Research Capabilities and Facilities

What is isotope hydrology?

The Water Cycle:

The water cycle describes the migration of water from a source, normally the sea surface, through the atmosphere and terrestrial ecosystems and back to the oceans. A complete description includes information of the mass of water in each compartment (oceans, ice, groundwater, lakes, rivers etc) and time scales for the transport processes.

Isotopes and the water cycle:

Isotopes contributing to the study of the water cycle are listed below.

ISOTOPES AND THE WATER CYCLE

* The isotopes H-2 (deuterium), oxygen-18 and tritium (H-3) are incorporated within the water molecule H₂O. Carbon-14 (bicarbonate) and Cl-36 (chloride) are dissolved in the natural waters

Stable isotopes

The water molecules HDO and H₂¹⁸O have vapour pressures which are slightly less than normal water H₂O. There is therefore some separation of isotopes associated with processes of evaporation and condensation within the water cycle.

These processes are very small yet measurable. This is illustrated in the schematic diagram of the water cycle. Typical values for the ¹⁸O/¹⁶O ratios for ocean water, water vapour and surface and groundwater are shown. The actual numbers are the deviations in parts per thousand (per mille) of the oxygen isotope ratio in the water sample from that in standard ocean water.

These are representative values. In practice the stable isotope composition of each sample is different and reflects the source of the water (for instance the tropical oceans) and its subsequent history of evaporation, condensation and mixing.

Environmental radioactive isotopes

Environmental radioactive isotopes can be used to age date groundwater; ie to estimate the time that has elapsed since the recharge water infiltrated underground. The isotopes commonly used are listed in the table. The maximum age of the groundwater depends on the half-life of the isotope. Tritium can be used to identify waters up to about 40 years; carbon-14 up to 30,000 years and chlorine-36 up to about 1.5 million years.

The practical importance of age dating is related to the fact that the time scale for replenishment of the groundwater is commensurate to its age. Young groundwater is efficiently replaced by natural recharge. On the other hand, the use of old (or fossil) groundwater is analogous to mining the resource. This situation is of particular concern in arid zones.

The Great Artesian Basin:

In an early collaborative study, groundwater in the Great Artesian Basin was dated up to 1.5M years using chlorine-36. These samples may reflect the oldest flowing groundwater on the planet.

How ANSTO can help

Background:

ANSTO (formerly the Australian Atomic Energy Commission) has been involved in isotope hydrology studies for over 30 years. Study sites have included the Burdekin Delta, the Namoi Valley, the Mereenie Sandstone aquifer (Alice Springs), the Great Artesian Basin, the Murray Darling Basin and many others.

Skills:

ANSTO currently focuses on the application of a wide range of techniques to address priority problems including:

• Improving strategies for the sustainable use of groundwater through better understanding of the dynamics of groundwater flow;
• Refining estimates of water balance in large catchments such as the Murray Darling Basin;
• Assessing the impact of climate change on groundwater recharge in arid zones; a contribution to developing strategies for groundwater sustainability;
• The impact of salinity in urban and regional areas;
• The impact of contaminant release to coastal dune systems

Capabilities:

ANSTO has a range of analytical and isotope measurement capabilities including:

Isotope measurements: These include the environmental radioisotopes tritium, carbon-14 and stable water isotopes.

Advanced analytical capability: Techniques include neutron activation analysis; particle activation analysis (PIXE, PIGME); gamma spectrometry; X-ray fluorescence; ICPMS etc;

Water tracing: Both radioactive and non-radioactive tracers are available;

The skills and capabilities are best illustrated with case studies. Some are presented below; others may be accessed through the links provided.

Case Studies

Evaporation in the Darling River during the 2002 El Niño Drought

Underpinning the sustainable management of water resources in large inland basins is an adequate knowledge including the extent of evaporative losses. As an example, ANSTO has used stable isotopes to quantify losses in the Darling R due to evaporation in drought.

Water samples were collected monthly from nine stations July 2002 to Jan 2003 and weekly from Burtundy. Evaporation leads to the progressive accumulation of water with heavier isotopes (HDO and H₂¹⁸O). Estimates of the cumulative evaporation can be made from the relationship between the two isotopic enrichments.

For instance at Burtundy it was found that the cumulative loss from 8 July 02 to 7Jan 03 was 22.5% indicating the extremity of the El Niño drought.

Click on image for larger view

Evaluation of climate change models in the arid zones

Isotopes are being used to independently evaluate General Circulation Models GCMs and hence improve climate change prediction.

An invaluable dataset has been assembled over four decades by the IAEA under the Global Isotopes in Precipitation (GNIP) program which is accessible at http://isohis.iaea.org. These data are being used to evaluate isotope enabled climate models.

At ANSTO we have used isotopes to evaluate model predictions of changes of water balance within the Amazon basin during the period of deforestation.

Further we are using isotopes to calculate the threshold rainfall intensity for effective recharge in Alice Springs. These studies are linked to the development of strategies for the sustainable use of groundwater in arid regions challenged by climate change.

Sustainable groundwater recharge in arid zones

ANSTO is developing a methodology for assessing the impact of climate change on groundwater recharge in warm arid zones. The approach will be illustrated with data from Alice Springs and Melbourne.

Alice Springs rainfall shows a much wider distribution in the HDO/H₂O ratios than that of Melbourne as shown in the figure.

Click on image for larger view

In both cases the isotopes in the groundwater are well mixed and can be represented by the vertical lines. If all the local precipitation infiltrated underground and contributed to recharge, the isotopic composition of the groundwater would match the weighted mean of the precipitation. This match is clearly much closer for cool temperate Melbourne than for warm arid Alice Springs. The groundwater (from the Mereenie sandstone aquifer) is significantly different (in fact more depleted) than the rainfall average.

An analysis of the data shows that in the Alice Springs region, effective rainfall only occurs if the rainfall intensity (mm/month) exceeds a threshold value and that this value is about 80 mm/month.

A more detailed analysis of the data suggests that the threshold value was somewhat lower a few thousand years ago suggesting that the region was less arid.

These threshold values may be calculated using isotope enabled GCMs and applied to strategies for the sustainable use of groundwater in arid regions challenged by climate hange.

Future Opportunities

Mixing of surface and groundwater

As shown in the Water Cycle diagram, the isotope composition of river and groundwater are sufficiently different to allow estimates to be made of the extent to which river water may be seeping into the banks and mixing with groundwater. There exist opportunities for applications to conjunctive water management.

Conjunctive water management is the integrated management of hydraulically connected surface and groundwater to the long term benefit of all water users (please refer to http://www.brs.gov.au/connectedwater ). Isotopes can contribute to assessing the degree of connectivity of surface and groundwater which is essential to developing management strategies.