Since they were available, pitless adapters have been strongly recommended for all wells which had "pits" - dug 2 to 3 foot in diameter pit that a well was sunk into as much as 20 feet. They are usually lined with stome, to prevent colapse and almost alwats had water in them, cover the actually well head. The pits were considered a major way that surface water could get into the well.
The truth be told, in 5,000 some tests, a surprising few alctually had problems, even in the worst of conditions.
Now pitless adapters are a good thing, they raise the well head above the ground so that water can not (or should not) penetrate, allowing normal soil to do its thing and clean the water that is taken from a well.
Today, I had two examples were the work done to setup a pitless adapter was so poor, that it made the situation worse.
One a unevenly cut casing had a loosely fitting cap place on top of it. bacteria was found in the well.
The other, the pitless adapter was loosely attaches to the main casing, so that the seal rotted away, allowing soil bacteria to get into the well.
How do you prevent such things? The only way I can think of is to test the water for bacteria in a heavy rain situation to see if the work holds up.