A construction dewatering assessment estimates how much groundwater an excavation will draw in, how far the drawdown will spread, and what that means for nearby wells, buildings, natural features and the water you discharge. In Ontario it is the technical basis for an EASR registration or a Permit to Take Water, and for the sewer or discharge approvals that follow. The assessment is completed by a licensed hydrogeologist (P.Geo.) or professional engineer (P.Eng.).
What triggers it
A construction dewatering assessment is triggered when an excavation is expected to take more than 50,000 litres of groundwater, stormwater or both on any day, or earlier when the municipality asks for one at the planning or building permit stage.
The provincial trigger is volume. Construction site dewatering that takes more than 50,000 litres of groundwater, stormwater or a combination of both on any day must be registered on the Environmental Activity and Sector Registry (EASR), according to the ministry's water taking permissions page. Groundwater is not the only water counted: the registration has to allow for storms, because rain falling into an open excavation adds to the daily total.
Municipalities often ask for the assessment earlier, at the planning or building permit stage. Hamilton's urban hydrogeological study guideline lists official plan and zoning amendments, plans of subdivision, site plan control, building permits and stand-alone construction dewatering applications as triggers, and requires the study to state construction dewatering volumes in litres per day and whether an EASR registration is needed.
Typical projects that end up here:
- Mid-rise and high-rise buildings with one or more underground parking levels
- Deep servicing trenches, pumping stations and tanks
- Infill houses with basements in areas of shallow groundwater
- Linear projects such as pipelines and utilities, where one registration can cover several sites
EASR dewatering or a Permit to Take Water: which applies
The predicted daily taking decides the route: more than 50,000 litres of groundwater on a day means an EASR registration, stormwater alone needs only a discharge report, and some takings need another permission such as a Permit to Take Water.
The assessment's predicted daily taking decides the route. The rules below come from the ministry's EASR water taking user guide.
| Predicted taking | What is needed |
|---|---|
| Groundwater (alone or mixed with stormwater) more than 50,000 L on at least one day | EASR registration, with a water taking report and a discharge report uploaded at registration |
| Stormwater only, any volume | No EASR registration or permit, but a discharge report prepared and retained before taking water |
| 50,000 L per day or less of groundwater | No provincial water taking permission; municipal discharge approval may still apply |
| Discharge to the natural environment inside a WHPA-A (100 m around a municipal well) | EASR registration is not available for that taking; another permission applies |
The regulation itself, O. Reg. 63/16 section 7, sets the same 50,000 litre floor with no upper limit, and lets two or more construction sites for the same linear project, such as a pipeline, utility corridor or transit line, be treated as one site. It does not apply where the groundwater is taken for human consumption, agriculture or an industrial process, or where a Permit to Take Water for the taking was already in effect when the 2025 amendments (O. Reg. 66/25) came into force.
Use the EASR or Permit to Take Water decision tool to test your own numbers.
For the site team, the EASR dewatering registration steps guide sets out the notices, daily records and reporting that follow.
What the dewatering report has to cover
The dewatering report for an EASR registration describes the taking and the hydrogeological setting, sets out the expected area of influence, and assesses contaminant movement, settlement, other water users and natural features within it.
For an EASR registration the ministry sets out the content in detail. The report must describe the rate and volume of the taking and the hydrogeological setting, including a well survey, and identify the expected area of influence of each dewatered work area with every calculation and assumption shown. Within that area it must then assess:
- Contaminant movement. Whether drawdown could pull existing contamination toward the excavation or neighbouring properties.
- Settlement. Soil settlement and land subsidence, and their effect on buildings, roads and services.
- Other water users. Whether the taking would limit water available to nearby wells.
- Natural features. Effects on conservation land, fish and wildlife habitat and ecosystem functions.
It ends with the monitoring, mitigation and contingency measures, a protocol for written notice to potentially affected water users and the ministry district office at least 48 hours before pumping starts, and a signed statement from the qualified person. The separate discharge report assesses the quality and quantity of the water, how and where it will be discharged, and any treatment needed.
The water taking report and discharge report together form the dewatering plan the contractor works to on site, including the monitoring, contingency and shutdown measures.
Field work behind the numbers
Field work for a construction dewatering assessment usually combines monitoring wells, repeated water level readings, hydraulic conductivity tests, groundwater sampling and a survey of nearby wells and potential contamination sources.
The scope depends on depth, soil type and what is nearby, but most assessments draw on:
- Boreholes with monitoring wells screened at and below the planned excavation base
- Repeated water level readings to establish the seasonal high water table
- Single-well response tests or a pumping test to measure hydraulic conductivity
- Groundwater samples analysed against the sewer use by-law or discharge criteria
- A survey of nearby wells and a review of potential contamination sources
Monitoring periods are often set by the municipality. Hamilton, for mid-rise and high-rise developments, asks for biweekly water levels in all on-site monitoring wells for at least three months, with longer monitoring where highly permeable deposits are expected.
Construction dewatering assessment cost
A construction dewatering assessment costs least for a shallow infill house with one basement level and most for a tower with three or more levels on a large footprint that takes the Permit to Take Water route.
