Walter A. Illman

Three‐dimensional numerical inversion of pneumatic cross‐hole tests in unsaturated fractured tuff: 2. Equivalent parameters, high‐resolution stochastic imaging and scale effects

In paper 1 of this two-part series we described a three-dimensional numerical inverse model for the interpretation of cross-hole pneumatic tests in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. Our model is designed to analyze these data in two ways: (1) by considering pressure records from individual borehole monitoring intervals one at a time, while treating the rock as being spatially uniform, and (2) by considering pressure records from multiple tests and borehole monitoring intervals simultaneously, while treating the rock as being randomly heterogeneous. The first approach yields a series of equivalent air permeabilities and air- filled porosities for rock volumes having length scales ranging from meters to tens of meters, represented nominally by radius vectors extending from injection to monitoring intervals. The second approach yields a high-resolution geostatistical estimate of how air permeability and air-filled porosity, defined on grid blocks having a length scale of 1 m, vary spatially throughout the tested rock volume. It amounts to three-dimensional pneumatic “tomography” or stochastic imaging of the rock. Paper 1 described the field data, the model, and the effect of boreholes on pressure propagation through the rock. This second paper implements our inverse model on pressure data from five cross-hole tests at ALRS. We compare our cross-hole test interpretations by means of the two approaches with earlier interpretations by means of type curves and with geostatistical interpretations of single-hole test data. The comparisons show internal consistency between all pneumatic test interpretations and reveal a very pronounced scale effect in permeability and porosity at ALRS.

Three‐dimensional numerical inversion of pneumatic cross‐hole tests in unsaturated fractured tuff: 1. Methodology and borehole effects

We describe a three-dimensional numerical inverse model for the interpretation of cross-hole pneumatic tests in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. The model combines a finite volume flow simulator, FEHM, an automatic mesh generator, X3D, a parallelized version of an automatic parameter estimator, PEST, and a geostatistical package, GSTAT. The tests are simulated by considering single-phase airflow through a porous continuum, which represents primarily interconnected fractures at the site. The simulator solves the airflow equations in their original nonlinear form and accounts directly for the ability of all packed-off borehole intervals to store and conduct air through the system. Computations are performed in parallel on a supercomputer using 32 processors. We analyze pneumatic cross-hole test data, previously conducted by our group at ALRS, in two ways: (1) by considering pressure records from individual borehole monitoring intervals one at a time, while treating the rock as being spatially uniform, and (2) by considering pressure records from multiple tests and borehole monitoring intervals simultaneously, while treating the rock as being randomly heterogeneous. The first approach yields a series of equivalent air permeabilities and air-filled porosities for the rock volume being tested, having length scales of the order of meters to tens of meters. The second approach yields a high-resolution geostatistical estimate of how air permeability and air-filled porosity, defined on grid blocks having a length scale of 1 m, vary spatially throughout the tested rock volume. It amounts to three-dimensional pneumatic “tomography” or stochastic imaging of the rock, a concept originally proposed by one of us in 1987. The first paper of this two-part series describes the field data, the model, and the effect of boreholes on pressure propagation through the rock. The second paper implements our approach on selected cross-hole test data from ALRS.

Type curve interpretation of a cross-hole pneumatic injection test in unsaturated fractured tuff

Cross-hole pneumatic injection tests have been conducted in 16 vertical and inclined boreholes in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. Their purpose was to characterize the bulk pneumatic properties and connectivity of fractures at the site on scales ranging from meters to several tens of meters. We describe the design, conduct, and type curve interpretation of one of these tests. Our cross-hole type curves are modified after Hsieh and Neuman [1985] to consider single-phase airflow and extended to consider the effects of storage and skin in monitoring intervals. Cross-hole type curves of pressure derivatives and recovery are included for improved pneumatic characterization of the site. We analyze recorded pressures in each monitoring interval separately from those in other intervals while treating the fractured rock as a uniform, isotropic porous continuum. Each record yields an equivalent directional air permeability and air-filled porosity for fractures that connect the corresponding monitoring and injection intervals. Both parameters are found to vary considerably from one monitoring interval to another, reflecting the nonuniform nature of pneumatic rock properties at the ALRS. The geometric mean of these equivalent permeabilities is found to be larger by a factor of 50 than that obtained from single-hole pneumatic injection tests on a nominal scale of 1–3 m (single-hole tests yield only limited information about porosities, which therefore cannot be meaningfully compared with cross-hole results). Our results find support in numerical inverse modeling of cross-hole tests at the site by Vesselinov [2000; see also Illman et al., 1998; Vesselinov et al, 2000]. Vesselinov [2000] has further demonstrated [see also Chen et al., 2000] that a similar scale effect is exhibited by fracture porosity at the ALRS and that both scale effects disappear when cross-hole tests at the site are interpreted by means of a numerical inverse model, which resolves heterogeneities down to a scale of 1 m. When considered jointly with these and other studies of the site, our analysis implies that the pneumatic pressure behavior of unsaturated fractured tuffs at the ALRS can be described quite accurately by means of linearized single-phase airflow equations; this behavior can be interpreted by treating the rock as a continuum on scales ranging from meters to tens of meters: the continuum is representative primarily of interconnected fractures; as these fractures are filled primarily with air, their pneumatic permeabilities and porosities are close to the bulk intrinsic properties of fractures at the site: these intrinsic properties vary randomly with location and direction across the ALRS, and they depend strongly on the scale at which they are determined.

