Temporal dynamics of AVS and SEM in sediment of shallow freshwater floodplain lakes

Acid volatile sulfide (AVS) is an operationally defined sulfide fraction, which is considered important for trace metal fate in reduced sediments. Understanding AVS formation rates is important for the management of metal polluted sediment. However, little is known about the fate and dynamics of AVS in spatially and seasonally variable freshwater environments. The authors monitored in situ AVS formation and degradation and simultaneously extracted metals (SEM) in two floodplain lakes and compared this to AVS formation rates in laboratory experiments with the same sediment. In the laboratory experiments, the formation rates of AVS were studied at 20 °C for initially oxidized sediments that were: (a) untreated; (b) enriched with extra SO42-; and (c) treated with sodium-azide (biocide). In the field, AVS concentrations were highly variable and were significantly correlated to surface water temperature and O2 concentrations as well as to sediment composition. Between February and August, AVS formation was approximately linear at a rate of 0.07 μmol g−1 d−1. Degradation rates differed drastically between the lakes due to different degradation mechanisms. In one lake AVS removal was caused by reworking and oxygenation of the sediments by bream (Abrami brama), at a rate of 0.25 μmol g−1 d−1. In the other lake AVS removal was caused by desiccation, at a rate of ±2.6 μmol g−1 d−1. This illustrates the large differences that can be found between similar lakes, and the importance of biological processes. In the laboratory, concentrations of AVS with and without SO42- addition were similar during the first weeks, and increased at a rate of 0.15 μmol g−1 d−1. However, SO42- addition increased the AVS concentration at the end of the experiment, whereas sodium-azide eliminated AVS formation, as expected. This suggests that AVS formation was SO42--limited in the laboratory as well as in these shallow freshwater lakes.