Origin and distribution of carbon dioxide in the unsaturated zone of the southern High Plains of Texas

Partial pressures of CO₂, O₂, N₂, and Ar were monitored at two locations in the Ogallala aquifer system on the Southern High Plains of Texas. Samples were collected monthly during parts of 1980–1981 from nine depths ranging from 0.6 to 36 meters below land surface. PCO₂ was observed to be greater at depth than in the active soil zone and thus appears to contradict the normal process in which CO₂ is generated in the soil zone and diffuses upward to the atmosphere and downward to the water table. The δ¹³C of the CO₂ gas was quite uniform and averaged −17.9 per mil. PO₂ declined with depth, suggesting in situ generation of CO₂ by the oxidation of carbon. Several hypotheses were considered to explain the origin of the CO₂ at depth. It was concluded that the most probable hypothesis was that dissolved and particulate organic carbon introduced by recharging water was oxidized to CO₂ by the aerobic microbial community that utilized oxygen diffusing in from the atmosphere. This hypothesis is consistent with the CO₂ concentration profile, calculated production profile of CO₂, δ¹³C values of CO₂ gas, caliche, soil humic acid fraction, and dissolved carbonate in groundwater. The abundance of CO₂, its concentration profile, and its probable origin provide information for evaluating the observed complex sequence of caliche dissolution and precipitation known to occur in the aquifer.