Harmful effluents from a degraded peatland catchment following rainstorms

Peatlands are a large store of carbon and nutrients. In Ireland, these cover 21% of the country’s area and have been extensively drained mainly for peat extraction (both industrial and domestic). This has intensified decomposition and water flow from peat-dominated catchments. Consequently, there has been an increase in the discharge of harmful contaminants downstream that may violate the EU Water Framework Directive requirements for good ecological status of surface waters, and ultimately disturb aquatic ecosystems. Climate variations also affect the water quality by mediating the release of nutrients and modifying flows. Mitigation measures that overcome the resulting regulatory, ecological and climate challenges are warranted. For industrial peat extraction in Ireland, ponds have been excavated at the edge-of-field to provide treatment of outflows. However, little is known about the dynamics of the contaminants, and the efficiency of currently implemented measures. We hypothesize that the quality of water from degraded peatland catchments is highly dynamic and is harmful to surface waters all year round. Prolonging the hydraulic retention time on site with ponds is not sufficient for good treatment due to high proportion of soluble nutrients and unfavorable biogeochemical conditions. We show this with an experiment at the edge of an Irish degraded raised bog subjected to peat extraction where a drainage network allowed water to flow through ponds at the edge-of-field. A monitoring station was installed at the outlet with an automatic sampler which captured water samples during 14 storm events. Moreover, a one-year grab sampling campaign was conducted at the pond inlet and outlet. The samples were analysed for pH, electrical conductivity, and nutrient and ion concentrations. The station also directly measured water quality parameters and flow, for one and a half years. Both meters were then moved to the pond inlet for another year while a flow was monitored at the pond outlet. The amount of sediment deposited in the pond was estimated by counting the number of filled excavator buckets to clean it and then accounting for sediment moisture. Nutrients in effluents were mostly in soluble forms and varied greatly between storm events. These were particularly high at low flows suggesting a dilution effect. Large nutrient exports occurred only momentarily during extreme high flows. All water quality parameters varied widely throughout the seasons showing significant differences (p < 0.05). pH, nitrate and total ammonia often exceeded environmental water quality standards. Flow and temperature significantly explained the variability of nearly all water quality parameters, and temperature had a greater effect. These had inverse (generally monotonic) and direct (generally linear) relationships, respectively, with water quality parameters, except that flow was directly and linearly related to turbidity. Therefore, warm periods appear to produce nutrient and ion-rich effluents, whereas cold and rainy periods appear to produce acidic and turbid effluents. The dissolved water quality at the pond outlet was similar to the inlet indicating minimal treatment by the pond. However, it retained around 23 L of sediment per day. The results corroborate our hypotheses highlighting the need for more appropriate mitigation measures.