Abandoned boreholes - additional source for CO2 emissions?

Boreholes and wells create complex boundaries at the Earth-atmosphere interface connecting the hydrosphere, lithosphere and biosphere from below and the atmosphere from above. It is important to understand and quantify the air exchange rate in the presence of at these geometries and subsequently their potential role as a source for greenhouse gases (GHGs) emissions to the atmosphere. Here, we investigate the effect of atmospheric conditions, namely atmospheric pressure and temperature, on air, CO2 and radon transport inside a 110-m deep and 0.5-m wide borehole. Temperature, relative humidity, CO2 and radon (alpha detector) sensors were placed within a cased borehole in northern Israel, and a standard meteorological station was located nearby. All borehole data were logged at high 0.5-min resolution for nine months. Results show that climatic driving forces initiated advective air transport mechanisms that had a similar effect on the CO2 and radon trends within the borehole. Diurnal atmospheric pressure oscillations controlled the daily air transport (barometric pumping) whereas temperature differences between borehole and atmosphere were important on the seasonal scale (thermal-induced convection). In addition, air velocities inside the borehole and CO2 emissions to the atmosphere were quantified, fluctuating from 0 and up to ~6 m/min and ~5 g-CO2/min. respectively. This reveals the role of boreholes as additional drivers for GHG emissions.