In my area, shared wells sometimes arose from this situation: A family owns a piece of land and builds two houses on it, both of which share a well. Over time, one or both houses are sold. A few generations ago, easements were not as common as they are today. So many times, a home was sold without an easement in place.
If you are thinking of buying a home with a shared well, I suggest that you add a contingency to the offer that it is "subject to satisfactory legal review of the shared well agreement". And then have your real estate attorney review this. It is impt to not just have an agreement, but that it is recorded.

A shared well is generally thought of as a private well, that is, it's not owned by a public utility. In addition a shared well is a well that is shared by more than one user. Each user is responsible for the payment of shared expenses and the maintenance, including the environmental water quality analysis of the well water as prescribed by your county. Hope this helps.

A shared well can supply up to five homes in Maricopa County depending upon the flow rate (gpm) of the well. Other counties may allow more homes on a shared well. If you are buying a house on a shared well, be sure to read the shared well agreement and obtian the well registration information. Your agent should be able to point you in the right direction for all of this information.

A shared well is a well that supplies water to more then one home. The homes share in the expense of the cost and upkeep of the well.