Seep wells are best dug near a water source where the ground is already fairly saturated. Mud, lake shore, and river bank or sand bar are all good locations. It can also be dug where a water source is not visible but where plant life indicates a water source may be just below the surface of the ground.
There is a difference between filtering water and purifying water in outdoor terminology. Filtering involves removing particulates from the water. This would be removing things like leaves, dirt, bugs, and algae. An exceptional filter would also remove bacteria and other microorganism. After being filtered water is generally safe-er to drink, but not by definition, safe. Purifying is specifically used to describe water that has been cleansed to the point where it is safe to drink. A seep well is a method of filtration, not purification. It improves the quality of risky water and reduces the likelihood of ingesting something that will make you sick.
As I said, the water you gather using this method is filtered. It is cleaner that drinking straight from a pond or stream, but it is still safer to boil this before drinking it.