An industry is illegally discharging benzene into a river. The concentration in the river at the point of discharge is 0.2 mg/L. A drinking water treatment plant (DWTP) is located 1.5 km downstream. The water velocity in the river is 0.35 km/day.
A. What is the hydraulic residence time of the river between the discharge point and the DWTP?
B. If benzene degrades with a first-order rate constant k = 0.75 d^-1, what is the benzene concentration at the DWTP at steady state?
C. The oral potency factor for benzene is 2.9×10-2 (mg/kg-d)^-1 (text, Table 4.9). If a 70 kg person drinks 2 L of the water from the DWTP every day, is the lifetime cancer risk greater than 1 in one million? Assume no removal in drinking water treatment.