According to z :

my knowledge on this is pretty much lab rather than real world, but my
first guess would be a leak from a sewage/septic system, or some other
source of poop like bears in the woods getting in. you killed off all
the bugs, which is why the e. coli are now gone, but the coliform are
getting restocked. ??

Some friends had to get a new well drilled (deep one this time -
the shallow one ran out of head room - water table dropped a bit
too far).

The flow rate was very high, and they'd been unable to completely kill
off the TC by shocking. No FCs tho.

Assumption is that the well hit an underground stream that has
a persistent TC level, and no amount of shocking will make it
go away.

So they now have a UV sterilizer too.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.