Origin of shallow saline groundwater on the Island of Læsø, Denmark

Abstract

Shallow saline groundwater has a patchy occurrence in superficial postglacial marine sediments beneath coastal meadows and salt marshes on the southern shores of the Island of Læsø, Denmark. The postglacial sequence consists of 1–2 m of marine sands and gravels resting on a slightly undulating platform of practically impermeable 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 abstract water 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.