A Comprehensive Analysis of Water Chemistry and Greenhouse Gas Emissions in a Treatment Wetland System for Diffuse Agricultural Runoff

Abstract

Agricultural activities are the main cause of eutrophication of waterbodies downstream of farmed lands. Water protection measures such as treatment wetlands (TW) are highly effective in reducing diffuse agricultural pollution and therefore reduce the risk for eutrophication. This study presents a long-term (7-year period) overview of a well-established in-stream free water surface (FWS) TW system to reduce diffuse agriculture pollution in a temperate climate zone in southern Estonia. The wetland system consists of two subsequent in-stream FWS TWs (W1 and W2) with a catchment area of 2.2 km2. The wetlands are mainly vegetated with cattail (Typha latifolia) and common reed (Phragmites australis).   Water parameters have been monitored biweekly from 2017 to 2023 and greenhouse gas emissions have been monitored biweekly from 2018 to 2023. Water temperature, oxygen concentration, dissolved oxygen, electrical conductivity, pH, redox potential, and turbidity were measured on-site using a portable device (YSI ProDSS). Flow rate was measured with SonTek FlowTracker handheld Acoustic Doppler Velocimeter. Total carbon (TC), total inorganic carbon (TIC), total organic carbon (TOC), dissolved organic carbon (DOC), total nitrogen (TN), total phosphorus (TP), nitrite-nitrogen (NO2-N), nitrate-nitrogen (NO3-N), phosphate-phosphorus (PO4-P), sulfate (SO4-2) and chloride (Cl-) concentrations were analyzed in the laboratory. CO2, CH4 and N2O fluxes were measured using a manual closed chamber method (samples analyzed with GC-2014, Shimadzu on years 2018-2022) and from 2022 using portable LI-7810 and LI-7820 trace gas analyzers (LICOR Biosciences).   Results are showing positive removal efficiency for the whole study period in W1 for TOC (average 6.2±9.9%), DOC (average 16.9±13.9%) and TP (average 23.1±32.4%); and in W2 for TC (average 2.3±6.8%), TOC (6.9±10.4%), TN (average11.9±27,8%), NO3-N (average 8.7±28.2%), TP (average 1.3±61.2%) and PO4-P (average 20.4±301.0%). CH4 and CO2 emissions from the wetlands showed an increasing trend with a clear seasonal dynamic. Over the years the mean CH4 flux increased from 88 µg CH4-C m−2 h−1 in 2018 to 2505 µg CH4-C m−2 h−1 in 2021. The large increase in the emissions was mainly due to the more extensive vegetation growth that provides more carbon into the system. Nitrous oxide flux on the other hand showed a slight decrease over the years. However, about half of the annual N2O emission originated from very small shallow areas (less than 5% of the total area) in the wetlands that acted as hot spots throughout the study period. This study provides insights into the development of treatment efficiency of the wetland system and concordant change in greenhouse gas emissions. The long-term monitoring shows that overall, the water treatment efficiency is increasing but there is a clear trade-off related to the increase in CH4 emissions.