The cooling system is a major culprit. Especially if they use a zero-waste recycling cooling system. There are all sorts of toxic materials inside the computers being cooled, and those tend to leach out into the cooling fluid in the case of Vera Rubin class processors that must be liquid cooled. And those cooling systems themselves have a lot of toxic materials inside of them, to begin with. So, you have to filter out some of those toxic materials and remove them from the cooling fluid. That’s called “blowdown”.

There are some blowdown processes that leave you with a thick nasty brine that has to be trucked off site. Some blowdown processes take it all the way to creating little dried pucks of super toxic materials, and likewise have to be trucked off site.

If there is a leak of any of these toxic materials, then that can affect nearby creeks, and potentially run off into rivers and lakes that are used for intake of drinking water for cities, in addition to killing or otherwise harming all sorts of local creatures that might be endangered.

Leaks can also potentially leak into the karst geography below the facility, and into the aquifers. Which are also used as inputs to city drinking water sources.

The problem there is that a surface leak might be able to be remediated by scooping up a bunch of poisoned dirt or other materials, and trucking it off site. But you can’t do that with toxic materials that leak into the karst geography and aquifers underground. You can’t just dig up billions of tons of soil and rock to get down to the stuff that’s been poisoned and then remove that.

Likewise, many datacenters have diesel generators on site, and that means they also need diesel fuel storage. If that leaks, it’s also toxic.

Sadly, many datacenters will supposedly “treat” their blowdown and just dump it into the city sewers. Which has about the same effect as leaks.

I’m sure there are other sources to be concerned about, these are just the two main ones I’ve come across so far.