Groundwater has been contaminated with three chemicals: benzene (log Kow 2.13), pentachlorobenzene (log Kow 5.03), and naphthalene (log Kow 3.36). The aquifer (rs = 2.3 g/cm3; h = 0.4; foc = 0.02) has a gradient of 0.004 and hydraulic conductivity of 500 m/day.
a. Which chemical will be transported the farthest downgradient? Draw an overhead snapshot (planar view) of the plume(s) at a representative time.
b. Calculate the retardation coefficient for the three chemicals.
c. Find the Darcy velocity and the average linear groundwater velocity.
d. Ignoring dispersion and the other chemicals, how long will it take the benzene plume to reach a drinking water well 1000 m downgradient?
e. Given an initial benzene concentration of 20 ?g/L and degradation constant in the aquifer of 0.25 yr-1, assess the lifetime cancer risk to an adult of drinking 2 L/day of benzene-contaminated well water for 30 yrs. Assume residential setting