The map: https://www.ewg.org/interactive-maps/2025-chromium6-cooccurance/map/
One thing that's not clear from either the article or the website is whether there is any data on water systems that are not contaminated. For instance, Jacksonville, FL appears clear - is that because there was no contamination, or because there was no testing data available?
It also seems likely that the contamination is clustered around urban areas because there is data available for those water systems, not because the urban areas are more likely to be contaminated. I seriously doubt there is much regular testing data available for rural residential well water systems.