BIOGEOCHEMICAL CHANGES IN GROUNDWATER-INFILTRATION SYSTEMS - COLUMN STUDIES

A laboratory continuous flow-through sand reactor was used to study qu alitatively 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 b y microorganisms and can be described by the classical sequence of ino rganic redox reactions in aquatic systems. In the steady state situati on, the nitrate and sulfate reduction follow a pseudo first-order kine tics, with respective rate constants of (2.7 +/- 0.2) . 10(-1) min-1 a nd (2.3 +/- 0.3) . 10(-2) min-1. These rate constants are within the s ame range of other laboratory and field studies when taking the popula tion 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 almos t unchanged in the range of 30 nM/min. The important inorganic geochem ical 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) form ing FeS. The last reaction consumes more than 80% of the S(-II) produc ed, i.e., controls the concentration of the enzymatically produced S(- II). The calculated alkalinity, carbon, and electron balance matched w ell with the measured concentration changes in the reactor. This shows that the microbially mediated redox processes can be expressed by a s et of simple chemical reactions.