Another important distinction between unconfined and confined aquifers is the way they respond when water is pumped from them. When water is pumped from a well in an unconfined aquifer, the pumped water is replaced by air entering the drained pores from above as illustrated in the before and after images of Figure 32. In contrast, when water is pumped from a confined aquifer, air does not enter the pores, rather the water pressure is relieved and the geologic layers compact (especially the clayey layers within or between aquifer layers). This occurs because the high-water pressure has been supporting the particles by bearing some of the weight of the overlying geologic layers and water (Figure 33).
Because pores drain when unconfined aquifers are pumped and pores depressurize when confined aquifers are pumped, the response to pumping propagates outward much more rapidly in confined aquifers as compared to unconfined aquifers. Decreasing water levels in response to pumping from a confined well typically are manifested at distances of 100s of meters to kilometers within a few hours or days (depending on the aquifer properties), even though the groundwater near the well may have moved only a few meters in that time. This is somewhat analogous to a wave in the ocean, the wave travels rapidly while the molecules of water stay in essentially the same location because the wave is a transfer of energy between the molecules of water, not a journey of the molecules.
An important consequence of the difference between pores draining in unconfined aquifers while confined aquifers compress is that an equal decline of water levels in an unconfined aquifer will yield far more water than that of a confined aquifer. Typically, a one‑unit decline of water level (e.g., 1 m) in an unconfined aquifer will yield 1000s of times more water than the same water level decline in a confined aquifer.
In some regions of the world where large volumes of groundwater have been pumped from a confined aquifer, there has been significant compaction of geologic formations, often revealed as sinking of the land surface called subsidence. An example is shown for the Central Valley of California where decades of groundwater pumping to irrigate water‑intensive crops in the hot and dry summers caused the valley floor to sink as much as 10 meters from 1925 to 1977 as shown in Figure 34. Such sinking of the land due to release of groundwater pressure in confined aquifers has occurred not only in the western United States, but in other locales, notably in Mexico City, Mexico; Jakarta, Indonesia; Venice, Italy; and Beijing, China. When subsidence occurs near coasts, it accelerates local saltwater intrusion into coastal aquifers. When the pumping rate is reduced such that aquifer levels no longer decline, the land stops subsiding. However, if further reduction in pumping occurs so that aquifer water levels rise, the subsidence is not fully reversed (that is, the land surface does not rise) because the clay grains cannot return to their prior arrangement as shown in Figure 34c and d.