Depth and footprint drive the field program, and permit support is added on top. The table is generated from the same pricing data the estimator uses.
| Answer | Indicative range |
|---|---|
| Starting point (1 basement level (under 4 m), Under 0.25 ha, Assessment only) | $9,000 – $14,000 |
| Excavation depth | |
| 1 basement level (under 4 m) (starting point) | $9,000 – $14,000 |
| 2 levels (4 to 8 m) | $12,500 – $19,500 |
| 3 or more levels (over 8 m) | $18,000 – $28,000 |
| Excavation footprint | |
| Under 0.25 ha (starting point) | $9,000 – $14,000 |
| 0.25 to 1 ha | $11,500 – $18,000 |
| Over 1 ha | $15,500 – $24,000 |
| Permit support | |
| Assessment only (starting point) | $9,000 – $14,000 |
| EASR registration | $11,500 – $18,000 |
| Permit to Take Water application | $17,000 – $29,000 |
| Full span across all answers | $9,000 – $62,500 |
Each row changes one answer and leaves the others at the starting point. Indicative ranges, rounded to the nearest $500. They exclude well drilling, government application fees and HST. Final scope is confirmed by the hydrogeologist after reviewing your site.
What pushes the price up
- Three or more basement levels, or an excavation reaching into bedrock
- Contaminated groundwater that needs treatment design before discharge
- Sensitive neighbours: heritage buildings, private wells, wetlands or a watercourse
- A Permit to Take Water instead of an EASR registration
What keeps it down
- Existing geotechnical boreholes and monitoring wells that can be reused
- A watertight (bathtub) design that avoids long-term drainage
- Early access to the site for monitoring well installation
Timeline
The timeline for a construction dewatering assessment runs from monitoring well installation and water level monitoring through testing, the reports and EASR registration to written notice at least 48 hours before pumping.
Calendar time is set more by monitoring and approvals than by report writing. Fixed points worth planning around:
- Monitoring well installation and water level monitoring, for whatever period the municipality requires.
- Hydraulic testing, sampling and laboratory analysis.
- Water taking and discharge reports, then EASR registration.
- Written notice to affected water users and the ministry district office at least 48 hours before pumping.
- Toronto's sewer discharge applications are due at least eight to 12 weeks before the planned start of discharge, per the City's discharge permit page.
What to have ready
For a construction dewatering assessment, have the drawings with the lowest slab and footing elevations, the shoring concept, any geotechnical report and environmental site assessment, and the construction schedule ready.
- Architectural or structural drawings showing the lowest slab and footing elevations
- The shoring concept (for example, caisson wall, secant piles or open cut)
- The geotechnical report and borehole logs, if one exists
- Any Phase One or Phase Two environmental site assessment
- The construction schedule, including how long the excavation will stay open
Why submissions stall
Dewatering submissions stall when monitoring is too short to show the seasonal high, storms are left out of the daily volumes, water quality data is missing, or the drawings change after the report is written.
- Too little monitoring. One round of water levels taken in a dry month rarely satisfies a reviewer looking for the seasonal high.
- Storms left out. Daily volumes that ignore rainfall into the open excavation understate the taking.
- No water quality data. Without lab results against the sewer use limits, the discharge approval cannot proceed.
- Drawings changed. A deeper footing or extra parking level after the report is written means the inflow estimate has to be redone.
Worked cost examples
Each example runs the estimator's own calculation on a realistic set of answers.
-
Infill house, one basement level$9,000 – $14,000
Excavation depth: 1 basement level (under 4 m) · Excavation footprint: Under 0.25 ha · Permit support: Assessment only
Shallow and small, so a few monitoring wells and response tests usually characterize the site. If the predicted taking stays at or below 50,000 L on every day, no registration is needed and the assessment supports the building permit and any sewer discharge application.
-
Mid-rise with two parking levels, registered on the EASR$19,000 – $29,500
Excavation depth: 2 levels (4 to 8 m) · Excavation footprint: 0.25 to 1 ha · Permit support: EASR registration
Depth and footprint both multiply the base, because more wells, deeper testing and a wider area of influence have to be assessed. The EASR registration adds the separate water taking and discharge reports.
-
Tower with three or more levels on a large footprint, Permit to Take Water route$38,500 – $62,500
Excavation depth: 3 or more levels (over 8 m) · Excavation footprint: Over 1 ha · Permit support: Permit to Take Water application
The top of the range. Deep excavations draw from a wide area, settlement and contaminant movement need detailed analysis, and a permit application (for example, where the discharge would go to the natural environment inside a WHPA-A) adds the largest permit-support cost.
What the reviewer checks
Reviewers of an EASR dewatering registration look for the qualified person, a daily maximum that includes storms, the area of influence with every calculation, the impact analyses, the contingency plan and the discharge report.
