Page 4: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Total Coliforms

Appendix A: Decision Tree for Routine Microbiological Testing of Municipal Scale Systems

Decision Tree for Routine Microbiological Testing of Municipal Scale Systems
figure 1 Decision Tree
Figure 1 text description

The recommended actions for responding to routine drinking water samples in public systems that are positive for E. coli and/or total coliforms are presented as a decision tree. The decision tree is made up of boxes that are connected to each other using single direction arrows. Each box in the decision tree contains a recommended action. The path taken through the decision tree, and therefore the recommended actions, are dependent on the results from the microbiological sampling. The left hand side of the decision tree presents the actions to be taken when samples are positive for E. coli. The right hand side of the decision tree presents the actions to be taken when samples are positive for total coliforms but negative for E. coli. The middle of the decision tree presents the actions to be taken if a boil water advisory has been issued.

On the left hand side of the decision tree, if E. coli is detected at multiple sites during routine sampling, the recommended action is to issue a boil water advisory in consultation with the responsible authorities. If only a single site is positive for E. coli, the recommended action is the positive site and adjacent sites should undergo a first re-sampling for E. coli and total coliforms (Footnote *). If any re-sampled site is positive for E. coli, the recommended action is to issue a boil water advisory in consultation with the responsible authorities. Depending on the jurisdiction, a boil water advisory may be issued on a single site contamination. If the re-sampled site was negative for E. coli and total coliforms, the recommended action is to carryout a second re-sampling at the positive site and at the adjacent sites. If any of the second re-sampled sites are positive for E. coli, the recommended action is to issue a boil water advisory in consultation with the responsible authorities. If the second re-sampled sites are negative for E. coli and total coliforms, the water is considered safe and the recommended action is to continue routine sampling.

The right hand side of the decision tree should be followed if a total coliform sample is positive in the absence of E. coli. Therefore, if total coliforms are detected during the first or second re-sampling for E. coli, or, if a routine sample is positive for total coliforms, in the absence of E. coli, the recommended action is to re-sample the positive site and adjacent sites for E. coli and total coliforms (Footnote **). If E. coli is detected, the recommended action is to issue a boil water advisory in consultation with the responsible authorities. A boil water advisory may be issued based on a positive total coliform, in the absence of E. coli, if deemed necessary by the responsible authority. If the re-sampled sites are negative for both total coliforms and E. coli, the recommended action is the positive sites and the adjacent sites should undergo a second re-sampling for E. coli and total coliforms. If these sites are negative for E. coli and total coliforms, than no further action is required, the water is considered safe and the recommended action is to continue with routine sampling. If total coliforms are detected in any of the re-sampled sites, in the absence of E. coli, the recommended action is to investigate the source of the contamination, implement corrective actions, and then return to the top of the decision tree, to the box recommending the first re-sampling of the positive sites and adjacent sites (Footnote ***).

In all instances where the decision route leads to issuing a boil water advisory, the centre of the decision tree should be followed (Footnote ****). After issuing the boil water advisory, the recommended actions are to investigate the source of the contamination, implement corrective actions and undergo a first re-sampling of the positive sites and adjacent sites. If E. coli is detected, the arrow directs the decision route back to investigating and implementing corrective actions. If the first re-sampling is negative for E. coli, a second re-sampling at the positive sites and adjacent sites is recommended. If the second re-sampling is positive for E. coli, the decision route is directed back to investigating and implementing corrective actions. If the second re-sampling is negative for E. coli, the water is considered safe and the recommended action is to rescind the boil water advisory in consultation with the responsible authorities, and to then continue with routine sampling.

Footnotes

Footnote 1

A boil water advisory may be issued on a single site contamination if deemed necessary by the responsible authority

Return to footnote * referrer

Footnote 2

A boil water advisory may be issued based on a positive total coliform, in the absence of E.coli, if deemed necessary by the responsible authority.

Return to footnote ** referrer

Footnote 3

If a total coliform positive sample is detected during resampling for E.coli, the decision route for detection of a total coliform positive sample, in the absence of E.coli, should be followed (right-hand side of the decision tree).

Return to footnote *** referrer

Footnote 4

Depending on the jurisdiction, "boil water order" may be used in place of, or in conjunction with, "boil water advisory."

