FPTD: Tracer test measurements often consist of one-dimensional (averaged) tracer concentration breakthrough curves, as a function of time t, at selected distances from the tracer source. The breakthrough usually refers to the plane of exiting particles, and the (non-cumulative) curve corresponds therefore to what is known historically as a "first passage time distribution" (FPTD). In terms of the CTRW formulation, the FPTD is defined as the probability per time for a tracer particle to reach the measurement plane at time t for the first time. Averaging over the coordinates parallel to the exit plane yields the average distribution of tracer per unit time. We note that this probabilistic formulation of tracer distribution is equivalent to that of flux-averaged concentration values.