Hydraulic conductivity, K, represents the relative ease of groundwater movement though an earth material as stated in Section 4. It represents of the combination of the intrinsic permeability, k, and fluid properties (Equation 31). If the fluid properties are constant, then hydraulic conductivity will increase as intrinsic permeability increases, because K is directly proportional to Cd². K will increase if C (which reflects pore shape and size distribution, as well as the tortuosity of pore connections) becomes larger and if the characteristic length, d, increases. In Section 4, the investigation of the relationship between K and k described by Freeze and Cherry (1979) used the diameter of glass spheres for d as a surrogate for the size of the pore spaces. Conceptually, for a group of uniform spheres, if the squared diameter of the spheres increases the squared diameters of the pores also increases (Figure 30). The doubling of the pore channel diameter increases the cross-sectional area of pores by a factor of four; thus, the capacity of the porous medium to transmit water increases.

As the size of grains become more uniform, the number of pores decreases, thus reducing tortuosity (flow path length divided by the straight-line distance), and pore diameters become larger, so hydraulic conductivity increases. In contrast, if grain sizes are less uniform, finer grains fill large pores, so the number of pores increase, thus increasing tortuosity; and pore diameters become smaller, so hydraulic conductivity decreases. The effect of the magnitude of grain size and uniformity of grain size on hydraulic conductivity values is illustrated in Figure 31.

Figure 31 provides an example of how the hydraulic conductivity of a uniform coarse sand is impacted by the addition of finer particles. A mix of 75% coarse sand and 25% fine sand alters the K to be much closer to the value of the fine sand. In fact, for mixtures of more than 30% of fine sand, the hydraulic conductivity is less than that of 100% fine sand because the larger grains of the coarse sand occupy some of the pore space available for water flow in the 100% fine-sand sample.

Primary and Secondary Hydraulic Conductivity

As with porosity, earth materials have an initial hydraulic conductivity when originally formed and secondary hydraulic conductivity that develops after their original formation. Hydraulic conductivity can increase as a result of weathering, fracturing, jointing, and faulting; and can decrease as a result of mineral precipitation, and at times as a result of weathering. Like porosity, the hydraulic conductivity of a formation near the surface may be higher than the same formation buried at some depth below the surface. This is because lithostatic pressure (weight of the overlying earth) acts to reduce pore openings and interconnections. Snow (1968, 1970), observed that, in general, hydraulic conductivity of fractured rocks decreases with depth. Davis and Turk (1964) found hydraulic conductivities of crystalline rocks were significantly lower at depths greater than 100 m, whereas D’Agnese and others (1997) noted decreased hydraulic conductivity of sedimentary and volcanic rocks of Death Valley, USA, was most pronounced between 330 and 1000 m. Changes in fractured rock hydraulic conductivities with depth are described in Krasny et al. (2003) and Krasny and Sharp (2007).