I have been following this topic with great interest. Leo has made some points with respect to DO2 in ground water that were new to me. It inspired me to do a bit of extra reading on the subject and a quick google search revealed that there is LOTS of information on this topic. A brief perusal of the literature demonstrates the axiom "It all depends" is correct. It appears that there are multiple variables at play including the location in the aquifer (both vertically and horizontally), rate of flow from the surface, rate of oxygen respiration (often determined by the concentration of organic compounds to be metabolized) and a myriad of other factors that would not be readily apparent without intensive study of the system in question.
Thanks to all the posters above for starting a very interesting discussion. If anyone is interested some of the sources are posted below.
Mike


Ground-water microbiology and geochemistry
By Frank Chapelle

http://info.ngwa.org/gwol/pdf/880245629.PDF

Study and interpretation of the chemical characteristics of natural water
(Google eBook)