Aqueous species oscillations within transport calculation

["Fiona Hunter" <Fiona.Hunter@xxxxxxxxxxxxxxxxxx>]

Dear David
I've been using Phreeqc to model experimental data which mimics the
advance of an alkaline fluid (such as that released from the cement
which would surround a nuclear waste repository) and its interaction
with the surrounding geology (clay sediment).

I have tried calculations in which the minerals are in thermodynamic
equilibrium in each cell, and also where the dissolution of primary
minerals is kinetically controlled.  For simplicity, I have carried out
advection only transport calculations (no diffusion).

1. Equilibrium controlled dissolution
To a first approximation, the Phreeqc calculation shows similar values
to the experimental concentrations of the effluent water collected the
the end of the laboratory core.  However, the aqueous species
concentrations oscillate rapidly over short timescales,  despite the
amounts of minerals remaining constant.  We are puzzled by this and
wonder if you can help with the explanation of this problem: Is there an
unstable mathematical answer to this oscillation, or have we overlooked
a problem with the chemistry?

Here is some more explanation of the underlying experiment:
A highly alkaline fluid (Solution 0) is advected slowly (3e-12 m3s-2)
through a column containing Boom Clay ( a clay composed of 7 different
(primary) minerals in equilibrium with its pore water and initially at
pH ~8; Solutions 1-32).  The mass of water in each cell is kept small to
mimic the experimental data.  The minerals are allowed to equilibrate
thermodynamically at each shift, and likely secondary minerals are
allowed to precipitate out.

I have included the input file (Ec64a3)  and an excel file (Ec64a3)
containing all the selected output data plotted up.  You can see from
the Excel file that the Na, K, Si, Ca concentrations oscillate rapidly,
while the primary and secondary minerals remain constant over the same
interval.  I used the Llnl.dat database throughout.

2. Kinetically controlled mineral dissolution
I have tried the same calculation with kinetically controlled mineral
dissolution.  In this case, Phreeqc does not converge, despite
alteration of the KNOBS parameters.  Have you any suggestions for the
cause of this?  I have attached an input file (k17) which shows this
calculation attempt.

Thanks in advance for any suggestions you may have to explain these
difficulties!

Best regards
Fiona Hunter

~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr Fiona Hunter    MRSC
Serco Assurance
Building 150
Harwell International Business Centre, Didcot
Oxfordshire OX11 0QJ
Telephone: +44 (0)1635 280418
Facsimile: +44 (0)1635 280305
~~~~~~~~~~~~~~~~~~~~~~~~~~





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<<<<GWIASIG 0.07>>>>(See attached file: k17)(See attached file: Ec64a3)

Attachment: k17
Description: Binary data

Attachment: Ec64a3
Description: Binary data