oil tank containment (pools)

I regularly walk by a 500(?) gallon kerosene tank that is surrounded by a concrete secondary containment tank. This, I’m sure, is to hold the kerosene should the metal tank spring a leak. What I don’t understand is how these concrete tanks deal with rainwater that would naturally collect in them. When I walk by, there is often a fair amount of water in the concrete tank, which means if the metal tank leaked, it would just overflow the (already filled with water) secondary containment.
How is this (supposed to be) prevented?
Additionally, for those really big multi thousand gallon tanks I’ve seen surrounded by earth berms. Presumably the berms and bottom of the structure are impervious to liquid; what prevents these structures from filling up with water then failing to contain the liquid should the tank rupture?

Those are called bundings or bund systems.

Rainwater in a lot of areas will evaporate. The bundings have to be able to hold not only the complete volume of the tank (plus some extra as a safety factor) but also the full amount of rainwater that can be expected to be in there, worst case. Since storage tanks often leak small amounts of material into the bunding, any rainwater that gets in there is often treated as hazardous waste. They usually don’t just let it drain away.

If evaporation isn’t sufficient, the bunding may be pumped. Some places will have pumps built into the facility. Others may use trucks to pull out the rainwater and other stuff so that it can be taken away and processed.

Some pumps can discriminate between oil and water, for example, and can filter off the nasty stuff so that only the water gets pumped out, and if that water is then in compliance with environmental regulations, it may just be dumped back into the environment at that point.

Some bundings have coverings over them to keep out rainwater.

ETA: If the tank you are looking at would overflow if the kerosene tank ruptured due to rainwater, then the bunding is likely not in compliance with environmental regulations.

The secondary containment berms for such tanks are fitted with drains that are normally left capped. Some might even be equipped with sump pumps.

Once a particular water level has been exceeded the accumulated water is sampled and, if found acceptable, drained. If hazardous constituents are detected the water can be sucked up with a vacuum truck for further processing.

Also note that Federal Regulation 40 CFR 112.8©(2) requires that the berm be designed with adequate freeboard to accommodate both the entire contents of the tank plus a given amount of rainfall (e.g. 100-year flood):

http://www.ecfr.gov/cgi-bin/text-idx?SID=17c653b5f0d0b4aed525a1a4d98325b4&mc=true&node=pt40.22.112&rgn=div5#se40.22.112_18

Oil tank containments are secondary containment systems. They are used to catch any leaking oil, overfilling or other spills from a primary container, such as a tank, and its pipework. example for SCS ( secondary containment systems) are Bunds and drip trays.

So this tells me the bund systems need to be monitored every so often and pumped out to make sure they perform correctly in case of rupture.

I walked by the kerosene tank last night. Its 3/4 full of ice and snow. I don’t think they could pump it out if they wanted to, much less needed to.

I’ve got a 20,000 gallon methanol tank in a containment area. It has a rainwater sump with a pump to the outside. Rising liquid in the sump doesn’t automatically start the sump pump, it triggers an alarm, then the operator has to walk out to the tank and verify that the liquid is, in fact, water before starting the sump pump locally. There’s a certain amount of grumbling about having to do that in the midst of an inch an hour rain event, which is mainly when it needs to be done, but better than pumping a bunch of flammable liquid to the storm sewer.