Steady-state analysis of cross-hole pneumatic injection tests in unsaturated fractured tuff

Abstract Numerous single-hole and cross-hole pneumatic injection tests have been conducted in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) near Superior, Arizona. Steady-state analyses of single-hole tests conducted by Guzman et al. [Summary of Air Permeability Data From Single-Hole Injection Tests in Unsaturated Fractured Tuffs at the Apache Leap Research Site: Results of Steady-State Test Interpretation (1996)] have yielded values of air permeability at various locations throughout the tested rock volume on nominal scales from 0.5 to 3.0 m. Analyses of transient data from single-hole and larger-scale cross-hole tests were performed by Illman and Neuman [Ground Water 38 (2000) 899; Water Resour. Res. 37 (2001) 583] using type-curves and by Vesselinov and Neuman [Ground Water 39 (2001) 685] and Vesselinov et al. [Water Resour. Res. 37 (2001a) 3001; Water Resour. Res. 37 (2001b) 3041] using numerical inversion. These have yielded bulk fracture permeabilities and porosities on scales ranging from a few meters to several tens of meters. We complement the latter results by steady-state analysis of a relatively large set of cross-hole tests. Steady-state analysis (a) allows interpreting cross-hole tests that were not interpreted using type-curves due to weak or noisy signals, (b) is relatively easy to perform, and (c) yields results comparable with those obtained from transient analyses. We analyze the results statistically and discuss their implications vis-a-vis the pneumatic properties of unsaturated fractured tuff at the ALRS. Our results strengthen the evidence for a previously surmised permeability scale effect at the site.

Hydraulic Tomography for Detecting Fracture Zone Connectivity

Fracture zones and their connectivity in geologic media are of great importance to ground water resources management as well as ground water contamination prevention and remediation. In this paper, we applied a recently developed hydraulic tomography (HT) technique and an analysis algorithm (sequential successive linear estimator) to synthetic fractured media. The application aims to explore the potential utility of the technique and the algorithm for characterizing fracture zone distribution and their connectivity. Results of this investigation showed that using HT with a limited number of wells, the fracture zone distribution and its connectivity (general pattern) can be mapped satisfactorily although estimated hydraulic property fields are smooth. As the number of wells and monitoring ports increases, the fracture zone distribution and connectivity become vivid and the estimated hydraulic properties approach true values. We hope that the success of this application may promote the development and application of the new generations of technology (i.e., hydraulic, tracer, pneumatic tomographic surveys) for mapping fractures and other features in geologic media.

Theoretical interpretation of a pronounced permeability scale effect in unsaturated fractured tuff

Received 10 May 2001; revised 8 January 2002; accepted 8 January 2002; published 27 June 2002. [1] Numerous single-hole and cross-hole pneumatic injection tests have been conducted in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) near Superior, Arizona. Single-hole tests have yielded values of air permeability at various locations throughout the tested rock volume on a nominal scale of � 1 m. Cross-hole tests have yielded equivalent air permeabilities (and air-filled porosities) for a rock volume characterized by a length scale of several tens of meters. Cross-hole tests have also provided high-resolution tomographic estimates of how air permeability (and air-filled porosity), defined over grid blocks having a length scale of 1 m, vary throughout a similar rock volume. The results have revealed a highly pronounced scale effect in permeability (and porosity) at the ALRS. We examine the extent to which the permeability scale effect is amenable to interpretation by a recent stochastic scaling theory, which treats the rock as a truncated random fractal. INDEX TERMS: 1869 Hydrology: Stochastic Processes; 1875 Hydrology: Unsaturated Zone; 3260 Mathematical Geophysics: Inverse Theory; 3250 Mathematical Geophysics: Fractals and Multifractals; KEYWORDS: scaling, permeability, fractals, fractured rocks

Performance Assessment of Bioremediation and Natural Attenuation

Bioremediation and monitored natural attenuation are among the most cost-effective approaches to manage soil and groundwater contamination by hazardous organic pollutants. However, these remediation alternatives are not universally applicable and may be marginally effective for recalcitrant pollutants if the necessary microbial catabolic capacity is not present or expressed. Thus, regulatory and public approval of bioremediation and natural attenuation requires documentation of the efficacy of microbial degradation of the target pollutants. Performance assessment generally consists of three components: documented contaminant mass loss, geochemical fingerprints associated with biodegradation, and microcosm studies that show direct evidence of biodegradation. More recently, new molecular and isotope fractionation techniques have emerged to complement existing technologies for the forensic analysis and the demonstration of bioremediation and natural attenuation. This critical review examines the current state-of-art in performance assessment methods and discusses future research directions.

