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 ~~~~~~~~~~~~~~~~~~~~~~~~~~ ****Disclaimer*********** This e-mail and any attachments may contain confidential and/or privileged material; it is for the intended addressee(s) only. If you are not a named addressee, you must not use, retain or disclose such information. Serco cannot guarantee that the e-mail or any attachments are free from viruses. The views expressed in this e-mail are those of the originator and do not necessarily represent the views of Serco. Nothing in this e-mail shall bind Serco in any contract or obligation. Serco Group plc. Registered in England and Wales. No: 2048608 Registered Office: Serco House, 16 Bartley Wood Business Park, Bartley Way, Hook, Hampshire, RG27 9UY, United Kingdom. <<<<GWIASIG 0.07>>>>(See attached file: k17)(See attached file: Ec64a3)
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