Recovery of water quality from drained forested peatlands with biochar

Ireland has 21% of its territory consisting of peatlands of which more than a third has been afforested. Irish bogs are naturally treeless, therefore afforestation and reforestation require drainage of the peat causing substantial changes in the biogeochemical and hydrological conditions. This includes release of nutrients and increased runoff ultimately disturbing aquatic ecosystems downstream. Thereby, measures that recover the water quality of these effluents are warranted. Biochar has sorption properties representing a potential solution as supported by previous studies. However, application in this context needs verification. We hypothesize that (i) biochar significantly reduces the concentration of nutrients in peatland forest drains and (ii) flow dynamics regulate treatment performance. The experiment takes place in two forested Irish blanket bogs named Lake Atorick (53°01'50.4"N 8°31'40.7"W) and Annalaka (53°03'44.5"N 6°24'24.9"W). Previous water quality analyses in the drainage ditches revealed pH generally between 4 and 5, and carbon, nitrogen and phosphorus concentrations generally below 40, 1 and 0.10 mg/L, respectively. We installed a pile of biochar bags (1.5-2.0 m long) in a drain in such a way as to allow permeation of the flow. Grab samplings were conducted once a week before and after the bags for 8-9 weeks (Aug-Nov/21 in Lake Atorick and Oct-Dec/22 in Annalaka). The samples have been analyzed for carbon, nitrogen and phosphorus compounds as well as pH and electrical conductivity. A V-notch weir was installed downstream the bags to allow flow measurements. Here a Raspberry Pi ultrasonic sensor HC-SR04 preceding the weir is allowing automatic measurements of water level variation every 30 min (Nov-Dec/22 to date), which will be converted into flow data. Rainfall data were obtained from regional meteorological stations on dates close to the grab samplings. Data analysis performed to date reveals no significant difference (p > 0.05) in water quality before and after the biochar bags. However, trends are graphically visible: pH rises while dissolved nitrogen concentration and carbon aromaticity decrease. The results suggest that successful application of biochar probably depends on favorable conditions for sorption, e.g. high concentrations of incoming nutrients, near neutral pH and/or good flow distribution. This highlights the need for design optimization.