Preamble

This document clarifies how the ministry interprets section 18 (1) and (4) under the Safe Drinking Water Act, 2002 and Schedules 13 and 16 of Ontario Regulation 170/03 so that all drinking water system owners/operating authorities and laboratories have a common understanding of:

  • Testing requirements for trihalomethanes and haloacetic acids
  • Reduced sampling requirements
  • The requirements when calculating the running annual average
  • When and to whom to report exceedances

Source law

Provincial standards for water quality are set out in:

Introduction

Trihalomethanes (THMs) and haloacetic acids (HAAs) are disinfection by-products (DBPs) that are formed when dissolved organic matter reacts with chlorine in drinking water. THMs are the sum of chloroform, bromoform, dichlorobromomethane, and dibromochloromethane. HAAs are the sum of monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid and dibromoacetic acid.

Schedules 13-6 and 13-6.1 of Ontario Regulation 170/03 require owners and operating authorities of municipal residential and non-municipal year-round residential drinking water systems that provide chlorination or chloramination to sample for THMs and HAAs. They must take at least one sample, for both THMs and HAAs, in each calendar quarter, from a location in their distribution system that is likely to have an elevated potential for the formation of these disinfection by-products.

Ontario Regulation 170/03 defines the calendar quarter to mean, in relation to a year, the three-month period that begins on January 1, April 1, July 1 or October 1:

Definition of quarters
QuarterDate range
FirstJanuary 1st to March 31st
SecondApril 1st to June 30th
ThirdJuly 1st to September 30th
FourthOctober 1st to December 31st

Calculation

According to Ontario Regulation 169/03, the standard for THMs and HAAs is expressed as a running annual average (RAA), where RAA is defined as the running annual average of quarterly results for THMs or HAAs for a drinking water system.

Drinking water system owners/operating authorities and laboratories are responsible for calculating the RAA.

Since the Ontario Regulation 169/03 standard is based on the RAA, all results from samples taken and tested during the calendar quarter must be included in the RAA quarterly calculation (including resamples required by the local health unit).

The RAA for both parameters shall be calculated after each calendar quarter by using the following formula:

[A + B + C + D] ÷ 4

where:

  • "A" is the average of all the results from samples tested in that calendar quarter
  • "B" is the average of all the results from samples tested in the most recent calendar quarter preceding the calendar quarter referred to in A in which testing was carried out
  • "C" is the average of all the results from samples tested in the most recent calendar quarter preceding the calendar quarter referred to in B in which testing was carried out
  • "D" is the average of all the results from samples tested in the most recent calendar quarter preceding the calendar quarter referred to in “C” in which testing was carried out

Note: Under exceptional circumstances, there may be no result in a quarter that a system is open. In this situation, the average of all results for the same parameter obtained in the same quarter in the previous year must be used to calculate the RAA. This will ensure the calculation maintains the seasonal variation in the RAA.

Example calculations

Example 1a:
QuarterSample 1 result μg/LSample 2 result μg/LSample 3 result μg/LSample 4 result μg/LQuarterly average μg/L
"D" April 1st – June 30th, 202282.365.287.6102.584.4
"C" July 1st – September 30rd, 202292.672.3NANA82.5
"B" October 1st – December 31st, 2022108.5NANANA108.5
"A" January 1st – March 31st, 202392.375.7101.3NA89.8
RAA (#1) – calculated April 2nd, 2023000091.3

NA – Sample not taken

  • In this example, (RAA #1), the RAA calculation is (89.8 + 108.5 + 82.5 + 84.4)/4 = 91.3 μg/L
  • If the samples analysed were for THMs, this result would not be above the THM standard and would not be reported as adverse
  • If the samples analysed were for HAAs, this result would be above the HAA standard of 80 μg/L and is reportable
Example 1b:
QuarterSample 1 result μg/LSample 2 result μg/LSample 3 result μg/LSample 4 result μg/LQuarterly average μg/L
April 1st – June 30th, 2023140.4152.897.899.6122.7
RAA (#1) – calculated July 3rd, 20230000100.9
  • Extending the example for another quarter, at the end of the next quarter (April 1 – June 30, 2023), the results for the quarter are averaged ((140.4 + 152.8 + 97.8 + 99.6)/4) and a result of 122.7 μg/L is obtained
  • The RAA (#2) is calculated using the current quarter average (April 1 to June 30, 2023) and the previous 3 quarters average
  • The RAA (#2) calculation is (122.7 + 89.8 + 108.5 + 82.5)/4 = 100.9 μg/L

In this example, the RAA is adverse as it is above the standard for THMs and HAAs and reporting is required (see ‘Reporting’ section below).

Seasonal systems example calculation

Seasonal drinking water systems are not expected to collect samples during those quarters where the system is closed for the complete quarter. In this case, all quarterly results should be summed and divided by the number of quarters the system was operating.

