What a pile of tripe.

First of all, as others have indicated, the scattering of human remains is a "white people" thing.

Secondly, the chemical properties of human ash are easily mitigated prior to scattering.

But thirdly, and most importantly, the dilution of the ash in the environment is not considered at all. I did some quick calculations - which I'll post separately later - and I'm pretty sure we could dissolve the ashes of the entire world's population in just Lake Ontario. And the earth has about 1 million times more water than Lake Ontario.

But wait, there's more!

It's not just about volumetric dilution, but also temporal dilution. Less than 1% of the world's population die each year, and only a fraction of those people are cremated, and only a fraction of those have their ashes scattered. Each scattering on land only affects a few square metres of land.

Even if there is some environmental damage locally to the immediately area, the Earth is plenty capable of remediating it on its own.

The earth has about 5x10^8 km^2 of land; that's 5x10^14 m^2. If we needed, say, 25 m^2 to safely scatter a person's ashes, then we could safely scatter the ashes of everyone who dies in a year over about 1% of 1% of the Earth's surface. And over a few years, any damage caused would be remediated by the Earth itself.

Human ashes just aren't significant. It's just not a problem!