$4.$ About $200$ m$3$ industrial liquid waste with contaminant concentration $($C$0)$ of $469$ kg$/$m$3$ is injected into a homogeneous unconfined aquifer through an illegal deep disposal well. The ground water flows along the positive x$-$axis. The aquifer characteristics are: porosity $=0\mathrm{.}1,$ hydraulic conductivity $=10$ m$/$day$,$ hydraulic gradient $=0\mathrm{.}003,$ hydrodynamic dispersion coefficients for x$,$ y and z directions $=1\mathrm{.}2$ m$2/$day$,0\mathrm{.}1$ m$2/$day and $0\mathrm{.}04$ m$2/$day$,$ respectively. The contaminant is$,$ however, biodegradable and the decay coefficient $(\backslash$lambda $)$ is $0\mathrm{.}003$ d$-1.$ After $45$ days of the injection, estimate the equivalent concentration at pt$.$ A and pt$.$ B if the location of injection point is at the origin $(0,0,0).$ Assume no retardation. Pt$.$ A $=(16,-2,1),$ Pt$.$ B $=(20,0\mathrm{.}2,-0\mathrm{.}1).$

$5.$ Given the flow situation in Question $4,$ how long does it take for the contaminant to migrate from the injection point to a sampling point located at $(40,0,0)$ with $2\mathrm{.}8\%$ of its original concentration?