Example 2a:
QuarterSample 1 result μg/LSample 2 result μg/LSample 3 result μg/LSample 4 result μg/LQuarterly average μg/L
"D" October 1st – December 31st, 202220.3NANANA20.3
"C" January 1st – March 31st, 2023
Closed
NANA0
"B" April 1st – June 30th, 202342.145.7NANA43.9
"A" July 1st – September 30th, 202336.5NANANA36.5
RAA (#1) – calculated October 6th, 2023000033.6

NA – Sample not taken

  • In the example above, the drinking water system is closed each year from January 1st - March 31st)
  • The RAA (#1) calculation is (36.5 + 43.9 + 20.3)/3 = 33.6 μg/L
  • This RAA result is below the standard and is not reported as adverse
Example 2b:
QuarterSample 1 result μg/LSample 2 result μg/LSample 3 result μg/LSample 4 result μg/LQuarterly average μg/L
October 1st – December 31st, 2022Required sample not takenNANANANA
RAA (#2) – Calculated January 6th, 2023000033.6

Notes:

  • In this example, the required sample was not taken in the October 1 – December 31, 2022 quarter
  • The quarterly average of 20.3 μg/L from the same quarter from the previous year is used to calculate the RAA #2
  • The RAA (#2) calculation is (36.5 + 43.9 + 20.3)/3 = 33.6 μg/L
  • This RAA result is below the standard and is not reported as adverse

Reporting

In accordance with Schedule 16-6 and 16-7 of Ontario Regulation 170/03, the owner or operating authority of the drinking water system is responsible for calculating the RAA and reporting an adverse test result by written notice (using the Notice of Adverse Test Results and Issue Resolution (Schedule 16) form) within seven calendar days of the completion of the quarter that produced the adverse test result.

Licensed laboratories are also responsible for calculating the RAA and reporting an adverse test result by written notice within seven calendar days of the completion of the quarter that produced the adverse result, if the laboratory does not meet the calculation exemption requirements as outlined in Schedule 16-6 (3.2) of Ontario Regulation 170/03.

This subsection provides an exemption to licensed laboratories from reporting any drinking water system’s RAA if the lab uploads the test results to the ministry’s Drinking Water Information System (DWIS) and provides the results to the clients within 48 hours of the test results being authorized at the lab.

At the end of each calendar quarter, laboratories and drinking water system owners/operating authorities must calculate the RAA using all results received to date in that quarter. If the resulting RAA exceeds the Ontario Drinking Water Quality Standard, reporting to Spills Action Centre and the local Medical Officer of Health is required via written notice within seven calendar days of the completion of the quarter that produced the test result using the calculation outlined in schedule 13-6. Laboratories and drinking water system owners/operating authorities must complete and submit the Notice of Adverse Test Results and Issue Resolution (Schedule 16) form.

Note: Small municipal residential systems and non-municipal year-round residential systems that serve designated facilities must also report the adverse result to the operator of each designated facility served by the system.

Once the issue that gave rise to the adverse test result is resolved, drinking water system owners/operating authorities must complete and submit the Issue Resolution Section 2(C) of the Notice of Adverse Test Results and Issue Resolution (Schedule 16) form.

The follow-up written notice must summarize the action taken and the results achieved.

  • Send the notice to the local medical officer of health, and the ministry’s Spills Action Centre within seven calendar days of resolving the issue
  • Send the notice to the interested authority for any designated facility (if applicable) within 30 days

Corrective actions

Although the regulation does not require any resampling for adverse THM or HAAs results, drinking water system owners and operating authorities must follow any corrective actions required by the local Medical Officer of Health.

Reduced sampling for THMs/HAAs

Small municipal residential and non-municipal year-round residential drinking water systems may qualify for reduced THM sampling if requirements of Ontario Regulation 170/03 (subsection 13-6 (4) of Schedule 13) are met.

Specifically, if these drinking water systems have taken samples for at least 12 consecutive quarters, they may cease to sample for eight consecutive calendar quarters provided:

  1. No test result in the previous 12 consecutive calendar quarters indicated that the concentration of THMs was greater than 0.050 mg/L (50 μg/L)
  2. The drinking water system’s raw water supply has not changed
  3. No alterations have been made to the drinking water system
  4. The ministry has not provided written direction to sample each calendar quarter

After eight consecutive calendar quarters with no sampling, the drinking water system must resume sampling and testing for THMs for four consecutive calendar quarters. At this point, provided the criteria listed above continue to be met, the system may continue to rotate between four consecutive calendar quarters of testing and eight consecutive calendar quarters with no testing.

Note: When systems resume sampling after eight consecutive calendar quarters with no testing, the results from the most recent three calendar quarters in which testing was carried out and the current calendar quarter will be used to determine the HAAs (see subsection 13-6 (5) of Schedule 13).

Contact us

If you have additional questions or would like more information about drinking water, contact the ministry’s drinking water hotline: