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. In September 2026, amendments to Ontario Regulation 137/25 came into effect and the regulation now also prescribes storm water management works servicing electricity generation, transmission, distribution and battery energy storage sites described by NAICS code 2211, for registration in the EASR. For all 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 (SDWT). 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

Refer to the 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 SDWT.

Storm water management works may be SDWTs 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 SDWTs:

• 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 SDWTs

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 SDWTs 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 SDWT.

SWM report requirements

If the storm water management works are assessed to be a SDWT, the SWM report must describe the considerations that are included in the report that are intended to achieve the objective of ensuring that the activity ceases to be or never becomes a SDWT. Basically , the LEP should be describing how the requirements set out in the SWM report, which will be implemented by the proponent, will manage the activity (the establishment and operation of storm water management works) in a way that sufficiently reduces the likelihood that the activity will contaminate the drinking water source. 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 storm water management works and the activities would be a SDWT, 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. However, the requirement to achieve a minimum of enhanced protection does not apply to a spill containment works or underdrain system.

Significant drinking water threat requirements polices

Before the establishment of a new storm water management works that is assessed to be a SDWT can be registered on the EASR, it must first be confirmed that the relevant source protection plan does not include a policy that has the effect of prohibiting the establishment of new  storm water management works at the proposed location.

Below are the steps to follow to check for whether this type of source protection plan policy affects a storm water management works:

1. Find the source protection plan on Source Protection Plans for the source protection area where the new storm water management works is proposed.
2. Review List C in the appendix, which refers to Significant threat policies that affect prescribed instrument decisions
3. Determine whether any of those policies would have the effect of prohibiting the establishment of new storm water management works. For example, a policy may prohibit the establishment of new storm water management facilities and drainage systems where those facilities or systems would be a SDWT.

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 .

If the plan does contain a prohibition policy, the new storm water management works that would be a SDWT cannot be registered on the EASR (and an environmental compliance approval cannot be obtained for the storm water management works either). However, if for example the new storm water management works were to be established in a different location where the activity would not be a SDWT, the policy would not prohibit the establishment of that storm water management works.

Notification requirements

If the assessment indicates your works are or would be a SDWT and there is no policy that prohibits new storm water management 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 storm water management works
   • a statement that the storm water management works is or would be a SDWT
   • your EASR registration confirmation number for your storm water management 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 storm water management 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 storm water management 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 storm water management 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 is 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  storm water management 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 storm water management works

Storm water management facilities are a type of storm water management works. A 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 storm water management 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 are in effect at the time of the assessment.

Currently, the only storm water management works that can be a SDWT involve one of the following:

• 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 your storm water management works does not include one of these components, it is not a SDWT, and would not need to be assessed further.

• 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 (such as, Escherichia coli, total coliform, or both)

Note: If any part of your storm water management works activity (outfall or the infiltration component) overlaps with an IPZ-ICA or WHPA-ICA, the storm water management works activity is automatically considered a SDWT.

In addition, if the storm water management works activity is in an EBA for storm water management 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 storm water management works.

For more details about the IPZ-ICAs, or WHPA-ICAs or EBAs and their associated parameters, refer to the sSource pProtection pPlans and or contact the local source protection authority.

• if no to all, the storm water management 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.
• determine whether the infiltration component, or part of it, is 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 the infiltration component, or part of it, is located in one of the vulnerable areas listed above (ii.a, ii.b, and/or ii.c), determine whether the activity is a SDWT.

To complete iii. and assess whether the activity involving an infiltration component is a SDWT:

• determine the predominant land use of the area serviced by the stormwater management worksinfiltration component by first identifying all land uses within the drainage area (such as agricultural, parkland, outdoor recreational, commercial, residential, institutional, community usee, or rural). Then compare these land uses to determine which one occupies the largest portion of the drainage area. The land use with the greatest coverage is considered the predominant land use. For example, if the area serviced by the works includes the following land uses:

Example: if the area serviced by the works is composed of the following land uses:

• Agricultural land: 55%
• Residential land: 30%
• Commercial land: 10%
• Institutional: 5%

Agricultural land would be considered the predominant land use because it occupies the largest portion of the drainage area, even if another land use (such as commercial) may present a higher drinking water risk.

