Deuterium labeling of unsaturated zone water movement: An example from sandy-soil dunes at Langeoog Island / Germany

The recharge rate and hydrochemistry of an aquifer is determined mostly during rain water infiltration into soils and the subsequent movement through the unsaturated zone. Very often, the environmental isotope input signals of precipitation are not pronounced enough to follow them through the unsaturated zone, e.g., because seasonal temperature variations are not pronounced, rainfall occurs only infrequently, or at coastal locations where isotope signals are usually dampened. In such cases artificially enriched deuterium might be a possibility to create a clearer input signal. Deuterium was not used for labeling experiments very often because it is rather expensive and advanced laboratory extractions and measurements techniques were necessary. New laser techniques allow an easier detection of soil water isotopes after water extraction or directly from soil matrix material with additional equipment. We conducted a labeling experiment at the Island of Langeoog in Northern Germany during 2012/2013 with a deuterium application in October 2012 and consequent collection of soil profiles in January and May 2013. For labeling we used about 1.2 kg of a 70% D₂O tracer solution applied in 100 ml aliquots on 13 points on a 1 m² study plot (equals 1.3 mm of rain input). Soil samples were collected with a hand auger (Eijkelkamp Inc.) to a depth of up to 6 m in 10 cm steps and analyzed for deuterium using an induction module (IM) and a L2120-i cavity ring down laser from Picarro Inc. The results show a deuterium peak at 200 cm after 3 months and almost background values over the entire soil depth after 6 months, indicating a rapid water movement at sand dunes of Langeoog Island.