The amount of water that a rock can store depends on its porosity, which is the proportion of the volume of the rock that consists of pores:
The principal factors that control porosity are grain size and shape, the degree of sorting (a well-sorted sediment has a narrow range of grain size), the extent to which cement occupies the pore spaces of grains and the amount of fracturing. Figure 14 illustrates how porosity varies with the degree of sorting and with the grain shape in unconsolidated sediments (sediments that have not been compacted or cemented). Unconsolidated sediments with rounded grains of uniform size (i.e. perfectly sorted) are the most porous (Figure 14a). Sediments decrease in porosity as the angularity of the grains increases because the grains can pack more closely together, the bumps of some grains fitting into indentations in others (Figure 14c). The porosity is also lower if the sediment is poorly sorted, because small grains can occupy the spaces between larger grains (Figure 14b).
b.Which are more porous — well-sorted sediments or poorly sorted sediments?
c.Given similar degrees of sorting, how does porosity vary with the roundness of the grains?
d.Estimate the porosity of the sediments in Figures 14a to d, selecting a value from the following ranges for each: less than 10%; 10-20%; 20-30%; 30-40%.
a.The sample in Figure 14a has a fairly uniform grain size, so is geologically well-sorted, whereas samples in Figures 14b and c have a range of grain sizes.
b.Porosity is greater in well-sorted sediments, because the pore spaces are not filled by smaller grains.
c.Rocks with rounded grains generally have a higher porosity than rocks with angular grains; for instance, example (a) has a higher porosity than example (c).
d.Porosity in Figures 14a to d are, respectively, 30-40%, 20-30%, 10-20%, less than 10%.
Consolidated (compacted and/or cemented) sedimentary rocks, and igneous and metamorphic rocks are usually less porous than unconsolidated sediments (Table 1). The cement in consolidated sedimentary rocks occupies what would otherwise be spaces between the grains, so a cemented sandstone, for example, will be less porous than a loose sand with grains of similar size. Igneous and metamorphic rocks generally have very low porosity, because of their interlocking crystals. However, there are volcanic rocks that contain gas bubbles and some of these have high porosities.
The porosity of rocks may be increased by processes that occur after the rocks have formed. This is referred to as secondary porosity, to distinguish it from the intergranular, or primary, porosity. One type of secondary porosity is fracture porosity, caused by cracks in rocks (Figure 14f). Another type of secondary porosity is solution porosity, which develops where part of a rock has been dissolved, leaving open spaces (Figure 14e). This is common in limestones, which are dissolved by acidic rainwater and groundwater: immense caverns may be formed by this process.
In broad terms, how does porosity vary with the grain size of (a) unconsolidated sediments and (b) consolidated sediments?
The porosity will vary with grain size in the following ways:
a.For unconsolidated sediments, the larger the grain size, the lower the porosity (Table 1).
b.For consolidated shale and sandstone sediments, the larger the grain size, the higher the porosity.
So how fast does water flow underground? Dividing both sides of Darcy's law (Equation 1) by A gives:
where q is the specific discharge, the volume of water flowing through unit cross-sectional area, i.e. Q/A. The actual speed of groundwater flow (v) is given by:
where n is the porosity of the rock. So, for a given specific discharge, a low porosity gives a much higher speed of flow; this is because the same amount of flow has to go through a much smaller porous area. For example, water flowing through a porous sandstone flows more slowly than water flowing through a granite or limestone when the porosity is provided by just one or two narrow fissures.
The speed of flow in rocks is extremely slow in comparison with surface flow, even for rocks with high hydraulic conductivities. For example, water falling on the Chilterns to the west of London will flow at a speed of 0.1 to 1 m s−1 in a river, taking a few days to reach London. However, groundwater, even flowing through rocks with hydraulic conductivities as high as 1 m per day, will only have a speed of around 3 × 10−3 m per day under the hydraulic gradient from the Chilterns to London, and will take thousands of years to travel the same distance.