User "AllAboutKeeley" voiced a valid point, "There would be no way to monitor this. Even if they can put something on my well to monitor how much we use, there would be absolutely no way they could know if we collect rain water or how much of it we do collect."
Inadvertantly I found this little morsel:
Granted this isn't to tax the water itself, BUT they have imposed a tax for any impermeable surface (ie hard surface) on your property that cause run-off and therefore pollution. "ten counties in Maryland are being levied with the Rain Tax, even though not all counties are responsible for runoff pollution in the Chesapeake Bay."
So, even though they can't point their finger and say - "YOU are collecting rainwater" - they CAN tax your hard surfaces that would allow you to collect rain water efficiently.