Water in Fuel Tanks: Causes, Risks, and Solutions

Points of Entry

1. Damaged or Missing Caps and Seals on Dip, Fill, VR and Other Tank Penetrations
Caps and seals on dip points, fill risers, and vapour recovery (VR) connections are critical for keeping tanks watertight. Over time they degrade, crack, or are fitted incorrectly after use. When this happens, rainwater, washdown water, or surface runoff can enter directly into the system. Because these points are accessed often, human error is also a common factor. Even small gaps allow water to enter tanks, leading to immediate fuel contamination and making it necessary to remove water from fuel tanks before damage spreads.

2. Manway and Tank Lids
Manways and lids are among the largest exposed openings on an underground tank. Gaskets harden with age, lids corrode or warp, and bolts loosen under heavy forecourt traffic. Poorly sealed manways allow stormwater pooling on the surface to seep directly into tanks. In flood situations, entire volumes of water can bypass a manway cover and cause sudden, large-scale underground fuel tank water contamination. These events often require emergency tank cleaning and water removal.

3. Leaks in Vent and VR Lines
Vent and vapour recovery systems maintain safe pressure balance, but damaged vent caps, corroded fittings, or cracked pipes create unseen entry points for water. Missing vent caps allow rain to drip directly into lines and into tanks. Because vents are less visible, this type of water in fuel tank contamination often goes unnoticed until filters clog or ATGs trigger alarms. In coastal or cold climates, salt corrosion and frost damage make this issue even more common.

4. Groundwater via Tank Wall Integrity Issues
Underground tanks are constantly under hydrostatic pressure from surrounding groundwater. Corroded steel walls or cracked fibreglass structures allow groundwater to seep in continuously. Even pinholes create slow but relentless underground fuel tank water contamination that builds over weeks or months. This not only contaminates fuel but also weakens the structural integrity of tanks, leading to costly replacements.

5. Condensation
Partially filled tanks trap humid air. As temperatures fluctuate, moisture condenses on the interior surfaces and drips into the fuel. This gradual process accumulates a water layer at the base of tanks. Over time, condensation alone can create serious water in fuel tank problems, encouraging microbial growth, corrosion, and reduced fuel quality.

Why Water in Fuel Tanks Matters

Phase Separation in Petrol
In ethanol-blended fuels like E10, ethanol absorbs water. Once saturated, ethanol separates from petrol, creating a water-ethanol layer at the bottom of the tank. This leaves the remaining fuel with reduced octane and unsafe for use. Vehicles running on separated fuel may experience poor performance, stalling, or long-term damage. This is one of the most common reasons operators must remove water from fuel tanks quickly.

Microbial Growth in Diesel (“Diesel Bug”)
Water in diesel tanks creates a perfect breeding ground for microbial colonies. Known as “diesel bug,” these organisms thrive at the fuel-water interface, producing acids and sludge that corrode tanks and clog filters. Left untreated, microbial contamination spreads quickly, forcing deep cleaning and chemical treatment. This is one of the most damaging forms of underground fuel tank water contamination.

To learn more click here Microbial Growth

Microbial Growth in Diesel

Corrosion of Tanks and Lines
Water accelerates corrosion in steel tanks, pipelines, and fittings. Even small amounts trigger rust that spreads over time, weakening containment and risking leaks. Fibreglass tanks are not immune; cracks can develop under chemical and structural stress. Corrosion-related failures often result in environmental breaches and major remediation costs.

Blocked Filters and Reduced Flow
As water and sludge accumulate, they clog dispenser filters. This reduces fuel flow rates, frustrates customers, and increases operational costs from constant filter replacements. Persistent blockages are a clear sign that it’s time to remove water from fuel tanks.

Customer Complaints and Reputational Damage
Fuel contaminated with water leads to rough running, loss of power, and breakdowns. Customers affected by poor fuel quality may stop using the site or demand compensation. Reputational damage spreads quickly, especially with social media, and can take years to repair.

Regulatory and Compliance Risks
Australian UPSS regulations require operators to prevent leaks and contamination. Failing to control underground fuel tank water contamination exposes sites to fines, remediation orders, or shutdowns. Maintaining dry tanks is essential to compliance, safety, and business continuity.

