PRODUCTION OF CO2 IN THE CAPILLARY-FRINGE OF A DEEP PHREATIC AQUIFER

CO2 profiles obtained along a 30-m-thick unsaturated zone under land i rrigated with sewage effluents show two production regions: a seasonal one in the root zone and another, at steady state, near the water tab le (29 m). On an annual basis the CO2 flux from the deep source toward the atmosphere (6.3 g C m(-2) yr(-1)) is balanced by a similar influx of soluble organic carbon (SOC) from sewage effluents. The delta(13)C values of soil CO2 indicate that CO2 is produced from plant material in the root zone and from biodegradation of total sedimentary organic carbon in the capillary fringe. High CO2 concentration in the capillar y fringe (up to 2%) is likely to reflect a decrease in diffusivity rel ative to the unsaturated zone due to increase in both water content an d tortuosity induced by the capillary fringe structure. The long resid ence time of SOC and CO2 in the unsaturated zone (29 years at the stud y site) suggests that the unsaturated zone of deep aquifers may have a significant storage capacity for carbon and may act as a temporary ca rbon sink.