So far it does look like hearsay, but even if it was-——
From:
SPECIAL REPORT OF THE MOUNTAIN MEADOW MASSACRE BY J. H. CARLETON, BREVET MAJOR; UNITED STATES ARMY, CAPTAIN, FIRST DRAGOONS.
Camp at Mountain Meadows,
Utah Territory, May 25th, 1859
Mr. Rodgers, United States Deputy Marshal, who accompanied Judge Cradlebaugh in his tour to the South, told me that the water in the spring referred to runs with such volume and force “a barrel of arsenic would not poison it.”
http://www.law.umkc.edu/faculty/projects/ftrials/mountainmeadows/carletonreport.html
I’ve run across some indications that there may have been a contagious disease in SW Utah at the time, leading to a number of people falling sick shortly after the wagon train went through. Possibly one of the people in the train was a carrier.
The locals promptly jumped to the conclusion that the wagon train must have poisoned the springs, somewhat as during the Middle Ages the Jews were accused of poisoning the wells and causing the plague. Hey, a Jew passed through town just before the plague struck, therefore...
Just a supposition.