For an EASR registration, section 8 of O. Reg. 63/16 and the ministry guide set the content. Reviewers and auditors look for:
- The qualified person's credentials and a summary of their experience
- Rate and volume of the taking, with storm events included in the daily maximum
- Hydrogeological setting, geology and a well survey
- The expected area of influence for each dewatered work area, with every calculation and assumption
- Separate analyses of contaminant movement, settlement, other water users and natural features
- A contingency plan, including whether a shutdown protocol is needed
- The 48-hour notification protocol for water users and the district office
- A discharge report covering water quality and quantity, discharge method and location, and treatment
- Signed statements from the qualified persons
Dewatering assessment or sewer discharge permit?
A construction dewatering assessment supports the provincial permission to take the groundwater, while a sewer discharge permit is the municipal approval to put that water into a municipal sewer.
Owners often expect one approval. Most deep excavations need both, built on the same field data.
| Aspect | Dewatering assessment (EASR or PTTW) | Sewer discharge permit |
|---|---|---|
| Covers | Taking the groundwater | Putting it into a municipal sewer |
| Authority | Province (MECP) | Municipality |
| Key numbers | Daily volume, area of influence, impacts | Volume, flow rate, water quality against by-law limits |
| Trigger | More than 50,000 L of groundwater on any day | Any private water discharge to the sewer |
See groundwater discharge permits for the municipal side.
Where this study is commonly required
Location pages with local requirements, groundwater data and the agencies involved, by region:
Greater Toronto and Hamilton Area
- Hydrogeological study Toronto
- Hydrogeological study Mississauga
- Hydrogeological study Hamilton
- Hydrogeological study Vaughan
- Hydrogeological study Richmond Hill
- Hydrogeological study Markham
- Hydrogeological study Brampton
- Hydrogeological study Milton
- Hydrogeological study Clarington
- Hydrogeological study East Gwillimbury
- Hydrogeological study King Township
Central Ontario
Eastern Ontario
Dewatering questions
Does every basement excavation need a dewatering assessment?
Not every basement excavation needs a dewatering assessment: if it stays above the seasonal high water table, there may be little or no groundwater to manage. The trigger is groundwater: once a site expects to take more than 50,000 litres of groundwater (or groundwater mixed with stormwater) on any day, the taking must be registered on the EASR, and that registration needs a water taking report and a discharge report. Many municipalities also ask for a hydrogeological assessment of dewatering at the planning or building permit stage, whatever the volume.
Who can sign the water taking report for an EASR registration?
A practising geoscientist, or a professional engineer who meets the Professional Geoscientists Act, 2000, can sign the water taking report for an EASR registration. Under the ministry's EASR water taking guide, the qualified person for that report must be a practising member of the Association of Professional Geoscientists of Ontario (now Professional Geoscientists Ontario), or a professional engineer who meets the requirements of the Professional Geoscientists Act, 2000. The discharge report has a separate, broader qualification requirement. For how to check either kind of licence, see our guide to who can sign.
We only pump rainwater out of the hole. Do we need anything?
Taking only stormwater does not require an EASR registration or a Permit to Take Water. The ministry guide still requires a qualified person to prepare and retain a discharge report before the water is taken, and the measures in that report must be followed.
What happens if dewatering runs longer than a year?
If an EASR water taking is expected to continue for more than 365 days, written notice must go to the local municipalities and any conservation authority with jurisdiction. If a taking first expected to be shorter runs past 365 days, the registration must be updated and the same notice given.
Can the dewatering water go into the municipal sewer?
Only with the municipality's approval, and usually only if it meets the sewer use by-law limits. Toronto, for example, requires a sewer discharge permit or agreement before any private water, including construction dewatering, is discharged. The groundwater quality results from the assessment are what the municipality reviews.
Is a pumping test always part of the assessment?
Not always: on smaller sites, hydraulic conductivity from single-well response tests in monitoring wells may be enough to estimate inflow. Deep excavations, permeable sand and gravel, or nearby water users usually justify a pumping test so the predicted area of influence rests on measured aquifer response.
Related studies
- Permit to Take Water & EASR
Work out whether a water taking needs an EASR registration or a Permit to Take Water.
- Groundwater Discharge Permits & Foundation Drainage
Approvals for discharging pumped groundwater to municipal sewers, during construction and over the long term.
- Development Hydrogeological Study
Groundwater assessments that support subdivision, site plan and zoning applications.
- Hydrogeological study cost guide
Ranges for every study type, side by side.
Sources
- Environmental permissions for water taking (opens in a new tab), Government of Ontario (MECP).
- Hydrogeological Study (Urban): development application guidelines (last updated February 13, 2026) (opens in a new tab), City of Hamilton.
- Water taking user guide for the Environmental Activity and Sector Registry (opens in a new tab), Government of Ontario (MECP).
- O. Reg. 63/16: Registrations under Part II.2 of the Act - Water Taking (consolidated to e-Laws currency date October 9, 2026) (opens in a new tab), Government of Ontario (e-Laws).
- Discharge permits and agreements for private water (opens in a new tab), City of Toronto.
- New business name: Professional Geoscientists Ontario (since June 2019; the legal name in the Professional Geoscientists Act is unchanged) (opens in a new tab), Professional Geoscientists Ontario.