Re: Limestone I.D.
Do some google searching on 'end member testing'. Streams do have telltale chemistry that gets diluted downstream. I think Limestone would provide buffered (ph neutral or slightly basic) water that is high in calcium, carbonate, and/or calcium carbonate. You would either need to be good at hand titrations to determine amounts of metals, etc, or have access to a mass spectrometer (ie if you work in a mass spec lab).
End member testing is used to determine pollution sources and dilution patterns, and it can also be used to date water that is being added to streams from aquifers.
Limescale deposits (like hardwater on pipes) and actual limestone outcrops would be a much simpler identifier.
Simplest limestone test is to drop weak HCL acid on the rock. It should fizz as the acid releases carbon dioxide from the calcium carbonate. Only rocks containing limestone and marble will fizz under HCL.