There is abundant evidence of induced electrical field in horizontally placed objects like pipelines, wire fences, semi-trailers, etc. resulting in problems like enhanced corrosion of metals when high-voltage lines (e,g., 450,000 V) are emplaced above the objects. The US has not established uniform standards for health protection for these types of fields as yet.
Your concern, in the hundreds of volts, is not insignificant from the standpoint of electrical protection, but is unlikely to result in corrosion of a vertically placed well casing if they are metallic, and in all events has no lasting effect, if any, on the quality properties of the supplied water. If the casing is more recent and is of PVC, the effects are pretty much non-existent. Jerry's resources below from the Health Physics Society and Wikipedia are enlightening about the subject. Also, check the following where standards have been established (UK is a bit ahead of us in this area) for more information: