A review of non-invasive imaging methods and applications in contaminant hydrogeology research

Abstract

Contaminant hydrogeological processes occurring in porous media are typically not amenable to direct observation. As a result, indirect measurements (e.g., contaminant breakthrough at a fixed location) are often used to infer processes occurring at different scales, locations, or times. To overcome this limitation, non-invasive imaging methods are increasingly being used in contaminant hydrogeology research. Four of the most common methods, and the subjects of this review, are optical imaging using UV or visible light, dual-energy gamma radiation, X-ray microtomography, and magnetic resonance imaging (MRI). Non-invasive imaging techniques have provided valuable insights into a variety of complex systems and processes, including porous media characterization, multiphase fluid distribution, fluid flow, solute transport and mixing, colloidal transport and deposition, and reactions. In this paper we review the theory underlying these methods, applications of these methods to contaminant hydrogeology research, and methods' advantages and disadvantages. As expected, there is no perfect method or tool for non-invasive imaging. However, optical methods generally present the least expensive and easiest options for imaging fluid distribution, solute and fluid flow, colloid transport, and reactions in artificial two-dimensional (2D) porous media. Gamma radiation methods present the best opportunity for characterization of fluid distributions in 2D at the Darcy scale. X-ray methods present the highest resolution and flexibility for three-dimensional (3D) natural porous media characterization, and 3D characterization of fluid distributions in natural porous media. And MRI presents the best option for 3D characterization of fluid distribution, fluid flow, colloid transport, and reaction in artificial porous media. Obvious deficiencies ripe for method development are the ability to image transient processes such as fluid flow and colloid transport in natural porous media in three dimensions, the ability to image many reactions of environmental interest in artificial and natural porous media, and the ability to image selected processes over a range of scales in artificial and natural porous media.

Figure 1

Dual-energy gamma system at Pacific Northwest National Laboratory with source holder, detector, and vertical movement configuration. In the picture, the gamma rays are traveling through a compartmental cell used for the determination of fluid attenuation coefficient.

Figure 2

An illustration of X-ray microtomography system components (adapted from www.ctlab.geo.utexas.edu/overview/index.php).

Figure 3

Illustration of an MRI pulse sequence, associated perturbations of the net magnetic moment and nuclear spins, and the corresponding MRI signal.

Figure 4

Images of a) DNAPL (white) and water (light gray) in a silicon-etched regular pore structure, b) DNAPL (red) and water in the pores of a column filled with glass, c) colloids accumulating at the interface of an air bubble in a water filled glass-etched pore structure, d) DNAPL (white) migration pathway in a heterogeneous sand-packed flow cell, e) colloid travel paths (exposure time was long relative to travel distances) in water flowing around a cylinder in a silicon-etched pore structure, f) the product (higher intensity region along horizontal centerline) of a reaction between calcium and a calcium sensitive fluorophore in the same pore structure, and g) the product of a reaction between Trion and molybdate in a flow cell packed with circular inclusions of small glass beads in a bed of larger glass beads. Cylinders in 3.1-1a, c, e, and f are several hundred microns in diameter, images were taking using fluorescent microscopy, and water flow is from left to right. The images were adapted from a) Chomsurin and Werth (2003), b) Montemagno and Gray (1995), c) Wan and Wilson (1994), d) Glass et al. (2000), e) Baumann and Werth (2004), f) Willingham et al. (2008), and g) Oates and Harvey (2006) (PERMISSION BEING OBTAINED).

Figure 5

(a) Picture (b) and aqueous phase saturation gamma scan of unsaturated flow cell before carbon tetrachloride injection, and (c) picture (d) and carbon tetrachloride saturation gamma scan of the same flow cell at the end of the fluid redistribution period. The images were adapted from Oostrom et al. (2005) (PERMISSION BEING OBTAINED).

Figure 6

a) Pore network structure extracted from SMT data, b) distribution of wetting fluid at residual saturation in a bead pack, c) distribution of colloids (red) in a glass-bead pack (blue), d) residual organic-liquid saturation for a three-phase system: organic liquid is white, the aqueous phase is dark gray, air is black, and porous-medium grains are light gray, and e) 3D renderings of the organic-liquid blobs entrapped in the packed column as a function of water flushing (time sequence is left to right). The images were adapted from a) Al-Raoush and Willson (2005a), b) Turner et al. (2004), c) Gaillard et al. (2007), d) Schnaar and Brusseau (2006a), e) and Schnaar and Brusseau (2006b) (PERMISSION BEING OBTAINED).

Figure 7

Magnetic resonance images of (a) octanol (white) trapped in glass beads (dark) initially saturated with water (grey); (b) dodecane invasion (red) of a rock fracture at selected time points; (c) nonaqueous phase trifluorobenzene ganglia at the pore scale trapped in a column packed with water saturated silica gel; d) nonaqueous phase trifluorobenzene trapped as ganglia and pools in a spatially correlated random permeability field. The images were adapted from (a)Johns and Gladden (1999); (b) Becker et al.(2003); (c) Zhang et al. (2002); (d) Zhang et al. (2007) (PERMISSION BEING OBTAINED).

Figure 8

Magnetic resonance images of (a) flow paths of a paramagnetic tracer injected into a spatially correlated random permeability field; (b) colloids advecting through a homogeneously packed column after 15 minutes of flow; (c) adsorptive retention of Cr³⁺ ions (in green) from a 3 mg/L solution flowing through 0.1 – 0.4 mm quartz sand at selected time points. The images were adapted from (a) Yoon et al. (2008); (b) Baumann and Werth (2005); (c) Nestle et al. (2003) (PERMISSION BEING OBTAINED).