Can you be a bit more specific? There are a few I can think of off the top of my head. When you sample you want to filter in 0.45 um or less to remove any suspended particles. You need very good temperature control. You need experimental blanks. You need some agitation. The SSA needs to be known either by BET or geometrically. The mineral or rock has to be very well characterized. The batches need to be large enough to get a good number of samples from. And of course, you will need to run preliminaries just to figure out how often to sample. It's a tedious process. If you have specific questions, I might be able to answer them. There is a paper on phosphate posted in my profile - I think the online supplement to that article might help you out. Of course, those were done by initial rate method - you might not be doing that,
Chemical kinetics play a important role For rock-water interaction study. Disintegration, dissociation, dissolution are the main terms which take part in the study. Laboratory study has been carryout in particular for solubility of fluoride in groundwater..Results are very much informative.For detailed study please go through our papers :
V.K.Saxena and S. Ahmed 2001
Dissolution of fluoride in groundwater: A water -Rock interaction Study, Environmental Geology, 40(9) 1084-1087.
V.K.Saxena and S.Ahmed (2003 Inferring the chemical parameters for the dissolution of fluoride in groundwater. Environmental Geology,43( 6),731-736.
I answered the questions I could in your message. I forgot to say that the best way is to use methods that have worked for others (in the literature) on similar materials. And, just understand your first few experiments will likely not work as you figure out the best parameters. That's just the way it goes.
I am agreed to the purpose of my colleagues. When you want any information about a solid phase, the first question could be : what is this chemical behaviour?
If you are interested in silicate phases, your sample preparation will be different from amorphous phases. So this is your first step : silicate or amorphous phases. Then you could find geochemical treatment for other phases on the Net.