Water people came back out to do a top-to-bottom video inspection of the well. They wanted to see if there is water coming in anywhere but that one dirty source at 90'.
As the county water inspector rightly surmised, blocking that source would leave the whole 600 feet of the well without any water at all.
The people who drilled the well probably knew this all along. When drilling it, they would've found that one source pretty quickly, noticed that it wasn't the best looking water, and kept drilling for another 500 feet, hoping to find something better.
So, our options are to keep drilling farther down in the existing well or drill an entirely new well. A new well will probably cost north of $10K.
Water people have some new slick radar/sonic/magic device that can help identify water sources in rock without having to drill into them. That should increase the chances of hitting water in either situation. They've never used it though. We volunteered our house for them to train on, if they'll give us a discount!
If we do drill another one, what happens to the existing well? We'd probably want to fill it in so that nothing important falls down it. It's close to 600 feet deep, so you might think it would take a lot of stuff to fill it up. But, is it? The hole is only about six inches in diameter. Which is 339 cubic inches per foot. 203,575 cubic inches for the whole well. There are 46,656 cubic inches in a cubic yard, so that makes 4.4 cubic yards of hole to fill. That's about a third of an average dump truck's carrying capacity. Maybe $150 worth of gravel (plus delivery charge) would do it.