Radiator sludge on garden

[PartyFops]

I had some rads replaced the other week and I now have black sludge on my lawn.

It was just in one patch but my dd decided to walk it around!

It has rained and rained since but is still there.

Any ideas?

It's iron oxide. Not toxic and you are allowed to empty radiator water down drains. Don't know what effect it will have on the grass, but stains are permanent on clothes and rough skin. You might try hosing or brushing it onto a flowerbed and forking it in to the soil.