LEAKY AQUIFER TESTING
(Non-technical. Click link for a technical web page on leaky aquifer testing.)
By Darrel Dunn, Ph.D, PG, Hydrogeologist
Leaky aquifers are layers of water-bearing permeable material, such as sand or gravel, that are in the subsurface and are confined between low-permeable layers, such as silt. The water level in wells completed in such aquifers usually rises above the top of the aquifer and the wells are called artesian. When water is pumped from such aquifers via a well, the pressure in the aquifer is reduced and seepage is induced from the overlying and underlying low-permeable layers (aquitards). This seepage affects how much water a well in the leaky aquifer can produce. One way to estimate how much water can be produced from such aquifers by a water well is to perform a test that involves pumping the well at a constant rate. As the well is pumped, the water level in the well declines in order to draw water into the well from the aquifer. The rate of this decline can be analyzed to obtain useful information on the water well and the aquifer. The rate of decline is determined by making successive measurements of water level in the well as it is pumped. This decline due to pumping is called drawdown. In addition, it is useful to measure drawdown in a nearby well that is not being pumped (observation well).
Professional analysis of drawdown data usually involves comparing the observed drawdown in real wells and aquifers with unknown properties to calculated drawdown in hypothetical aquifers with known properties. The calculated drawdown is obtained from equations that are derived by assuming values for the unknown properties and assuming relatively simple aquifer and aquitard characteristics. The unknown properties of interest include (1) the ability of the aquifer to transmit water toward the well (transmissivity, permeability), (2) the amount of water yielded by the aquifer due to compression of the aquifer materials as the water pressure declines (storativity), (3) the ability of the aquitards to transmit water to the aquifer (aquitard hydraulic conductivity), and (4) the amount of water yielded to the aquifer due to compression of the aquitard materials (aquitard specific storage). The effect of well construction on the ability of the water to pass from the aquifer into the well (skin effect) is another parameter that may be unknown and can be estimated from pumping test analysis.
Modern methods of pumping test analysis involve successive trial calculations of drawdown in hypothetical wells and aquifers with trial values of aquifer and aquitard properties and well skin until the calculated drawdown in the hypothetical aquifer and well matches the actual drawdown in the real aquifer and well. This match yields estimated values of the actual aquifer properties (such as transmissivity and storativity), aquitard properties (hydraulic conductivity and specific storage), and the well skin effect.
DP_LAQ is a computer program that generates hypothetical drawdown curves (type curves) used for analyzing pumping test data from leaky aquifers. Figure 1 is an example of a DP_LAQ match between calculated drawdowns (type curves) and actual drawdown in a pumped well and an observation well in a real aquifer. The values used for the horizontal and vertical axes in this type of matching are somewhat complicated and a reference that explains them is given on the technical web page for leaky aquifer testing. Considerable professional knowledge of mathematics, computer software, groundwater hydrology, and water well technology is needed for effective use of this advanced pumping test analysis software.
Figure 1. Example of DP_LAQ type curve matching.
Leaky aquifer test analysis is not only used in water supply investigations, it can also be used in injection well investigations. For example, estimating how much water can be pumped into injection wells for disposal of produced water associated with oil and gas extraction and mine drainage.