Dissolved Inorganic Carbon in the Tucson Basin Aquifer

The concentration of dissolved inorganic carbon (DIC) species and the other major ionic constituents were determined at 45 sampling locations within the Tucson basin aquifer. The distribution and concentration of the various inorganic carbon species were related to the hydrogeological characteristics of the principal recharge zones within this alluvial basin. Bicarbonate alkalinity concentrations were highest subjacent to a floodplain where calcite and other relict salts likely precipitated in the shallow subsurface under past pluvial conditions. Calculated partial pressure of carbon dioxide (PCO 2 ) values were highest where the water table is sufficiently elevated to support phreatophyte growth. The regional distribution of PCO 2 suggests that the generation of carbon dioxide is limited to the oxidation of influent plant detritus below the stream channels. The alkalinization of Tucson basin ground water is ultimately constrained by this limited production of CO 2 . The dissolution of calcite in young ground water (<200 years) and the precipitation of calcite in deeper, more chemically evolved ground water likely represent other important controls upon DIC concentrations.