Understanding Water Contamination in Stored Diesel and Gasoline

Water is the most common contamination problem in stored fuel, and it shows up in two forms: free water and emulsified water. Both create conditions that lead to corrosion, microbial growth, and filter problems if left unaddressed.

Where the water comes from

Fuel tanks accumulate water from a few predictable sources: condensation as temperatures cycle, small leaks around fittings or tank seams, and water intrusion during fuel delivery. None of these require the tank to be old or the equipment to be in poor condition — condensation alone can introduce water into a tank that's otherwise well maintained.

Free water vs. emulsified water

Free water settles to the bottom of the tank, since it's denser than fuel. It's usually the easier of the two to address. Emulsified water has mixed into the fuel itself, often after sitting for an extended period, and gives the fuel a hazy or cloudy appearance. Emulsified water is harder to separate and is more likely to be accompanied by microbial growth, since bacteria and fungus thrive at the fuel-water interface.

Why it matters

Water in fuel causes injector damage, accelerates corrosion in tanks and fuel system components, feeds microbial growth, and plugs filters faster than they should wear out. For generators and other standby equipment, water contamination is one of the more common reasons stored fuel fails to perform when it's actually needed.

Fuel polishing with water-separation equipment removes both free and emulsified water from fuel while it stays in the tank. If you're seeing cloudy fuel, recurring filter changes, or you simply haven't checked tank condition in a while, a fuel sample and assessment is the place to start.