probably continue to a rate of about 0.4 m per year for many years, and could eventually reach a total subsidence of tens of meters in the thickest part of the Chalco Plain. Pore water in much of the aquitard is saline; however, release of salts and other chemical constituents to the underlying aquifer has not yet significantly impaired the aquifer water quality. A better understanding of the behavior of the aquitard under the influence of aquifer pumping is needed to assist in long-term management of ground water and land use in the Chalco Plain.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1745-6584.1993.tb00841.x

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