Do I need soil testing for a wastewater system?

Yes. Before designing an onsite wastewater system, we need to understand how the soil on your land will handle the water coming from your home.

We like to carry out our own soil checks alongside reviewing your geotechnical report. These checks are part of our free site visit service. We usually test two or three locations because soil can vary across a property.

Your treatment system cleans the wastewater from your toilets, showers, kitchen and laundry. That treated water then goes to a disposal field, where it is distributed into the ground. The soil also helps with further treatment.

Different soils handle that water differently. Clay generally drains slowly, so water can build up if too much is applied in one place. Sandy ground usually drains faster, but water moving too quickly through the soil can carry contaminants towards groundwater before enough treatment occurs.

We need to know both how well the ground drains and how safely it can receive wastewater. Looking at the grass on top will not tell us that.

Your geotechnical report gives us useful information about the ground. However, a report prepared for house foundations may focus on the building platform rather than the area where wastewater will be discharged.

Some reports include a wastewater assessment; others do not. We review yours to see what has already been investigated and what information is still needed.

Our own soil checks help us assess the locations being considered for your disposal field. The ground beneath your future house may be quite different from the ground further down the section.

These checks complement the engineering information and help us work out whether your preferred wastewater layout is suitable.

We look below the grass and topsoil to examine the ground underneath.

A wastewater soil assessment commonly involves using a hand auger, which brings soil up through a narrow hole, or an inspection pit where needed. Examining different depths shows how the soil changes below the surface.

We look at:

• Soil type: whether it is mainly clay, silt, sand or a mixture.

• Soil structure: whether it has spaces and channels for water to move through, or forms dense layers.

• Changes with depth: whether freely draining topsoil sits above clay, rock or another restrictive layer.

• Signs of seasonal wetness: moisture, groundwater and soil colour patterns that indicate the ground stays wet for part of the year.

These observations help us assess drainage and select suitable design allowances. Where necessary, a separate soakage or permeability test can measure how water moves into the ground. Not every property needs the same type of testing.

Because one hole does not necessarily represent the whole section.

We usually check two or three potential disposal locations to compare the ground conditions. More investigation may be needed on a complex property.

For example, one area could have suitable soil, while another has dense clay close to the surface. A lower corner might collect water in winter, even though it looks dry during a summer visit.

Comparing locations can help us find a better position for the field. It also helps avoid designing around an area that turns out to be unsuitable.

Most soil is a mixture, and its structure matters as well as its ingredients. These are the main differences we look for:

These characteristics help determine the field size, disposal method and treatment requirements. Slow drainage does not automatically rule out a system, and fast drainage does not automatically make a site straightforward.

There are broad regional patterns, but they cannot tell us exactly what is beneath your proposed disposal field.

Regional information gives us a starting point. The checks on your property tell us what we actually need to design for.

Soil is one part of the assessment. We also consider the slope, surface drainage, access, proposed buildings and distances from features such as boundaries and waterways.

We talk through how you want to use the property, too. You might want to keep the flat lawn clear, incorporate the field into planting or leave room for a future shed. Those preferences are useful before the layout is decided.

Tell us about winter puddles, springs, old drains, previous earthworks or areas where fill has been placed. Your experience of the land can reveal things that are not obvious on the day.

The findings help us answer three practical questions.

Where should the disposal field go?
We compare potential locations to find suitable ground that also works with your property plans.

How much land does it need?
The field must accommodate the home’s design wastewater flow. If the soil can only receive a small amount per square metre each day, more land may be needed.

How should the water be distributed?
Drip irrigation, soakage beds and other methods suit different conditions. The design needs to match the method to the ground.

We then recommend a treatment system that works with the disposal design. For example, fine-outlet drip irrigation requires suitably treated wastewater to help prevent its small outlets becoming blocked.

That is why choosing the tank before assessing the land can put the decisions in the wrong order.

Poor drainage means we need to understand the limitation and assess whether it can be managed.

Depending on the findings, we may consider another location, a larger field or a different land application method. In some cases, further specialist investigation is needed.

Higher treatment can reduce the solids and organic waste reaching the field, helping limit the build-up that can clog it. But cleaner water does not make clay drain faster. The field still needs enough suitable land to receive the daily volume without becoming waterlogged.

Your disposal area can be planted out with lots of native plants and grasses. The irrigation will actually help it thrive. But the layout will need to account for how you want the property to look and how servicing staff will reach the components they need to maintain.

However, it is not spare building land. Vehicle traffic, grazing and future construction can damage a field or interfere with its operation. Ask what uses are suitable before planning a driveway, shed or other development over it.

Don't reserve your best flat ground for wastewater, let us look at the options.

Send WaterFlow your property address, proposed bedroom count, site plans and any existing soil or engineering reports. Tell us where you would prefer the field and what you want to keep available for outdoor living, landscaping or future buildings.

We can review the available information, identify whether a site visit or further soil assessment is needed, and help work out a suitable disposal method and location.