A critical component of the National Energy Technology Laboratory's Sequestration Program is the development of tools that can reliably monitor and quantify the amount of CO2 that leaks to the surface. One major requirement for the commercial application of geologic sequestration is accurate leak detection; i.e., leak monitoring and accurate estimation of leak volumes through continued monitoring. This is essential to assure that long-term sequestration is achieved. Significant leakage from the sequestration reservoir defeats the purpose of sequestration, which is to stabilize and then reduce atmospheric concentrations of CO2 for several hundreds to thousands of years. Multiple investigators have attempted to estimate the amount of leakage that is acceptable (e.g., Pacala, 2002; Hepple and Benson, 2002; Dooley and Wise, 2002; Herzog, 2002). Their estimates vary considerably, and range from 1% to 0.01% per annum which leads to leakage of 50% of the injected CO2 volume in 70 to 7000 years, respectively.