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.