Return to footnote **** referrer

Appendix B: Decision Tree for Routine Microbiological Testing of Residential Scale Systems

Decision Tree for Routine Microbiological Testing of Residential Scale Systems
Figure 2 - Decision Tree for Routine Microbiological Testing of Residential Scale Systems
Figure 2 - Text description

The recommended actions for responding to routine drinking water samples in residential  systems that are positive for E. coli and/or total coliforms are presented as a decision tree (Footnote *). The decision tree is made up of boxes that are connected to each other using single direction arrows. Each box in the decision tree contains a recommended action. The path taken through the decision tree, and therefore the recommended actions that should be taken, are dependent on the results from the microbiological sampling. The left hand side of the decision tree presents the actions to be taken when samples are positive for E. coli and a boil water advisory has been issued (Footnote **). The right hand side of the decision tree presents the actions to be taken when samples are positive for total coliforms but negative for E. coli.

On the left hand side of the decision tree, if E. coli is detected during routine sampling the recommended action is to issue a boil water advisory in consultation with the responsible authorities.

The right hand side of the decision tree should be followed if a total coliform sample is positive in the absence of E. coli. For this decision route, the recommended action is to undergo a first re-sampling of the positive sites and at adjacent sites for E. coli and total coliforms (Footnote ***). If E. coli is detected, the recommended action is to issue a boil water advisory in consultation with the responsible authorities. A boil water advisory may also be issued based on a positive total coliform result in the absence of E. coli, if deemed necessary by the responsible authority. If the first re-sampled sites are negative for both total coliforms and E. coli, the recommended action is the positive sites and the adjacent sites should undergo a second re-sampling for E. coli and total coliforms. If E. coli is detected in the second re-sampling, the recommended action is to issue a boil water advisory in consultation with the responsible authorities. If the second re-sampling sites are negative for E. coli and total coliforms, than no further corrective actions are required, the water is considered safe and the recommended action is to continue with routine sampling. If total coliforms are detected in any of the re-sampled sites, in the absence of E. coli, the recommended action depends on the extent of the contamination, the history of the system, the jurisdictional requirements and if there is an outbreak in progress (Footnote ***). The recommendation can be to either investigate the source of the contamination, implement corrective actions, and carryout a first re-sampling of the positive sites and adjacent sites, or to issue a boil water advisory in consultation with the responsible authority. If the decision route taken was to carryout the first re-sampling, and the sites are negative for both total coliforms and E. coli, the recommended action is to undergo a second re-sampling for E. coli and total coliforms. If these sites are negative for E. coli and total coliforms, than the water is considered safe and the recommended action is to continue with routine sampling. If either the first or the second re-sampling results in a total coliform or an E. coli positive sample, the recommended action is to issue a boil water advisory in consultation with the responsible authority.

In all instances where the decision route leads to issuing a boil water advisory, the recommended action is to then investigate the source of the contamination, implement corrective actions and undergo a first re-sampling of the positive sites and adjacent sites. If E. coli or total coliforms are detected, the arrow directs the decision route back to investigating and implementing corrective actions. If the first re-sampling is negative for E. coli and total coliforms, a second re-sampling at the positive sites and adjacent sites is recommended. If the second re-sampling is positive for E. coli or total coliforms, the decision route is directed back to investigating and implementing corrective actions. If the second re-sampling is negative for E. coli and total coliforms, the water is considered safe and the recommended action is to rescind the boil water advisory in consultation with the responsible authorities, and to then continue with routine sampling.

Footnotes

Footnote 1

Private systems (eg. an individual well serving a rural home) are responsible for the microbiological quality of the water serving the system. Nevertheless, health authorities should be willing to provide advice on remedial actions, when necessary.

Return to footnote * referrer

Footnote 2

Depending on the jurisdiction, "boil water order" may be used in place of, or in conjunction with, "boil water advisory."

Return to footnote ** referrer

Footnote 3

A boil water advisory may be issued based on a single positive TC result, if deemed necessary by the responsible authority.

Return to footnote *** referrer

Appendix C: List of acronyms

ANSI
American National Standards Institute
AOC
assimilable organic carbon
CFU
colony-forming unit
EPA
Environmental Protection Agency (U.S.)
GUDI
groundwater under the direct influence of surface water
HPC
heterotrophic plate count
MAC
maximum acceptable concentration
MCL
maximum contaminant level (U.S.)
MCLG
maximum contaminant level goal (U.S.)
MF
membrane filter
MPN
most probable number
MTF
multiple tube fermentation
NSF
NSF International
P-A
presence-absence
QA
quality assurance
QC
quality control
UV
ultraviolet

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