MODULE I: WRITTEN ASSIGNMENTS
SESSION I-1. NAPL PROPERTIES AND BEHAVIORComplete questions I-1 through I-8 by writing short answers using sketches where necessary.
I-1. Explain the difference between LNAPLs and DNAPLs. (5 pts)
I-2. Give examples of two NAPLs which are composed of a “mixture” of different compounds; give one example of a “pure phase” NAPL. (5 pts)
I-3. Which diagram in Figure 2.1, pg 29, does not include a NAPL phase? Explain briefly. (5 pts)
I-4. What are the BTEX compounds and why are they important? (5 pts).
I-5. List the chlorinated ethenes and their abbreviations (5 pts).
I-6. Explain the term “source area” as presented on pages 33-39. Use sketches. (10 pts)
I-7. Comment on using the thickness of LNAPL observed in monitoring wells as an indicator of the actual amount of recoverable LNAPL actually in the formation. (5 pts).
I-8. With reference to Figure 2.14, pg 64, comment on the effects of subsurface heterogeneities on NAPL distribution. (5 pts).
SESSION I-2. NAPL DISTRIBUTION
Complete questions I-9 through I-11 by writing short answers using sketches where necessary. Before answering questions I-9, I-10 and I-11 carefully read the first three paragraphs under section 2.4. Remember that this material applies only to scenario 1 wherein contaminant is present only in the dissolved phase (i.e. no NAPL phase is present), such as dissolved organics released directly from process units or industrial waste water systems. Note also that the scenario 1 condition could also arise after the NAPL phase has been completely removed from the source area and only the dissolved phase remains.
I-9. With reference to Figure 2.8 and Equation 2.1 explain the concept of the “pore volume”. Explain how this concept may be useful in the context of an active remediation strategy wherein contaminated ground water is pumped to the surface and treated (i.e. pump and treat). (10 pts).
I-10. Briefly explain the simple flushing model presented in Equation 2.2. Draw a plan view sketch which illustrates the source area and the dissolved plume extending in the direction of ground water flow. Show all variables in Equation 2.2 on sketch. (5 pts).
I-11. After reviewing the development of Equation. 2.3 – 2.8, along with the information in Figure 2.9, rework Example 2.1 using a maximum benzene concentration of 500 micrograms per liter. (5 pts). (Note that the concepts underlying Equations 2.2 – 2.8 will be covered in more detail later in Module I of this course).
I-12. Briefly explain the purpose, assumptions and limitations for Equation. 2.9, pg 47. (5 pts)
I-13. Explain the concept behind the effective solubility relationship (Raoult’s Law) given by Equation 2.11. If the aqueous solubility of benzene is 1760 mg/l what is the benzene effective solubility if gasoline with a 0.02 mole fraction of benzene comes into contact with water for a long enough time to achieve chemical equilibrium? (10 pts)
I-14. With reference to Figure 2.13, pg 62, discuss in depth the how both LNAPLs and DNAPLs are distributed and dissolve into groundwater. Organize your answer by discussing the site-wide, core, pore and sub-pore scale sketches shown in Figure 2.13, pg 62. (20 pts).
I-15. Define residual saturation in the context of a NAPL released into the vadose zone. Now briefly explain Figure 2.20, pg72. What implication does the NAPL residual saturation, along with the information in Figure 2.2, have on the ability to pump NAPL or “free product” directly out of the ground? (20 pts)
SESSION I-3. EQULIBRIUM PARTITIONING
I-16. Locate the attached (hard copy) 10-page Partitioning Theory Problem which analyzes the equilibrium partitioning of three components of gasoline (Benzene, Toluene, and ethylbenzene) inside a 1 cubic meter control volume containing air, water, solid and gasoline in the NAPL phase. This problem has been set up and solved so that you can see the computational approach needed to determine phase distribution of organic contaminants in the subsurface. As you can see the problem isolates a small volume of gasoline floating on the surface of the water table and uses the portioning relationships (Raoult’s Law, Soil-water Partitioning formula, and Henry’s Law) which you have been studying in Session I-4. You should begin by working through the problem solution until you are satisfied that you understand the conceptual approach as well as the individual calculations.
