UNCONFINED AQUIFER TESTING
(Non-technical. Click link to see a technical web page on unconfined aquifer testing.)
By Darrel Dunn Ph.D., PG, Hydrogeologist
Water table aquifers (aka unconfined aquifers) are layers of permeable material, such as sand or gravel, that are present at the land surface and are capable of yielding useful supplies of water to water wells. 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 often useful to measure drawdown in one or more nearby wells that are not being pumped (observation wells).
Professional analysis of drawdown data from a pumping test usually involves comparing the observed drawdown in real 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 characteristics that make it possible to derive the equations starting with principles of physics. The unknown properties of interest include (1) the ability of the aquifer to transmit water toward the well (transmissivity, permeability), and the amount of water drained from the aquifer due to decline in water table elevation (specific yield). A non-technical (and slightly inaccurate) description of the water table is that it is the upper surface of underground water that completely saturates the aquifer material. This underground water is referred to as groundwater.
Current methods of analysis often involve successive trial calculations of drawdown in hypothetical aquifers with trial values of aquifer properties until the calculated drawdown in the hypothetical aquifer matches the actual drawdown in the real aquifer. This match yields estimated values of the properties (such as transmissivity and specific yield) for the real aquifer. The methods have evolved over many years as the equations used to calculate drawdown have become more realistic and consequently more complex. These complex equations require computers for their solutions. WATEQ is a computer program that provides solutions for such complex equations. Figure 1 is an example of a WATEQ match between calculated drawdown and drawdown in the 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.
Figure 1. Example of WATEQ type curve matching.
WATEQ allows hypothetical aquifer characteristics to include different resistances to flow in the vertical and horizontal direction (vertical and horizontal hydraulic conductivity), and various ways to simulate delayed drainage at the water table. It allows treatment of the effects of characteristics of the pumped well and the observation wells. These characteristics include (1) only partial penetration of the aquifer, (2) water pumped from storage in the pumping well, and (3) increased resistance to flow into the pumped well due to effects of well construction. WATEQ offers a choice of options for the method used to match real and calculated drawdown to estimate values for transmissivity, specific yield, and other values that affect the behavior of the pumped water table aquifer system. 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.
Posted: December 2014