Ground water quality can change, too. Especially, if it is a shallow well, surface conditions can affect it. Road salt, contaminents, etc.
El Paso, TX gets its water (mostly) from really deep wells. There's salt water underneath the fresh, and they're approaching the salty layer since they've had uncontrolled expansion and poor water conservation. Kind of tough in the desert, but it happens more often than it should.
Just got back from an Alaska vacation. Found out that the fresh water melt from the glaciers is often so strong that you can drink the fresh water on top of the salt in many places before it mixes. Lots of animals drink from the bay, which would normally kill them with the salt.