Why use impervious area to determine the fee?

When rain falls on impervious areas, the stormwater is unable to be absorbed and picks up pollutants such as oil, grease, fertilizers and sediment along the way.  There is a correlation between the quantity of pollutants contained in stormwater and the amount of impervious area.  The use of impervious area is also consistent with the methodology in stormwater permit calculations and pollutant load reductions.