What Are Isotopes and What Can Be Learned From Them?



Elements consist of atoms of different mass called isotopes. During the evaporation and condensation of water, the concentration of oxygen and hydrogen isotopes that make up the water molecule undergoes small changes. Modern instruments can measure these with great precision.

The history and pathway of water in different parts of the hydrological cycle (click here for figure 2) can be followed by the abundance of the stable heavy isotopes of hydrogen (2H) called deuterium, and oxygen (18O). In this way, water in different environments develops isotopic "fingerprints" with which it can be identified and its origins traced.

Some isotopes are radioactive, which means that they decay away with time. For example, those of tritium (3H) and radiocarbon (14C), are naturally produced in tiny amounts in the atmosphere. Large amounts of tritium and radiocarbon were also injected into the atmosphere by nuclear bomb tests in the 1960s.

These isotopes are carried into groundwater by infiltrating rain and they can also be measured with specialised, sensitive equipment. The known "half-life" (click here for figure 3) of these radioactive isotopes allows a measurement of their concentrations to be interpreted as an "age", or residence time, of groundwater. Residence time indicates the replenishment rate as well as the rate of movement of groundwater.

In summary, isotope techniques:

Isotope techniques cannot "find" groundwater. The initial steps must be taken by geologists and geophysicists. With some springs, however, isotopes can provide preliminary indications of the flow paths and origins of the water. This information is important to access the groundwater during drilling. Radioactive and non-radioactive or "stable" isotope information is thereafter, integrated with other available data to produce a comprehensive hydrological concept of the groundwater system. Hydrologists can then develop a water management model, and strategies for sustainable development can be formulated.

The movement of surface and groundwater can also be investigated by artificial tracing, where a substance is purposely injected into the water and then detected at some other point in the system. Numerous substances may be employed, such as ordinary table salt or dyes. However, in most applications the sensitivity of measurement required can only be achieved by injected radioisotopes, such as artificially produced 3H or radioactive iodine, (131I).

Isotopes measure water recharge

Recharge of groundwater is one critical aspect in resource management, and isotopes can help determine both the area and the rate of recharge. The area can be identified by measuring 2H and 18O concentrations and correlating them to the altitude at which precipitation could have infiltrated the ground. The rate can be measured by tracing levels of radioactive tritium in soil at various depths. In many instances, the tritium "peak" can be found at considerable depths, which indicates the distance travelled by the moisture since being deposited as tritium fallout in 1963.

The tritium peak method has been applied all over the world, and in many different climates (click here for figures 4 and 5). In moister climates, where infiltration is high, artificial tritium can be injected as a tracer to determine the rate of recharge. Profiles of either environmental or artificial tritium can also give a measure of the movement of pollutant, such as nitrates and pesticides from agriculture.

Isotopes Help Manage Drinking Water Supplies

Groundwater is heavily utilized by densely populated communities in the Nile Valley in Egypt and Sudan. But its availability and sustainability in supplying the needs of growing populations is a problem. An IAEA Isotope Hydrology-assisted investigation, through the Technical Cooperation Programme, focussed on claifying the role of Nile River in the replenishment of the local groundwater aquifers being exploited for domestic and agricultural purposes.

Using 18O, 2H, 3H and 14C, it was possible to distinguish between fresh water originating from Nile River infiltration and paleowater representing a non-renewable resource. It has been demonstrated that the influence of the Nile water is seen up to 60 km from the river bank. The information about the relative contribution of fresh and old water in the exploitation of wells helped to design adequate management strategies for local water supply systems.

As in all other investigations stable isotopes, environmental tritium, chemistry and other tools will be applied to produce a comprehensive picture of the hydrological system.

Isotopes help track pollution

Pollution of surface water may be remedied by concerted prevention and controls, but it is more serious when pollution enters the groundwater. Polluted groundwater may remain in aquifers for centuries, even millennia, and is very difficult if not impossible to clean up.

Isotope techniques can assess the vulnerability of groundwater to pollution from the surface by determining how rapidly it moves and where it is being recharged. Surface sources of pollution can then be determined, e.g. natural, industrial, agricultural, or domestic. Isotope techniques can also identify incipient pollution, providing an early warning when the chemical or biological indicators do not give cause for concern. (Click here for figure 6).

Isotopes help tap renewable energy sources

When rain water moves deep into the earth's crust, it is heated to high temperatures and stored in deep reservoirs as a geothermal energy resource. Sometimes it emerges at the surface as hot springs or geysers. In many countries, this geothermal energy resource is harnessed by drilling into the reservoir and using the hot water to generate electric power.

Isotope techniques can determine the origin and flow paths of geothermal fluids and thus aid in the management of the energy resource. The isotopes 18O and 2H, as well as artificial tracers can determine the effects of exploitation and contribute to strategies for siting wells both for exploration and geothermal energy production; and for reinjection of cooled geothermal fluids which can extend the production period of the reservoir. (See figures 7 and 8).

Isotopes monitor global warming

The rapidly increasing concentration of carbon dioxide (CO2) and methane (CH4) in the atmosphere may be leading to global warming as a result of the so-called "greenhouse effect". Isotope techniques are also proving to be effective tools in unraveling this complex environmental phenomenon.

Analysis of carbon isotopes helps to explain what happens to the man-made "greenhouse" gases (CO2) and (CH4) in the atmosphere. Nitrogen and sulphur isotopes can reveal the connections between industrially produced oxides and acid rain. The oxygen and hydrogen isotopes in water are also very useful indicators of climate-related parameters such as surface air temperature, relative humidity and amount of precipitation.

Isotopes document climate changes

The isotopic composition of water can provide information about past climates. For example, radiocarbon measurements of groundwater in the Syrian desert indicate widely varying residence time or "age," ranging from very recent to nearly 40,000 years ago. Deuterium values of this very old water provide a striking record that the climate then was cooler than today (click here for figure 9).

The presence of an enormous freshwater lake that filled the entire Dead Sea Valley about 20,000 years ago confirms that the coolest period revealed in the deuterium record was also the wettest one during the past 40,000 years. The isotopes showed that present day recharge in the region is negligible and that groundwater is not being replenished -- information that is invaluable to water resource management strategies.

Isotope Analysis Protects Groundwater Quality

Groundwater in the Guayas alluvial aquifer underlying a major river basin in Ecuador has been heavily exploited for irrigation. Fears arose that sea water intrusion and salination of the groundwater could result. Thus, IAEA conducted an investigation under its Technical Cooperation Programme.

An isotope and hydrochemical study showed that groundwater becomes more confined, immobile and brackish towards the river mouth, but that sea water does not as yet influence groundwater quality. In the higher-lying parts of the delta, groundwater was found to be rapidly recharged by rain and river water. The study showed that groundwater extraction should be restricted to the upper part of the delta, leaving groundwater pressure near the river mouth undisturbed in order to prevent seawater intrusion. (Click here for figure 10).