All unconsolidated sediment s possess a significant amount of porosity. Porosity can be as high as 60 percent of the total sediment volume in clays, but the individual pores are microscopic in size — often no larger than a few water molecules — and are poorly interconnected, resulting in low permeability. On the other hand, the porosity of sand and gravel seldom exceeds 30 percent, but the pores are large and well interconnected, resulting in high effective porosity and permeability. The permeability differences are manifested by the contrasting colors in the photograph: the poorly permeable , clay-rich till has a gray color, indicative of prolonged waterlogging, which leads to reducing conditions caused by a lack of oxygen, whereas the permeable sand below has a light-brown color with many oxidized clasts, reflecting rapid drainage and plentiful oxygen. Photo by A. H. Fleming.
Porosity and permeability are the key physical properties that determine the ability of a body of rock or sediment to store and transmit water (fig. 1). Although closely related, the two properties are inherently different, as described below.
Porosity is defined as the volume of open spaces, or pores, between the solid mineral components that make up a rock or unconsolidated sediment, and is expressed as a percentage. Thus, in a rock or sediment with 25 percent porosity, a quarter of the total volume is made up of open spaces, or pores. All unconsolidated sediments possess a significant volume of porosity, which ranges from 25 to 35 percent in well-sorted sand and gravel to as much as 60 percent in some silts and clays. Porosity is inversely proportional to grain size, with sediment composed of finer grains, such as silt and clay, having a substantially greater volume of open spaces than those composed of coarse grains, such as sand and gravel. This relationship may at first seem counterintuitive in light of the fact that sand and gravel transmits water far more readily than silt and clay. But total porosity itself is not the determining factor in the ability to transmit water; instead, several other characteristics of the sedimentary particles and the pore spaces between them control the permeability of a rock or sediment.
Foremost among these characteristics are the sizes and shapes of the individual pores. All other things being equal, the size of the individual pores in a body of well-sorted sediment is directly proportional to grain size, thus the individual open spaces in a sand or gravel body, for example, are orders of magnitude larger than in a glacial till composed predominantly of silt and clay. Sand grains range up to 2 mm in diameter, and gravel up to several centimeters; the shapes of pebbles and sand grains tend to be broadly equidimensional and are typically somewhat to strongly rounded. As a result, the pore spaces between grains of sand or gravel are similar in shape and size to the surrounding particles, and tend to be quite large relative to the size of water molecules. In contrast, individual clay grains are microscopic in size and consist of tiny, electrically charged plates: a million grains of clay will fit in the space occupied by one average grain of sand. As a result, the pore spaces between the clay minerals are also microscopic, generally only a few times wider than a molecule of water. The small pore dimensions, coupled with the electrical charge on the surfaces of the clay minerals, greatly retard the passage of water and instead tend to attract and cling tightly to water molecules, resulting in exceedingly slow permeabilities. The term “effective porosity” refers to how well interconnected the pore spaces are in a block of rock or sediment; like porosity, effective porosity is expressed as a percentage of the total rock or sediment volume. Higher effective porosity results in higher permeability.
The popular term “permeability” is generally understood to mean the ability of a fluid to pass through another object or substance. Permeability is an intrinsic property that is difficult to measure, however, and it is being used in this discussion as a proxy for “ hydraulic conductivity ,” the more precise term that geologists use to describe the ease with which a specific fluid, such as water or petroleum, will flow through a body of rock or sediment under saturated conditions and a uniform hydraulic gradient . Hydraulic conductivity can be directly measured relatively easily in the laboratory or in the field, using standard methods that allow the resulting values to be compared from one type of geologic material to another. Hydraulic conductivity is expressed as a rate (units of distance per unit of time) and is measured in familiar units, such as centimeters per second, inches per day, or feet per year. Hydraulic conductivities range over more than ten orders of magnitude in nature. Many clays and fine-grained glacial tills have hydraulic conductivities measured in inches per year, whereas the hydraulic conductivities of some coarse gravel deposits are as great as several hundred feet per day. Some glacial outwash deposits in Marion County have hydraulic conductivity values approaching this upper range, whereas many of the glacial tills may transmit water at rates of only a few inches to a few feet per year.
Figure 2 illustrates how fractured glacial till and bedrock units can possess dual porosities and permeabilities. Left - Groundwater is discharging from a prominent, open fracture in this otherwise poorly sorted, fine-grained, and slowly permeable glacial till. The hydraulic conductivity of the matrix of this till was measured in the laboratory at less than 3 inches per year, but the secondary hydraulic conductivity associated with the network of fractures is about 5 orders of magnitude higher, as measured by field testing. Note the orange-brown staining around the fracture, produced by the oxidation of dissolved iron as the groundwater encounters oxygen in the atmosphere. Right-A large stream of groundwater issues from a fracture in limestone that has been enlarged by solution, a characteristic particular to carbonate rocks. Weak acids present in precipitation and shallow groundwater cause carbonate minerals, such as the calcite that makes up this limestone, to gradually dissolve. Carried to its full extent, the solution of limestone results in karst topography, with numerous caverns, sinkholes, and solution channels capable of hosting sizable underground rivers. The Silurian and Devonian carbonate rocks below Marion County contain solution-enlarged fractures similar to this one.
