Effects of Groundwater Nitrate and Sulphate Enrichment on Groundwater-Fed Mires: a Case Study

Mires and peatlands in general are heavily influenced by anthropogenic stressors like acidification, eutrophication, desiccation and fragmentation. Groundwater-fed mires are, in contrast to rainwater-fed mires, often well protected against desiccation due to constant groundwater discharge. Groundwater-fed mires can however be influenced by groundwater pollution such as groundwater nitrate enrichment, a threat which has received minor attention in literature. The present case study demonstrates how groundwater nitrate enrichment can affect the biogeochemical functioning and vegetation composition of groundwater-fed mires through direct nitrogen enrichment and indirect nitrate-induced sulphate mobilisation from geological deposits. Biogeochemical and ecohydrological analyses suggest that the Dutch groundwater-fed mire studied is influenced by different water sources (rainwater; groundwater of local and regional origin) with differing chemical compositions. The weakly buffered and nitrate-enriched groundwater leads, where it reaches the uppermost peat, to nitrogen enrichment, enhanced isotopic nitrogen signatures and altered the vegetation composition at the expense of characteristic species. Nitrate-induced sulphate mobilisation in the aquifer led to enhanced sulphate reduction, sulphide toxicity and elemental sulphur deposition in the mire. Despite sulphate reduction and nitrate enrichment, internal eutrophication did not play an important role, due to relatively low phosphorus concentrations and/or low iron-bound phosphorus of the peat soil. Future management of groundwater-fed mires in nitrate-polluted aquifers should include the reduction of nitrate leaching to the aquifer at the recharge areas by management and ecohydrological restoration measures on both a local and landscape scale.