While I'd agree that 10ft of sand and gravel wouldn't necessarily purify water, I'd argue that, at least in my neck of the woods, deep wells are now common because those deeper aquifers are more dependable. Coliform bacteria in wells is mostly the product of the bacteria 'spilling' into the well. Wells in pastures or too near septic systems are usually at fault. Coliform bacteria cannot survive a trip through the soil to reach even a relatively shallow aquifer. Once it penetrates even a few feet into the soil there is no longer sufficient oxygen for it to survive.
Originally Posted by HAB
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted. - Emerson