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Highlights in Isotope Hydrology

Isotopes in water resources management

Lothar Wedekind, Chief Editor, with contributions from JoAnne Ford and Rodolfo Quevenco

March 20, 2000

Shortage of water is a key development issue in much of Africa. In 1998, technical co-operation activities to promote the use of isotope hydrology, in combination with other techniques, to improve the development and management of water resources were implemented in 16 countries.

Activities in Egypt, Ethiopia, Morocco, and Senegal were completed with tangible achievements. In Egypt, for example, the Agency supported national efforts to reclaim new lands on the fringes of the Nile flood plain. The project enabled the preparation of a comprehensive hydrogeological map to be used in future management of water resources of the areas investigated at Wadi Qena and Esna.
Concerned by the alarming increase in the level of Lake Beseka, the Ethiopian Government asked the Agency for help in finding the cause. This lake, about 190 km east of Addis Ababa, has expanded from its original 3.3 km² in 1964 to over 35 km², thus putting a highway and railway under serious threat. Under a Programme Reserve project, isotope investigations appear to validate previous hydrogeological data that the problem is likely due to an increased flow to the lake of deep groundwater from fractured aquifers. On the basis of these finding, the national authorities are considering various options to alleviate this problem.

Isotopes help track groundwater resources

Boy with water hose Improved knowledge of groundwater and aquifer systems is vital to effective sustainable management, especially in those areas where water is scarce. Isotopes can be used to track the movement of surface and groundwater and investigate the groundwater recharge, critical to managing scare resources.

In the Middle East region, major aquifer systems have been investigated using isotopes to map the aquifers and understand how the groundwater is replenished. Results of two projects in the region have shown that groundwater in certain areas is "paleowater"- water that infiltrated the ground thousands of years ago - and is not currently being replenished. Paleowaters are often encountered in such arid and semi-arid regions and, because they are not being replenished, must be exploited with caution.
Groundwater is the most valuable resource for future generations, but in many parts of Latin America it has been exploited "on demand" without sufficient knowledge of the exploitation capacity of the aquifers or their vulnerability to pollution. Under the project, Aquifer Characterization for Sustainable Management, counterparts in the region have acquired the ability to evaluate aquifer characteristics using conventional and isotopic techniques. Using these techniques where information is scarce, and compiling it with historical data available in the region, participants will develop a conceptual model for the hydrodynamics and water quality of the aquifers.
Isotope hydrology field investigations were conducted in the unsaturated zone in the Saudi Arabian Peninsula as part of the project, Isotope Hydrology Techniques in Water Resource Management. This investigation indicated a recharge rate of 15 mm/year in sand dune areas, in contrast to the result given by water balance that indicated almost no recharge. This implies that in sand dune covered parts of the peninsula, an average renewable rate of about 15,000 m3/year for each sq. km of surface area can be expected in shallow groundwater beneath sand dunes, thus increasing the amount of safely extractable water.
Key practical problems addressed by isotope studies in West Asia include estimating the natural recharge rate as a basis for assessment of available groundwater safe-yield, mapping of fossil groundwater resources with no natural recharge under present climatic conditions, assessing the effectiveness of artificial recharge to groundwater at selected sites, and identifying the sources and transport dynamics of pollutants, including salinity.

Isotopes for investigating water contamination

Isotopes are also used to investigate groundwater contamination, a growing concern in several regions of the world. Polluted groundwater may remain in aquifers for centuries, or longer, and is very difficult, if not impossible, to clean up. Isotope techniques can not only assess the vulnerability of groundwater to pollution from surface sources, but also provide an early warning when other indicators do not give cause for concern.

Agency investigations into the causes of high nitrate contamination observed in some of the West Asia’s main aquifer systems were initiated in 1998 with the intent of that the results will contribute to proper management decision for minimizing or preventing further water quality deterioration.

In Bangladesh, millions of people are at risk of exposure to arsenic through contaminated drinking water. One of the mitigation strategies is to develop deep groundwater as an alternative source of arsenic-free drinking water. Isotope techniques have been integrated into the hydrologic characterization of shallow and deep groundwater to determine the movement of water in the different aquifers. Such information is vital to managing deep aquifers over the long term and ensuring that they remain arsenic free.

Isotopes to study and trace water leakage

In recent years, the use of isotope techniques to study the origin and path of water leakage from dams and reservoirs has been given specific emphasis.

In 1998, Specialized Teams, established to investigate the origins of dam leakage in Africa, achieved a great deal of success in less than one year. Teams consisting of four experts in hydrogeology, hydrochemistry, and isotope hydrology investigated three dams in Algeria, Morocco, and Namibia. The origin of the leakage in one of them has already been identified and appropriate solutions recommended, while in the case of the others, additional tests are underway to confirm initial results. The projected savings directly attributable to the timely identification of the origin of leakage in the Moroccan dam are estimated to be in the range of several million dollars.
In Venezuela, isotopic investigations pinpointed the area of leakage at the La Honda dam, resulting in millions of dollars of savings in remedial measures.
The flow rate of the Arenal river in Costa Rica increases dramatically downstream of the El Arnel dam. If this increase was due to leakage or seepage from the reservoir, the stability of the dam- accountable for 65% of the country’s electricity production- would have been endangered. An isotopic study showed that the river flow increased due to groundwater discharge and not leakage from the reservoir.

Another important use of isotope techniques is in investigating the effectiveness of surface reservoirs.

In Jordan, for example, millions of dollars have been invested to construct surface reservoirs in order to capture flood water for replenishing underground aquifer systems. Isotopic investigations have concluded that these surface reservoirs do not function properly, since no effective recharge to the aquifers could be substantiated. Any loss of water in the reservoir could be accounted for by surface evaporation. In view of these findings the use of such reservoirs should be reviewed by national water authorities in order to incorporate the engineering measures that will make them function cost effectively.