Mercury poisoning is a debilitating disease that is often fatal. In the human body, Mercury reacts with esstenial enzymes leading to irreversible inactivity of these enzymes. If the amount of mercury in a polluted lake is 0.4 ug Hg/mL, what is the total mass in kilograms of mercury in the lake?
(The lake has a surface area of 100mi^2 and an average depth of 20ft.)