It's not supposed to leach into the groundwater, but nobody is enforcing the companies that are handling these chemicals to ensure they don't get into the groundwater supply. Many of the worst contaminations in the area were caused by big tech companies such as HP who didn't realize their underground TCE tanks were leaking whoops.

As of 2023 only two states have banned TCE (Minnesota and New York), and the federal government has yet to do anything to control it. It has and will continue to be used extensively in industrial application such as at electronic assembly lines, dry-cleaners, mechanics, air force bases, coffee decaffeination, textile industry, and the list goes on. The best you can do is live in a highly residential area which is far from the locations where any of these business could operate.

It's not allowed. These are old plumes and they stay around for a long time and are very expensive to clean up.