Different continuous freshwater contributions to submarine groundwater discharge at a coastal peatland, southern Baltic Sea

The impact of freshwater sources like surface river runoff and submarine groundwater discharge (SGD) on coastal waters is currently in focus of intense debate and investigation. One of the ongoing challenges in SGD research is the characterization and quantification of the freshwater endmember contributions to the subsurface mixing zone and their influences on element balance and biogeochemical transformations. Long-term investigations of the sediment porewater composition provide characterization and understanding of the physical, hydrological and biogeochemical processes controlling the substance exchanges. In this study, we focus on the hydrochemical and stable isotope (δ2H, δ18O) compositions of sediment porewaters along the coastline of a southern Baltic Sea peatland. Coastal surface water and groundwater dynamics were monitored at two coastal sites using 5-m-long stationary lances over a 5-year period.
The vertical compositional gradients were used to extrapolate to zero-salinity (ZS) components applying a binary mixing model on the salinity and water isotope composition. The results characterize a subterranean estuary (STE) with three potential mixing endmembers: two fresh groundwaters and the brackish Baltic Sea. Tritium–helium (3H–3He) porewater dating gave ages of more than about 20 years for the freshwater components. The ZS components were compared with other SGD sites along the southern Baltic Sea and North Sea and highlight the importance of local SGD studies for a proper groundwater endmember characterization as basis to understand hydrological and biogeochemical developments at the land–ocean continuum in times of current climate change.

地表河流径流、海底地下水排泄（submarine groundwater discharge, SGD）等淡水水源对近岸海域的影响，当前已是学界激烈讨论与深入研究的热点课题。海底地下水排泄研究中一项长期存在的挑战，在于如何表征并量化淡水端元对地下混合带的贡献，以及其对元素平衡与生物地球化学转化过程的影响。长期开展的沉积物孔隙水组成研究，有助于阐明控制物质交换的物理、水文与生物地球化学过程。本研究聚焦波罗的海南部泥炭海岸带沿线沉积物孔隙水的水化学与稳定同位素（δ²H、δ¹⁸O）组成。研究团队在两处海岸站点，通过5米长的固定式采样测杆，开展了为期5年的近岸地表水与地下水动态监测工作。
研究基于盐度与水同位素组成的二元混合模型，利用垂向组成梯度外推得到零盐度（zero-salinity, ZS）组分。结果表明，该区域存在包含三类潜在混合端元的地下河口（subterranean estuary, STE）：两处淡水地下水与咸水波罗的海水体。氚-氦（³H-³He）孔隙水定年结果显示，淡水组分的年龄超过约20年。研究将零盐度组分与波罗的海南部及北海沿岸的其他海底地下水排泄站点数据进行对比，结果凸显了开展局地海底地下水排泄研究的重要性——唯有以此为基础，才能准确表征地下水端元，进而理解当前气候变化背景下陆-海连续体的水文与生物地球化学演化过程。