Means for tracking sequestered CO2, using tracer gases, are under development.

Tracking fluids beneath the subsurface is not nearly so difficult as tracking injected gases, so I’m back to my earlier suggestion: require that hydrofracking fluids have tracers.

I’d think the gas companies and manufacturers of fracking fluids would be all for this approach, as it would help them prove the efficacy of their methods, while providing long-term monitoring to help prevent adverse effects.