John L. Wilson

Colloid transport in unsaturated porous media

This paper explores the significance of the gas-water interface on colloid sorption and transport. Three types of common saturation conditions were simulated in packed sand columns: (1) a completely water-saturated condition, (2) gas bubbles trapped by capillary forces as a nonwetting residual phase (15% gas), and (3) gas present as a continuous phase (46% gas), or in other words, a vadose zone situation. Different saturations provided different interfacial conditions. Two types of polystyrene latex particles (0.2 μm), hydrophilic and hydrophobic, were used in each of the three saturations. Relative surface hydrophobicity of latex particles was characterized by contact angle measurements. Each experiment was repeated five times. A total of 30 columns with good reproducible packing and gas content gave reproducible particle breakthrough curves. The retention of both hydrophilic and hydrophobic colloids increased with gas content of the porous medium. Colloids preferentially sorbed onto the gas-water interface relative to the matrix surface. The degree of sorption increased with the increase of colloid surface hydrophobicity. The results validate and quantify direct microscopic scale visualization observations made in two-dimensional pore network glass micromodels (Wan and Wilson, 1994). These findings suggest an additional mechanism for filtration and for particulate transport in the subsurface environment, whenever more than one fluid phase is present. The results also give clear evidence that the presence of inadvertently trapped residual gas could help explain the discrepancy between water-saturated laboratory column experiments and related theory.

Cross‐correlated random field generation with the direct Fourier Transform Method

This paper presents a computer algorithm that is capable of cogenerating pairs of three-dimensional, cross-correlated random fields. The algorithm produces random fields of real variables by the inverse Fourier transform of a randomized, discrete three-dimensional spectral representations of the variables. The randomization is done in the spectral domain in a way that preserves the direct power and cross-spectral density structure. Two types of cross spectra were examined. One type specifies a linear relationship between the two fields, which produces the same correlation scales for both variables but different variances. The second cross spectrum is obtained from a specified transfer function and the two power spectra, and it produces fields with different correlation scales. For both models the degree of correlation is specified by the coherency. A delay vector can also be specified to produce an out-of-phase correlation between the two fields. The algorithm is very efficient computationally, is relatively easy to use, and does not produce the lineation problems that can be encountered with the turning bands method. Perhaps most important, this random field generator is capable of co-generating cross-correlated random fields.

Adjoint method for obtaining backward-in-time location and travel time probabilities of a conservative groundwater contaminant

Backward location and travel time probabilities can be used to determine the prior location of contamination in an aquifer. For a contaminant particle that was detected in an aquifer, the backward location probability is the probability of where the particle was located at some prior time. Backward travel time probability is the probability of when the particle was located at some position upgradient of the detection. These probabilities can be used to improve characterization of known sources of groundwater contamination, to identify previously unknown contamination sources, and to delineate capture zones. For simple model domains, backward probabilities can be obtained heuristically from a forward model of contaminant transport. For multidimensional problems and complex domain geometries, the heuristic approach is difficult to implement and verify. The adjoint method provides a formal approach for obtaining backward probabilities for all model domains and geometries. We formally show that the backward model probabilities are adjoint states of resident concentration. We provide a methodology for obtaining the governing equations and boundary and final conditions for these probabilities. The approach is illustrated using a one-dimensional, semi-infinite domain that mimics flow to a production well, and these results are compared to equivalent probabilities derived heuristically.

Internal architecture, permeability structure, and hydrologic significance of contrasting fault-zone types

The Sand Hill fault is a steeply dipping, large-displacement normal fault that cuts poorly lithified Tertiary sediments of the Albuquerque basin, New Mexico, United States. The fault zone does not contain macroscopic fractures; the basic structural element is the deformation band. The fault core is composed of foliated clay flanked by structurally and lithologically heterogeneous mixed zones, in turn flanked by damage zones. Structures present within these fault-zone architectural elements are different from those in brittle faults formed in lithified sedimentary and crystalline rocks that do contain fractures. These differences are reflected in the permeability structure of the Sand Hill fault. Equivalent permeability calculations indicate that large-displacement faults in poorly lithified sediments have little potential to act as vertical-flow conduits and have a much greater effect on horizontal flow than faults with fractures.

