What's below the granite? In other words, what kind of structure is producing the water now? Hydrofracing is more effective in some types of layers and not effective in others.
If radon is not present now, its more likely that you won't get any. Radon, as far as I know, usually has a general distribution rather than being totally contained in particular pockets with none to be found in the surrounding zones. Chances are that, if you haven't found it yet, you won't find it anywhere near enough to affect your well after fracing, if you've seen no traces of it so far.
Water quality may be affected by the type of structure that you attempt to frac. Its possible that quality may be temporarily affected while fractured materials are cleared and return to better levels afterwards. Water may be purposely introduced and circulated in the frac affected zone to attempt to clear fractured particles immediately.
Sometimes fracs don't work. Overfracing can sometimes destroy the water bearing properties of the formation by causing the existing fissures to nearly, or entirely collapse once fracing pressure is removed.
At the depths you are talking about, there is little likelyhood of affecting anything at the surface, however again depending on the formation, damage to surrounding wells could be at least a slight possibility. You'll need to assure that your contractor is very "well" experienced with the formation you're fracing and that he has done many more than just one frac into that formation with similar characteristics and at similar depths. Do not accept his word alone, if he is unknown personally to you. Get his previous work references and make the rounds to at least check a couple of random cases he has given you. There may be additional records of his work at the state or county water control authority.
Your contractor should be able to determine to what extent the surrounding formation will be affected. If there is another well in the area that could be affected, you should probably consider abandoning your fracing plan, or at least take extreme measures in advance to prevent potential legal troubles and try to obtain evidence now that could help you later. Personally, I would recommend that you do nothing that has any possibility to affect a neighbor's wells, unless you can somehow get his firm agreement first and then approved by your very good personal lawyer, but it still could be dangerous. If a critical well loses production, or quality, you can bet on a fight of some kind, maybe not totally via legal means only.
Obtain signed and verified statements, or even better, previous metered records of produced water from closeby wells before your fracing and save them to compare with production records afterwards, should they be needed. You should try to have solid evidence that there was no change in the production of surrounding wells after you have concluded your fracing operation.
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"The problem isn't working out the equation,
its finding the answer to the real question." BigInch