Recitation 9:  Modeling the evolution of rainwater through a carbonate aquifer

Today we are going to chemically trace a drop of rainwater as it becomes equilibrated with minerals commonly found in aquifers.

Purpose:

This will put you at in a good place to really start analyzing your well waters. After doing this, you should be able to get a feel for which waters are in equilibrium with which mineral phases and why.

Procedure:

Speciation model 1:

Begin by downloading a rainwater analysis from the NADP website. (http://www.geo.utexas.edu/courses/376m/chem_hydro_links.htm)

Put the rainwater analysis into PhreeqC as your initial solution. Speciate the water using PhreeqC.

Reaction model 2:

Equilibrate the water with atmospheric CO₂.

Check the PhreeqC database you are using and the “getting started with modeling” (http://www.geo.utexas.edu/courses/376m/phreeqc_example_input_files.htm) PDF file, also take a look at the example input files that come with PhreeqC,

particularly example 3.

Reaction model 3:

Equilibrate the resultant water with calcite and CO₂.

Reaction model 4:

Equilibrate the results of Reaction model 3 with calcite, dolomite, and CO₂.

Reaction model 5:

Take the resultant water from Reaction model 4 and equilibrate it with gypsum, calcite, dolomite, and CO₂.

Reaction model 6:

Repeat reaction model 5, at PCO₂ of 10^-2.5

Reaction model 7:

Repeat reaction model 5, at PCO₂ of 10^-1.5

Deliverables for each step include:

1)      Modeled composition of water: pH, alkalinity, major 8 ions, carbonate speciation.

2)      Amount of each mineral was dissolved or precipitated in order to attain equilibrium.

3)      A brief report describing the changes that occurred between each equilibration step. Give an explanation as to what you think is going on. For example, if you notice a significant rise in alkalinity between step 2 and 3, give me a reasonable hypothesis for why this is occurring.

Conclusions:

The hypotheses you generate will be useful in directing the rest of your work (i.e. what plots to generate and what other models to run) and will eventually go into your final project. If you have a reasonable hypothesis and it turns out to be false, don’t leave it out of the report, it’s still good data. Explain your hypothesis, demonstrate how it was tested, write your results. Some further directions to go in are listed below:

1)      Modeling of evaporation to make the rainwater match the recharge water (see example 4 in PhreeqC)

2)      Modeling mixing of waters.

3)      Modeling ion exchange to account for some of the differences between your water analysis and your model results. What happens when a water in equilibrium with calcite encounters clay?

4)      Equilibrating with other mineral phases (for example, silicates) to see if you can model some of the other constituents in your water such as K, Si, and Mg. (Remember that Al values should come from equilibrating the solution with kaolinite and using the resultant Al concentration in your input. )

5)      Speciating the metal complex in your water and how they might be affected by pH changes.