Isotope hydrology
The stable isotopes of water and its constituents are determined in the laboratory of isotope hydrology. Of particular interest are the stable isotopes of the elements hydrogen (H), oxygen (O), carbon (C), nitrogen (N) and sulphur (S), as these are used to trace the water molecule and other important substances (such as pollutants, impurities and natural components).
Main areas of work
Stable isotopes can be used to address specific issues in hydrology and hydrogeology, or to complement research in other disciplines, for example in the following areas:
- Groundwater recharge via isotope variability in the unsaturated zone;
- Catchment area characterisation (e.g. catchment elevation of springs and groundwater);
- Interaction between groundwater and surface water, especially during hydrological extremes (flood, drought);
- Mixing proportions of water (separation of baseflow);
- Paleoclimate research (fossil groundwater, ice cores);
- Layering, circulation and groundwater inflow in lakes;
- Artificial tracer tests (with enriched deuterium or oxygen-18);
- Sustainability with increasing use of groundwater resources;
- Processes and balances in the nitrogen, carbon and sulphur cycles.
Current projects
Analytical methods
At BGR’s isotope hydrology laboratory, the stable water isotopes (oxygen-18, δ¹⁸O and deuterium, δ²H) are measured with laser spectrometers from Picarro and Los Gatos. Isotope-ratio mass spectrometry (IRMS) from Thermo (high temperature conversion elemental analyser – TCEA) and Elementar (IsoTOC, Isoprime) are used as well. The isotopes of carbon (¹³C/¹²C), nitrogen (¹⁵N/¹⁴N) and sulphur (³⁴S/³²S) are measured after conversion via GasBench and elemental analysis (EA) in the gas phase.
During isotope analysis using laser spectrometers following the cavity ringdown method (Picarro L2120-I, L2130-I, and L2140-i) as well as for Los Gatos instruments, water molecules are measured in vapor phase above 100 °C and show characteristic absorption lines in the near infrared range. With known wavelength characteristics and under controlled test conditions (pressure, temperature), the light absorption (peak height) can be used to determine the isotope concentration.
Additionally, dissolved gases (nitrogen, argon, oxygen, helium and krypton) can be measured in the field using a MiniRüdi from Gasometrix and fluorescent tracers (fluorescein, rhodamine, naphtionat etc.) with a Perkin Elmer LS 55 analyser in the laboratory.