How is it that you know that the appraiser knew? I find your posts to be quite confusing. Your "One Christmas Eve" comment seems to imply one or more years in the past. Your comments about what "They" said they would sell for seems to imply a proposed sale between you and "they." We have no idea what the appraisal report was created for, what was the intended use of obtaining a real estate appraisal and who obtained it? Who hired the appraiser and why do you have a copy of the appraisal report?
Regardless of your answers to my questions I tend to lean strongly towards total incompetency of the appraiser involved. This does NOT sound like some sudden problem that literally nobody in the market area knew about and it suddenly just popped up out of nowhere a week before the appraiser was hired. If I am correct, and this was a known problem in the general market area going back for many months, or even years, then this appraiser has created, intentionally or not, a seriously misleading appraisal report. So seriously misleading that if the appraiser knew, or should have known, the appraisal board of that state should be having a sit down coming of the Lord with this appraiser. So yes, the appraisal report should have had an entire section of it dedicated to addendum discussing the market reaction to water sources and how any and all of that was considered by the appraiser. Unless, of course, this all just happened last week, there was no news media coverage of it, and the appraisal report was based upon no site inspection or property owner lies or omissions to the appraiser.
The appraisal problem, properly labeled, is one of "burden of ownership." In someways similar to a house too large for its market place, the extra costs of maintenance, insurance, heating, etc., become measurements of discounts that must be considered due to the not needed extra space. In your case, the burden(s) are not only the costs involved in having to pay for a substitute for a market acceptable steady water source, but also the life inconveniences and fear (insecurity) that represents. Not an easy appraisal assignment when the property is located in an area where the market has always (recent decades) demanded an easy and steady water source.