Dear Mr Parkhurst, I am a Dutch soil scientist working on my PhD-thesis. My special interest is the formation of sodic soils in Hungary. I use phreeqc a lot to analyze my data, and I really appreciate its capabilities. I have followed the phreeqc course of Tony Appelo in Amsterdam. Presently I want to use phreeqc to do calculations on my concepts of soil formation. I'm afraid I got stuck with phreeqc's transport capabilities. To clearify my problem, I think it is best to describe how I experience the present TRANSPORT module in phreeqc. My concept of transport in phreeqc is as it behaves like a freight train in which wagons carry the solutions along the flow line. Along the flow line, there are a number of platforms which serve as the precipitates and exchangeers. The idea is depicted by the following (cells 1-5 belong to the column, cell 0 contains SOLUTION 0, which contains the initial solution, furthermore, `o' implies the original solution, `n' the new solution, and `X' the front between `o' and `n'): cell_no: 0 1 2 3 4 5 step 0: nXo o o o o step 1: n nXo o o o step 2: n n nXo o o As the transport process proceeds, wagons with either `n' or `o' move along the different plantforms, equilibrate, and diffusion is taken into account by mixing the contents of adjoining wagons (here the idea of a freight train fails a little). This model produces good results in case of saturated flow: the different solutions `n' and `o' are carried in their wagons (and the front `X' may be regarded as the engine of the train). The wagons before and behind the engine will proceed and there is nothing against nearly empty wagons (in the phreeqc examples I have seen, waggons tend to be filled, or, flow is always saturated flow). I can depict this in the following way. Instead of the symbols `n' and `o' I'll write down the water saturation in the cell (which for this simple case is trivial of course, for the water saturation equals 1 in case of saturated flow). cell_no: 1 2 3 4 5 step 0: 1 1 1 1 1 step 1: 1 X 1 1 1 1 step 2: 1 1 X 1 1 1 etc. However, this concept produces unwanted effects in a sub-saturated soil which becomes saturated as a water front infiltrates the soil. For the type of work I do, there is presently nothing against a mixed cell concept (I mean, I do not presently need Darcy equation with K(h) relations that relate the conductivity to the water suction in the soil). However, the TRANSPORT model needs a (little?) modification. What I need is a system in which water before the front (so initially present water) accumulates to saturation 1 before it starts moving. For example, a column in which the initial moisture saturation is a half, the column develops in the following way when in each step a volume of water equal to half the saturation in cell is added: cell_no: 1 2 3 4 5 step 0: .5 .5 .5 .5 .5 step 1: 1 X .5 .5 .5 .5 step 2: 1 1 X 1 .5 .5 step 3: 1 1 1 X 1 1 This has the consequence that the solution in cell 4 starts moving in step 3, and that only after step 3 water will leave cell 5. It is in principle also possible to deal with a slighly more complicated example in which the initial column has different saturation values: cell_no: 1 2 3 4 5 step 0: .2 .3 .1 .5 .2 step 1(+.8) 1X.3 .1 .5 .2 step 2(+.6) 1 1X 1 .5 .2 step 3(+.3) 1 1 1X 1 1 One advantage of such an approach is that there is no chance that a highly concentrated solution at a certain depth starts moving too early and producing unwanted effects, or disappears from the system even though it is not reached by the infiltration front. There is, however, an even more important advantage to such an approach. It allows me to simulate capillary rise due to which considerable amounts of salt may accumulate at the soil surface, because all the water which enters the soil at the bottom boundary eventually evapotranspirates somewhere in the soil column. I think, I can achieve such a behaviour by means of a KINETICS and RATES couple which extracts part of the water from the upper cells, near to the soil surface. I hope I succeeded in describing the requirement I think I need to fully exploit phreeqc chemical capabilities. Now, here are the questions I would like to ask: 1 Is such an approach reasonable? I'm afraid I'm not a (numerical) mathematician, so I find it difficult to judge. There is a problem I foresee, which has to do with the additional mixing of solutions in adjoining waggons to allow for the water accumulation at the front between sub-saturation and saturation. This might interfere with the control on hydrodynamic dispersion. Maybe this can be solved by the use of finer grids. 2 If it is a reasonable approach, is it possible to implement it in phreeqc? 3 If that is the case, can it be done by modification of the present TRANSPORT module, or is it better to define a new module, e.g.~SOIL_COLUMN, and to what extend do you think it can make use of the code already present in TRANSPORT. 4 Are you aware of any such modifications, or modifications that take into account the aforementioned K(h) relationships? I am also not a programmer, although I have experience with C. If it helps and if the possibilities are there I am happy to do the programming, as I think phreeqc is a good programme and the way it is offered to the (scientific) community serves as an example for the way scientific work should be treated. If I can contribute to that, I would gladly do so. Thanks for your time, Chris van Uffelen Sub-dept of Soil Quality Dept of Environmental Sciences Wageningen University The Netherlands
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