> This untreated salt water can’t just hang around in ponds—or, in worst-case scenarios, go into oceans or sewers. Disposal depends on geography, but typically the waste does go into oceans or sewers, if not injected into wells or kept in evaporation ponds. The high concentrations of salt, as well as chemicals like copper and chlorine, can make it toxic to marine life.
> “Brine underflows deplete dissolved oxygen in the receiving waters,” said lead author Edward Jones, who worked at the institute and is now at Wageningen University in the Netherlands, in a press release. “High salinity and reduced dissolved oxygen levels can have profound impacts on benthic organisms, which can translate into ecological effects observable throughout the food chain.”
It's an actual waste product that has to actually be responsibly handled. It's not "just the salty water from the ocean that we can dump back in however we want."