It's no longer uncommon to (carbon) filter drinking water at the tap or in your fridge, which then makes a hot shower likely a major avenue into your body by way of your lungs unless you filter water for the whole house plumbing system.

These developments are not pulling local groundwater.

Doesn't matter - it's in the air, too. Read the article.

There are shower heads that can be installed with carbon filters. Can be done with a wrench. No plumber required.

If you read the TFA it says, "TCE is highly persistent in soil and groundwater; inhalation through vapor from these hidden sources is likely the prime route of exposure today."

Because water evaporates and TCE is particularly good at transmission through water vapor. That's my guess.

Water vapor is co-distilled with water vapor in room temperature evaporation. Humid air a 70F has a partial pressure of about 0.2 PSI - a substantial amount. I would have to look into tabular data on the H2O TCE system to see, but it is substantial = the smell you get. It also has it's own presence on dry air, but it is higher if water present due to low temperature co-distillation. These soils need to be dug up -washed with super-critical CO2 ($$$), then baked dry with the vapor condensed. This has been done in Toronto at a few old factories. Later they started to create a large underwater landfill area - the 'spit' and created a number of clay and membrane lined lagoons(embayments) where they buried it and covered it with clean fill to make a wild life area. Started about 50-75 years ago and it was well built and monitored since then and has stayed sealed.https://en.wikipedia.org/wiki/Leslie_Street_Spit

Because it is volatile.