, Volume 269, Issue 1-2, pp 109-115

Water table fluctuations and groundwater supply are important in preventing phosphate-eutrophication in sulphate-rich fens: Consequences for wetland restoration

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Nitrate leaching from agricultural land leads to oxidiation of FeS x in FeS x -containing subsoils resulting in SO 4 2− mobilisation. Pollution of the groundwater with SO 4 2− causes a higher availability of o-PO 4 3− , eutrophication and loss in biodiversity in groundwater fed fens with stagnating surface water. Under natural conditions, fens along the river Meuse are continuously fed by groundwater that besides SO 4 2− mostly also contains high concentrations of NO 3 and bivalent cations (Ca2+ and mg2+). During summer groundwater input is restricted resulting in periodic drought. Under these conditions no SO 4 2− induced o-PO 4 3− eutrophication occurs. Periodic drought and a high discharge of NO 3 , have a strong effect on S and P biogeochemistry in sulphate-rich fens. NO 3 inhibits SO 4 2− reduction and concomitant o-PO 4 3− mobilisation in fen sediments by being an energetically more favourable electron acceptor. In addition, NO 3 is capable of oxidising reduced Fe compounds, including FeS x , increasing the amount of oxidised Fe in the sediment capable of binding o-PO 4 3− . Periodic drought is important in reincreasing the concentration of oxidised Fe in the top layer of S-rich sediments preventing o-PO 4 3− mobilisation and an undesirable vegetation development. Damming of surface water, in order te restore desiccated sulphate-rich fens, prevents periodic drought and decreases groundwater input. This leads to NO 3 depletion, stimulation of SO 4 2− reduction, Fe depletion, o-PO 4 3− mobilisation and, in contrast to what was hoped for, in massive growth of algae, lemnids and fast growing wetland grasses. Therefore discharge of NO 3 – rich groundwater and the fluctuation of the water table are vital for succesful restoration of desiccated sulphate-rich fens. Successful rewetting of these type of fens, without causing stagnation of surface water and without preventing periodic drought, can be achieved by raising the water table to levels below the potential groundwater table using a controllable dam.