If I had a dime for every time I heard this question. It’s a perfectly normal question to ask, and it often comes up when someone is contemplating selling or purchasing a property. Actually the question is typically posed as, “What are the chances that my tank is leaking?”
Unfortunately, there’s really no simple or definitive answer that can be given. Sure, the older the tank is, the more likely it is to be leaking. If it’s a tank that was constructed to modern standards, it will be protected from corrosion and therefore it is less likely to have leaked. But let’s forget about modern tanks for now, and consider just bare steel tanks without corrosion protection. Within that subset of tanks, here are some facts to consider:
- Tanks come in different shapes and sizes, and have different wall thicknesses. A larger volume tank will have thicker walls, and a thicker wall will take longer to corrode through. Additionally, I’ve seen old riveted tanker railcars used as underground tanks, which have exceptionally thick walls, and small heating oil tanks with thin walls made for aboveground service that were buried and used as underground tanks.
- The backfill material can be highly variable. Quality tank installations have gravel or sand backfill surrounding the tank to draw moisture away. Backfill with silt or clay will trap moisture on the sides of the tank. This can accelerate corrosion tremendously.
- The quality of the tank installation can be critical. I saw one tank installation where the tank sat directly on a concrete pad. Instead of stresses being distributed along the entire underside of the tank, they were distributed along a line where the tank contacted the pad, resulting in accelerated corrosion. There was a straight line of corrosion holes along the bottom centerline of the tank, and significant leakage had occurred.
In summary, I’ve seen 50-year old tanks come out of the ground in great shape with manufacturer’s printing still fully legible, and others that come out of the ground after 20 years riddled with holes. Unless there is documentation of the tank installation procedures and tank type, which there almost always is not, there’s really no reliable way to predict whether a tank has leaked or not simply by knowing its age. It is therefore advisable, at the very least, to completely empty an old underground tank and remove it from service as soon as practicable. This will at least reduce the potential for a future release. Historical releases, of course, will remain unknown until a soil investigation is completed.