Purification efficiency of a peatland-based treatment wetland during snowmelt and runoff events

•Purification efficiency of constructed wetland was studied during snowmelt.•Effect of pumping regime and evaporation on purification efficiency was defined.•Effect of direct evaporation and precipitation on water quality was negligible.•Snowmelt contributed largest proportion of nutrient leaching.

Purification efficiency of a treatment wetland for a peat extraction area in Northern Finland was studied March–October 2012. The focus was on the snowmelt season, with inflow and outflow sampled twice a day. All samples were analysed for stable isotopes of water and various water quality parameters. The wetland inflow and outflow were continuously monitored which in combination of precipitation and evaporation data allowed to study the effect of direct precipitation (dilution) and evaporation (enrichment) on the wetland water balance and water quality. The evaporated water volume was approximately 20% of the inflow on days with small inflows, whereas the proportion of precipitation was more variable. Negative correlations between hydraulic load and water quality at the outlet were found, whereas evaporation, precipitation from/to wetland had a negligible impact on outlet water quality. The observation year was rainier than normal, which complicated application of isotope techniques as the evaporation signal was weak. However, the heavy rain provided ample data on wetland performance under different flow conditions. The wetland reduced concentrations of e.g. suspended solids, phosphate-phosphorus, ammonium and iron, but reduction rate of other dissolved nutrients and organic carbon was low or negative. The observed retention of suspended solids load (28% during snowmelt, 61% during the whole observation period) shows that treatment wetlands can reduce the environmental impact of peat extraction areas.