How Underground Pool Leaks Happen

 

Underground pool leaks are one of the most common — and most misunderstood — causes of water loss in residential and commercial pools. Because the plumbing is buried beneath concrete, soil, or decking, leaks often go undetected for long periods of time, quietly wasting water and causing hidden damage.

Understanding how underground pool leaks develop helps homeowners recognize warning signs early and avoid unnecessary repairs or excavation.

What Is an Underground Pool Leak?

An underground pool leak occurs when buried plumbing lines carrying water to and from the pool develop cracks, separations, or failures below ground level. These leaks are not visible from inside the pool and rarely show surface water until significant damage has already occurred.

Most underground leaks involve:

  • Return lines

  • Suction lines

  • Cleaner lines

  • Spa plumbing

  • Water features or auxiliary lines

The Most Common Causes of Underground Pool Leaks

Soil Movement and Ground Shifting

Soil beneath pools is not static. Over time, ground movement caused by:

  • Expansive clay

  • Limestone subgrade

  • Erosion

  • Improper compaction

can place stress on rigid PVC plumbing. Even small shifts can crack pipes or loosen glued fittings underground.

Freeze and Thaw Cycles

In colder climates, underground plumbing is exposed to seasonal freeze and thaw cycles. When water freezes, it expands. Repeated expansion and contraction can:

  • Split pipes

  • Weaken fittings

  • Create hairline cracks that worsen over time

Many underground leaks appear in spring after winter freezes.

Aging Plumbing and Glue Joints

Most pool plumbing relies on solvent-welded PVC joints. Over years of pressure, vibration, and temperature changes:

  • Glue joints can become brittle

  • Pipes can fatigue

  • Microscopic leaks can form

Older pools are especially prone to underground plumbing failures.

Poor Original Installation

Not all underground plumbing issues are caused by age. Installation-related problems include:

  • Inadequate pipe bedding

  • Pipes laid directly on rock

  • Improper glue application

  • Excessive stress on fittings

These issues may take years to surface but often result in premature leaks.

Hydraulic Pressure and System Stress

Pools operate under constant pressure when running. Over time:

  • Pressure-side leaks can worsen when the pump is on

  • Suction-side leaks may pull air into the system

  • Small cracks can turn into significant failures

Leaks that only occur while the system is operating are a strong indicator of underground plumbing issues.

Why Underground Pool Leaks Often Go Unnoticed

Autofill Systems Mask Water Loss

Automatic water levelers can continuously replace lost water, making it appear as though the pool is not leaking. While the water level stays normal:

  • Salt levels may drop

  • Chemical use increases

  • Water bills rise quietly

Many underground leaks remain hidden for months or years due to autofill systems.

Water Escapes Into Soil, Not the Surface

Unlike visible leaks, underground water loss often:

  • Soaks into surrounding soil

  • Travels away from the pool

  • Never reaches the surface

By the time surface signs appear, significant erosion may already exist.

Warning Signs of an Underground Pool Leak

Homeowners should watch for:

  • Unexplained water loss

  • Pump losing prime

  • Air bubbles in returns

  • Pool losing water only when running

  • Frequent chemical or salt additions

  • Soft or settling ground near the pool

  • Cracks in decking or nearby concrete

  • Higher-than-normal water bills

Any combination of these symptoms warrants professional testing.

Can Underground Pool Leaks Cause Structural Damage?

Yes. Left unaddressed, underground leaks can:

  • Wash out soil beneath decks

  • Cause concrete slabs to settle or crack

  • Undermine pool walls

  • Create voids beneath the pool shell

  • Accelerate corrosion of nearby metal components

Early detection prevents expensive structural repairs.

How Underground Pool Leaks Are Properly Diagnosed

At Four States Leak Detection, underground pool leaks are located using non-destructive diagnostic methods, including:

  • Pressure testing of individual plumbing lines

  • Acoustic listening equipment

  • Isolation of specific circuits

  • Dye testing where applicable

  • In-pool testing without draining whenever possible

This approach eliminates guesswork and minimizes unnecessary excavation.

Why Guessing Repairs Often Fails

Replacing parts or cutting concrete without proper testing often leads to:

  • Missed leaks

  • Repeated repairs

  • Higher overall costs

  • Continued water loss

Accurate leak detection identifies the exact location and cause before repairs begin.

When to Schedule Underground Pool Leak Detection

Professional testing is recommended if:

  • Water loss exceeds normal evaporation

  • Leaks occur only while the system is running

  • Autofill systems run frequently

  • Soil feels soft or unstable near the pool

  • Multiple repair attempts haven’t solved the issue

Underground Pool Leak Specialists in the Four States Region

Four States Leak Detection provides professional underground pool leak detection services throughout Missouri, Kansas, Oklahoma, and Arkansas. We work on vinyl liner, concrete, fiberglass, residential, commercial, and HOA pools.

Our focus is simple: find the leak accurately, explain the results clearly, and prevent unnecessary damage.