There is considerable research into adsorption of PFAS by water filter media. A recent study found that granular activated carbon$$s $($GAC$)$ Kp $($partitioning coefficient$)$ for PFOA, a type of PFAS, is around $1$ mL$/$g$.$

$($a$)$ What concentration of GAC $($in mg$/$L $$ pay attention to units!$)$ is required to lower the highest detected drinking water PFOA concentration of $186$ parts per trillion $($ppt$)$ to the $20$ ppt limit proposed by the state of Wisconsin?

$($b$)$ Does this seem like a reasonable real$-$world treatment concentration? $($c$)$ Why or why not?