2.1 Pumping Tests

When a well is pumped, water levels in the pumping well and adjacent groundwater system decrease over time. This decrease is referred to as drawdown (Figure 1). Drawdown is defined as the difference between the beginning (initial or static) water level in a well and the water level at some time after pumping has begun (a positive value). Events unrelated to the pumping may affect water levels in the aquifer, thus the so‑called static water level needs to be adjusted to the level that would exist if the other events did not occur, so in this book the static water level is referred to as the corrected static water level. Drawdown generally increases as the well is pumped until the system reaches a steady state or withdrawal from other wells in the vicinity decreases. In some pumping tests, water is pumped into the test well (injected) at a prescribed rate instead of water being pumped from the well. In this case, water levels in the formation rise from initial levels—and instead of drawdown, the water levels drawup (i.e., have negative drawdown). Pumping tests are typically conducted in one well while monitoring water levels in one or more unpumped observation wells, but sometimes pumping tests are conducted using a single well with no monitoring wells.

Time‑drawdown data are collected from each well included in the test, and analytical or numerical methods are used to generate information on the hydrogeologic properties of the system. Pumping tests can be conducted using constant and/or variable pumping rates, or a constant drawdown. Though packer testing within an open borehole can include pumping, they will be described in a section on packer methods (Section 15).

Constant‑rate tests (the most common type of pumping test) are designed to maintain discharge at a continuous rate throughout the test, which simplifies data analysis (as discussed in later sections of this book). Although it is practically difficult to maintain a constant discharge rate, this approach is preferable because using a variable pumping rate requires a more complex analysis. Specifically designed variable‑rate pumping tests are used to estimate formational properties and quantify how the well design and pumping rate affect the magnitude of drawdown in the pumping well. In some settings, such as when a well flows under natural conditions at the surface and discharge declines over time, or when water is extracted at a variable rate to maintain a constant water level over time, then a constant drawdown method is used to analyze the results for estimation of hydrogeologic properties (e.g., Lohman, 1972).

Hydraulic testing sometimes involves pumping multiple wells to investigate how other pumping wells or observation wells respond to stresses located over a specific area. For example, given a complex fractured‑bedrock groundwater system in which mine dewatering is planned or potential water supply is being explored, use of multiple pumping wells can evaluate the interconnectedness of the system.