Mountain‐Block Hydrology and Mountain‐Front Recharge

In semiarid climates, a significant component of recharge to basin aquifers occurs along the mountain front. Traditionally called “mountain-front recharge” (MFR), this process has been treated by modelers of basins as a boundary condition. In general, mountain-front recharge estimates are based on the general precipitation characteristics of the mountain (as estimated, e.g., by the chloride mass balance and water balance methods), or by calibration of a basin groundwater model. These methods avoid altogether the complexities of the hydrologic system above the mountain front, or at best consider only traditional runoff process. Consequently hydrology above the mountain front is an area ripe for significant scientific advancement. A complete view would consider the entire mountain block system and examine hydrologic processes from the slope of the highest peak to the depth of the deepest circulating groundwater. Important aspects above the mountain front include the partitioning of rainfall and snowmelt into vegetation-controlled evapotranspiration, surface runoff, and deep infiltration through bedrock, especially its fractures and faults. Focused flow along mountain stream channels and the diffuse movement of groundwater through the underlying mountain block would both be considered. This paper first defines some key terms, then reviews methods of studying MFR in arid and semiarid regions, discusses hydrological processes in the mountain block, and finally addresses some of the basic questions raised by the new mountain-block hydrology approach, as well as future directions for mountain-block hydrology research.

Adjoint‐derived location and travel time probabilities for a multidimensional groundwater system

Backward location and travel time probabilities can be used to determine the former location of contamination in an aquifer. For a contaminant parcel that was detected in an aquifer the backward location probability describes its position at some time prior to sampling, and the backward travel time probability describes the amount of time required for it to travel to the sampling location from some upgradient position. These probabilities, which can provide information about the source of contamination, are related to adjoint states of resident concentration. The governing equations of the backward probabilities are adjoints of the forward governing equation, e.g., the advection-dispersion equation. We derive these backward governing equations and their boundary and final conditions for both location and travel time probabilities in a multidimensional system. Each governing equation contains the adjoint of the advection-dispersion operator and a load term that defines the particular adjoint state (probability). The load term depends on both the type of probability (location or travel time) and the sampling device (pumping well or monitoring well) with which the contamination was detected. The adjoint equation can also be used to efficiently determine forward location and travel time probabilities describing the future location of groundwater contamination, a feature most useful for delineating pumping well captures zones. We illustrate the use of the backward model for obtaining location and travel time probabilities in a hypothetical two- dimensional domain.

Visualization of residual organic liquid trapped in aquifers

Organic liquids that are essentially immiscible with water migrate through the subsurface under the influence of capillary, viscous, and buoyancy forces. These liquids originate from the improper disposal of hazardous wastes, and the spills and leaks of petroleum hydrocarbons and solvents. The flow visualization experiments described in this study examined the migration of organic liquids through the saturated zone of aquifers, with a primary focus on the behavior of the residual organic liquid saturation, referring to that portion of the organic liquid that is trapped by capillary forces. Etched glass micromodels were used to visually observe dynamic multiphase displacement processes in pore networks. The resulting fluid distributions were photographed. Pore and blob casts were produced by a technique in which an organic liquid was solidified in place within a sand column at the conclusion of a displacement. The columns were sectioned and examined under optical and scanning electron microscopes. Photomicrographs of these sections show the morphology of the organic phase and its location within the sand matrix. The photographs from both experimental techniques reveal that in the saturated zone large amounts of residual organic liquid are trapped as isolated blobs of microscopic size. The size, shape, and spatial distribution of these blobs of residual organic liquid affect the dissolution of organic liquid into the water phase and the biotransformation of organic components. These processes are of concern for the prediction of pollution migration and the design of aquifer remediation schemes.

