Stable-isotopic composition of soil carbon dioxide in the Tucson basin, Arizona, U.S.A.

Abstract

Most models to correct 14C groundwater ages require the δ 13C-value of soil CO 2 in the recharge areas of the aquifer at the time of recharge. Ninety-one soil gas samples were collected at three long-term sites in the Tucson area from July 1980 to June 1981 to determine the extent of seasonal variations and the influence of vegetation on the δ 13C and CO 2 concentration of soil gases. Variations were observed in both δ 13C and CO 2 concentrations. The highest CO 2 concentrations, up to 1.3% at 60-cm depth, were observed during March and April; the lowest, 0.1% CO 2 at 12-cm depth, were observed during the winter months. The δ 13C-values varied by ~ 5‰ during the sampling period. The lowest 13C contents were found in early spring and summer. Part of the δ 13C variation is due to exchange with atmospheric CO 2. Other causes could be decomposition of organic matter with different isotopic composition and different decay rates and/or to a seasonal displacement of the period of maximum activity among different varieties of plants. The mean δ 13C for samples at the long-term sites was ~ −18‰. In addition, fifteen soil gas samples were collected in the bottom of washes which are the assumed recharge areas of the Tucson aquifer. Both CO 2 and 13C concentrations showed changes with time and location. High concentrations of CO 2 of up to 4.8% were found. The mean δ 13C for these samples was ~ −20‰. A major source of CO 2 in the riverbeds is believed to be decomposition of organic matter carried out by the runoff water during the periodic flooding of the washes.