Some general aspects of the transit-time distribution for a passive tracer released in a reservoir flow between localized sources and sinks are discussed. Because of hydrodynamic dispersion, tracer particles which sample different regions of the reservoir tend to arrive at different times, and the resulting distribution encodes information about the flow and the large-scale geometry of the reservoir in a simple way. In particular, the transit-time distribution has a region of power-law decay, whose exponent depends only on the multipole order of the distribution of fluid sources and sinks, an exponential decay region, whose characteristics are related to stagnation points in the flow field, and a shoulder whose location is related to the system size. Tracer measurements may thus be used as a non-invasive surface diagnostic tool to characterize subsurface reservoirs.