Significant drinking water threat assessment

This guidance was written to provide information on Environmental Activity and Sector Registry (EASR) requirements. It should not be construed as legal advice. While all efforts are made to ensure the accuracy of the requirements summarized below, if there is any discrepancy between this summary and the acts or regulations, the provisions of the acts and regulations take precedence.

Purpose

Ontario Regulation 137/25: Registrations under part II.2 of the Act - Storm water management works requires that a significant drinking water threat (SDWT) assessment be completed for storm water management works. For works that are being registered in the EASR, the assessment must be completed by a licensed engineering practitioner (LEP) as part of the information that is required to be included in the storm water management (SWM) report. In some cases, the works may need to be re-assessed after registration and in these cases, the assessment may be completed by a person engaging in the activity. This document is intended to provide guidance on how to complete a significant drinking water threat assessment for the purposes of Ontario Regulation 137/25.

The Clean Water Act, 2006, protects Ontario’s sources of drinking water as an overall commitment to safeguard human health and the environment. Under the Act, communities across the province protect their sources of drinking water from prescribed drinking water threats, such as storm water management works, through prevention and mitigation. This includes implementing collaborative, watershed-based source protection plans that are locally driven and science-based. Source protection plans apply within 38 source protection areas across Ontario, covering 95% of Ontario’s population.

Learn more about Ontario’s source protection framework.

Ontario Regulation 137/25 prescribes certain storm water management works for registration in the EASR and sets out the requirements that must be followed to register and engage in the activity. Ontario Regulation 137/25 includes provisions that relate to the potential for a storm water management works to be a significant drinking water threat. These include requirements related to:

• completing a significant drinking water threat assessment
• providing notification to a source protection authority
• preparing and implementing spill contingency plans

See Storm water management works guidance for the Environmental Activity and Sector Registry for more information about activity requirements that apply to storm water management works that are subject to registration.

Vulnerable areas

The source protection plans include maps that identify vulnerable areas around municipal drinking water sources (such as wells and intakes) where activities (such as the operation of storm water management works) may pose a risk of contamination and be a significant drinking water threat.

Storm water management works may be significant drinking water threats in these vulnerable areas:

• surface water intake protection zones (IPZs)
• wellhead protection areas (WHPAs)

Some source protection plans, as part of delineating IPZs and WHPAs, also delineate these specific types of IPZs and WHPAs where storm water management works may be significant drinking water threats:

• issue contributing areas, which are a type of IPZ or WHPA (also known as WHPA-ICAs or IPZ-ICAs), related to chemicals or pathogens like nitrogen or e-coli
• event-based areas, which are a part of IPZs, addressing spill risks in intake protection zones for surface water intakes in large water bodies, related to specific drinking water threats 

Drinking water threats

Ontario Regulation 287/07 (General) under the Clean Water Act, 2006, lists 22 drinking water threats, which are activities that can contaminate or deplete a drinking water source. These activities include the establishment, operation or maintenance of a system that collects, stores, transmits, treats or disposes of sewage (such as storm water management works). The establishment, operation or maintenance of works that discharge storm water to the natural environment through an outfall or infiltration can be assessed as either a low, moderate or significant drinking water threat activity depending on circumstances outlined in Part XII - Tables of drinking water quality threats in the Technical Rules.

Ontario Regulation 137/25 and significant drinking water threats

The section below summarizes some of the ways Ontario Regulation 137/25 addresses the potential for a storm water management works to be a significant drinking water threat (SDWT).

SWM report requirements

If the storm water management works are assessed to be a SDWT, the SWM report must describe how the activity is managed to sufficiently reduce the likelihood that the activity will contaminate the drinking water source. Considerations included in the report are expected to achieve the objective of ensuring that the activity ceases to be or never becomes a SDWT. This description may refer to any requirements or considerations included in the following sections of the SWM report that are expected to achieve this objective:

• design
• operation and maintenance manual
• assessment of monitoring
• erosion and sediment control plan

In addition, if your activities involve a new SWM works and the activities would be a significant drinking water threat, the water quality design criteria that are set out in the design section of the SWM report must include achieving a minimum of enhanced protection, as described in the Stormwater Management Planning and Design Manual, 2003.

Significant drinking water threat requirements

Before you assess your SWM works, confirm that the relevant source protection plan does not include a policy that has the effect of prohibiting the establishment of new SWM works at the proposed location.

Below are the steps to follow to check for a source protection plan policy:

1. Find the source protection plan on Source Protection Plans for the source protection area where the new SWM works is proposed.
2. Navigate to the appendix of the relevant source protection plan and find List C: Significant threat policies that affect prescribed instrument decisions.
3. Review each policy in the list to identify which policies apply to environmental compliance approvals for storm water management works.
4. Determine whether any of those policies would have the effect of prohibiting the establishment of storm water management works. For example, storm water management facilities and drainage systems that would be a significant drinking water threat.

