Persistent coliform in new wells
HOUGD at MDH-BEMIDJI.HEALTH.STATE.MN.US
Thu Jul 6 11:12:21 EST 1995
I have a perplexpling situation that I would like to solicit this
group's opinions on. I will describe the scenario, list some
specifics knowns, and finish with a list of questions. All replies
We are frequently coming across newly drilled wells that test
positive for coliform (absent for E. coli) bacteria despite repeated
disinfections. These wells seem to remain positive for up to one
year after the installation and then produce coliform free water
after this time. They are typically 4"-6" diameter wells drilled with
a rotary rig in unconsolodated glacial till. Typically 50 to 150
feet deep and usually penetrating significant amounts of clay. We
are in the process of speciating these coliforms and have data from
only one well so far, more coming shortly. 2 species have been
identifed: Enterobacter sakazakii and Enterobacter cloacae.
An interesting aside is that these species of coliform are being
detected by the Coliert (TM) presence/absence method that utilizes
the ONPG/MUG indicator nutrients. The membrane filter technique
based on Millipore's endo growth media doesn't appear to pick up
these organisms. This is leading to great frustration between us
(using Colilert) and the parties we regulate who often use MF in
Here are my questions:
1. Does anyone have information on the selectivity of different
methods for coliform detection as used in drinking water analysis?
Specifically Colilert (TM) vs. membrane filter vs. MPN?
2. What is the pathogenic potential of Enterobacter sakazakii or
cloacae? Background info on these organisms?
3. Are these organisms more resistant to chlorine than other
organisms or is their presence after repeated disinfections
indicating a recharge source that is not being reached by chlorinated
4. Have others experienced this problem and is there a literature
citation I could be directed to?
Minnesota Department of Health
Internet Address: hougd at mdh-bemidji.health.state.mn.us
Fax #: 218/755-3823
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