Identifying the dominant hydrochemical processes post wetland restoration along stream valleys, Denmark

Restoration and rewetting of wetlands previously drained for agriculture, is currently used to decrease net greenhouse gas (GHG) emissions, while improving biodiversity. Wetland hydro(geo)logy is known to exert a key control on GHG-retention and on conditions facilitating improved biodiversity. Yet knowledge of the major hydrochemical processes that occur in wetlands prior to drainage and after restoration is limited, although links between wetland hydrochemistry, GHG-retention and biodiversity might well exist. To reduce the knowledge gap, we sample surface waters, precipitation, and shallow (<1 m) groundwater from 61 wells. The sampling sites are either near-natural or restored wetlands of the riparian zone, and are distributed along three separate stream valleys, with subsurface geologies consisting of carbonate rock, glacial till or sand from glacial outwash. Furthermore, the wetlands are categorized based on management (grazed or unmanaged). Surface and groundwater samples are analyzed for dissolved major ions, methane (CH₄), organic carbon (DOC), fluorescence, and all samples are analyzed for stable water isotopes (δ¹⁸O, δD) and electrical conductivity (EC). EC, pH, dissolved oxygen (O₂) and temperature are measured in the field using a flow cell. Initial results from the groundwater wells in the wetlands indicate EC values between 101-5300 μS/cm (the high end due to marine influence), O₂ between 0.1-6.7 mg/L, and that the pH varies from acidic (min. 5.0) to alkaline (max. 7.7). The groundwater’s Fe(II) concentration appears to be significantly elevated in restored stream valley sites versus the near-natural sites. The results suggest differences in redox conditions that in turn may control production of GHGs, such as CH₄. In addition, the hydrochemistry and subsurface geology seem to be a key factor in the development of the present vegetation in the various field sites. With shifting climate, terrestrial wetness will change too, under influence of hydrogeochemical-vegetation interactions. To understand the associated climate feedbacks, a detailed understanding of wetland hydrology and ecology is needed. Through a method-independent approach, this study helps clarify the response related to hydrochemistry, geology, and time. The increased understanding could also contribute to fine-tuning of current and future restoration programs, thus increasing their success.