Folks have been dealing with this paper making process since 1879.
( https://en.wikipedia.org/wiki/Kraft_process )

As to the MSDS:

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=MSDS+on+black+liquor

Avoid it if it is hot. It no longer is, nor likely was it when it reached the surface.

Don’t breathe it as mist. It did not come down as mist.

Gas concentration was initially typically less than 1%. Much lower by the time it reached the surface, much lower than that after an hour, much lower than that the next day,…

Hitting it with acids will produce H2S which is toxic at high enough levels, would mostly just stink in amounts encountered. (same as rotten eggs)

The main danger was when it was boiling hot. That ended several seconds after the digester blew.

Biodegrades naturally. Has an oxygen demand while doing so. H2S is released quickly and gone, other components are released slowly and in low concentrations unless you agitate it. Not likely to be a problem here.

Most common way of getting rid of it is simply using it again in paper making. That likely won’t work here, too dispersed.

None of the components are listed under Cal. Prop. 65. (Proposition 65 requires the State to publish a list of chemicals known to cause cancer or birth defects or other reproductive harm.)

Don’t drink it. It is full of wood fibers.

Don’t boil it and pour it on yourself. Boiling liquids are harmful.

David for doomsayers