Tropico 4

You will eventually you will get an oil spill - (This is random ) - I previously thought you needed to have a refinary built, but after reading The Globilizers comment, 10minutes later i had a tanker capsize and spill its oil into the sea... When it happens you will get a short cut-scene showing the spill - You will first get an option to either spend 300 credits a month, 100 a month, or nothing you will contribute towards cleaning the spill, i chose 300 a month, then clicked on the rig, and there will be an option at the bottom to clean the spill immediately - This cost me around 48,500 credits. Do this and the achievement will unlock as long as you do this within the 4 month period.

There are 2 scenarios which can occur.

One is an oil tanker starts spilling oil. This is not connected to an oil refinery, it can just happen.

The second scenario is an oil platform from an oil refinery starts spilling oil.

When either of these occur it will zoom to them so you know it happened. At this point you can select any option it gives you and then click on the spill and press X to fix it. If you do not have the cash to fix it then issue the print money edict up to five times to get the cash. This is under the Economy and Tourism tab.

I found a good way to improve the chance of this occurring is do a sandbox (God mode Off) game with Plenty of Random Events set. I have had 3 oil spills in 50 years.