Steady-state modelling of biogeochemical processes in columns with aquifer material .1. Speciation and mass balances

A laboratory column containing aquifer sand was used to study biogeochemical processes occurring in groundwater. As a carbon source for the microbially mediated processes, lactate was introduced into the system. To describe quasi-steady-state conditions of the experiment, kinetically controlled biogeochemical processes were modelled together with chemical equilibrium reactions. The laboratory column was treated conceptually as a series of six boxes according to the position of the sampling ports used for the chemical analysis. The model calculations focused on the turnover of carbon and sulfur that are related to calcium and iron, respectively. The comprehensive approach allows the quantification of all important processes and the verification of the stoichiometries of the reactions considered. The electrons provided by lactate and its reaction products amount to approximate to 4 meq/l, which are consumed mainly by the electron accepters oxygen (0.8 meq/l), nitrate (1.3 meq/l) and sulfate (1.9 meq/l). Because all of the oxygen and nitrate was reduced in the first centimetre of the column, the formation of carbon dioxide was highly concentrated there. In the first centimetre, this led to a lower pH value (7.45) and to undersaturation with respect to calcium carbonate. In the rest of the column, the pH was approximate to 7.65 and CaCO3 saturation was attained. Hydrogen sulfide, which was produced by ''reduction of sulfate'', reacted mainly with Fe(III) oxides in order to form FeS(s). A systematic sensitivity analysis was used to quantify the influence of the rate coefficients on the acid-base behaviour of the system. According to these calculations, the pH is most sensitive with respect to ''disproportionation'' of lactate into propionate, acetate and bicarbonate as well as to the processes ''aerobic respiration'' and ''denitrification''.