• The first tank shown here has seepage at its fill and vent fittings, a rather typical and trivial problem which is hard to avoid unless meticulous sealant application is used during assembly of the fill and vent piping - the machining at the tank top and on larger diameter piping is often a bit rough. The second tank shown here is an older one which shows the same seepage around the tank top fittings. But what about those oil stains on the lower left tank body? The tank may be leaking at its seams or from perforations. Further investigation is warranted.
• Water accumulating in the tank from bad oil deliveries or from wide temperature swings and in-tank condensation. Test the tank for water and remove water from the oil tank by pumping (large amounts) or by absorbing chemicals or pads (small amounts of water).
• Poorly-made plumbing connections. Inspect and correct any leaky fittings.

• Include the causes listed above and add the following:
• Exposure to wider temperature swings, especially in cold and humid climates can increase in-tank condensation
• Exposure of the tank filler or vent pipes to rain, especially to roof runoff for tanks mounted under the eaves of a home and especially if a proper cap is not kept on the tank filler pipe or if the vent pipe is not properly installed and shielded.
• External rust due to exposure to the weather. Many small tanks, 250g or 275g, were never rated for outdoor use, as you may easily ready by examining the embossed UL label affixed to most tanks. Manufacturers of newer oil storage tanks in this size range appear to have removed this "indoor use only" wording from the UL label, as you can see in the photo shown here. We have not learned whether the standard was changed, the tanks were changed, or simply the label was changed. An outdoor tank should be protected from the weather and in freezing climates should be given shelter and warmth.
• Improper installation / installation of improper oil tank type: the second photo above shows an old, indoor-oil tank which was moved outside and placed onto the soil surface below a deck, and then was half buried. What is this tank, a buried tank or an above ground tank? It was not rated for outdoor use at all and is at extra risk of leakage due to placement of the tank body directly in contact with the soil.
• Improper tank supports, such as failure to keep the outdoor tank off of the ground, to install it at the proper pitch and direction of pitch, and to install it on level, secure piers can lead to a tank tipping over, ripping open an oil line, and obvious catastrophes of oil spillage and heat loss.

We discuss the inspection of above ground oil storage tanks at ABOVE GROUND OIL TANK INSPECTIONS.

• Include the causes listed above and add the following:
• Exposure of the tank filler to surface runoff (as shown in the photograph): some oil tank installers or homeowners insist on installing the tank filler flush with or even below ground level, perhaps for aesthetic reasons. This detail increases the risk of snow melt or surface runoff entering the oil storage tank, particularly if the filler cap is left off or if its o-ring or gasket seal is not perfect.
• Smaller tanks which have been buried, such as a 250g or 275g tank, especially if more than 10 years old, are suspect: we have not ever found one of these that was labelled as approved for outdoor below-ground use. If you have a buried oil tank in this size range further investigation is warranted.
• Mechanical damage to the tank during installation, such as scraping its protective coating (if there was one), or omission of any protective coating on the buried tank. Damage might occur for example, by scraping the tank with equipment used to move it, or by setting the tank into a hole atop a rock or other debris. Fiberglass or plastic tanks may be more vulnerable to serious mechanical damage during installation.
• Corrosive soils - soil corrosivity may be a significant factor in buried steel oil tank life in areas of soils with a high corrosivity index. Plastic or fiberglass tanks are less likely to be affected by soil corrosivity.
• Amount of ground water in soil: wet soils increase the risk of corrosion on buried steel tanks and increase the risk of leaks into a damaged (or poorly plumbed) tank for both steel and plastic/fiberglass oil storage tanks.