Warning Signs

The first indication of water ingress is often an alarm on the Automatic Tank Gauge (ATG). These alarms trigger when water levels exceed preset thresholds, signalling a problem. Physical sampling also reveals warning signs—fuel that appears cloudy or hazy usually indicates water contamination, while a clear separation layer in sample jars confirms the presence of free water. Frequent filter changes are another indicator, as filters quickly clog when water or microbial sludge is present. Operators may also notice that dispenser flow becomes weaker over time, which is a sign of restricted filters caused by contamination. Finally, customer complaints about poor fuel quality, reduced vehicle performance, or mechanical issues after refuelling are serious red flags that contamination has already impacted consumers and must be addressed immediately.

Dip your tanks regularly for water

Preventative Measures

• Regular Seal and Cap Replacement: Inspect dip points, fill points, and vapour recovery connections frequently. Replace cracked, worn, or missing seals and caps immediately. Use manufacturer-approved parts to ensure watertight barriers.

• Manway and Lid Inspections: Check gaskets and lids for warping, corrosion, or loose fittings. Reseal lids and replace gaskets as required. For traffic-exposed manways, install reinforced covers to reduce stress and seal failure.

• Vent and VR Line Maintenance: Inspect vent and VR lines for cracks, corrosion, or loose joints. Replace damaged or missing vent caps promptly. Consider upgrading to weatherproof vent caps with rain deflectors to reduce rainwater entry risk.

• Tank Integrity Testing: Perform regular integrity testing to identify cracks, pinholes, or corrosion in tank walls. Precision tank testing are especially important.  Tank Testing in Australia – Accurate, Fast, and Beyond Compliance

• Spill Bucket Management: Keep spill buckets clean and empty. Water and debris in spill buckets often flow directly into tanks during filling if seals are not tight.

• Tank Filling Practices: Train delivery drivers to check for proper sealing during deliveries and to avoid overfilling, which can compromise seals and allow water entry.

• Condensation Control: Maintain tanks as full as operationally possible to minimise headspace and reduce condensation. In high-humidity areas, increase monitoring frequency.

• Routine Sampling and Monitoring: Use water paste and ATG systems to regularly check for water presence. Conduct manual sampling to verify electronic readings.

• Preventative Biocide Treatment: For diesel tanks, consider periodic preventative dosing with approved biocides to reduce the risk of microbial growth if trace water enters.

• Record Keeping and Compliance Audits: Maintain detailed records of inspections, seal replacements, and integrity tests. This not only helps in early detection but also ensures compliance with UPSS regulations.

Reactive Measures: Tank Cleaning & Fuel Restoration

Even with strong preventative maintenance, water ingress and fuel contamination can still occur. When that happens, swift and professional action is critical to protect your customers, your equipment, and your compliance obligations.

Flowco provides comprehensive reactive services to restore fuel quality and bring storage systems back into safe operation.

• Emergency Water Removal – We deploy specialised equipment to quickly remove free water from underground tanks, stopping further contamination and protecting fuel stock.

• Tank Cleaning & Fuel Polishing – Contaminated product can often be saved. Our filtration and polishing systems remove water, sludge, and particulates, restoring fuel to meet specification. This reduces costly fuel disposal and protects your bottom line.  For tanks affected by microbial growth (“diesel bug”) or heavy sludge, Flowco’s technicians perform deep cleaning to remove contamination and restore tank integrity. Read more at Tank Cleaning & Fuel Restoration Process

• Phase Separation Management – In ethanol-blended fuels, phase separation renders petrol unusable. Flowco isolates, removes, and safely disposes of unusable product while recovering the remaining clean fuel.

• Testing & Certification – Once cleaned, tanks and fuel are tested and certified to meet Australian standards, giving operators confidence and proof of compliance for regulators. Read more at Tank and Line Testing

• Rapid Response Teams – With national coverage and a fleet of specialised cleaning trucks, Flowco responds quickly to minimise downtime, customer disruption, and reputational damage.

When contamination strikes, Flowco gets your site back on track—safely, efficiently, and with minimal loss.

Water finds its way into underground petrol storage tanks through damaged seals, lids, vent systems, groundwater pressure, and condensation. With regular inspection and maintenance, operators can dramatically reduce the risk of contamination, protect fuel quality, and extend the life of their storage systems.

If you have any questions please Contact Us.