Fractured media, such as the limestone and dolostone bedrock below parts of the county, and some glacial tills, are a special case involving dual porosities and permeabilities. Whereas the solid bedrock or till matrix will have very low hydraulic conductivity (for all intents and purposes, the permeability of an unfractured block of crystalline limestone is virtually negligible), a system of open, interconnected fractures can result in a secondary permeability that is much, much greater (fig. 2). Even though the fractures themselves may produce a net porosity of less than 1 percent, the essentially continuous, planar shape of the fracture openings results in a near-perfect effective porosity, and thus high permeability in an otherwise poorly permeable mass of rock or sediment. The overall permeability of a fractured rock or sediment is commonly called “ bulk permeability ” (or bulk hydraulic conductivity), a term that acknowledges the effects of both the slow primary permeability of the rock matrix and the higher secondary permeability associated with fractures and other secondary openings.
Aquifer s are characterized by relatively large, interconnected pore spaces between solid material, with gravel, sand, and bedrock having large, open fractures being prime examples. A well-fractured rock can have a bulk permeability approaching that of coarse sand, and some fractured limestone aquifers are every bit as productive as large sand and gravel aquifers.
Confining unit s, on the other hand, typically consist of fine-grained, poorly sorted sediment or solid, poorly fractured bedrock in which the openings between the larger particles are filled either with smaller grains or cementing minerals, resulting in mostly small, poorly interconnected pore spaces. The degree of sorting is thus another key rock or sediment property that affects the ability to transmit fluids: well-sorted sand or gravel, in which all the grains are about the same size, is vastly more permeable than a poorly sorted version of the same sand or gravel in which the spaces between the larger grains are partially filled by smaller grains, such as fine sand, silt, and clay. Glacial till typically is one of the most poorly sorted sediments in nature; the tills in Marion County consist of roughly equal proportions of gravel, sand, silt, and clay, resulting in very little effective porosity. It is suspected that most groundwater recharge through thick sequences of otherwise slowly permeable glacial till is accomplished through a network of fractures, root channels, thin partings of sand, and other types of secondary permeability.
Left-Bowling balls aligned in a bowling ball return chute represent well-sorted gravel. Right-Overlapping sheets of newspapers in stacks represent clay minerals.
A useful way to visualize the effects of porosity and permeability in unconsolidated sediments is to imagine a large, auditorium-size chamber. One side of the chamber is filled with bowling balls, representing well-sorted gravel, while the other side is filled with overlapping sheets of newspaper, representing clay minerals (in reality, the scale of the experiment is such that each character of newsprint would still be much larger than a typical clay mineral, relative to the bowling balls, but the overall effect of the analogue is still valid) (fig. 3). Now imagine that a million gallons of water is instantaneously introduced uniformly over the top of the chamber. It doesn’t take much imagination to foresee the outcome: the vast majority of the water, probably 99.999 percent of it, will flow through the bowling balls and out the bottom of the chamber before most of the newspaper even becomes damp. Now repeat the experiment, but this time mix in a sufficient number of golf balls to occupy most of the spaces between the bowling balls, thereby emulating a typical sand and gravel deposit. While the presence of the golf balls will undoubtedly reduce the permeability of the bowling balls somewhat, the overall result is largely unchanged: the effect of adding the golf balls is nominal because the openings in the golf ball/bowling ball side of the chamber are still quite large and well interconnected, leading to a high effective porosity. By the time the water is done flowing through the bowling ball/golf ball side of the chamber, it is unlikely that more than the upper few feet of newspaper will even be damp!
Let’s repeat this experiment one more time. We will add wheat flour to the side of the chamber with the balls, such that most of the spaces between the balls are now occupied by flour. Such a mixture simulates a poorly sorted sediment, such as glacial till or muddy sand and gravel. On the other side of the chamber, we stack bundles of newspaper such that the edges of the bundles all line up across the chamber in two directions, thereby simulating a network of open, vertical fractures. When we introduce the water, it may momentarily pool up on the top of the newspaper bundles, but will quickly find the planar openings between the bundles and rapidly drain downward, wetting the sides of the bundles in the process. The centers of the bundles will probably still be dry by the time all the water has drained through the “fractures” between the bundles. On the other side of the chamber, the water will drain very slowly as it makes its way through the tortuous, small pores between the grains of flour. Flow will be faster in a few places where the flour hasn’t completely filled the spaces between the balls, but overall, the permeability of the balls has been severely reduced. If the flour/ball mixture was compacted before the experiment (similar to the hard consistency of glacial till that was once under hundreds or thousands of feet of ice), it is likely that almost no water will have reached the bottom of the chamber by the time the side of the chamber with newspaper bundles has completely drained.