In a press release from Floyd County Board of Commissioners:
Drinking Water Safety from Private Wells
If you get your water from a private and the well was overtopped with flood waters the following are things you might consider doing first when power is available is to pump.
Step one: Pump minimum of 2 to 3 times the well water volume out of the well to help get flood water out as well as potential bacteria.
To calculate the volume of water that should be pumped use the following calculation:
Divide the diameter of the well (in inches) by 2 and then multiply that number (the radius of the well) by itself, then multiply that number by 3.1416 then multiply by the depth of water in inches (not feet) in the well. Once you have the value from this calculation (this is the amount of cubic inches in the well) divide it by 231 to get gallons of water in the well. To exchange 2 or 3 times the well volume of water, run your pump long enough to pump that calculated volume from the well.
An example of what the calculation looks like for a 4 inch well and a 100 foot water column can be found on the water at UGA Blog https://blog.extension.uga.edu/water/2017/09/information-for-pre-and-post-hurricane-preparedness/ .
Discarded this water from an outside faucet and not from an inside faucet. Reason for discarding from an outside faucet: water should bypass most of the internal plumbing and this volume of water should not be put into the septic tank. After pumping this 2-3 volumes of water, the well should be shock chlorinated.
Step two: shock chlorination of the well water should be done.
How to shock chlorinate well water: https://secure.caes.uga.edu/extension/publications/files/pdf/C%20858-4_1.PDF
Step three: A bacteria test should be done after shock chlorination and all the chlorine is dissipated by pumping out another 3 or 4 times the well volume out of the well. Again, water pumped after the shock chlorination should be discarded from an outside faucet with only a small portion being discarded through the inside faucets. This highly chlorinated water if discharged to the septic tank could cause problems with the bacterial colonies in the septic tank.
Bacteria test kits can be picked up at your local Cooperative Extension office