Subsidence is a Problem Everywhere

Subsidence is a global problem and, in the United States, more than 17,000 square miles in 45 States, an area roughly the size of New Hampshire and Vermont combined, have been directly affected by subsidence. More than 80 percent of the identified subsidence in the Nation has occurred because of exploitation of underground water , and the increasing development of land and water resources threatens to exacerbate existing land-subsidence problems and initiate new ones. In many areas of the arid Southwest, and in more humid areas underlain by soluble rocks such as limestone, gypsum, or salt, land subsidence is an often-overlooked environmental consequence of our land- and water-use practices.

Image source: USGS

Analyzing subsidence for the following specific situation:

(a) A horizontal aquifer is overlain by 15m of saturated clay. The density (dry) of clay is 1900 kg/m³. Calculate the total stress acting on top the aquifer.

(b) If the pressure head in the aquifer is 30m, calculate the effective stress in the aquifer. 

(c) If the aquifer is pumped and the hydraulic head at some point is reduced by 3m, what will be the resulting changes in the pressure head, the pressure, the effective stress, and the total stress?

(d) If the compressibility of the aquifer is 2x10^-8 m²/N and its thickness is 10m, how much compaction will the aquifer undergo during the head reduction in part c?

(e) If the porosity and hydraulic conductivity of the aquifer are: 0.3 and 5x10^-6 m/s, calculate the transmissivty and storativity for the aquifer.