You can tell from my avatar that I'm not interested in hurting any cats but it sounds like we're talking about different glycols, based on what @moey posted. Antifreeze (which is mostly ethylene glycol + some other stuff) has an NFPA health hazard rating of 2 (moderate), while polypropylene glycol is rated 0 (insignificant). It's also biodegradable so, if it gets out, there's a chance it will break down before it goes too far.I guess I've read too many stories of dead cats from drinking anti-freeze left outside of garages. I'm not sure what the "safe" concentration would be.
. Its also used in a lot of food products included carbonated beverages and cake icing. Given the volume in geothermal loop even if the entire loop dumped down the hole and into the aquifer it would be diluted quickly.
