ISOTOPE HYDROLOGY OF THE GROUNDWATERS OF CENTRAL MEXICO.

The environmental isotope (**1**8O, D equals **2H) composition of the groundwater of central Mexico divides these into three distinctive groups: groundwaters whose isotope composition fits the contemporary meteoric-water line with d equals 10**0///0//0; these are from shallow aquifers, mainly in volcanic and alluvial rocks; water whose isotope data plot to the right of the meteoric-water line, describing a new meteoric-water line of d equals 5 minus 7**0///0//0; this group is made up of waters from the confined limestone aquifers which are of regional dimensions; groundwaters with marked oxygen excess which emerge in regions of recent/subrecent volcanic activity and are geothermal in nature. The water sources of central Mexico can be fitted to a general hydrogeological model, which is presented.