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Hi, You can set most basis entries as bulk or free constraints. For a bulk constraint, the value you set refers to the entire thermodynamic component, whereas for a free constraint, it refers to the particular species only. The HCO3- component, for example, might include the HCO3- species, as well as CO2(aq), CO3--, NaHCO3, etc. If you set 10 “mg/kg” HCO3-, the concentration is for the sum of those species. If you set 10 “free mg/kg” HCO3-, though, the concentration refers only to the individual HCO3- species. An additional amount of carbon exists as the other species. Likewise, for H2O, “free kg” refers to solvent H2O only. If you change the setting to simply “kg”, the mass you specify refers to the sum of solvent H2O plus species composed of water (e.g., Al(OH)4- + 4 H+ = Al+++ + 4 H2O). For more information, please see 7.1 Example Calculation in the GWB Essentials Guide. Log refers to the base 10 log of a number (e.g., 1 free kg H2O = 0 free log kg H2O). A more common use is for setting the CO2 fugacity or partial pressure using logs (e.g. log f CO2(g) = -3.5). In the future, please don’t post topics multiple times. Regards, Brian Farrell Aqueous Solutions