When water test results are reported as <10 total coliforms per 100mL of water or <1 total coliforms per 100 mL of water, this is based on the sensitivity of the testing method and the dilution rate used in the accredited lab. The lab can only detect results to <10 or <1 total coliforms per 100mL of water; they cannot give a more specific reading. This essentially means that no organisms were found in the test but there is a possibility that if a larger sample were used (1000 mL vs. 100 mL of water) they may have found something.

Results reported this way are acceptable and the water is considered potable.

If an accredited lab uses the method where they can actually give a number under 10 (e.g., 7 total coliforms), then results above 0 are not acceptable and the water is not potable based on the microbiological parameters of Health Canada’s Guidelines for Canadian Drinking Water Quality.

The lab must report total coliforms using one of these two options (i.e., either an exact number such as 5, OR a “less than” count). It is not acceptable for the lab to report the results simply as “acceptable” or “negative”, since this prevents comparing the value to the Guidelines for Canadian Drinking Water Quality.

November 3rd, 2017 at 04:25 pm