Underground and Buried Water Tanks: What You Need to Know

Underground, underfloor, underdeck, or buried water tanks are great space-savers and popular for hiding the tank(s) out of sight. But while they may be out of sight, they shouldn’t be out of mind.

These types of tanks come with their own set of quirks that homeowners need to be aware of to avoid maintenance issues, contamination risks, or even structural problems.

Whether your tank is tucked under a deck or buried beneath the backyard lawn, here’s what you need to know to keep things running smoothly.

1. Ensure Your Access to the Tank Sits Above Ground Level

One of the biggest challenges of inground tanks is ensuring there is no ground water incursion through the access hatch. If the riser (the access point to your tank) sits flush with or below the ground level, ground water can pool and flow straight into your tank.

What to do:

• Make sure the riser sits at least a few inches above the ground. This helps minimise the chances of surface water flowing over the access hatch and into the tank in rain events.
• Resist all temptation to plant on or around the tank and build up the soil level around the access hatch.

You would be amazed at how plants seek out the water in tanks and roots get into the tank and compromise the tank’s integrity as well as the quality of your water.

2. Know Where Your Tank and Its Lid Is Located

It might sound obvious, but it’s surprisingly common for people to lose track of where their underground tank is. Over time, grass can overgrow the tank, or landscaping changes might cover it completely.

This becomes a real issue when:

• You need to check water levels or perform maintenance
• There’s a leak or blockage
• A vehicle like a septic truck unknowingly drives over it

We’ve seen tanks damaged simply because no one realised they were underneath until it was too late. Concrete tanks are strong but not indestructible.

What to do:

• Mark out where your tank and lid are.
• Avoid placing anything heavy over the lid or tank footprint.
• If your tank sits beneath a lawn, we recommend adding a riser to the hatch. This lifts the lid above ground level and helps prevent surface water from entering the tank during heavy rain.

3. Know Your Tank’s Capacity

Unlike above-ground tanks where the size is obvious, buried tanks can be trickier to understand.

Why does capacity matter?

• It helps you manage your water use, especially in summer or during drought conditions.
• It allows you to plan maintenance or filtration requirements.
• If your tank needs cleaning, it helps us quote accurately and bring the right equipment.

Tip: If you don’t have the specs from when your tank was installed, you can measure the internal dimensions (length × width × depth) and use these formulas to estimate capacity:

For rectangular tanks:
📏Volume (L) = Length (m) × Width (m) × Depth (m) × 1000

For circular tanks: 
📏Volume (L) = π × Radius (r)² × Height (h)

You can also visit Volume Calculator and add the basic measurements for rectangular or cylindrical tanks to get the volume calculated for you.

Even rough measurements help give a better idea of what you’re working with.

4. Make Sure the Lid Is Accessible

Underground tanks are already more difficult to access than above-ground ones, so ensuring the lid is clear and easy to reach is key – especially for cleaning.

If the lid is:

• Buried under garden beds or furniture
• Blocked by overgrown plants or fencing
• Tucked up against a retaining wall or post
• Under a deck

When this is the case, cleaning becomes more complicated (or even impossible). For best results, our equipment needs to be able to move freely inside the tank. If the access point is tight against a wall or placed at one end of the tank, it can limit the ability to reach the entire tank base surface area, where sediment settles.

What to do:

• Keep a 1m+ clearance zone around and above the hatch.
• Avoid heavy landscaping or structures right beside it.
• When building decks or placing a tank under a house, if there is not 1m clearance above the hatch, then include a dedicated access hatch to the lid in the deck, or locate the tank so the hatch is just outside the overhang of the deck/house.
• Ideally, having the lid more central in a purpose-built inground tank (rather than at one end) improves cleaning access.

Our tank cleaning operators are great at working with all sorts of setups, but a bit of planning around lid placement and access goes a long way to making the cleaning process both possible and more efficient – and thorough.

5. Beware of “Popping” Tanks

Yes, it sounds strange – but it’s a real thing. Underground tanks can pop out of the ground if the water level inside gets too low.

Here’s why:

• When a buried tank is full, the weight of the water helps hold it in place. But if it’s empty it can become buoyant.
• The surrounding inground pressure can actually push the tank upward (especially if the ground around it is saturated with rain and the water table is high), popping the tank(s) out of the ground, cracking concrete or damaging pipework in the process.

This is more common:

• When tanks are empty or have low water levels
• With high water table areas
• After extended wet weather
• If there has been improper installation

The good news?

At Safe H2O, our tank cleaning process is designed to conserve as much of your water as possible.

Unlike traditional methods that drain the entire tank before cleaning, our vacuum-based system removes sediment and contaminants without fully emptying the tank, before filtering the remaining water.

This means the weight of the water remains during the clean – making our method safe to use on underground or buried tanks at all times.

Our tip:  If you’re planning to empty your tank (for cleaning or repairs), make sure it’s done during a dry spell, and that it will be refilled as soon as possible afterwards.

Final Thoughts

Underground tanks can be a smart long-term solution, but they do require a little extra attention. By knowing where your tank and lid are, keeping your lid above ground level, understanding its size, ensuring proper access, and protecting it from structural risks, you’ll avoid costly surprises and help your tank last longer.

If you’re unsure about the condition of your buried tank or it’s been a few years since your last clean, get in touch with Safe H2O. We specialise in safe, non-invasive water tank cleaning and can give you peace of mind your system is in good shape.

👉 Need help? Click here to get a quote or book your next clean.