Mining in the area would explain it.
You are correct. The high ground water level, as shown in the pic, was the tip off. When I first saw the pic I thought the object hit a spectic leach field and that
whatever sulfur gases were released bothered the locals.
Copper and gold mining in the area and the associated release of chemicals into the ground water were the cause.
I believe there was arsenic in the area but have difficulty accepting that the vapor cloud hung around long enough above ground to sicken these people. Seems unlikely to me. Gasses disperse real quick unless they are trapped somehow.