Hi
I'm using GRASS for ground water modelling, I'm using it
to hold and manipulate the data for the groundwater model
modflow. I wouldn't say it was ideal, lack of good three
dimensional capabilities is restricting. Fortunately
the area I'm interested in has fairly simple geology so
I can model the various parameters, such as transmissivity
etc as 2 1/2 dimensional layers.
On the plus it is very easy to write new applications
if you know a bit of C.
mail me with any specific questions.
cheers Tom 
Tom Charnock O--O
Dept Civil Engineering (~~~~)
Aston University ( __ )
Birmingham B4 7ET UK /|\ /|\
charnotw@sun.aston.ac.uk