Chris,
Thanks for the update. I'd be curious to know if your date analysis of the 
stratification in the deposit justifies your assertion that the cistern was 
open and receiving deposits over a twenty year period. Kent Schneider 
reported that a local informant told him that the majority of the deposit 
was created by a single clean-out event  of the basement at the time the 
house was hooked up to city water. Don't know if he documented this, l but 
sometimes it's hard to sort out the baloney from the Spam (tm). Either way 
the safety advice is still good.

Tim T.