• determine the sum of the impervious areas within the total drainage area 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. For more details, please contact the local source protection authority
• review Part XII - Tables of drinking water quality threats of the Technical Rules and reference 2.4 Storm water management facilities and drainage systems: Storm water infiltration facility
• for each of the 10 circumstance types (for example, chemical C2.4.1 to pathogen P2.4.1) listed in rows under the column circumstance type and number, you need to consider each of the 2 circumstances under the corresponding column circumstances”, and the vulnerable areas under the corresponding column areas of SDWT. An infiltration component is or would be a SDWT if all of the circumstances corresponding to a particular circumstance type and number apply to it and it is or will be located in the type of WHPA or IPZ specified in the areas of SDWT column corresponding to the same particular circumstance type and number

Document the results of the assessment and the rationale, such as including the calculations of the sum of the impervious areas,  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 under threat 2.4 under the Part XII - Tables of drinking water quality threats.

The assessment of whether the infiltration component is a SDWT is complete. if the SWM works activity would not be a SDWT activity, then the assessment is complete.

Please note that if you are proposing to establish a new infiltration component that would be a SDWT based on the assessment above, you will also need to confirm whether the source protection plan for the source protection area in which the activity is located contains policies in list C of the appendix of the plan that prohibit the establishment of new storm water management works. Refer to the significant drinking water threat requirements.

• if yes, the proposed activity to establish a new infiltration component (storm water management works) 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. Generally, this will be a policy that requires the director issuing an ECA for the storm water management works to ensure that their decision to issue or amend an ECA is designed to achieve the objectives set out in paragraph 2 of subsection 22 (2) of the Act ( for example, ensure that the activity ceases to be or never becomes a SDWT)

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 to any of the above (i, ii, and/or iii), determine whether the activity is a SDWT

To assess whether the activity involving an outfall component is a SDWT:

• determine the predominant land use of the area serviced by the outfall component by first identifying all land uses within the drainage area (such as agricultural, parkland, outdoor recreational, commercial, residential, institutional, community use, or rural). Then compare these land uses to determine which one occupies the largest portion of the drainage area. The land use with the greatest coverage is considered the predominant land use.

Example: if the area serviced by the works is composed of the following land uses:

• Agricultural land: 55%
• Residential land: 30%
• Commercial land: 10%
• Institutional: 5%

Agricultural land would be considered the predominant land use because it occupies the largest portion of the drainage area, even if another land use (such as commercial) may present a higher drinking water risk.

• determine the percentage of the impervious areas of the total drainage area serviced by the outfalls. Include roads, sidewalks, parking surfaces, aisles and driveways in the calculation, exclude roofs
  • the percentage of impervious areas may be calculated as the total impervious surface areas within the drainage area divided by the total drainage area that contributes storm water to the storm water management worksoutfall component. For more details, please contact the local source protection authority
• review Part XII – Tables of drinking water quality threats of the Technical Rules and reference table 2.3 Storm water management facilities and drainage systems: Outfall from a storm water management facility or storm water drainage system
• for each of the 10 circumstance types (for example, chemical C2.3.1 to pathogen P2.3.1) listed in rows under the column circumstance type and number, you need to consider each of the 2 circumstances under the corresponding column circumstances and the vulnerable areas under the corresponding column areas of SDWT. An outfall component is or would be a SDWT if all of the circumstances corresponding to a particular circumstance type and number apply to it and it is or will be located in the type of WHPA or IPZ specified in the areas of SDWT column corresponding to the same particular circumstance type and number

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.

The assessment of whether the outfall component is a SDWT is complete.

Please note that if you are proposing to establish a new outfall component that would be a SDWT based on the assessment above, you will also need to confirm whether the source protection plan for the source protection area in which the activity is located contains policies in list C of the appendix of the plan that prohibit the establishment of new storm water management works. Refer to the SDWT requirements.

• if yes, the proposed activity to establish a new outfall component (SWM works) 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. Generally, this will be a policy that requires the director issuing an environmental compliance approval (ECA) for the storm water management works to ensure that their decision to issue or amend an ECA is designed to achieve the objectives set out in paragraph 2 of subsection 22 (2) of the Act ( for example, ensure that the activity ceases to be or never becomes a SDWT)

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

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