15.1 Introduction

In Section 5 we discussed the response of a confined aquifer to pumping from a well. To review, when a well is pumped in a confined aquifer, the pore spaces stay filled with water, and the water comes from the elastic response of the aquifer. More specifically, as pressure is reduced in the pores the pores get a little bit smaller and the overall pore volume decreases slightly. When we discussed this earlier, we did not consider the fact that changing the volume of the individual pores might change the overall volume of the aquifer.

Well…, it does change the overall volume of the aquifer. As the fluid pressure decreases, the grains expand, the pores get smaller, and the overall volume of the aquifer decreases. This is called consolidation. Just as different rocks react to tectonic stresses in different ways, different aquifers and hydrostratigraphic units react to changes in fluid pressure in different ways. Change in aquifer volume in response to change in pore fluid pressure is the subject of this section.