Long-term behavior of groundwater chemistry in a periodically rewetted fen area covered with macrophytes.

We studied the long-term changes in groundwater composition in the context of peat restoration at a degraded water-table managed peatland site typical for many agriculturally used fen areas in the northern hemisphere. At the study site, peatland rewetting with groundwater control and pumped canal water was carried out in two periods: from 1997 to 2002 and from 2011 to 2013. The site was not managed between 2002 and 2011, which led to an unstable groundwater table that had declined in part. The aim of this study was to investigate the consequences of rewetting and desiccation on groundwater chemistry. We pursued a multivariate approach using nonlinear principal component analysis (Isomap) to identify the prevailing processes that control the groundwater quality in this system. Sixteen years after peatland restoration, the groundwater quality had significantly improved. Principal component analysis revealed that hydrological processes had a major impact on groundwater quality, i.e. fluctuations between upwelling of local, salt-influenced groundwater and downwelling of surface and rainwater (first principal component) as well as upwelling of regional groundwater from deeper layers (second principal component) which originated from the catchment. In particular, the upwelling of regional deep groundwater had a strong positive impact on the groundwater quality of upper layers at the Biesenbrow site. Another major impact on groundwater quality was nutrient withdrawal by macrophytes and incorporation into organic matter. In the upper groundwater layer, peat mineralization processes resulted in substantially increased SO4 concentrations. We concluded that potential matter release after rewetting is buffered by hydrological barriers, and seems to be marginal with little impact on adjacent environments in the long term. The ecosystem is sustainably stabilized, and therefore has no negative impact on groundwater quality during periods of water shortage. Due to the strong influence of regional groundwater, management measures in the catchment are very important for maintaining and improving groundwater quality in peatlands.