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, with seepage rates of 1.05 cm/d (upper discharge region) and 0.16 cm/d (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 to 0.293 m²/d. 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.

中文翻译：

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波罗的海沿岸非潮汐沿海泥炭地的海底地下水排放

海底地下水排放 (SGD) 是陆海过渡区内水和物质的重要途径，可影响沿海环境和海洋生物。尽管近年来对沙质海岸线的研究进展迅速，但对以低导水率为特征的沿海地区（例如富含碳的沿海泥炭地）知之甚少。本研究的目的是确定位于波罗的海沿岸非潮汐低洼沿海泥炭地的陆地 SGD 的大小和位置，并使用数值模型了解控制因素。我们使用 HYDRUS-2D 建模包来模拟稳态条件下从沙丘堤坝分离的再湿沼泽延伸到浅海的断面中的水运动。土壤物理性质，水力梯度、地质分层和地形各不相同，以描绘沿海泥炭地所遇到的各种性质。我们的结果表明，陆地 SGD 以 0.080 m²/d 的通量发生在研究地点，渗流率为 1.05 cm/d（上部排放区域）和 0.16 cm/d（淹没泥炭层上方的下部排放区域）。这些计算出的渗流率与波罗的海其他湿地环境和 SGD 站点的观测结果相比较。地下水主要来自沙丘堤坝——通过降水和泥炭地表水的渗透补给——在较小程度上来自沙地含水层。情景模拟产生了 0.008 至 0.293 m²/d 的潜在 SGD 通量范围。他们揭示了陆地 SGD 的位置是由延伸到海底的泥炭层的屏障功能决定的。然而，它对体积通量几乎没有影响，因为大多数 SGD 发生在海岸线附近。SGD 的大小主要由水力梯度和泥炭和沙滩/沙丘的导水率驱动。水平方向的各向异性、含水层和泥炭厚度以及泥炭地高程对 SGD 的影响很小。我们得出结论，SGD 最有可能来自具有高水位、大 K 和/或沙丘堤防或带的沿海泥炭地，这可能是通过 SGD 途径获得碳和其他材料的重要来源。SGD 的大小主要由水力梯度和泥炭和沙滩/沙丘的导水率驱动。水平方向的各向异性、含水层和泥炭厚度以及泥炭地高程对 SGD 的影响很小。我们得出结论，SGD 最有可能来自具有高水位、大 K 和/或沙丘堤防或带的沿海泥炭地，这可能是通过 SGD 途径获得碳和其他材料的重要来源。SGD 的大小主要由水力梯度和泥炭和沙滩/沙丘的导水率驱动。水平方向的各向异性、含水层和泥炭厚度以及泥炭地高程对 SGD 的影响很小。我们得出结论，SGD 最有可能来自具有高水位、大 K 和/或沙丘堤防或带的沿海泥炭地，这可能是通过 SGD 途径获得碳和其他材料的重要来源。