Transit time distributions are not L-shaped

A probability density function f(t) with origin at t = 0 is defined here as being 'L-shaped' if f(t) The distinction between the inclusion or not of particles already at the observation point might seem pedantic but does have an important implication: if transit time distributions in the hydrological environment are defined to not include non-transiting particles, they must have the property f ( 0) = 0. This negates the possibility of L-shaped transit time distributions. The exclusion of non-transiting particles also has the implication that transit time distributions must have at least one mode at t > 0, which may or may not be identifiable from field measurements.