A well has a depth of 306.1 ft. If the depth to water is 124.7 ft, what is the pressure in psi at 5.8 ft above the bottom? Disregard the additional atmospheric pressure in the well.

• A. 60 psi
• B. 76 psi
• C. 129 psi
• D. 150 psi

Answer: (B)

Hydrostatic pressure grows with the height of water above the point where you measure it. In freshwater, you gain about 0.433 psi for every extra foot of water above the point.

First find how deep the measurement point is from the surface: the bottom is 306.1 ft deep, and the point is 5.8 ft above the bottom, so its depth from the surface is 306.1 − 5.8 = 300.3 ft. The water surface sits at 124.7 ft depth, so the water column above the measurement point is 300.3 − 124.7 = 175.6 ft.

Pressure at that point (gauge pressure, ignoring atmospheric pressure) is 175.6 ft × 0.433 psi/ft ≈ 75.97 psi, about 76 psi.

Other choices would come from using the full bottom depth or the surface water depth instead of the actual water column above the point, which isn’t correct for the pressure at that location.