Freshwater lenses as archive of climate, groundwater recharge, and hydrochemical evolution: Insights from depth‐specific water isotope analysis and age determination on the island of Langeoog, Germany

Abstract

The age stratification of a freshwater lens on the island of Langeoog, Germany, was reconstructed through depth-specific sampling and groundwater dating using the tritium-helium method. The stratification is strongly affected by the land use and resulting differences in recharge rates. Infiltration at the dune tops is significantly lower than in the valleys, due to repellency of the dry sand. Dune valleys contribute up to four times more groundwater recharge per area than other areas. Housing development in dune areas might therefore significantly decrease the available fresh groundwater. The freshwater column shows a distinct increase of stable isotope values with decreasing depths. Hence, the freshwater lens contains a climate archive which reflects changing environmental conditions at the time of recharge. Combined with tritium-helium dating, this pattern could be matched to climate records which show an increase of the temperature at the time of recharge and rainfall rates during the last 50 years. The spatial and temporal developments of water chemistry during the passage through the lens follow a marked pattern from a sodium and chloride-dominated rainwater of low conductivity to a more mineralized sodium bicarbonate water type, caused by dissolution of carbonate shells close to the surface and subsequent ion exchange of calcium for sodium in the deeper parts.

Observations and analytical modeling of freshwater and rainwater lenses in coastal dune systems

“…The stratification pattern of groundwater ages as depicted in Fig.7 was confirmed by sand tank experiments by Stoeckl and Houben (2012). Field evidence of groundwater age stratification in coastal dunes is available from Stuyfzand (1993), Röper et al (2012) and Houben et al (2014).…”

Review on soil water isotope-based groundwater recharge estimations

“…() and Houben et al . (). Groundwater recharge estimations using this method will benefit from in situ field measurements because time‐intense laboratory extractions are not necessary anymore.…”

“…Because an application of lumped parameter model approaches and in-depth interpretations of isotope-derived transit times are beyond the scope of this review, we do not discuss further details here and refer to discussions in e.g. Steward and McDonnell (1991), Kendall and McDonnell (1996), McGuire et al (2006), Leibundgut et al (2010) and Houben et al (2014). Groundwater recharge estimations using this method will benefit from in situ field measurements because time-intense laboratory extractions are not necessary anymore.…”

Oblique Porous Composite as Evaporating “Cap”: Do Desert Dunes Preserve Moisture by Capillary Barriers and Tilt of Their Slopes?

“…Plants can also take water from condensing and accumulation of soil moisture due to diurnal temperature variations near a sand surface; Danin, .) in application to a moderate climate has been investigated (see, e.g., Houben et al, ; Lehmann et al, ; Lehman and Or, ; Shahraeeni & Or, ; Shokri et al, ; Shokri & Or, , ; the composite soil studied in our note differs from the Swiss Federal Institute of Technology Zurich ones in a slanted, rather than vertical, interface between flow zones of contrasting texture).…”