NGWA Air Rotary Practice Exam

In gravel-well screening, percentage retention describes how effective the screen is at holding back solids from entering the well. It’s the portion of gravel or formation particles that the screen prevents from passing through its openings. Thinking of the screen as a sieve, a higher retention means more particles are blocked and kept out of the well, which helps protect the pump and control sand production. At the same time, too high a retention can reduce water flow or increase the risk of screen clogging, so designers balance retention with permeability.

So, the percentage retention is about the fraction of solids prevented by the screen, not about the screen’s length, the aquifer’s yield, or how fast gravel is produced.