Submarine Groundwater Discharge From Non-Tidal Coastal Peatlands Along the Baltic Sea

Manon Janssen,Erwin Don Racasa,Miriam Toro,Bernd Lennartz

doi:10.3389/feart.2021.665802

Abstract

Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.

Highlights

Submarine groundwater discharge has been recognized as an important land-ocean route for water and materials (Burnett et al, 2003; Burnett et al, 2006; Moore, 2010; Taniguchi et al, 2019) and often has higher concentrations of carbon, nutrients, and metals than river waters (Moore, 2010)
Water originates from 1) atmospheric input to the dune and the beach and 2) peatland surface water that infiltrated into the dune base, as deduced from velocity vectors
Coastal peatlands are widespread along the German Baltic Sea coast

Summary

Introduction

Submarine groundwater discharge has been recognized as an important land-ocean route for water and materials (Burnett et al, 2003; Burnett et al, 2006; Moore, 2010; Taniguchi et al, 2019) and often has higher concentrations of carbon, nutrients, and metals than river waters (Moore, 2010). The vast majority of these coastal peatlands are degraded due to a long history of drainage for agricultural usage (Baird, 1997) and bordered by coastal protection measures (Bollmann et al, 2010). They are characterized by a low hydraulic conductivity (Ks) and low hydraulic gradients. In organic-rich subterranean estuaries, high concentrations of remineralized forms of organic matter have been reported (Taniguchi et al, 2019) Coastal peatlands with their large stores of carbon, organic matter, and nutrients from decaying plants could be a potentially overlooked source of water and materials for the Baltic Sea via the SGD pathway

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