If you are proposing to establish a new SWM works, and the plan includes a policy under List C that prohibits new SWM works that are a SDWT, you must continue to assess if your works are a SDWT. If the threat assessment result is that your SWM works is a SDWT, the SWM works activity cannot be established at the proposed location and cannot be registered in the EASR.

If you require support in determining which plan policies apply to your activity or if a prohibition policy applies, please contact the local source protection authority.

Notification requirements

If the assessment indicates your works are or would be a SDWT and there is no policy that prohibits new SWM works that are a SDWT, you must:

1. Notify in writing the source protection authority for the area where your works are located before engaging in your activities (or as soon as reasonably possible if you’re already engaged). Include:
   • a description of your SWM works
   • a statement that the SWM works is or would be a significant drinking water threat
   • your EASR registration confirmation number for your SWM works activity
2. Ensure the information entered in the Registry regarding your works reflects the results of this assessment. This may include updating the information in the registry for SWM works that have already been registered.
3. Prepare, maintain and implement a spill contingency plan, and make available an inventory of response and clean-up equipment.

Re-assessment

If your SWM works are already registered in the EASR and were originally assessed as not being a SDWT, you may receive notice from the local source protection authority about a source protection plan amendment or update. If the amendment or update could result in your works becoming a significant threat, you must re-assess your SWM works once the plan amendment or update is approved and in-effect. If your works are re-assessed to be a SDWT, follow the requirements in Ontario Regulation 137/25 for works that are a SDWT.

You must update information filed in the registry within 30 days if you know it’s no longer complete or accurate. The SWM report, prepared by a Licensed Engineering Practitioner (LEP), must be filed in the registry and indicate whether your SWM works are a SDWT. If a re-assessment reveals that only this threat-related information has changed, no update to the SWM report is required unless other information also becomes incomplete or inaccurate.

The re-assessment can be completed as an addendum to the SWM report by a LEP, or you can re-assess the threat yourself. In either case, the threat assessment must be done by following the Technical Rules that is, Part XII - Tables of drinking water quality threats , in effect at the time of the assessment.

How to assess the SWM works

Storm water management facilities are a type of storm water management works. Storm water management facility is defined in Ontario Regulation 525/98 as a facility for the treatment, retention, infiltration or control of storm water.

The following steps explain how to use the ministry’s Source Protection Information Atlas (SPIA) and the Threats Tool to assess whether SWM works are a SDWT. The threat assessment must be done by following Part XII - Tables of drinking water quality threats under the Technical Rules that  in effect at the time of the assessment. 

Currently, the only SWM works can be a SDWT involve either:

• a storm water management facility outfall
• a storm water drainage system outfall
• a storm water infiltration facility

Refer to the Technical Rules for the definition of these terms.

For the purposes of this guidance:

• a storm water infiltration facility is referred to as an infiltration component
• storm water management facility outfall and a storm water drainage system outfall are referred to as outfall components

• if yes, continue to Step 2
• if no, the SWM works activity would not be a SDWT activity and the assessment is complete

This can be done by mapping the outfall or the infiltration component, to determine if it overlaps with any of the following:

• a wellhead protection area (WHPA) or intake protection zone (IPZ) with a vulnerability score of 8 or higher
• an issue contributing area (WHPA-ICA or IPZ-ICA) associated with a chemical parameter, for example, chloride, sodium, nitrogen, phosphorus, or a pathogen parameter, for example, Escherichia coli/total coliform or both
• an event based area (EBA) associated with storm water management works activities and a chemical parameter; for example, chloride, sodium, nitrogen, phosphorus, or a pathogen parameter (i.e., Escherichia coli, total coliform, or both) 

Note: if your SWM works activity overlaps with an IPZ-ICA or WHPA-ICA, you must consider the chemical and pathogen parameters associated with those areas. For ICAs, if the facility may contribute any parameter (for example, sodium from road salt) that is also associated with the ICA, the SWM works activity is automatically a SDWT. In addition, if the SWM works activity is in an EBA for SWM works, then the activity is automatically a significant threat, provided the minimum spill volume threshold is met. At the time this guidance was written, there were no EBAs identified in source protection plans for SWM works.

