.. _example-phreeqc-manual-14:

14 - Transport Cation Exchange and Surface Complexation
=======================================================

The one that puts everything together: advective transport with mineral
equilibria, cation exchange and surface complexation all active at once, on a
real aquifer.

The Central Oklahoma aquifer has naturally high arsenic and high pH, and the
question is where they come from. The model follows water through the aquifer
and lets the three kinds of reaction compete.


Three reaction types, one column
--------------------------------

.. raw:: html
   :file: study_14.html

* **Mineral equilibria** -- dissolution and precipitation of the phases
  present.
* **Cation exchange** -- the clays, holding and releasing the major cations.
* **Surface complexation** -- the iron oxides, holding arsenic.

Each has had an example of its own (:ref:`example-phreeqc-manual-11` for
exchange, :ref:`example-phreeqc-manual-08` for surfaces). Here they interact,
and the interaction is the result: the arsenic is not controlled by arsenic
chemistry but by what the other two do to the pH.


The result
----------

.. figure:: Concentrationevolution.svg
   :alt: Arsenic, calcium, magnesium, sodium and pH against pore volumes through the aquifer
   :align: center

   Arsenic in ppb, the major cations in molal, and pH, against pore volumes.
   Note the logarithmic concentration axis -- arsenic and the major ions
   differ by orders of magnitude and would not otherwise share a figure.

   Read it as a chain. The **cation exchange** replaces calcium and magnesium
   on the clays with sodium, which softens the water. Losing calcium lets
   carbonate equilibria push the **pH** up. And arsenic sorbs to iron oxide
   surfaces less and less well as pH rises, because the surface becomes more
   negative while arsenate is an anion -- so the **arsenic** is released.

   Arsenic rises because of what sodium did to the clays. Nothing about
   arsenic's own chemistry changed.


What it shows
-------------

That a model with one process cannot find this. Exchange alone would predict
softening; surface complexation alone would predict arsenic sorbing at
whatever pH it was given. Coupling them produces the mechanism, and the
mechanism is what tells you which aquifers to worry about.

This is also the practical argument for reactive transport over a batch
calculation: the sequence of waters a parcel of rock sees is what drives the
release, and that sequence only exists if the water is moving.


Source
------

* Parkhurst, D. L. and Appelo, C. A. J. (2013). *Description of input and
  examples for PHREEQC version 3.* U.S. Geological Survey Techniques and
  Methods, book 6, chapter A43. This is Example 14 of that manual.
* The Central Oklahoma aquifer study it is based on is Parkhurst, D. L.,
  Christenson, S. and Breit, G. N. (1996), *Ground-water-quality assessment
  of the Central Oklahoma aquifer, Oklahoma -- geochemical and
  geohydrologic investigations*, U.S. Geological Survey Water-Supply Paper
  2357-C.