When listing a home with a water treatment system, be sure to check when the system was last serviced & whether or not a regular service regime was maintained. This is important because a neglected system can actually contaminate an otherwise clean water source.
Aside from yielding poor quality water; a poorly maintained water treatment system has the potential to harm people who consume the water or occupy the home in which the system is located. Ground water is "ever changing" & hidden dangers such as bacteria, nitrates, voc's, arsenic, radiologicals including radon can all of a sudden become present. Oftentimes, treatment systems are subjected to contaminants not originally accounted for when the system was originally installed.
As an example; a chlorination / filtration (C/F) system that was installed to treat an iron, manganese, color or odor problem can present a radiation exposure danger to the occupants of the structure in which the system is located. Here's how & why;
Let's assume iron is the targeted contaminant; A C/F system is set up to inject chlorine into the incoming well water stream to oxidize dissolved iron in the water. As the chlorine & water mix; the iron is oxidized into particulate form which filtered out. Now that the iron has been removed, there is still a chlorine residual that needs to be addressed. This is accomplished by the installation of a granular activated carbon(GAC) filter in the treatment process.
GAC has an affinity for chlorine & readily removes it from water. Problem is; GAC also has an affinity for radon...so if the water was never tested for radon...& the system was installed without a thought given to what the radon content might be...you may have a GAC tank adsorbing hundreds of thousands of picocuries of radon daily! All the adsorbed radon stays on the carbon bed - it doesn't get removed during backwash. The collected radon will naturally decay & as this occurs...gamma rays emit from the bed of carbon. If this system is located in close proximity to living areas...whomever occupies those areas will be exposed to those gamma emissions.
Consider for a moment the consequence of someone getting sick & finding this was an exposure source they were not made aware of. While there are no fool proof approaches to issues like these, you'll be better served having experienced technicians assess the current status of a water treatment system. This is but one example of how a poorly maintained system can negatively impact the quality of life in a given home.