Jorgensen (2001) reported that shallow saline groundwater has a patchy occurrence in superficial post-glacial marine sediments beneath coastal meadows and salt marshes on the southern shores of the Island of Læsø, Denmark. The post-glacial sequence consists of 1–2 m of marine sands and gravels resting on a slightly undulating platform of low permeability interglacial marine clay, which makes up a confining layer or aquiclude. The saline groundwater primarily occurs beneath the vegetated fringe and associated minor unvegetated sand pans of coastal meadows and salt marshes. The salinity varies from 2% to approximately 17% and analyses of major ions indicate a prevailing marine composition.
The saline groundwater is supersaturated with respect to carbonates in the entire salinity range and slightly supersaturated with respect to gypsum with salinities above 10%. However, aragonite is the only precipitate recorded so far. Stable isotope analyses (2H and 18O) plot fairly well along the local groundwater–seawater mixing line indicating mixing of seawater with fresh meteoric groundwater in the proportion 0–50%. In contrast, the stable isotopic compositions show no correlation to the wide range of salinities recorded. It is proposed that the thin cover of superficial deposits resting on a clay platform results in a relatively closed hydrochemical system in which increasing concentration of dissolved solids in the interstitial water is caused by transpiration which is known to draw moisture from groundwater without leading to isotopic
fractionation.
Periodical precipitation/dissolution of salt in topsoils of sand pans and from salt secreting halophytes may also be contributory causes of increasing salinity in the groundwater. The spatial distribution of the saline groundwater most likely depends on the microtopography of the slightly undulating clay platform. The dense, saline shallow groundwater, emerging from the vegetated fringe and sand pans and flowing toward the depressions in the clay platform, percolates downward displacing lower salinity seawater and groundwater.
The hydrochemistry of shallow saline groundwater in the superficial post-glacial marine deposits at Rønnerne on the Island of Læsø indicates that the saline groundwater and brine predominantly have a marine origin. However, oxygen and hydrogen isotope signatures reveal a significant correlation to the local groundwater–seawater mixing line indicating a contribution of fresh groundwater in the range from 0% to 50%. The salinity of the saline groundwater is in the range from 2% to 17%. Calculated saturation indices indicate significant supersaturation with respect to calcite and aragonite in the entire salinity range and slight supersaturation with respect to gypsum with salinities above 10%. However, authigenic aragonite cement that occurs in the most saline lower part of the aquifer is the only precipitate recorded so far.
The high salinities of up to 17% do not show corresponding enrichments in the isotopic compositions of the saline groundwater and this rules out any kinetic evaporative effect on the formation of the saline water. The isotope distributions suggest that the transpiration that takes place in the dense vegetation on coastal meadows and salt marshes results in increasing concentration of dissolved solids in the shallow groundwater–seawater mixture. Additionally, precipitation/dissolution of salt from salt-secreting halophytes and from the surface of the sand pans may be a contributory cause for salinity enrichment of the groundwater in the shallow stratified aquifer.
The dense saline water emerging from the vegetated fringe and flowing toward the depressions in the clay platform, percolates downward displacing lower salinity seawater–groundwater mixtures. The stratified water body, low flow regime and stagnant conditions in the depressions in the clay platform most likely prevent subsequent flushing and dilution of the saline waters by meteoric water and/or seawater.
The results of this study demonstrate an important biological/physiological interaction between the vegetation of the coastal meadows and salt marshes and the shallow groundwater–seawater system.
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