This update provides clarification on how the Ministry interprets section 18 (1) and (4) under the Safe Drinking Water Act, 2002 and Schedules 13 & 16 of O. Reg. 170/03. Licensed/eligible testing laboratories and drinking water system owners/operating authorities are required to calculate and report results to the appropriate authorities.

About this document

This document provides information and rules for owners and operating authorities of municipal residential drinking water systems, non-municipal year-round residential drinking water systems and laboratories.

Please note the following document is rescinded:

  • A newsletter for OWWA, OMWA and Drinking Water Systems Owners/Operators – Issue #3
  • Revised THM Calculation Procedure Relating to Re-samples – Memo to Licensed/Eligible Laboratories – January 21, 2011
  • Total Trihalomethane (TTHM) Reporting Requirements UPDATE Bulletin – December 2015

Preamble

To help ensure safe drinking water in Ontario, this update provides clarification on how the ministry interprets certain sections of the act and regulations, so that all drinking water system owners/operating authorities and laboratories have a common understanding of:

  • The requirements when calculating the running annual average (RAA)
  • When and to whom to report exceedances
  • New testing requirements for haloacetic acids (HAAs), and
  • Clarification to trihalomethane (THM) reduced sampling requirements

The Drinking Water Systems Regulation (O. Reg. 170/03) and the Ontario Drinking Water Quality Standards (O. Reg.169/03) made under the Safe Drinking Water Act, 2002 (SDWA) came into effect in 2003. O. Reg. 169/03 states that the standard for THM is 0.100 mg/L (100 µg/L) and this standard is expressed as an RAA.

O. Reg. 170/03 was amended in 2015 and requires drinking water systems to change the method of calculating the RAA. As of January 1, 2016, the RAA is assessed using calculations outlined in subsection 13-6 of Schedule 13 of O. Reg. 170/03.

As of January 1, 2017, drinking water system owners/operating authorities are required to take samples quarterly and have them tested for HAAs as outlined in O. Reg. 170/03 (subsection 13-6.1 of Schedule 13).

Effective January 1, 2020, a standard for HAAs will be introduced. The standard will be 0.08 mg/L (80 µg/L) and will be expressed as an RAA.

Source law

Provincial standards for water quality are set out in:

Introduction

Schedules 13-6 and 13-6.1 of O. Reg. 170/03 require owners and operating authorities of municipal residential drinking water systems and non-municipal year-round residential drinking water systems that provide chlorination or chloramination to take samples from a location in their system that is likely to have an elevated potential for the formation of THMs and HAAs and ensure that samples are tested for these parameters.

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

Licensed laboratories are also responsible for calculating the RAA and reporting an adverse test result by written notice within 7 calendar days of the completion of the quarter that produced the adverse result for THMs, if the laboratory does not meet the calculation exemption requirements as outlined in Schedule 16-6(3.2) of O. Reg. 170/03.

Note: The O. Reg. 169/03 standard for HAA does not come into effect until January 1, 2020.

Definition

THMs are defined as the sum of chloroform, bromoform, dichlorobromomethane, and dibromochloromethane.

According to O. Reg.169/03, the standard for THM is expressed as a RAA, where RAA is defined as “the running annual average of quarterly results” for THM for a drinking water system.

HAAs are defined as the sum of monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid and dibromoacetic acid.

On January 1, 2020, the O. Reg. 169/03 standard for HAA (80 μg/L) will come into effect and be expressed as a RAA, where RAA is defined as “the running annual average of quarterly results” for HAA for a drinking water system.

O. Reg. 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
Quarter Date Range
First January 1st to March 31st
Second April 1st to June 30th
Third July 1st to September 30th
Fourth October 1st to December 31st

Calculation

Drinking water system owners/operating authorities and laboratories are responsible for calculating the RAA for THMs. Owners/operating authorities and laboratories must ensure that the data used to produce the RAA calculation is available if requested.

Since the O. Reg. 169/03 standard is based on the RAA, all THM 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).

