This is a practice exam for the midterm.  The problems in the actual exam will be selected from, and be almost identical to, those shown here, except that the actual mineral will change (although a carbonate will be replaced with another carbonate), or the pH, or the Pco2, or the temperature.  If you can work each of the problems on this practice exam without the book, you can work the actual exam problems just as easily.  DO NOT MEMORIZE the answers to this exam - that is a waste of time.  Learn the method of solution.  The point totals shown here are for illustration - they will be slightly different for the actual exam.  

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CHEMICAL HYDROGEOLOGY

Practice Midterm Exam

You have 2 hours to complete this exam. State all assumptions. Circle final answers - no circle?, no credit!!. Mark each page with your name, and each page  with the problem # you are working on! Use a fresh scratch sheet with each problem - please do not use the back page of the exam.  Turn in all work sheets stapled to your test. If you do not know a constant, you may obtain it from the T.A., but you will loose a point or two. Scan the exam and answer the easy ones first, then return for the tougher ones.  Mark the skipped questions clearly so you can find them later.  Check to be sure you did not accidentally skip a problem.  Good luck.  Partial credit will be given for incomplete answers that are even related to the correct approach. 

  1. Using data from the tables,

    2. a) Write out the reaction of rutherfordine (UO2CO3) dissolving in water, and calculate the solubility constant in water at  25°C.   (20)

    b) Is rutherfordine more or less soluble than calcite? (5)

    c) If a water in contact with and equilibrated to gypsum is exposed to rutherfordine, what will happen? Be specific, i.e. D[Ca]; D[UO2], D[alk], DpH, Dphases (20)

    d) What will happen to these constituents if you add NaCl to this system after it has equilibrated with gypsum and rutherfordine? (10)

  2. A well water in Kansas has the following partial analysis:

    3. pH = 7.5
    [Ca2+] = 88 mg/l
    [Alk] = 320 mg/l
    [Mg2+] = 7.3 mg/l
    [Silica] = 24.0 mg/l
    [SO4=] = 6.7 mg/l
    [Na+] = 19 mg/l
    [Cl-] = 13 mg/l

    a) Convert all analyses to milliequivalents/liter (millimoles for neutral species). Convert alkalinity to meq bicarbonate. Write down beside the mg/l concentration. (10)

    b) What volume of 0.1 N HCl is required to titrate a 50 ml sample to the total alkalinity endpoint. (15)

    c) What is the concentration of carbonate (CO3=)? (a real number is needed here, "insignificant" is not an acceptable answer) (15)

    d) What is the equilibrium PCO2 of this water (10)

    e) What is the value for CTCO3 (i.e., DIC) of this water? (10)

    f) Is this water charge balanced? What is the %error?  YOU MUST SHOW YOUR CALCULATION!! (10)

  3. What is the pH and the concentration of all three carbonate species in a water at equilibrium with an open system of PCO2 of 10-0.3 Atm. (open system, no solid). (35)

    4.  

  4. The rate of dissolution of quartz in water (SiO2 + 2H2O <--> H4SIO4) at 15oC is 10-17.5 moles/cm2/sec.) Assuming a density of 2.65, how far would the faces of a 2x1x1 cm cube of quartz retreat after dissolving into a cubic meter of water (Ksp = 10-4 mol/l), and how long would it take to reach equilibrium? (30)


  5. What is the concentration of Ca2+ in a water in equilibrium with calcite and an atmosphere of 10-1 atm Pco2  at pH 5 and 25oC?