$2.$ As a result a car accident, $4$ m$3$ of liquid waste containing $9\mathrm{.}2$ kg of toxic chemical rapidly infiltrated into a shallow, uniform, sandy aquifer. The approximate spill area is $2\mathrm{.}2$ m$2.$ The spill is located at $(-1,2)$ m$.$ The average groundwater velocity in the aquifer is $0\mathrm{.}7$ m$/$day along the positive x$-$axis. The hydrodynamic dispersion coefficient for x$-$direction is $3$ m$2/$day and y$-$direction is $0\mathrm{.}2$ m$2/$day$.$ A drinking well of a small town is located at $(74,1)$ m$.$ Calculate the concentration $($g$/$m$3)$ of the chemical at the well after $($i$)90$ days, and $($ii$)120$ days of the spill. Assume the chemical is conservative and there is no retardation.

$3.$ Laboratory tests indicated that the well water in Q$2$ is unsafe for drinking if the chemical concentration is higher than $1500$ mg$/$m$3.$ The town mayor is considering the possibilities of well closure due to the groundwater contamination. Given the flow situation in Question $2,$ advice the town mayor on the period and duration of the closure. Plot the concentration $($y$-$axis$)$ with respect to time $($x$-$axis$).$

SOLVE Q$3$ ONLY!