This homework set will have you calculate some concentrations of different components of the water that you collected from sinkholes along the trace of the Santa Fe River, measure composition of the alkalinity of the samples, and make some interpretations of these data.  As in the previous geochemistry homework, you might want to look through a couple of books such as Drever, The Geochemistry of Natural Waters or Stumm and Morgan, Aquatic Chemistry in order to get an idea of what alkalinity is and how you go about measuring it.  I hope to cover most of this during the lecture.  Again, some of the calculations are repetitive, and thus it will be easiest for you to do the calculations in a spreadsheet such as Excel.  Send me an email if you want the data table emailed to you as an Excel attachment.

On this web site, you will find two data tables that you will need to bring into a spreadsheet program.  One table includes the data that we collected while in the field (pH, T, dissolved oxygen, and conductivity).  The other table has raw data of peak height and peak area for the major components in the water (Na, Ca, Mg, Cl, NO3 (if present), and SO4).  In addition, peak area and heights are listed for the calibration standards (Autocal1, Autocal2, Autocal3, Autocal4, and Autocal5), blanks, and a check standard (SFR) which was measured every fourth sample.  The concentrations of the standards are tabulated below:
 
 

Standard	Na	Mg	Ca	Cl	NO3	SO4
	(mM)	(mM)	(mM)	(mM)	(mM)	(mM)
Autocal1	0.00	0.00	0.00	0.00	0.00	0.00
Autocal2	0.20	0.20	1.25	0.15	0.03	0.30
Autocal3	0.40	0.40	2.50	0.30	0.06	0.60
Autocal4	0.60	0.60	3.75	0.45	0.09	0.90
Autocal5	0.80	0.80	6.25	0.60	0.12	1.20

 

Note that the samples were measured in triplicate, so that you will need to average the three values in order to determine the final concentrations.
 

(1) Construct a standard curve for each component and calculate the concentrations on the basis of both peak height and area.  Calculate a linear regression of the standard curve and show the equation of the line and the r² value for the equation.

(2) Calculate and include in the data table the concentration for each sample.  How much difference is there between the three individual measurements and the average value?  Is it significant?  Coefficient of variation is defined as the average of a standard deviation of a population divided by the arithmetic mean for that population times 100 (to make a percentage).  What is the coefficient of variation of the check standards?  Is this larger than the coefficient of variation of the three individual measurements?  What would you estimate the error of the measurements to be.

(3) Come to the lab (B109) in groups of three and measure using the Gran titration the alkalinity of the samples collected during the sampling trip.  Before you come to the lab, make sure that I will be around, or that the door is unlocked.  Also be sure to not overlap with other groups – there is only one titration apparatus.  Do not leave the tops off of the samples while titrating.  Include this data in the table.
 

NOTE:  B109  is my working lab and there are many people using it.  Be neat, clean up, do not destroy anything.  In general, just be careful.
 

(4) Plot the change in concentrations of Cl, Ca, and alkalinity with distance downstream.  You will need to look at the topographic maps in the Map Library in order to determine the distance between the sink and the sinkholes.  Interpret any observed changes in the concentrations considering the changes in concentrations that you observed in the previous homework set.  Things to think about:  (a) does the water chemistry change “downstream”?  Why or why not?  Does the water chemistry seem more similar to groundwater or river water (as defined in the previous homework set?  Does there appear to be exchange between the conduits and matrix rocks?