In the 1960s through 1980s, a medical filter manufacturer in Ann Arbor discharged 1,4-dioxane (an industrial solvent) directly into the environment, where it ultimately leached into underlying groundwater aquifers. A plume of 1,4-dioxane has been spreading under Ann Arbor and could pose water quality risks if it reaches Barton Pond, which supplies 85% of the city's drinking water.

a) Barton Pond contains 1.5 billion gallons (5.68 million m3) of water and initially contains no 1,4-dioxane. Assume the pond is perfectly mixed with one stream flowing in and out at a rate of 1 m3/s. Suppose the plume enters the pond at a rate of 0.1 m3/s and contains 50 mg/L of 1,4-dioxane, which has a first-order decay constant of 6.4*105 s1. What is the steady-state concentration of 1,4-dioxane in the pond?

b) Local water authorities find a way to divert the plume such that 1,4-dioxane is no longer entering the pond. How long will it take for the 14-dioxane concentration in the pond to reach the Michigan Department of Environmental Quality's new allowable drinking water level of 7.2 ug/L?

c) The EPA classifies 1,4-dioxane as a probable carcinogen with a potency factor of 0.011 (mg/kg/d)1. If the water contains the EPA's allowable limit of 7.2 μg/L of 1,4-dioxane, calculate the carcinogenic risk based on the default EPA exposure factors over a lifetime for ingestion of potable water in an average adult. Is the water safe to drink based on the "rule of thumb" discussed in class?