An air line 200 ft long is installed in a well for determining water level. With air pumped and the pressure gauge stabilized at 36 psi, what is the water level in feet?

• A. 83 ft
• B. 117 ft
• C. 200 ft
• D. 36 ft

Answer: (B)

The key idea is that hydrostatic pressure from a water column is directly related to how tall that column is. In water, roughly 1 psi supports about 2.31 feet of water, so the depth corresponding to a given pressure is pressure (psi) × 2.31 ft/psi.

With a gauge reading of 36 psi, the water depth from the surface to the water line is about 36 × 2.31 ≈ 83 ft. The well itself is 200 ft deep, so the remaining distance from the bottom of the well to the water line is 200 − 83 ≈ 117 ft. Therefore, the water level (measured from the bottom of the well) is 117 ft. If you were reporting depth from the surface, it would be about 83 ft, but the common way in this context is to state the water level from the bottom, which yields 117 ft.