Field Study of Subsurface Heterogeneity with Steady-State Hydraulic Tomography

Remediation of subsurface contamination requires an understanding of the contaminant (history, source location, plume extent and concentration, etc.), and, knowledge of the spatial distribution of hydraulic conductivity (K) that governs groundwater flow and solute transport. Many methods exist for characterizing K heterogeneity, but most if not all methods require the collection of a large number of small-scale data and its interpolation. In this study, we conduct a hydraulic tomography survey at a highly heterogeneous glaciofluvial deposit at the North Campus Research Site (NCRS) located at the University of Waterloo, Waterloo, Ontario, Canada to sequentially interpret four pumping tests using the steady-state form of the Sequential Successive Linear Estimator (SSLE) (Yeh and Liu 2000). The resulting three-dimensional (3D) K distribution (or K-tomogram) is compared against: (1) K distributions obtained through the inverse modeling of individual pumping tests using SSLE, and (2) effective hydraulic conductivity (K(eff) ) estimates obtained by automatically calibrating a groundwater flow model while treating the medium to be homogeneous. Such a K(eff) is often used for designing remediation operations, and thus is used as the basis for comparison with the K-tomogram. Our results clearly show that hydraulic tomography is superior to the inversions of single pumping tests or K(eff) estimates. This is particularly significant for contaminated sites where an accurate representation of the flow field is critical for simulating contaminant transport and injection of chemical and biological agents used for active remediation of contaminant source zones and plumes.

Hydraulic/partitioning tracer tomography for DNAPL source zone characterization: small-scale sandbox experiments.

Dense nonaqueous phase liquids (DNAPL) are prevalent at a large number of sites throughout the world. The variable release history, unstable flow, and geologic heterogeneity make the spatial distribution of DNAPLs complex. This causes difficulties in site remediation contributing to long-term groundwater contamination for decades to centuries. We present laboratory experiments to demonstrate the efficacy of Sequential Successive Linear Estimator (SSLE) algorithm that images DNAPL source zones. The algorithm relies on the fusion of hydraulic and partitioning tracer tomography (HPTT) to derive the best estimate of the K heterogeneity, DNAPL saturation (S(N)) distribution, and their uncertainty. The approach is nondestructive and can be applied repeatedly. Results from our laboratory experiments show that S(N) distributions compare favorably with DNAPL distributions observed in the sandbox but not so with local saturation estimates from core samples. We also found that the delineation of K heterogeneity can have a large impact on computed S(N) distributions emphasizing the importance of accurate delineation of hydraulic heterogeneity.

Hydraulic Tomography Offers Improved Imaging of Heterogeneity in Fractured Rocks

Fractured rocks have presented formidable challenges for accurately predicting groundwater flow and contaminant transport. This is mainly due to our difficulty in mapping the fracture-rock matrix system, their hydraulic properties and connectivity at resolutions that are meaningful for groundwater modeling. Over the last several decades, considerable effort has gone into creating maps of subsurface heterogeneity in hydraulic conductivity (K) and specific storage (Ss ) of fractured rocks. Developed methods include kriging, stochastic simulation, stochastic inverse modeling, and hydraulic tomography. In this article, I review the evolution of various heterogeneity mapping approaches and contend that hydraulic tomography, a recently developed aquifer characterization technique for unconsolidated deposits, is also a promising approach in yielding robust maps (or tomograms) of K and Ss heterogeneity for fractured rocks. While hydraulic tomography has recently been shown to be a robust technique, the resolution of the K and Ss tomograms mainly depends on the density of pumping and monitoring locations and the quality of data. The resolution will be improved through the development of new devices for higher density monitoring of pressure responses at discrete intervals in boreholes and potentially through the integration of other data from single-hole tests, borehole flowmeter profiling, and tracer tests. Other data from temperature and geophysical surveys as well as geological investigations may improve the accuracy of the maps, but more research is needed. Technological advances will undoubtedly lead to more accurate maps. However, more effort should go into evaluating these maps so that one can gain more confidence in their reliability.