Nutrient and organic matter dynamics in drained peatland forest and the impact of drainage ditch reconstruction on water quality.

Draining land to remove excess water in areas where precipitation exceeds evaporation is a common practice. At peatlands used for forestry, this facilitates accelerated tree growth but has significant environmental implications. Drainage exposes peat soils to oxygen, triggering peat decomposition and mineralization, which leads to the leaching of solids, organic matter, and nutrients to run-off water. This study monitored nutrient and organic matter dynamics, as well as changes in water quality, in a 507.6 ha actively managed peatland forest in western Estonia since July 2022, while the drainage system underwent reconstruction in 2025. To mitigate the negative impacts of reconstruction, ecological water protection measures - sedimentation ponds and hybrid systems combining sedimentation ponds with treatment wetlands were implemented in the studied peatland forest area during the reconstruction. Outflow from four of the implemented measures was monitored with a V-weir overflow combined with an automated water level logger (Solinst Canada Ltd.) to estimate the flow rates. Monthly water samples were collected, and during the collection, on-site measurements of water temperature, dissolved oxygen concentration, electrical conductivity, pH, redox potential, and turbidity were taken using a portable device (YSI ProDSS). Concentrations of total suspended solids (TSS), total inorganic carbon (TIC), total organic carbon (TOC), dissolved organic carbon (DOC), total phosphorus (TP), phosphate-phosphorus (PO₄-P), total nitrogen (TN), nitrite-nitrogen (NO₂-N), nitrate-nitrogen (NO₃-N), ammonium (NH₄-N), sulfate (SO₄^-2), magnesium (Mg⁺²), calcium (Ca⁺²), chloride (Cl^-) and total iron (Fe_TOT) analyzed in the laboratory. For continuous monitoring, starting in May 2025, one of the hybrid systems' inflows was equipped with an automated monitoring device (YSI EXO1), which recorded water temperature, dissolved oxygen concentration (DO), electrical conductivity (EC), pH, and fluorescent dissolved organic matter (fDOM) levels every 5 minutes. The reconstruction works elevated TP, TSS, and TIC release, with mean concentrations rising from 0.04 mg/L to 0.17 mg/L, 14.80 mg/L to 161.71 mg/L, and 13.82 mg/L to 17.47 mg/L, respectively. For TOC and TN, the effect was opposite, with mean concentrations decreasing from 54.63 mg/L to 51.26 mg/L and from 4.05 mg/L to 2.56 mg/L, respectively. Continuous monitoring revealed a severe short-term decrease in pH, fDOM, and DO levels and a slight short-term rise in EC in the inflow of the hybrid system as the released sediments passed through it. This indicates that the main release originates from the mineral soils underneath the peat that were disturbed during the works. To assess the volume of sediments retained by the ecological water protection measures, two bottom topography surveys were conducted in July 2025 (before) and October 2025 (after) using a Trimble^® R12 (Trimble Inc. USA) GNSS receiver with RTK mode. Measured GPS points were interpolated in QGIS, yielding results of low sediment accumulation in two of the systems and sediment release from two of the systems. The fine particle sediments released during the reconstruction require very low flow rates and long hydrological retention times in the system to settle. Additionally, long-term monitoring is necessary to determine whether these systems have a lasting positive impact during active peatland forest management.