An underground gasoline storage tank has leaked benzene into an unconfined aquifer over a period of several months. The aquifer characteristics are: Ambient temperature=8 oC Hydraulic conductivity = 123 ft/day Hydraulic gradient = 0.00091 ft/ft Effective porosity = 0.31 Total organic carbon = 198 mg/kg Bulk density = 1.65 g/cm3 What is the average velocity of the groundwater?