Certain geologic media are known to have small permeability; subsurface environments composed of these media and lacking well developed secondary permeability have groundwater flow sytems with many distinctive characteristics. Moreover, groundwater flow in these environments appears to influence the evolution of certain hydrologic, geologic, and geochemical systems, may affect the accumulation of pertroleum and ores, and probably has a role in the structural evolution of parts of the crust. Such environments are also important in the context of waste disposal. This review attempts to synthesize the diverse contributions of various disciplines to the problem of flow in low-permeability environments. Problems hindering analysis are enumerated together with suggested approaches to overcoming them. A common thread running through the discussion is the significance of size- and time-scale limitations of the ability to directly observe flow behavior and make measurements of parameters. These limitations have resulted in rather distinct small- and large-scale approaches to the problem. The first part of the review considers experimental investigations of low-permeability flow, including in situ testing; these are generally conducted on temporal and spatial scales which are relatively small compared with those of interest. Results from this work have provided increasingly detailed information about many aspects of the flow but leave certain questions unanswered. Recent advances in laboratory and in situ testing techniques have permitted measurements of permeability and storage properties in progressively “tighter” media and investigation of transient flow under these conditions. However, very large hydraulic gradients are still required for the tests; an observational gap exists for typical in situ gradients. The applicability of Darcy's law in this range is therefore untested, although claims of observed non-Darcian behavior appear flawed. Two important nonhydraulic flow phenomena, osmosis and ultrafiltration, are experimentally well established in prepared clays but have been incompletely investigated, particularly in undisturbed geologic media. Small-scale experimental results form much of the basis for analyses of flow in low-permeability environments which occurs on scales of time and size too large to permit direct observation. Such large-scale flow behavior is the focus of the second part of the review. Extrapolation of small-scale experimental experience becomes an important and sometimes controversial problem in this context. In large flow systems under steady state conditions the regional permeability can sometimes be determined, but systems with transient flow are more difficult to analyze. The complexity of the problem is enhanced by the sensitivity of large-scale flow to the effects of slow geologic processes. One-dimensional studies have begun to elucidate how simple burial or exhumation can generate transient flow conditions by changing the state of stress and temperature and by burial metamorphism. Investigation of the more complex problem of the interaction of geologic processes and flow in two and three dimensions is just beginning. Because these transient flow analyses have largely been based on flow in experimental scale systems or in relatively permeable systems, deformation in response to effective stress changes is generally treated as linearly elastic; however, this treatment creates difficulties for the long periods of interest because viscoelastic deformation is probably significant. Also, large-scale flow simulations in argillaceous environments generally have neglected osmosis and ultrafiltration, in part because extrapolation of laboratory experience with coupled flow to large scales under in situ conditions is controversial. Nevertheless, the effects are potentially quite important because the coupled flow might cause ultra long lived transient conditions. The difficulties associated with analysis are matched by those of characterizing hydrologic conditions in tight environments; measurements of hydraulic head and sampling of pore fluids have been done only rarely because of the practical difficulties involved. These problems are also discussed in the second part of this paper.

Groundwater Flow in Low‐Permeability Environments

Osmotic Flow and Volume Change in Clay Soils

Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay

Chemico-osmotic efficiency coefficients, ω, are determined from measured differential pressures across specimens of a geosynthetic clay liner (GCL) containing granular bentonite in response to applied differences in potassium chloride (KCl) concentrations under no-flow conditions. The results show that the GCL acts as a semipermeable membrane with ω values at steady state, ωss, ranging from 0.08 to 0.69 for KCl concentration differences ranging from 0.0039 to 0.047 M. The chemico-osmotic efficiency of the GCL decreases with increasing porosity and increasing KCl concentration. The decrease in ωss with increasing porosity is consistent with an increase in pore size reflected by an increase in measured hydraulic conductivity. The decrease in ωss with increasing KCl concentration is consistent with compression of the diffuse double layers surrounding the clay particles, and is reflected by a time-dependent decrease in the induced differential pressure as well as an increase in the hydraulic conductivity of t...

Chemico-osmotic efficiency of a geosynthetic clay liner

A laboratory apparatus for measuring the chemico-osmotic efficiency coefficient, ω, for clay soils in the presence of electrolyte solutions is described. A chemico-osmotic experiment is conducted by establishing and maintaining a constant difference in electrolyte concentration across a soil specimen while preventing the flow of solution through the specimen. The chemico-osmotic efficiency coefficient is derived from a measured pressure difference induced across the specimen in response to the applied concentration difference. The effective diffusion coefficient (D*) and retardation factor (Rd) of the electrolytes (solutes) also can be determined simultaneously by measuring the diffusive solute mass flux through the specimen until steady-state diffusion is achieved. Experimental results using specimens of a geosynthetic clay liner subjected to potassium chloride solutions indicate that the measurement of ω may be affected by soil-solution interactions, as well as by changes in the induced chemico-osmotic pressure difference due to solute diffusion. As a result, ω should be evaluated using the induced pressure difference at steady state. The time required to achieve a steady-state response in induced pressure difference is related to the time required to achieve steady-state diffusion of all solutes, and may be affected by the circulation rate at the specimen boundaries. The circulation rate should be sufficiently rapid to minimize changes in the boundary concentrations due to diffusion, but sufficiently slow to allow measurement of solute mass flux at the lower concentration boundary for evaluating D* and Rd.

https://www.engr.colostate.edu/ce/homepages/shackel/Papers/23.pdf

J. P. Quirk

Papers

Ion Mobilities and Electric Charge of External Clay Surfaces Inferred from Potential Differences and Osmotic Flow1