Water quality and greenhouse gas emissions from degraded forest drainage ditches on peat soil

Draining land from excess water is a common practice in forestry to accelerate tree growth, but it has significant environmental implications, particularly in the case of peatland forests. Drainage exposes nutrient-rich peat soils to oxygen, triggering peat decomposition and mineralization, which leads to increased CO₂ emissions from the soil and the leaching of solids, organic matter, and nutrients to water. While accelerated tree growth may partially offset CO₂emissions, unmanaged drainage ditches are possible hotspots for greenhouse gas (GHG) emissions.  

This preliminary study monitors water quality and quantity, and GHG emissions in unmanaged ditches of four sub-catchments of a 507.6 ha peatland forest drainage system in western Estonia. Ditch reconstruction works will be done in the summer of 2025. To mitigate the negative impacts, ecological water protection measures - sedimentation ponds and hybrid systems combining ponds with treatment wetlands, are used. From July 2022, once per month, water temperature, dissolved oxygen content, electrical conductivity, pH, redox potential, and turbidity are measured onsite from ditches entering mitigation measures. From grab samples total suspended solids (TSS), total inorganic carbon, total organic carbon (TOC), dissolved organic carbon, total phosphorus (TP), phosphate-phosphorus, total nitrogen (TN), nitrite-nitrogen, nitrate-nitrogen, ammonium, sulfate, magnesium, calcium, chlorides, and total iron are analyzed in the laboratory. Flow rates monitored from the outflows of mitigation measures with V-weirs combined with automated water level loggers are the basis for the estimation of potential sediment and nutrient loads. From April 2023, monthly CH₄ and CO₂ fluxes were measured on four 0.6 km sections of unmanaged ditches entering mitigation measures with a floating chamber and portable LI-7810 trace gas analyzer. In addition, an extensive random mapping of GHG emissions from unmanaged ditches of the whole drainage system was conducted in May 2024. 

The median concentration (range presented in parenthesis) of TSS 10.0 (2.0-200), TOC 53.0 (28-81), TP was 0.032 (0.012-0.281), and TN 2.70 (0.78-14.0) mg L^-1 are indicating that the studied ditches are a source of diffused water pollution, foremost for phosphorus. 

The median CH₄ and CO₂ flux emissions from unmanaged ditches entering mitigation measures were 0.30 (0.01-69.39) mg CH4-C m^-2h^-1 and 31.02 (0.39-644.38) mg CO2-C m^-2h^-1, respectively. The mapping resulted with median CH₄ and CO₂emissions of 1.40 (0.06-70.25, n=33) mg CH4-C m^-2h^-1 and 30.30 (-64.46-100.14, n=23) mg CO2-C m^-2h^-1. GHG emissions from unmanaged ditches show high seasonal variability, high emissions in summer, and relatively low mean emissions during autumn and spring.  

The performed monitoring gives unique information about the water quality and quantity, and GHG emissions in unmanaged ditches. This background data is the main input for evaluating the impact of reconstruction works of peatland forest ditches and the performance of mitigation measures.