$4.(12$ pts$)$ This question has two parts.

Three wells are located in an $x,y$ plane with the following coordinates $(x,y)$ and hydraulic head

information $($referenced to some common datum$)$:

$4a(6$pts$).$ Sketch the well field and find the direction of the groundwater flow and the hydraulic

gradient.

$4b.(6$ pts$)$ If the above problem has a hydraulic conductivity of ${10}^{-4}\frac{m}{s},$ and a porosity of $0\mathrm{.}30.$

i$.$ What is Darcy's velocity?

ii$.$ What is the average linear velocity $($true velocity$)?$

iii. For a non$-$conservative pollutant with a retardation factor of $3,$ how long will it take the

dissolved pollutant to travel $100m$ along the flow direction?