5.3 Hydraulic Conductivity Values for Earth Materials

Most groundwater textbooks contain tables of hydraulic conductivity values summarized from a large number of laboratory and field tests. The tables typically include ranges of values for a given type of earth material because they are formed by a wide variety of processes that influence the size and distribution of interconnected voids and complexity of pathways. A gravel that is infilled with sand, silt and clay will have a lower hydraulic conductivity than and open framework gravel found in a river bar. Though both are classified as gravel deposits, their hydraulic conductivity may differ by orders of magnitude. As explained in Section 5.1, the larger and more interconnected the pores of a material, the higher the hydraulic conductivity.

Freeze and Cherry (1979) provide a useful table of intrinsic permeability, k, and hydraulic conductivity, K, of unconsolidated material, sedimentary rocks, and igneous and metamorphic rocks similar to that shown as Figure 32. This figure presents the range of values of hydraulic conductivity and intrinsic permeability in three systems of units. The data show that hydraulic conductivity varies over a wide range. There are few other physical parameters that take on values ranging over 13 orders of magnitude. In practical terms, this wide range of values suggests that an order-of-magnitude knowledge of hydraulic conductivity can be useful. Conversely, it implies that if a third decimal place is reported for a hydraulic conductivity value, it is likely of little significance.