Dear Chris, I don't quite follow the mass balance on water. Seems like 1.0 pore volumes have been added to the column at step 2, instead of just 0.5? 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 Still, however the flow should be, it would take some programming to do it, but actually might not be too difficult. Advection is accomplished in one short loop in either transport or advection, where solutions are copied from one position to the next. It would be possible to modify that part of the program to advect in a different way. There is a variable in a solution structure for the mass of water. It is also possible to define solution 0 with a mass of water other than 1 kg. In concept, you can look at these masses of water (assuming fully saturated is 1 kg water) and determine how much to advect between cells. There is a routine add_solution that can be called multiple times to accumulate a solution composition; saving the mixture, memory. and storage would take a bit of work. There is a problem with the dispersion calculation. The mixing fractions are based on equal volume mixing. It would still work, but I don't think the dispersion would be correct when you have different volumes and that would be messier to try to reprogram. The dispersion calculation would change the mass of water in cells if the volumes were not equal. It would be simpler to work in the advection module and ignore dispersion, at least initially. As for K(h), one way to do it is to have an unsaturated zone transport model that sequentially transports each element in the system to make new solution compositions in each cell, then run PHREEQC on each cell. We've used this splitting approach in PHAST for 3D ground-water systems, but I don't see any reason it shouldn't work if an unsaturated transport code were used. I suppose if you knew the fluxes between cells from a 1D unsaturated flow code, it would be possible to use that information in PHREEQC at the point where advection is calculated. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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