A storm drain receives runoff from a street 500 m long and 12 m wide and an adjacent pedestrian walkway that has a total are of 0.5 ha in size. Estimate the lead (Pb) concentration in the runoff. The design conditions are:
(i) design storm intensity = 24.3 mm/hr,
(ii) design storm duration = 30 min and
(iii) design antecedent dry period = 3 days. Explain your calculations and state all assumptions made.