Tree with multiple trunks

[Achy]

I have a large chestnut tree in my garden with four main trunks, where the trunks meet at the bottom there is a 9 inch hollow, which just collects rainwater.

This rainwater is smelly, stagnant, and is full of slugs and gnats.

Is there anything I can do with this?

I was thinking along the lines of filling it with something... sand? concrete!?

Thanks for any help.

 

[bludger]

Get somebody knowledgeable to look at this. When branches grow together at too close an angle the contacting areas of bark and wood can compress each other. You can usually see the compressing ridges.

The areas are actually weakened and vulnerable to splitting apart when under stress such as a high wind. If this is the case with your tree, the rotting of the joint would make the vulnerability worse.