Surface controls on the characteristics of natural CO2 seeps: implications for engineered CO2 stores

CO₂ injected into rock formations for deep geological storage must not leak to surface, since this would be economically and environmentally unfavourable, and could present a human health hazard. In Italy natural CO₂ degassing to the surface via seeps is widespread, providing an insight into the various styles of subsurface ‘plumbing’ as well as surface expression of CO₂ fluids. Here we investigate surface controls on the distribution of CO₂ seep characteristics (type, flux and temperature) using a large geographical and historical data set. When the locations of documented seeps are compared to a synthetic statistically random data set, we find that the nature of the CO₂ seeps is most strongly governed by the flow properties of the outcropping rocks, and local topography. Where low‐permeability rocks outcrop, numerous dry seeps occur and have a range of fluxes. Aqueous fluid flow will be limited in these low‐permeability rocks, and so relative permeability effects may enable preferential CO₂ flow. CO₂ vents typically occur along faults in rocks that are located above the water table or are low permeability. Diffuse seeps develop where CO₂ (laterally supplied by these faults) emerges from the vadose zone and where CO₂ degassing from groundwater follows a different flow path due to flow differences for water and CO₂ gas. Bubbling water seeps (characterized by water bubbling with CO₂) arise where CO₂ supply enters the phreatic zone or an aquifer. CO₂‐rich springs often emerge where valleys erode into CO₂ aquifers, and these are typically high flux seeps. Seep type is known to influence human health risk at CO₂ seeps in Italy, as well as the topography surrounding the seep which affects the rate of gas dispersion by wind. Identifying the physical controls on potential seep locations and seep type above engineered CO₂ storage operations is therefore crucial to targeted site monitoring strategy and risk assessment. The surface geology and topography above a CO₂ store must therefore be characterized in order to design the most effective monitoring strategy.     

The Awkward Squad: Arts Graduates from British Tropical Africa Before 1940

ABSTRACT

Before the Second World War, most Africans from British Tropical Africa who studied abroad were seeking qualifications in three professions: law, medicine and the Church. In all these, they could be reasonably sure of employment on their return. True, government employment was problematic, but law and medicine did offer the alternative – often more lucrative – of private practice. Much more threatening to colonial authority were arts students: those who read philosophy, literature, politics or sociology. This article focuses on several men who graduated in Britain or the USA: Kobina Sekyi, J.B. Danquah, James Aggrey, Bankole Awoonor-Renner (Gold Coast); Eyo Ita, Nathaniel Fadipe, Nnamdi Azikiwe – and his (mostly Igbo) protégés (Nigeria); Hosea Nyabongo, Balamu Mukasa, Ernest Kalibala (Uganda); Peter Koinange (Kenya). It concludes by surveying the career of Nicholas Ballanta-Taylor (Sierra Leone), who studied music in New York and in 1927 won a fellowship from the Guggenheim Foundation for musicological research in Africa. Of these pioneer African Africanists, four wrote successful doctoral theses, Azikiwe published an academic study. Aggrey and Fadipe died prematurely. Danquah applied his intellect to essentially non-academic pursuits. Ballanta, worked as a practical musician in the 1930s, but his African research remains virtually unpublished.