Risk assessment of the phosphorus export from a re-wetted peatland

The peatland 'Mittleres Trebeltal' in Mecklenburg-West Pomerania (northeast Germany), a former valley mire, was re-wetted in 1995 in the course of an EU-funded conservation project. As there are recent concerns regarding water quality deterioration caused by phosphorus originating from re-wetted peatlands, the aim of this study was to develop a simple method based on existing data to conduct a risk analysis estimating minimum and maximum phosphorus export rates from the study area (993 ha) towards the river Trebel. Especially in the temporarily unsaturated zone of the peatland, high phosphorus concentrations were measured, while the water quality of the river was 'moderately polluted' according to the German water quality classification. Two approaches were applied: First, the geostatistics software Surfer® was utilised to derive an areal estimate of half-yearly groundwater contours, fluxes and transport rates on the basis of groundwater level data of 33 dipwells. Second, daily groundwater levels from a hydrological model applying the Boussinesq equation to geological cross-sections were used to calculate specific discharges and transport rates along transects following the general flow direction. With the software Surfer, the general hydrological conditions of the study area and the calculated phosphorus fluxes were plausibly illustrated. Daily transport rates calculated on the basis of the Boussinesq model were dynamic on a very low level, while flow directions frequently pointed towards the peatland. Even when assuming worst case conditions, very low phosphorus export rates (<0.001-0.003 kg SRP ha−1 a−1) from the study area via groundwater were estimated with both approaches. According to these results, there is no acute danger of water quality deterioration caused by remobilised phosphorus from this re-wetted peatland. Even though there is no phosphorus export or eventually accumulation within the peatland under favourable conditions, monitoring should continue.