Chapter 7: Water Pollution

Homework solutions

1.    b is true.
2.    Hg and As interact with bacteria in anaerobic sediments to form     methylated species which are volatile and toxic.
3.    Massive amounts may cause bone marrow shutdown, and loss of red blood cell production. Some nuclides are preferentially absorbed into certain organs (eg I in thyroid and Sr in bone) This makes the local dose much more intense. Ionizing radiation can break DNA strands and cause mutations which may lead to cancer at much lower doses than will cause radiation sickness symptoms.

6.     5 mg/sec in 10 liter of water per sec = 5 mg pesticide per 10⁷ mg of water (that’s 10 liters in mg). This is 0.5 ppm (mg pesticide/10⁶ mg water). So it is leaking at a sufficient rate to be toxic. The question then is how long will it take to empty the barrel—278 hrs at 5 mg/sec. Since the water moves at 1 km/hr, the contamination would extend for 278 km!

9.      136 liter/sec for 1 hr is 489600 liters. 0.25 ppm is 0.25 mg/liter.

           0.25 mg/l x 489600 liters is 122400 mg or 122 g.

11.     Soap is more easily biodegraded than ABS detergents and usually  precipitates in sewer treatment plant (with Ca⁺²), so does not cause foam in environmental waters or solubilize fats and oils in the treatment.

14. Use Eqn 7.13.5 to calculate the activity after 16 hours. (A_{o = 1000cpm, the rate constant, calculated from the given half life, using Eqn 7.13.6. = .029/hr, and t = 16 hr. A = 630 cpm. So the whole sample of radioactive isotope would have an activity of 630 cpm after 16 hrs. But in 16 hrs half the isotope would have been cleared from the body by normal biological processes. Therefore, the activity left after 16 hrs would be half of 630, or 315 cpm.}

17.   A low BOD relative to TOC indicates a biorefractory contaminant, so it is probably the less biodegradable ABS which is causing the problem.

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