Production of CO₂ in the capillary fringe of a deep phreatic aquifer

CO₂ profiles obtained along a 30-m-thick unsaturated zone under land irrigated with sewage effluents show two production regions: a seasonal one in the root zone and another, at steady state, near the water table (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 δ¹³C 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 capillary fringe (up to 2%) is likely to reflect a decrease in diffusivity relative to the unsaturated zone due to increase in both water content and tortuosity induced by the capillary fringe structure. The long residence time of SOC and CO₂ in the unsaturated zone (29 years at the study site) suggests that the unsaturated zone of deep aquifers may have a significant storage capacity for carbon and may act as a temporary carbon sink.