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Hello, I am using GWB X1t to simulate fluoride transport in a soil column. First, the column is contaminated with fluoride for a certain period of time and then the column was flushed with deionized water. In the simulation, first, I want to simulate the transport process during sorption, where fluoride sorption was described by surface complexation. For surface complexation, I used the general composite approach (I added a new basis species (X) with 1g/mol molecular weight, defined a new element and a mineral with the basis species). For the surface dataset, I defined the reactions in terms of a new surface species (>XOH). For the mineral,if I use the exact density (e.g., 3591 g/l, that I used for my column with 39% porosity) and a site density of 2.6e-5moles/g, the program ends with the message that the porosity is too small. Since the porosity at each grid block is calculated with each run, I decreased the mineral site density (e.g., 1g/l or 0.1 g/l) and accordingly increased the site density. Then the program doesn't converge. However, if I keep both site density and mineral density small, the program runs but the results are not satisfactory (fluoride appears in the outlet just after 1 pore volume because the sorption is small). I appreciate any suggestion on how to keep the desired site density? P.S.: My PHREEQC simulation gives good results with the same site density, mineral conc. and surface reactions. Padhi