After you are familiar with the problem and solution re-solve the problem (for Benzene only) assuming that the initial mole fraction of benzene in the 0.1 liter of gasoline is 0.04 instead of 0.02. Show all your work below. Use additional pages if needed.(20 pts)
The final answers you should get for mass of benzene in each phase are approximately:
Mwater = 203.1 mg (0.203 g)
Msoil = 1952.7 mg (1.952 g)
Mnapl = 30.9 mg (0.0309 g)
Mair = 45.6 mg (0.0456 g)
Mtotal = 2232 mg (2.232 g)
I-17. Now re-solve the partitioning theory problem (for Benzene only) assuming that the organic carbon content is 1% instead of 2%. Show all work below. Use additional sheets if necessary. (20 pts)
The final answers you should get for mass of benzene in each phase are approximately:
Mwater = 183.5 mg (0.1835 g)
Msoil = 881.9 mg (0.8819 g)
Mnapl = 27.9 mg (0.0279 g)
Mair = 41.2 mg ( 0.0412 g
I-18. The default input for the EPM is identical to the hard-copy Equilibrium Partitioning Problem which you have just worked by hand. Access the EPM and verify the calculations you made by hand in problems I-11 and I-12. (10 pts)
I-19. The value of “fraction of organic carbon, foc,” varies widely in different soil environments. Examine the “sensitivity” of the model to foc by changing its value over three orders of magnitude: 0.2, 0.02, and 0.002. In each case record the resulting value of “mass of benzene in the water phase (grams)” from the solution table. Which of these foc values is most favorable to conventional “pump and treat” remediation (i.e. pumping ground water to the surface and treating it)? Which foc value would favor excavation of the soil phase as a remediation strategy? Which foc might be most favorable for using soil vapor extraction?. (20 pts)
SESSION I-4. NAPL DISOLUTION AND PLUME FORMATION
I-20. There are 4 process-based models described on pages 83 – 96. List each by name (no equations please) then write a short description of each modeling approach. (10 pts).
I-21. Explain (using sketches and equations) the three methods of “source life time” calculation illustrated in Figures 2.37, 2.38, and 2.39. (20 pts)
SESSION I-5. MASS TRANSPORT PROCESSES AFFECTING PLUME MIGRATION
The following questions are intended to focus on key concepts and information.
I-22. Sketch the experimental apparatus used to illustrate Darcy’s law.
I-23. What are the fundamental units of volumetric flow rate (discharge), hydraulic head hydraulic gradient, and specific discharge (Darcian velocity).
I-24. Explain the concept of “average pore velocity” in the context of a conservative tracer moving through a one-dimensional porous media column.
I-25. Specific discharge (Darcian velocity), average pore velocity, and actual pore velocity all have units of velocity (L/T). Explain the relationship between these three variables.
I-26. The solution shown for Example 3.3 states that the second term in the analytical solution equation is “usually negligible”. Calculate the magnitude of this term using the data given and verify this assumption. What per cent error is involved? (5pts)
I-27. A residence is located 500 feet from the local gas station. A test of the homes’ well water reveals a benzene concentration of 70 mg/L. The concentration of benzene in the ground water directly below the source area is found to be 750 mg/L. Again use the analytical solution for advection-dispersion presented in Example 3.3, pg 136 (neglect the second term),.estimate how long the gasoline storage tank has been leaking. Use the following aquifer parameters. (10 pts)
Effective porosity = 0.3
Hydraulic Conductivity = 0.001 ft/sec
Hydraulic gradient = 0.0015 ft/ft
Longitudinal Dispersivity = 60 ft
I-28. Rework example 3.4, pg 149 using an foc of 2.3%. What is the value of R for this calculation? (10 pts)
I-29. Look over the calibration procedure for the Keesler AFB model as described on pg 395 and in Table 8.5 pg 396. Make sure you understand how and where all values are entered on the input page. Now adjust your input page will be exactly that shown in Figure 8.6. Example 8.3 uses the “instantaneous reaction model” to work through the calibration. Since we have not yet covered this approach to biodegradation (but will in Module II) let us instead focus on the BIOSCREEN results for the “first order decay” approach (i.e. the blue line appearing on the output for the “Run Centerline” solution. Run BIOSCREEN using the calibrated data file. Note the location of the blue line for the “Run Centerline” solution. Now Chance the value for “lamba” to 4.0E+0 and hit “Run Centerline”. Note how the blue line is adjusted compared to the previous solution. Now by trial and error adjust the value of lamba until the blue passes essentially equidistant between the observed data values at 32 and 64 feet down-gradient from the source. Summarize your results in a table and indicate which value of lamba gives the “best fit” to these two data points ( don’t spend a lot of time on this once you get the idea) (10 pts).
I-30. Now run the same calibrated Keesler file this time using the “Run Array” Option. When the solution screen appears select the “1st order decay” option (upper right) to display results. Here we see that concentration results displayed at 50 ft intervals in the transverse direction as well as the longitudinal direction—thus illustrating the effects of transverse dispersion. Note the “plume and Source Mass” calculations in the box at the lower right. Explain briefly how the 16.4 Kg removed by biodegradation is calculated. How much soluble mass is still left in the source zone after the 6 year simulation time? (5 pts).
I-31. To return to the default data set for Keesler AFB simply hit the “Restore Formulas….” bar to restore default values. Perform a sensitivity analysis on the non-degradation terms in the default Keesler AFB data file by systematically changing the default value for seepage velocity by plus and minus 10%. Repeat this process for the value of Longitudinal Dispersivity (keeping seepage velocity at its original default value. Now Raise R by 10%. In each case hit “Run Centerline” and record the value of concentration at 100 feet down-gradient form the source area using the red “No Degradation” line. Which variable is most sensitive? (i.e. 10% changes in which of these three variables results in the greatest change in calculated concentration at 100 feet?) List results and explain (10 pts).