Severance Hydrogeological Study (D-5-4 / D-5-5)

When a severance needs a hydrogeological study

A severance hydrogeological study is most often requested when the new lot will rely on a private well and a septic system, is under one hectare, or sits in an area known to be hydrogeologically sensitive.

The request usually comes with the consent application, or in pre-consultation before it is filed. D-5-4 applies formally to developments of more than five units on individual sewage systems, but it encourages municipalities to apply the same provisions to consents and severances, and many do. D-5-5 asks that the availability of a potable water supply be determined as early as possible in the severance approval process.

A study is most often requested when:

• the new lot will rely on a private well and a septic system;
• the lot is under one hectare, or the area is known to be hydrogeologically sensitive;
• nearby wells have known quality or quantity problems; or
• several lots are being created from the same parcel.

If you are still deciding whether a hydrogeological study for a severance applies to your lot, our guide does my severance need a hydrogeological study walks through the servicing, lot size and local policy questions before you file the consent application.

The septic side: the D-5-4 nitrate impact assessment

The septic side of a severance study tests whether nitrate-nitrogen from the new septic system stays within 10 mg/L at the property boundary, working through lot size, isolation and attenuation only as far as needed.

D-5-4 uses nitrate-nitrogen as the indicator of impact, with 10 mg/L at the property boundary as the limit. The assessment goes only as far as it needs to.

1. Lot size. Lots of one hectare or more are considered acceptable in most cases. Lots averaging one hectare, none under 0.8 ha, may not need a detailed assessment if a qualified consultant shows the area is not hydrogeologically sensitive.
2. Isolation. For smaller lots, the consultant tests whether the sewage effluent is hydrogeologically isolated from existing or potential supply aquifers. That can mean looking at land up to 500 m beyond the site.
3. Contaminant attenuation. If isolation cannot be shown, a detailed study predicts the nitrate concentration at the downgradient property boundary. The ministry assumes at least 40 grams of nitrate per lot per day for a residence and accepts only dilution as a quantifiable attenuation mechanism.

Background matters too: the ministry will normally not support development where background nitrate-nitrogen already exceeds 10 mg/L, unless the levels can be explained, for example by past farming on the site.

The well side: the D-5-5 well test

The well side of a severance study shows that each future well can supply enough water of acceptable quality, through a pumping test of at least six hours and physical, chemical and bacteriological analysis.

Each future well must supply enough water of acceptable quality. D-5-5 sets out how that is demonstrated:

For larger developments D-5-5 also sets a minimum number of test wells: three for sites up to 15 hectares, rising with area. Whatever the size, there must be at least one test well, new or existing, on the site, and existing wells nearby can be used when they meet the guideline's requirements.

What you receive

A signed report for the consent application that sets out the site geology and groundwater conditions, the D-5-4 assessment and its conclusion, the pumping test and water quality results, recommended well construction, and any conditions the municipality may want registered, such as treatment requirements or warning clauses.

Severance hydrogeological study cost

The cost of a severance hydrogeological study is lowest for one lot over a hectare in a non-sensitive area with an existing well, and rises with more lots, a new test well or a full nitrate assessment.

A lot over one hectare in a non-sensitive area, with an existing well that can be tested, sits at the low end. A new well drilled for testing is billed by the driller on top of the range. Several lots, or a site where the isolation step fails and a full nitrate assessment is needed, push the price up.

If the new lot will be on full municipal water and sewer, a hydrogeological study may not be required. Check with the planning department first.

Before you call

Before you call about a severance study, gather the severance sketch, well records and the septic permit or inspection report, the comments on the application and any past water test results.

• The severance sketch with proposed lot dimensions and building envelopes
• Well records for any existing well, and the septic permit or inspection report
• The municipality's or conservation authority's comments on the application
• Any past water test results from wells on the property

How the time adds up

A severance study schedule is usually set by the wait for a well driller, then a pumping test of at least six hours, recovery monitoring and laboratory analysis, plus any background nitrate sampling across seasons.

Arranging a well driller, if a new test well is needed, is often the longest wait. The pumping test itself is at least six hours of continuous pumping, then recovery monitoring until 95% recovery or 24 hours, whichever is less, followed by laboratory analysis of the samples. Where D-5-4 needs background nitrate sampling across seasons, the consultant has to justify how many rounds and over what period, which can extend the schedule.

For the other clocks, such as municipal review and how long a finished report stays usable, see how long a hydrogeological study takes.

Where severance studies run into trouble

Severance studies run into trouble when the pumping test cannot sustain the required rate, chlorine spoils the samples, background nitrate is high with no explanation, or lot lines move after the study.

• Pumping test below the required rate. A test that cannot sustain the calculated rate raises the question of long-term yield.
• Chlorine in the sample. A recently disinfected well gives unusable bacteriological results.
• High background nitrate with no explanation for it.
• Lot lines moved after the study, changing the area available for dilution.

Worked cost examples

Each example runs the estimator's own calculation on a realistic set of answers.

• One new lot, existing well, septic system$5,500 – $8,500

  Number of new lots: 1 lot · Water supply: Existing well · Sewage servicing: Septic system

  The base case: one pumping test on a well that meets D-5-5, water quality analysis, and a D-5-4 assessment that may stop at the lot size or isolation step.

• Two or three lots, new test well needed, septic systems$8,500 – $13,000

  Number of new lots: 2 to 3 lots · Water supply: New well needed (drilling extra) · Sewage servicing: Septic system

  More lots multiply the D-5-4 work, because the combined nitrate load from all systems is assessed at the boundary. A new test well adds supervision; drilling is billed separately by the well contractor.

• One lot on municipal water, with a septic system$3,500 – $5,000

  Number of new lots: 1 lot · Water supply: Municipal water · Sewage servicing: Septic system

  Only the sewage side remains, so D-5-5 well testing drops out and the range falls. If the lot will also be on municipal sewer, check whether a study is needed at all.

What the reviewer checks

A reviewer of a severance hydrogeological study checks the lot sizes and sensitivity opinion, background nitrate sampling, the boundary prediction against 10 mg/L, the pumping test, water quality and well construction.

• Lot sizes against the one hectare test, with a stated opinion on hydrogeological sensitivity
• If isolation is claimed, the lower boundary of the receiving groundwater and how it was defined
• Background nitrate sampling, with the number and timing of rounds justified
• A nitrate loading of at least 40 g per lot per day and dilution as the only quantified attenuation
• Predicted nitrate-nitrogen at the downgradient boundary against 10 mg/L
• Pumping test of at least six hours at a fixed rate meeting the calculated minimum
• Recovery to 95% or 24 hours
• Water quality, with a sample from the last hour of the test and a chlorine residual check
• Well construction recommendations that meet Regulation 903
• Land uses within 500 m and their potential effect on the supply

Severance study or large septic assessment?

Septic system size decides which study applies: individual household systems stay with a severance study, while larger or combined systems, or works crossing a lot line, need a large septic assessment.

See the large septic system assessment page for the larger case.

Where this study is commonly required

Location pages with local requirements, groundwater data and the agencies involved, by region: