ES8901 Chemical and Biological Pathways

1. You might have already recognized that DCM has relatively high solubility and a relatively high vapour pressure. What characteristics of this molecule (look at the diagram) account for its high solubility? What accounts for its volatility?

2. Let’s start at the warehouse. The pipe is 3 m below the surface. Based on the size of the warehouse, the quantity of DCM spilled, and the volume of the vadose zone (unsaturated soil with lots of air space), we can determine that the concentration of DCM in soil air is 204 g m^-3 or 204 mg L^-1, or 0.204 mg cm^-3. The diffusion coefficient of DCM in air is about 0.10 cm² s^-1.  Determine the flux of DCM from the soil air into the warehouse. I suggest expressing this in final units of g m^-2 h^-1.

3. OK, next let’s assume that the majority of the spilled material does not volatilize and enter the warehouse. Let’s assume that it enters the river. Let’s assume 130,000 g (or 130 kg) of DCM enters the river in a short period of time (again, we can essentially treat this as a single point spill).

The river has the following characteristics:

Mean depth: 1.8 m

Mean width: 75 m

Slope: 0.001

Manning coefficient: 0.035

A. Determine the mean velocity of water in the river (let’s pretend the river has a fairly uniform geometry, like a rectangular channel).

B. Determine the travel time for the DCM to reach the water intake 40 km downstream of the spill

4. Drinking water standards for DCM are a maximum concentration of 10 mg L^-1 in the short term (for a one-day exposure). Suppose that past studies on this river allow us to determine the longitudinal diffusion coefficient (D_L) as 38 m² h^-1.  (Just as a point of clarification, the units on D_L are m² h^-1, not m h^-1, as indicated in the PowerPoint)

A. What is the maximum concentration of DCM when the peak reaches the water intake for this small community?

B. Will the concentration exceed the maximum allowable concentration under the drinking water standards?

5. The calculation you just did for question 4 assumed that the DCM behaved conservatively. In other words, the total mass of DCM that was spilled into the river would still be present when the water reached the water intake. However, in reality it is likely that some DCM would no longer be in solution (in water).

A. What are the other potential fates of DCM (what might account for a decrease in total DCM load over time)?

B. Given the Henry’s Law constant and given the K_OW which of the processes you indicated above likely account for the greatest loss of DCM over time?

6. Based on the processes you indicated above, let’s imagine that a total of 100,000 g (or 100 kg) remains when the river enters into the lake. The lake is thermally stratified, it is the summer season.

A. Explain why a lake becomes stratified. How does the water in the hypolimnion differ from the water in the epilimnion?

B. Transport in the river was advective, gravity driven. DCM was well mixed throught the water column. In the lake, transport is much less advectively driven, and more driven by Fickian mixing. The water column in a thermally stratified lake is not mixed top to bottom. Review the physical properties of DCM (above). In what part of the water column of the lake is the DCM likely to collect? Why?