What is excess deuterium

Stable water isotopes as tracers of the surface water / groundwater interaction

Marc Schürch1, Ulrich Schotterer2, Ronald Kozel2
1 Hydrology Department, Federal Office for the Environment FOEN
2 University of Bern, Institute of Physics, Climate and Environmental Physics

O 8.9 inIsotope and tracer methods in hydrogeology

April 16, 2016, 10: 45-11: 00, Audimax A, building 30.95

The stable isotopes of the water molecule, deuterium and oxygen-18, as well as the corresponding deuterium excess mark the water cycle and its changes over time. As part of the National Groundwater Observation NAQUA, Module Water Isotopes ISOT, the Federal Office for the Environment FOEN in Switzerland has been collecting isotope data in precipitation (13 stations) and in surface water (9 stations) since 1992. The isotopes were analyzed from 2007-2013 at the 50 measuring points of the NAQUA module TREND (groundwater quality).

The isotope signal in precipitation is transmitted to surface water and groundwater. The stable isotopes can be used to check whether and how the local information from a precipitation station is modified by the various processes involved in the formation of runoff in a river. This has an impact on the seasonal fluctuations in river water. The stable water isotopes thus represent a natural tracer to describe the surface water / groundwater interaction in river valley levels. Depending on the groundwater conditions, the infiltration and exfiltration conditions change seasonally and spatially along the rivers, each with characteristic isotope signatures in the river water and groundwater. Many rivers in Switzerland have a higher-lying catchment area than the local groundwater, which means that the proportion of river water and rainwater infiltrate can be differentiated in the groundwater on the basis of the stable water isotopes.

In the Bern area, for example, the difference in oxygen-18 (i.e.18O) between the river Aare and the groundwater with 2 to 3 ‰ large enough to be able to follow the seasonal course of the interaction between river water and near-river groundwater. The stable water isotopes are observed here in the groundwater at the Kiesen measuring point. The extraction well is located in the Aare gravel with a hydraulic connection to the river. The deposits forming the aquifer are around 30 meters thick here. They have an average permeability of 4.2 * 10-3 m / s and a porosity of 15%. The mean flow velocity of the groundwater is between 8 and 15 m / day. The seasonal dynamics in the d18O is similar in the Aare and in the groundwater, with the negative d-values ​​of winter precipitation being shifted to summer due to the high importance of meltwater. This correspondence in the course of the isotope signature shows that the groundwater in this area - especially in summer when the snow and glacier melt - is largely fed by river water infiltration. The seasonal proportions of river and rainwater infiltrate determined on the basis of the stable isotopes agree well with the results of a large-scale tracer experiment. During extremely dry periods or flood peaks, however, the mixing ratios are significantly changed, which is then also reflected in the deviating isotope signature.

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