• if no to all, the SWM works activity would not be a SDWT activity and the assessment is complete
• if yes, to any of the above, continue to step 3

• if no, continue to Step 4
• if yes, identify:
  1. The types of infiltration component. For example, low impact development (LID), infiltration gallery, swale, or wet or dry pond.
  2. The GPS coordinates for the infiltration component.
• is the infiltration component located in:
  1. A wellhead protection area (WHPA-A or B) with a vulnerability score of 10, WHPA-E with a vulnerability score of 9 or higher, or intake protection zone (IPZ-1, -2 or -3) with a vulnerability score of 9 or higher?
  2. An issue contributing area (WHPA-ICA or IPZ-ICA) for a chemical parameter, pathogen parameter, or both? If so, also identify all chemical and pathogen parameters associated with the ICA.
  3. An event based area (EBA) for storm water management works activities for a chemical parameter, pathogen parameter, or both? If so, also identify all chemical and pathogen parameters associated with the EBA.
• If yes, the infiltration component is located in one of the vulnerable areas listed above (i, ii, and/or iii), determine whether the activity is a SDWT

To do so:

• review Part XII - Tables of drinking water quality threats of the Technical Rules
• find table “2.4 Storm water management facilities and drainage systems: Storm water infiltration facility”
• you need to consider the circumstances and areas set out in the columns titled Circumstances and Areas of SDWT for each circumstance type and number under threat number 2.4

What is the predominant land use of the area that is serviced by your storm water management works? For example, agricultural, parkland, outdoor recreational, commercial, residential, institutional, community use or rural that would be served by the stormwater infiltration facility.

What is the sum of the impervious areas draining to the storm water infiltration facility at the site in meters squared (m²)? Include roads, sidewalks, parking surfaces, aisles, and driveways in the calculation, exclude roofs.

Document the rationale for why the infiltration facility activities are or are not a SDWT, taking into consideration the vulnerability scores associated with the location of the infiltration facility and the circumstances from threat 2.4 under the Part XII - Tables of drinking water quality threats.

• If yes, your infiltration activity is a SDWT

Does the source protection plan for the source protection area in which the activity is located contain policies in List C of the appendix of the plan that prohibit the establishment of new storm water management works? See significant drinking water threat requirements.

• If yes, the proposed activity is not prescribed for the EASR and cannot register.
• If no, document the relevant source protection plan and policy (code or number) of the policy from List C that would otherwise apply to this activity if it were not subject to the EASR

You will need to enter information about the SDWT assessment in the Registry when you register your activities or update information about your activities.

Are the outfalls located in:

1. a wellhead protection area (WHPA-A or -B) with a vulnerability score of 10, WHPA-E with a vulnerability score of 8 or higher, or intake protection zone (IPZ-1, -2 or -3) with a vulnerability score of 8 or higher?
2. an issue contributing area (WHPA-ICA or IPZ-ICA) for a chemical parameter, pathogen parameter, or both? If so, also identify all chemical and pathogen parameters associated with the ICA.
3. an event-based area (EBA) for storm water management works activities for a chemical parameter, pathogen parameter, or both? If so, also identify all chemical and pathogen parameters associated with the EBA.

• If yes, determine whether the activity is a SDWT

To do so, review threat 2.3 Storm water management facilities and drainage systems: Outfall from a storm water management facility or storm water drainage system in Part XII - Tables of drinking water quality threats of the Technical Rules. You will need to consider the circumstances and areas set out in the columns labelled Circumstances, and Areas of SDWT, for each Circumstance type and number under threat 2.3. This will include determining the following information regarding circumstances:

• What is the predominant land use of the area that is serviced by your storm water management works? For example, agricultural, parkland, outdoor recreational, greenhouses, industrial, commercial, residential, institutional, community use or rural.
• What is the percentage of the impervious areas of the total drainage area to the outfalls. Include roads, sidewalks, parking surfaces, aisles, and driveways in the calculation, exclude roofs.

Note that for infiltration facility activities, the sum of impervious areas is a relevant circumstance, but for outfall activities, the percentage of impervious areas is a relevant circumstance.

Document the rationale for why the outfall activities are or are not a SDWT, taking into consideration the vulnerability scores associated with the location of the outfalls and the circumstances from the threat 2.3 under the Tables for Drinking Water Threats.

• If yes, your outfall activity is a SDWT, and you must confirm that the source protection plan for the source protection area in which the activity is located does not contain policies in List C of the appendix of the plan that prohibit the establishment of new storm water management works that are significant drinking water threat.
• If yes, the proposed activity is not prescribed for the EASR and cannot register.
• If no, document the relevant source protection plan and policy code or number of the policy from List C that would otherwise apply to this activity if it were not subject to the EASR.

You will need to enter information about the SDWT assessment in the Registry when you register your activities or update information about your activities.

See storm water management works guidance for Environmental Activity and Sector Registry for information about the requirements for activities that are a SDWT.