Not all swimming pool water starts out the same. Whether it comes from a garden hose, a well, a water truck, or the sky, each source brings its own chemical fingerprint and its own potential problems. The source of the pool water matters, and so does how you test it. When filling a pool with a new source, always begin with a full baseline test. It’s easier (and cheaper) to treat the water before issues appear than to fix cloudy water, stains, or scaling after the fact.

Understanding how to test and interpret different fill water sources is key to avoiding issues before they start.

Tap Water

Most residential pools are filled with municipal tap water, which is typically treated for drinking. This means it may contain chlorine or chloramine, and it’s generally free from pathogens. However, tap water isn’t perfect.

• Common issues: High pH, moderate total alkalinity, and variable calcium hardness.

• Hidden problems: Metals like copper or iron may be present, especially in older plumbing.

• What to test: pH, total alkalinity, calcium hardness, and metals if staining is suspected.

Tip: Some cities use chloramine instead of chlorine, which doesn’t dissipate easily. Always test combined chlorine in tap-filled pools, especially if you notice eye irritation or odor.

Well Water

Well water can vary widely depending on the region and depth of the well. It’s often untreated and can be rich in minerals.

• Common issues: High iron, manganese, or copper levels; low pH; high total dissolved solids (TDS).

• What to test: Metals (especially iron), pH, calcium hardness, TDS, and alkalinity.

Tip: Always pre-test well water before filling a pool. If metal levels are high, consider using a sequestering agent from the start to prevent staining and discoloration.

Truck-Filled Water Trucked-in water is often used for commercial pools or large residential pools in areas with water restrictions.

It may come from a municipal source or a private well, and quality depends on the source and how it’s transported.

• Common issues: Variable pH and sanitizer levels, possible debris contamination.

• What to test: Everything: pH, alkalinity, chlorine, calcium, and metals.

Tip: Ask the water hauler for source details.

Rainwater

Rain may look clean, but it’s naturally acidic and often carries organic contaminants or airborne pollutants. It also dilutes pool chemistry, which can disrupt carefully balanced water.

• Common issues: Low pH, low alkalinity, contamination from nearby trees or roofs.

• What to test: pH, alkalinity, and chlorine levels after a significant rain event.

Tip: After heavy rainfall, retest the pool and adjust as needed. Organic material can consume chlorine quickly, and low pH can corrode surfaces.