The full fuel drums are often stored lying down, particularly where there is dew or rain. This stops water being retained in the top of the drum by virtue of the raised 'lip'. The cap should be positioned as near to the top of the drum lid when it is lying down.
When required the drum is 'stood up' and the first action having removed the cap is to carry out a water test with a syringe and the little capsules. Ideally, the pipe inserted in the drum should not reach the bottom and again, ideally, anything left over should not be retained for aviation purposes. It is also sensible to 'flush' the cap with fuel before using it as a receptacle for the fuel drawn off for testing.
On airstrips in the Far East, one problem with any stacks was what creepy crawlies might have made their home amongst the drums.