Hysteresis and state-dependent anisotropy in modeling unsaturated hillslope hydrologic processes

This paper describes a series of soil water tracer experiments and approaches taken to numerically model the flow and transport behavior observed in the field experiments. These experimental and numerical results strongly suggest that current widely held views and commonly applied modeling approaches are flawed in many cases for unsaturated flow, and provide strong supporting evidence for a variable, state-dependent anisotropy in the hydraulic conductivity of an unsaturated medium. This phenomenon has been previously postulated in a number of independent theoretical and experimental investigations. In general, the previous studies identify layered heterogeneity as a primary cause of the macroscopic anisotropy. In addition, we show how hysteresis in the soil moisture characteristics (θ-ψ relationship) can cause a lexturally homogeneous porous media profile to behave anisotropically under transient unsaturaied conditions. Recognizing that both of these factors (layered heterogeneity and capillary hysteresis) contribute the anisotropic behavior observed in the tracer experiments, we attempt to quantify the relative magnitude of their contributions in a numerical modeling investigation. For the numerical modeling study we use a finite element flow and transport code, and introduce a simple procedure for incorporating variable anisotropy into a predictive numerical model. To determine the relative magnitude of textural heterogeneity and capillary hysteresis as causes of the observed macroscopic anisotropy, we employ a diagnostic modeling approach. The results of the diagnostic modeling study indicate that textural heterogeneity is by far the most important contributor to the variable macroscopic anisotropy observed at the field site. The diagnostic simulations further show that the variable anisotropy approach is well suited to modeling field-scale problems. Subsequently, a sensitivity analysis was performed to determine how climate, geologic and topographic structure, and media lithology affect flow and transport behavior when soils were specified to have a variable macroscopic anisotropy. The results of this study clearly indicate that variable state-dependent anisotropy is a real and significant process at the field site and that modeling with consideration of variable anisotropy strongly affects model predictions.

Architecture of the Sierra Ladrones Formation, central New Mexico: Depositional controls on the permeability correlation structure

Statistical models of hydrogeological heterogeneity are often used in aquifer and reservoir characterization. The number of data required to estimate objectively the spatial correlation structure of permeability, however, is often prohibitive. The objective of this study was to develop a better understanding of how information about depositional processes can be used to characterize hydrogeological heterogeneity. An outcrop of the fluvial/interfluvial Sierra Ladrones Formation of New Mexico was studied for this purpose. On the basis of previous studies of paleogeography and our own field observations, deposits of the Sierra Ladrones Formation are interpreted as marginal ancestral Rio Grande flood-plain and tributary deposits. Architectural elements were mapped over a 0.16-km[sup 2] peninsular outcrop of Pliocene-Pleistocene deposits of the central Albuquerque Basin. Geostatistical analysis of the architectural-element map data indicates non-orthogonal anisotropy in the horizontal direction. The orientations of the strongest (N30[degree]W) and weakest (N90[degree]E) correlation correspond to the orientation of the tributary system and the ancestral Rio Grande flood plain, respectively. In the vertical direction, the correlation structure exhibits exponential behavior corresponding to the average-element thicknesses. The results demonstrate that information about depositional environment can be used to help to quantify statistically subsurface heterogeneity. 28 refs., 9 figs., 1 tab.

Micromodel studies of three-fluid porous media systems: Pore-scale processes relating to capillary pressure-saturation relationships

An experimental approach to study physical processes affecting fluid behavior in three-fluid porous media systems was designed and implemented. These experiments were designed to provide quantitative evidence of important pore-scale displacement processes. A unique experimental approach using a two-dimensional synthetic porous medium (micromodel) and digital image analysis (DIA) enabled both measurement of three-fluid capillary pressure-saturation relationships and analysis of fluid behavior at the pore level. Experiments were pressure controlled, and were designed to mimic measurements typically performed on laboratory cores. Only quasi-static measurements were made, with a focus on capillary pressure-saturation relations. Different orders of fluid infiltration with respect to wettability were studied. DIA made it possible to obtain quantitative information about the experiments, including fluid saturations, saturation changes between pressure steps, and movement of apparently isolated fluids through films. The results provide insights into important pore-scale mechanisms and provide a basis for three-fluid pore-scale computations.