Effluent dynamics from a degraded peatland catchment and the influence of an on-site pond—a warning sign

• Large nutrient exports are infrequent and short-lived depending on heavy rainfalls. • Heavy rainfalls regulate nutrient and ion concentrations by a dilution effect. • Effluent water quality varies seasonally as controlled by climatic factors. • The pond assures particle retention but fails to treat soluble components. • Downstream aquatic communities are subject to harmful effects year-round. Peatlands are important stores of carbon and nutrients, but drainage for peat extraction disrupts hydrology and water quality, enhancing leaching of organic matter and nutrients. This study examined the seasonal dynamics of water quality from a 16.4 ha industrial cutaway peatland catchment in Ireland following storm events and evaluated the influence of an on-site sedimentation pond. Water quality, including carbon, nitrogen and major ions, was monitored over three years using automated storm-event sampling, grab sampling and continuous flow measurements to assess seasonal variability, pond treatment effects and relationships with climatic drivers (flow and temperature). High flows (≥25 L/s) were infrequent, whereas low flows (≤5 L/s) were common. Carbon and nitrogen concentrations varied markedly between storm events, with increasing flow generally causing dilution. Despite their rarity, short-lived high flows accounted for a large proportion (>70–85%) of annual nutrient export. Effluent water quality exhibited clear seasonal patterns and was strongly related to flow and temperature. Warm, dry periods produced more neutral, nutrient-rich effluents, whereas colder, wetter conditions generated more acidic waters with lower concentrations, regulated by pH. Oxygen availability within the catchment appeared to influence carbon production in effluents concurrently with ammonia production. The pond received subsurface inflows low in dissolved oxygen and exhibited highly variable hydraulic retention times (mean ± standard deviation 90 ± 551 days; median 15 days). While the pond retained particulate material, its influence on dissolved constituents was limited. These effluents pose risks to receiving waters, highlighting the need for improved mitigation strategies in extracted peatland catchments.