How this calculator works
A well is a cylinder, so the volume of water standing in it is the cross sectional area of the casing multiplied by the depth of water. The depth of water is measured from the water surface down, not from the top of the casing, which is why the static water level has to be taken before any pumping. The total well volume uses the full depth to the bottom. Three well volumes is the usual purge target after shocking a well or before sampling: pumping out three times the standing volume clears the treated water and the sediment that was disturbed, leaving fresh groundwater in its place.
Formula
area = pi x diameter squared / 4. Water in the well = area x depth of standing water. Three well volumes = 3 x water volume. Total well volume = area x total well depth.
Worked example
A 15 cm casing has a cross section of pi x 0.15 m x 0.15 m / 4 = 0.01767 m2. With 10 m of standing water the well holds 0.1767 m3, which is 176.7 L, so purging three volumes means 530.1 L. The full 30 m well holds 0.5301 m3, or 530.1 L.
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Frequently asked questions
Why three well volumes?
Three volumes is the common minimum in water well guidance for purging after disinfection or before sampling. It is enough to remove the standing water and the disturbed sediment while avoiding hours of unnecessary pumping.
Should I use casing diameter or borehole diameter?
Use the inside diameter of the casing, because that is the space the water occupies. The drilled hole may be wider, but the space outside the casing is filled with gravel or grout.
What if the well is partly filled with sediment?
Measure the water depth to the top of the sediment, not to the bottom of the drilled hole. The volume of water is what matters for purging, and sediment occupies space water would otherwise fill.
How do I find the static water level?
Lower a weighted line until it goes slack or a water level meter beeps, then measure the wet length. Take it after the well has rested, because the level recovers after pumping and a pumping reading will be too low.