Although not required until January 1, 2020, system owners/operating authorities are encouraged to calculate the RAA for HAAs to determine if there are elevated levels in the drinking water system. (HAAs may form if humic acids are present and tend to decline over time within the distribution system.)

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, and

“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 , and

“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:
Quarter Sample 1 Result μg/L Sample 2 Result μg/L Sample 3 Result μg/L Sample 4 Result μg/L Quarterly Average μg/L
"D" April 1st – June 30th, 2016 82.3 65.2 87.6 102.5 84.4
"C" July 1st – September 30rd, 2016 92.6 72.3 NA NA 82.5
"B" October 1st – December 31st, 2016 108.5 NA NA NA 108.5
"A" January 1st – March 31st, 2017 92.3 75.7 101.3 NA 89.8
RAA (#1) – calculated April 2nd, 2017         91.3

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 THM, 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 proposed HAAs standard of 80 μg/L and would be reportable after January 1st, 2020.
Example 1b:
Quarter Sample 1 Result μg/L Sample 2 Result μg/L Sample 3 Result μg/L Sample 4 Result μg/L Quarterly Average μg/L
April 1st – June 30th, 2017 140.4 152.8 97.8 99.6 122.7
RAA (#1) – calculated July 3rd, 2017         100.9
  • In this example, (RAA #1), the RAA calculation is (89.8 + 108.5 + 82.5 + 84.4)/4 = 91.3 μg/L.
  • Extending the example for another quarter, at the end of the next quarter (April 1 – June 30, 2017), 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, 2017) and the previous 3 quarters average.
  • The RAA (#2) calculation is (122.7 + 89.8 + 108.5 + 82.5)/4 = 100.9 μg/L

The THM RAA is adverse 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:
Quarter Sample 1 Result μg/L Sample 2 Result μg/L Sample 3 Result μg/L Sample 4 Result μg/L Quarterly Average μg/L
"D" October 1st – December 31st, 2016 20.3 NA NA NA 20.3
"C" January 1st – March 31st, 2017
CLOSED
NA NA  
"B" April 1st – June 30th, 2017 42.1 45.7 NA NA 43.9
"A" July 1st – September 30th, 2017 36.5 NA NA NA 36.5
RAA (#1) – calculated October 6th, 2017         33.6

NA – Sample not taken

  • In the example above, the drinking water system is closed down each year from Jan 1st -Mar 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:
Quarter Sample 1 Result μg/L Sample 2 Result μg/L Sample 3 Result μg/L Sample 4 Result μg/L Quarterly Average μg/L
October 1st – December 31st, 2017 No sample taken NA NA NA NA
RAA (#2) – Calculated January 6th, 2018         33.6

Notes:

  • In this example, the required sample was not taken in the October 1 – December 31, 2017 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

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 standard of 100 μg/L, reporting to Spills Action Centre and the local Medical Officer of Health is required via written notice within 7 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 also must report to the operator of each designated facility served by the system.

The O.Reg. 169/03 standard (80 μg/L) and the O.Reg. 170/03 reporting requirements for HAAs come into effect on January 1, 2020, as such drinking water system owners/operating authorities are not required to perform the RAA calculation for HAAss until 2020. Owners/operating authorities may wish to perform the calculation to determine if there are elevated levels of HAAs in the drinking water 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 days of resolving the issue.
  • Send the notice to the interested authority for any designated facility (if applicable) within 30 days.

Exemption

Subsection 16-6(3.2) of Schedule 16 under O.Reg. 170/03 provides an exemption to licensed laboratories from reporting any drinking water system’s RAA if the lab uploads the THM (and HAA effective January 1, 2020) test results from the drinking water system 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.

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

Small municipal residential and non-municipal year-round residential drinking water systems may qualify for reduced THM sampling if requirements of O.Reg. 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 8 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, and
  4. The ministry has not provided written direction to sample each calendar quarter.

After 8 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 8 consecutive calendar quarters with no testing.

Note: When systems resume sampling after 8 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).

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