CE 397 Environmental Risk Assessment
Final Exam
Spring 1998

This is a take-home exam that can be done at any time between 9AM Wednesday May 14 to 9AM Thursday May 15. Please turn in the completed exam by 9 AM Thursday to Cheryl Gamble in ECJ 8.6. You should work on this exam alone with no help from anybody else. Do all parts of all questions. Present your answers on the blanks spaces left on this exam paper. Note: All questions are equally weighted.


1. Groundwater Woes

A family is living in a home whose water supply is drawn from a groundwater well. The family is concerned that there is an industrial facility located ¼ mile away and that contamination from this facility may affect their drinking water. The facility began operations 40 years ago. Make the following assumptions: the piezometric head gradient is 0.01, the house is directly down gradient from the facility, the hydraulic conductivity is 1ft/day, the porosity is 35%, the porous medium has a particle density of 2.65 kg/l, the fraction of organic carbon is 1%, and the representative chemical to be studied is Benzene. (Benzene is used in this question and in Questions 2 and 3 because in some risk assessments Benzene is considered to be an indicator chemical such that if all is right with Benzene then the other chemicals are ok as well).


Determine whether or not the Benzene could have reached the water supply well in the 40 years of the facility’s operation. Use your best judgement to make any additional assumptions needed to solve this problem.











2. And Still More Groundwater Woes

(a). The water table at the facility described in Question 1 is 20ft below the surface. The owners of the facility in the early years weren’t too concerned about secure waste disposal and they put waste materials in barrels in a pit on one area of the plant. Subsequent investigation has shown that the barrels have corroded and leaked and waste materials have been leached into the soil under the pit. Assume that the annual rainfall is 20 inches per year of which 10% becomes infiltration. The effective porosity of the soil is 20%. How long would it take before these materials reached the water table assuming no retardation?













(b). A short distance down gradient from the waste disposal pit there is a manufacturing facility containing industrial workers. Suppose that the water table is 20 ft below the floor level of the facility and that the groundwater is contaminated with Benzene to a concentration of 10 mg/L. Estimate the concentration of Benzene in the air inside the manufacturing facility. Use the default parameter values in the TNRCC guidance manual where necessary. If you use a spreadsheet solution, please annotate a copy of the solution to show what formulas are applied to get the results at each step.


Extra page for problem 2 (if necessary)

3. Water ---- The Spice of Life?

(a). Suppose the family in Question 1 has been drinking groundwater contaminated with 20 mg/L of benzene for the last 20 years. Assuming that the mother was drinking 2 liters/day, estimate the increase in her risk of getting cancer that results from this exposure when averaged over a 70 year normal life span.




















(b). A state has established the following risk-based screening level (RBSL) of 0.32 mg/m3 for benzene in residential indoor air near "chemically obnoxious" sites.  The following characteristics may be assumed for a "typical" home:  Floor area = 600 m2,  Ceiling height = 3 m, ACH = 0.5/hr.  An average benzene emission rate associated with environmental tobacco smoke (ETS) is  435 mg/cigarette. Estimate the number of packs of cigarettes that a smoker would have to consume per day (20 cigarettes per pack) to achieve and maintain the RBSL for benzene at steady state. You may assume that cigarettes are smoked "uniformly" through a 16-hour day (so many cigarettes/hour) averaged over that period.









Extra page for problem 3 (if necessary)

4.  Better to Wear Rags??

A state has established a risk-based screening level (RBSL) of 72 mg/m3 for tetrachloroethene in residential indoor air near "chemically irreverent" sites.  "Reasonably" fresh dry-cleaned clothing has been observed to emit tetrachloroethene at a rate of 27 mg/m2/hr. Estimate the number of dry-cleaned sweaters necessary to reach the RBSL for tetrachloroethene within four hours of returning the sweaters home from the cleaners. Use the home described in problem 3(b). You may assume that the entire home is a well-mixed system. State all other assumptions. Comment on the significance of your results.







5. Little Ditty 'bout Jack and Diane.

A ground-level point source emits trichloroethene (TCE) at a rate E. Two hundred meters downwind of the source two teenagers (Jackie-Jim Johnson and Diane-Sue Duncan) from Borgers, Texas, are driving in an American-made pick-up truck perpendicular to the centerline of the plume (along the y-axis). The windows on the American-made pick-up are partially open.

(a)  Develop an equation that you would use as a "starting point" for estimating the TCE concentration in the American-made pick-up truck from Borgers as a function of time. Take the equation as far as possible without solving it. Identify all variables and assumptions.





















(b)  Suppose that the source of TCE is a groundwater well equipped with an air sparging (bubble) system. Compressed air is introduced to the groundwater at a rate of 10 m3/min in such a way that all of the air exits from the same well. Assume that the groundwater is at 10 C and contains TCE at a concentration of 10 mg/L. Air leaving the well is at equilibrium with groundwater in terms of TCE concentrations.
Now, assume that Jackie-Jim and Diane-Sue are driving in the American-made pick-up truck with their windows closed (no air intake) on a clear night with a ground-level (2 m above ground) wind speed of 2 m/s. Jackie-Jim parks his American-made pick-up truck at the exact location of the plume centerline at x = 200 m. After approximately 30 minutes of introspective political discussion, Jackie-Jim and Diane-Sue decide they need some "fresh" air and open the windows of the American-made pick-up. This allows an air exchange rate of 4/hr. After 15 minutes, Jack and Diane decide to close the windows of the American-made pick-up yet one more time for additional discussion of monetary complexities associated with the new Euro.


What is the concentration of TCE in the cab of Jack's American-made pick-up after 15 minutes of having open windows?


Does this seem alarming?


Do Jack and Diane care?



Extra page for problem 5 (if needed)

  6.  The Ball's in Your Court!

(a). Choose one of the Questions 1-5 and make a critical evaluation of your answer. What are the limitations of the methods that you have used? How do you think your answer could have been affected by these limitations?

















(b)  Suppose you were working in a professional environment where you had all the resources needed to make more detailed investigations than those you have been able to do so far. Describe briefly what you would do to reduce the limitations that you have described in part (a) of this question.




Extra page for problem 6 (if necessary)

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