Write the mass balance equation for a completely mixed lake with an impulse input of mass, M, added at time t = 0. Assume the lake is of constant volume, V, with continuous outflow, Q, and a conservative substance. Solve for the outlet concentration in terms of the initial concentration C0, the mean hydraulic detention time, td, and time t. Plot the normalized concentration, C/C0, versus normalized time, t/ td. How long does it take for 90% of impulse input of material to be depleted from the system?