To define a reaction and mass action constant for each surface species. In general, SURFACE_SPECIES is used in the file with the thermodynamic database, but additional information can be stored in the input file phreeqc.in. A specific block is available in the HYDRUS-1D GUI to provide additional thermodynamic data (Solute Transport – HP2 Definition: Additions to Thermodynamic Database).
SURFACE_SPECIES
association reactions
log_k
-cd_music
-add_constant
-add_logk
association reaction association reaction for a surface species. The defined species is the first species at the right hand side of the equal sign.