Laboratory Determination of Hydraulic Conductivity*

The most reliable method of determining the hydraulic conductivity and intrinsic permeability is from aquifer tests conducted in the field. These methods are discussed in Chapter V. Values of hydraulic conductivity, determined by carefully conducted experiments in the laboratory, are quite accurate and reproducible. By necessity, laboratory samples are extremely small compared to the aquifer as a whole, however. Furthermore, some degree of disturbance always accompanies the collection of the samples. For these reasons, it is extremely difficult to characterize the hydraulic conductivity of an aquifer, or even a small portion, by means of laboratory measurements.

Hydraulic conductivity and intrinsic permeability can be measured directly with permeameters. Two commonly used permeameters are shown in Fig. 3-6. Figure 3-6a depicts a constant-head permeameter in which steady upward flow through the sample is established. Darcy’s equation can be applied directly in this case to compute K,

The total head loss through the permeameter is indicated by the difference in elevation between the inflow and outflow water levels. In a properly designed permeameter, the head loss through the retaining screens and the inflow and outflow plumbing is negligibly small and the head loss through the sample is very nearly equal to the difference in the inflow and outflow water levels. Some constant head permeameters are equipped with piezometer taps located in the test section and the difference in piezometric head is indicated by the difference in water level in the two piezometers. This difference reflects the head loss between the two piezometer taps, regardless of the head loss in the remainder of the system.

The falling head permeameter is shown in Fig. 3-6b. The rate of discharge through the sample decreases with time because the driving head decreases as the water level falls in the inflow standpipe. It is left to the student to show that the equation for hydraulic conductivity is

in which ∆h₀ is the initial head loss (at t = 0) and ∆h(t) is the head loss at time t. Again, care must be taken to insure that the loss of head in the retaining screens and inflow-outflow plumbing is very small relative to that which occurs in the sample.

*McWhorter, D. B. and D. K. Sunda, 1977. Ground-Water Hydrology and Hydraulics. Water Resources Publications. Fort Collins, CO. pg. 79-80.