Could you just dig out the bad dirt? If it's just overspray maybe you'd only need to removed a few inches from the surface.

See if you can wet the area enough to create standing water and look for an oily sheen. If no sheen, the area may be safe for butterflies.

Or place a scoop of the sand in a container, add water, and look for evidence of oil. Sniff for an oily smell.

My understanding is that WD40 is made from fish oil, so it may not be toxic.

Fish oil? That's great, Sandy! I thought it was some type of typical motor oil.

Sherry

Sorry...fish oil is not and never has been one of the ingredients. :-( It's definitely a petroleum product, a mineral spirit....the company keeps the exact recipe secret though dismisses the fish oil rumor.

A 'sort-of' list of ingredients can be seen in the MSDS, easily found on-line.

Here is a link that might be useful: WD40 folklore

Whoops!

...plus, WD40 does not smell fishy.

When the heating oil tank was decommissioned here years ago, the surrounding soil was tested. At concentrations of less than one part per thousand, the soil smelled oily.