Watery world of underground chemistry

Have you tried the ELI 'The watery world of underground chemistry'?  The pH of water is used as the basis of a discussion of how water flows through, and interacts with, the rocks and soil during the underground part of the water cycle, so integrating aspects of the atmosphere, hydrosphere, biosphere and lithosphere.
This is one of many 'watery' Earthlearningideas which are listed in Teaching strategies on our website.