Office of Ground Water and Drinking Water 

March 16, 2010

MEMORANDUM 

SUBJECT: 	Basis for Expedited Approval of ReadyCult®, Chromocult®, and
Modified Colitag™ for the determination of Escherichia coli (E. coli)
under the Ground Water Rule

FROM: 	Jennifer Best, Microbiologist 

Technical Support Center 

TO: 		The Record 

The Ground Water Rule (GWR) was proposed in May 2000 (67 FR 65888) and
promulgated in 2006 (71 FR 65574).  Many of the the methods proposed for
use for E. coli determination under the GWR were the same as those
promulgated for use under the Total Coliform Rule (TCR) (40 CFR
141.21(f)(6)) at the time of the preparation of the GWR proposal.  

During the six years between the GWR proposal and promulgation,
additional methods for E. coli determination were evaluated and approved
for use under the TCR.  Because these methods were not included in the
GWR proposal, they were not included in the final GWR. 

Readycult® and Chromocult® are among the methods approved for E. coli
determination under the Total Coliform Rule (TCR) (40 CFR 141.21(f)(6)).
 Readycult® and Chromocult® were approved for use under the TCR in
2002 (67 FR 65888), after the proposal of the Ground Water Rule (65 FR
30194).  Prior to the approval of these methods under the TCR, these
methods were evaluated through the Alternate Test Procedure (ATP).  The
ATP process consists of a procedure  whereby new methods are evaluated
against a previously approved reference method.  Through ATP
evaluations, both Readycult® and Chromocult®  were found to be as
equally effective as Standard Methods 9221F for E. coli determination. 
Standard Methods 9221F had been previously approved for use under both
the TCR and the GWR.  For this reason, these methods would be equally
effective for use under the GWR.  

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Cmodified Colitag™ has been found to be as equally effective as
Standard Methods 9221F, which is approved under the TCR and under the
GWR,  modified Colitag would be as equally effective for use in E. coli
determination under the GWR.  

