rate of growth of a leaking oil tank

A gas tank on a dock has a small puncture and is leaking gas at the rate of into a lake. It forms a circular slick that is 1mm thick on the surface of the water.

so as a function of time the amount of gas leaked is but i also have to state the radius as a function of volume and as a function of time. im supposing i should approach the radius as volume as a very short cylinder, which i think i would be and as a function of a time as

this is my first time attempting a question like this and im not even sure its in the correct forum so any advice would be appreciated :)