Most of it is common sense, so to speak.
If you place the septic system (which could be a tank or some kind of infiltration system that let’s the polluted water trickle into the into the ground) it comes with the concept that the septic system should be placed downstream (gravitationally) from the well. Water flows downwards, and if the well is upwards, the pollution cannot end up reaching the well.
Another thing is that the septic system is placed just barely underground; A tank is just ridiculous to dig down deeper than you have to since you are going to need a manhole for repairs and inspections and an infiltration is SUPPOSED to trickle out pollution into the ground in a way that makes it meet a lot of material in the ground before it reaches the ground water, digging it down deep defies the point of it.
Most water wells are deep bored. When you risk pollution in the well because the surface ground water is of bad quality and what you really want to pump up is the deep ground water, it’s possible to add a large rubber gasket to the hose in the hole and fixate it at a depth where you want a barrier, so that the well pumps up the fresh water from a depth spring instead of a surface spring.
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