Original paper

Effects of stagnating sulphate-rich groundwater on the mobility of phosphate in freshwater wetlands: a field experiment

Lucassen, E.C.H.E.T. P.Smolders

Abstract

Water tables in the Netherlands have been greatly lowered in recent decades, largely in order to favour agricultural activities. Drought and increased nitrate (NO3-) leaching from agricultural land lead to oxidation of iron-sulphide (FeSx) in the subsoil, resulting in increased sulphate (SO42-) concentrations in the groundwater. In addition, increased atmospheric sulphur (S) deposition and leaching of SO42- from agricultural land have increased SO42- concentrations in the groundwater. Attempts to retain groundwater in desiccated wetlands by simply damming drainage ditches have resulted in greatly increased retention times of the water, die-back of the original vegetation and massive development of algae and lemnids in summer. In order to determine whether stagnation of SO42--rich groundwater plays an important role in eutrophication, nine bottomless enclosures were placed in the sediment of an open water body to block the supply of groundwater. Na2SO4 was added to three enclosures twice a year to simulate a minimised input of SO42--rich groundwater. In addition NaCl was added to three enclosures, while three enclosures were left untreated. (Pore) water quality and vegetation development were compared with those at seepage-fed sites outside the enclosures. The results show that stagnating SO42--rich groundwater led to increased SO42- reduction in the sediment, resulting in phosphate (o-PO43-) mobilisation from the sediment and eutrophication of the water layer, due to the interference of S2- with the iron-phosphorous cycle. This was accompanied by massive growth of common duckweed (Lemna minor) and filamentous algae and a decreased growth of broadleaved pondweed (Potamogeton natans). The consistently high input of SO42- via seepage did not lead to eutrophication. The results indicated that SO42- reduction rates at sites fed by seepage were very low. This was probably caused by the consistently high input of Fe and NO3- via the groundwater, which prevents reduction of SO42- by providing energetically more favourable electron acceptors. In fens with stagnating groundwater, Fe and NO3- will be rapidly depleted, stimulating SO42- reduction re1 sulting in development of toxic S2- concentrations and eutrophication of the water layer.

Keywords

eutrophicationsulphate pollutioniron-phosphorus cyclewater tablesdesiccationofwetlands.