Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 84 / 3 / b / Solution

After transverse equilibration, a localized release on a sufficiently long aquifer has the advection-diffusion heat-kernel solutionwhere $\mathcal{M}=\int Cdx$ is conserved. The actual tracer amount per unit source length is $\varphi h\mathcal{M}$ in this normalization. Its center moves at the mean interstitial velocity $U$, its variance is $2D_{\mathrm{eff}}t$, and its peak decreases as $t^{-1/2}$. A finite initial packet is convolved with this kernel.

Before times of order $h^2/D$, this effective Gaussian description need not hold. The parabolic velocity profile initially shears a packet, and particles initially at different depths acquire different along-flow displacements. Transverse diffusion subsequently erases depth memory and produces the long-time Taylor dispersion law. A short pulse can therefore have an initially asymmetric or strongly distorted longitudinal shape before entering the averaged regime.