ABSTRACT
Since 1973 11 deep well recharge experiments, each at a different site with intensive hydrochemical monitoring, were carried out in the Netherlands. At each test site the influent was oxic and the anoxic target aquifer contained pyrite. All experiments revealed a typical quality evolution in 7 phases due to sequential leaching of aquifer constituents. The main processes were, in mean order of increasing duration: mixing with native groundwater, cation exchange, SiO2 desorption, PO4 and F exchange, pyrite oxidation, acid-buffering by calcite dissolution or HCO3 -, and oxidation of organic matter. Differences in quality changes between the experiments relate to variations in aquifer geochemistry and influent quality. Pyrite oxidation is kinetically hindered, it takes about 10–100 days to deplete O2 and NO3 - in the influent. Oxygen is reacting faster with pyrite than with labile organic matter, for nitrate the reverse holds. Pyrite oxidation leads to mobilization of As, Co, Ni and Zn, of which only As may reach the recovery well.
