On the road with the Darcy Lecture.

I am in the midst of my 2008 Darcy Lecture year; so far, I have enjoyed every one of my visits to deliver the lecture. I have been to Europe twice, have completed a tour to China, and traveled throughout the United States. In the fall, I return to Europe, then head to Australia, and will make many additional visits in the United States. More often than not, I have lectured to large and quite mixed groups, which include not only hydrogeologists but also many others who have an interest in the carbon and climate problems. I have been fortunate to have terrific audiences in places like the Ecole Polytechnique Fédérale de Lausanne in Lausanne; the International Association of Hydraulic Engineering and Research International Groundwater Symposium in Istanbul; a number of universities in China, including Tsinghua and Peking; the MODFLOW conference in Colorado; the Annual Ground Water Summit in Memphis, Tennessee; and universities across the United States and parts of Canada. My lecture is titled Geological Storage as a Carbon Mitigation Option; the basic structure of the lecture follows. I begin with an overview of the carbon problem; I then consider feasible solutions using existing carbon mitigation technologies. The arguments borrow heavily from the work of my Princeton colleagues Steve Pacala and Rob Socolow; their classic paper (Pacala and Socolow 2004) has impacted the discussion of carbon mitigation more than any recent work I know. In particular, I introduce and define the concept of the ‘‘stabilization wedge,’’ which has become part of the general vocabulary in carbon mitigation discussions. After considering a range of technologies that can deliver at least one ‘‘wedge,’’ I focus on carbon capture and storage with a specific focus on geological storage. I describe the overall physical system associated with CO2 injection into deep geological formations, and I use concerns about possible CO2 leakage along old wells as motivation for a particular modeling strategy we have developed. That strategy is based on simplifications that focus on the most critical parts of the problem. Solution of the resulting equations allows for very efficient calculations. We have used this approach to model possible leakage along wells with concomitant calculation of the space-time evolution of both the CO2 plume and the associated larger-scale pressure perturbations. Our models allow us to predict the movement of both CO2 and brine over three-dimensional domains with many potentially leaky wells and many