Evaluating a Lot and Understanding Your Soil
Soil type is the single most important factor in whether a property can support a septic system and what that system will cost.
23 questions in this section
- Why does soil type matter so much?
- What is a test pit and why is one needed?
- How deep does a test pit have to be?
- Where must the test pit be located?
- How long does the test pit have to stay open?
- Is a test pit dangerous? Who is responsible for it?
- What do the soil categories A, B, C and D mean?
- What kinds of soil fall into Category D?
- What are the actual permeability numbers behind each category?
- How does the installer identify soil texture in the field?
- What is a ribbon test?
- Can soil texture alone be misleading?
- What are limiting factors?
- What is mottling and why does the inspector look for it?
- What is bedrock, in septic terms?
- How much clearance is needed above bedrock or groundwater?
- How does slope affect my system?
- What is an interceptor drain or diversion ditch?
- What should I consider when laying out a lot?
- Where should the well go relative to the septic system?
- Why do subdivisions with small lots cause septic problems?
- I am thinking of buying a vacant lot. What should I do first?
- I am buying an existing home with a septic system. What should I ask for?
Why does soil type matter so much?
The soil does the final treatment. It provides infiltration, dispersal and treatment of effluent before it reaches bedrock, groundwater or a setback boundary. Soil type determines whether a lot is suitable at all, and what design is required.
What is a test pit and why is one needed?
A test pit is an excavation dug so the installer and inspector can see the soil profile. A test hole must be dug for every application. The soil texture, density, structure, depth and colour in the pit let the licensee predict permeability and classify the soil.
How deep does a test pit have to be?
At least 1.8 metres (6 feet), unless bedrock is encountered first.
Where must the test pit be located?
Within approximately 3 metres (10 feet) of where the proposed disposal field will go. Digging a test hole directly in an existing field area is not recommended because it could contaminate groundwater.
How long does the test pit have to stay open?
It must be dug and left open, ready for assessment by the inspector, until the licensee has received an approval or a refusal.
Is a test pit dangerous? Who is responsible for it?
It is a shared responsibility of the licensee and the property owner to clearly flag the test pit so the inspector can find it and so nobody falls in. Test pits must be easy and safe to enter and exit, with stepped or sloped sides. WorkSafeNB has requirements for excavations and trenches.
What do the soil categories A, B, C and D mean?
Category A is good: coarse and medium sands. Category B is moderate: fine sand and loamy sand. Category C is fair: sandy loam. Category D is unacceptable - soil that either drains far too fast or far too slowly to be used to select a standard system.
What kinds of soil fall into Category D?
Two very different groups. Too slow: silt loam, loam, clay loam and clay. Too fast: very coarse sand, gravelly sand and sandy gravel. Both extremes require a raised mound or an engineered design.
What are the actual permeability numbers behind each category?
Type A is between 1x10-4 and 6x10-4 metres per second. Type B is 2x10-5 up to 1x10-4 m/s. Type C is 3x10-6 up to 2x10-5 m/s. Below 3x10-6 or above 6x10-4 is Category D.
How does the installer identify soil texture in the field?
Using the Flow Diagram for Estimating Soil Texture by Feel, which is Appendix A of the Guidelines. It involves moistening the soil and testing whether it forms a ball, flakes or ribbons, and whether it feels gritty, floury or sticky.
What is a ribbon test?
Moist soil is squeezed between thumb and finger to see how long a ribbon it forms. No ribbon suggests sand or loamy sand. A very short ribbon of 1.5 to 2.5 cm suggests sandy loam. A ribbon of 2.5 cm or longer means clay content and an unacceptable Category D soil.
Can soil texture alone be misleading?
Yes. Soils with a platy or massive structure, or that are very compact to dense, may not meet the permeability range that their texture suggests. An in-situ permeability test using a constant head permeameter can be done to verify.
What are limiting factors?
Bedrock, the groundwater or seasonal high water table, impermeable soil such as clay, and soil that is too permeable. These restrict how a system can be built and often force a raised mound design.
What is mottling and why does the inspector look for it?
Mottling is colour blotching in the soil profile that indicates a seasonal high water table. It tells the inspector the ground gets saturated at certain times of year even if the pit is dry on the day of inspection.
What is bedrock, in septic terms?
The solid rock underlying loose material such as gravel, sand, loam or clay. It includes limestone, sandstone and shale. Shallow bedrock is one of the most common reasons a mound system is required in New Brunswick.
How much clearance is needed above bedrock or groundwater?
At least 1.2 metres (4 feet) from the invert - the bottom - of the distribution pipe to the limiting factor. For concrete leaching chambers and plastic infiltrative chambers, at least 1.0 metre (3.3 feet) measured from the bottom of the unit or trench.
How does slope affect my system?
Slope affects how quickly water from the distribution piping moves through the soil downhill of the field. Steep slopes can limit which system types are possible, and may trigger the need for an interceptor drain.
What is an interceptor drain or diversion ditch?
A drain or ditch installed upslope of the disposal field to divert surface water and groundwater around it. This prevents water running down the hill from saturating the field soil during wet seasons.
What should I consider when laying out a lot?
Property size and dimensions including easements and rights of way; slope percentage and direction; the system layout; original and finished ground elevations; all separation distances; estimated daily sewage flow; soil characteristics from the test pit; limiting factors; interceptor drains and foundation drainage; and any future development such as a pool or garage.
Where should the well go relative to the septic system?
Wherever possible, the well should be located upslope of the on-site sewage disposal system, with the required separation distances respected.
Why do subdivisions with small lots cause septic problems?
Because separation distances between wells and septic systems become hard to maintain when many houses sit on minimum-sized lots. The Guidelines specifically flag this as problematic.
I am thinking of buying a vacant lot. What should I do first?
Consult a licensed septic system installer before you buy, to confirm the lot can actually support a system. Check separation distances, flood plains and room for future development. Also check with the local municipal or planning authority for by-laws that may apply.
I am buying an existing home with a septic system. What should I ask for?
The Guidelines advise obtaining details on the system's design, condition and location prior to purchase. Ask for the plan of installation, the Certificate of Compliance and the operating instructions, plus pumping records.
Have a question about your own property? General answers only go so far — your soil, slope, water table and setbacks are specific to your lot. Call (506) 233-6272 or send us the details.
Ready to talk about your septic system?
Serving homeowners, builders, and property owners throughout Greater Moncton and southeastern New Brunswick.