I do a lot of work with ...
Published by Ezra Boyd, DisasterMap.Net, LLC
I do a lot of work with GIS. GRASS is free and open source GIS software. It has a number of modules for terrain and surface water analysis that I have worked with a lot. I know that they have at least a few ground water modules, but i have not had an opportunity to work with them. If you have any GIS experience, GRASS is worth checking out.