Consider a solute$-$soil system with a distribution coeeficient $0\mathrm{.}55$ mL$/$g and volumetric water content of $0\mathrm{.}30.$ Tracer test results indicate that the contaminant of interest migrates through the aquifer at $20\%$ of the average linear velocity.

$($a$)$ what is the retardation factor?

$($b$)$ what is the dry bulk density of the soil?