BIOGEOCHEMICAL CHANGES IN GROUNDWATER-INFILTRATION SYSTEMS - COLUMN STUDIES

A laboratory continuous flow-through sand reactor was used to study qualitatively and quantitatively the biogeochemical processes resulting from an input of an easily degradable organic substance (lactate) into a model aquifer. The primary occurring redox processes are mediated by microorganisms and can be described by the classical sequence of inorganic redox reactions in aquatic systems. In the steady state situation, the nitrate and sulfate reduction follow a pseudo first-order kinetics, with respective rate constants of (2.7 +/- 0.2) . 10(-1) min-1 and (2.3 +/- 0.3) . 10(-2) min-1. These rate constants are within the same range of other laboratory and field studies when taking the population densitv of microorganisms into account. The dissolution of Mn(II) and Fe(II) follows pseudo zero-order kinetics. During the experiment (3 months) the respective rate constants for Mn(II) decrease from 100 nM/min to 1 nM/min, whereas the rate constant for Fe(II) remains almost unchanged in the range of 30 nM/min. The important inorganic geochemical processes induced by redox reactions are dissolution of CaCO3 by CO2, adsorption of Mn(II), precipitation of Fe(II) by sulfides (S(-II)), and the reductive dissolution of Fe(III)(hydr)oxides by S(-II) forming FeS. The last reaction consumes more than 80% of the S(-II) produced, i.e., controls the concentration of the enzymatically produced S(-II). The calculated alkalinity, carbon, and electron balance matched well with the measured concentration changes in the reactor. This shows that the microbially mediated redox processes can be expressed by a set of simple chemical reactions.