No abstract

Peat soils are heterogeneous, anisotropic porous media. Compared to mineral soils, there is still limited understanding of physical and solute transport properties of fen peat soils. In this study, we aimed to explore the effect of soil anisotropy on solute transport in degraded fen peat. Undisturbed soil cores, taken in vertical and horizontal direction, were collected from one drained and one restored fen peatland both in a comparable state of soil degradation. Saturated hydraulic conductivity (K s ) and chemical properties of peat were determined for all soil cores. Miscible displacement

No abstract

<p>Most research on submarine groundwater discharge (SGD) focuses on sandy beaches. Less studies have investigated environments with low hydraulic conductivity (K<sub>s</sub>) such as coastal peatlands, which are abundant along the southern Baltic Sea coast. Coastal peatlands, which have been drained for agricultural purposes, hold high quantities of carbon, nitrogen, and other compounds that could possibly be released to the sea upon rewetting of these sites. In this study, we simulated groundwater flow from a coastal rewetted fen with a peat layer extending out into the sea to understand the short– and long–term dynamics of SGD, quantify SGD water and matter fluxes, and assess the impact of a storm surge on SGD and seawater intrusion. Five-year (2016 – 2021) daily 2D numerical simulations of groundwater flow were based primarily on monitored groundwater and seawater level data and field-gathered soil hydraulic parameters. Hydraulic conductivities of geological layers were optimized against measured water levels. Manual seepage meter measurements were conducted and water samples were collected. The modeled seepage rates fitted the measured ones well. Our results reveal that SGD and seawater intrusion are highly dynamic and vary spatially and temporally. Two dominant submarine discharge areas were observed: 1) near the beach (up to ~30 m from shore) where mean seepage rates based on nodal water velocities reach up to 12.4 cm d<sup>-1</sup> with waters originating from the dune dike and recirculated seawater; 2) seeps from the aquifer at about 60 m distance from the coast with discharge rates of 1.1 cm d<sup>-1</sup> on average. Mean seepage rates from the discharge areas are comparable to other wetland and sandy environments. The low K<sub>s</sub> of the peat layer limits water exchange between the peatland and the Baltic Sea to these regions. The groundwater-seawater interface below the dune moves between the beach and the central dune on an hourly to weekly basis. However, the extent of the interface changes at a seasonal scale. Higher SGD fluxes occur in spring and summer while seawater intrusion increases during fall and winter, as a consequence of the seasonal variations of the peatland’s water level and the resulting hydraulic gradient. During storm surges, higher seawater intrusion fluxes are expected, while low seawater would lead to higher SGD fluxes. The mean daily net flux which represents land-derived SGD from the peatland is 0.15 m<sup>2</sup> d<sup>-1</sup> (range: -6.12 m<sup>2</sup> d<sup>-1</sup> to 1.63 m<sup>2</sup> d<sup>-1</sup>), with the highest intrusion occurring during the 2019 storm surge and the highest SGD occurring two days after the surge event. Our mean daily net flux compares well with previous studies but total SGD, which includes recirculated seawater, is likely underestimated. Nearshore carbon and nitrogen SGD concentrations are higher than ambient seawater concentrations demonstrating the potential impact of SGD on local biogeochemistry. Our findings show that SGD is an important coastal process even from low-lying and low K<sub>s</sub> coastal peatlands. We emphasize the importance of conducting more interconnection studies between peatland hydrogeology and geochemistry disciplines to better understand SGD processes in these environments.</p>

<p>Spatial variability of soil properties is important for hydrological studies. However, little information is available on the spatial variability of hydro-physical properties of peat soils. Three study sites: natural, degraded and extremely degraded peatland were selected for this study. At each site, 72 undisturbed soil cores were collected from 5m by 5m grid cells in an area of 40m by 45m. The saturated hydraulic conductivity (Ks), soil water retention curves, total porosity, macroporosity, bulk density and soil organic matter (OM) content were determined for all sampling locations. The van Genuchten model parameters (θs, α, n) were optimized using the RETC software package. A strong positive correlation between macroporosity and Ks was observed irrespective of the degradation stage of the peat. However, the relationships between macroporosity and Ks differed for the different sites. The soil physical properties (e.g. OM content and bulk density) exhibited different levels of spatial autocorrelation depending on the soil degradation stage. The cross-semivariograms showed a strong or moderate spatial dependency between soil physical properties and van Genuchten model parameters. The more a peat soil is degraded, the more likely it is that soil physical properties are spatially dependent. In conclusion, degradation stage plays an important role and should be considered more often in spatial analysis. The obtained cross-semivariogram may serve as a basis for 2D and 3D hydrological modelling. </p>