Fig. 1: Global Water Availability - Selected Regions
Figure 2: The hydrological cycle - a schematic representation of the movement of water both above and below the earth's surface. The figure indicates approximate oxygen-18 values [expressed in the delta notation in parts per thousand (0/00) deviation from the standard mean ocean water.] The oxygen-18 values change through environmental processes and can be used to trace the pathway and history of water.
Figure 3: Environmental isotopes used in hydrology.
Isotopes measure water recharge
Fig. 4: In low-rainfall, arid conditions, infiltrating rainwater moves intermittently, and may remain in the unsaturated zone for decades. The "peak" in tritium concentrations of precipitation worldwide may still be found in soil profiles, such as this one from Senegal, which shows how far down moisture has moved from the surface since it was deposited by rain in the early 1960's.
Fig. 5: In moist climates, water moves downwards through the soil faily steadily. When tritium is injected, just below the surface, it moves with the water. The artificial "peak" can later be traced, and the infiltration rate measured.
Isotopes help track pollution
Fig. 6: Sources of nitrates - among the most common and serious pollutants of groundwater - and their nitrogen isotope values expressed as delta in parts per thousand (0/00) deviation from the standard. Nitrogen isotope analysis of groundwater samples can assist in identifying the source of nitrate pollution which facilitates appropriate mitigation measures.
Isotopes help tap renewable energy sources
Fig. 7: Isotope and chemical methods complement each other in studying the Palinpinon geothermal field in the Philippines. Tritium and 18O identify the incursion of cooler "meteoric" water during exploitation.
Fig. 8: IAEA technical assistance on isotope hydrology produced a hydrological model of the Palinpinon field during exploitation. The components of geothermal fluids and the processes affecting them were determined using isotope and chemical techniques.
Isotopes document climate changes
Fig. 9: Groundwater "ages" in the Syrian desert range up to 40,000 years. More negative values of 2H indicate moister past climates with a very wet period about 10,000 to 20,000 years ago. More positive values, also found in the Sahara desert, indicate persistent aridity in the region. Isotope methods determined that the age of major groundwater reserves beneath the Sahara is greater than 10,000 years.
Isotope analysis protects groundwater quality
Fig. 10: Map of the Guayas acquifer showing lines of equal radiocarbon concentrations in percentage of modern carbon. Recent water (~100%)is found upstream in the delta, while older water (~10%) is found towards the river mouth and the sea.
Tracking the source of groundwater pollution
Fig. 11: Venezuela's Lake Valencia is heavily polluted by industrial sources. Together with the nearby Taguaiguay Reservoir, it is becoming increasingly saline. The connection of the two water bodies to groundwater resources posed a possible threat to drinking water and irrigation supplies. A study undertaken by IAEA's Isotope Hydrology Section compared chloride and 18O values. It concluded that the salinity in groundwater was derived from the reservoir, rather than from the lake thereby ensuring safe development of the groundwater in the vicinity of the Lake.
The IAEA-WMO Global Network for Isotopes in Precipitation
Fig. 12: Tritium levels in the atmosphere dramatically increased between 1953 to 1962 due to atmospheric nuclear weapon tests. Since the ban on thermonuclear tests, atmospheric levels have declined. Recharge in groundwater can be easily detected by measuring tritium levels and the vulnerability to pollution of an aquifer system can be assessed.
Fig. 13: Monthly composite samples of rain and snow are collected at stations around the world and analyzed for 18O, 2H and 3H. The Agency's laboratory in Vienna performs part of the analyses of the large numbers of samples while the remainder are analyzed in Member States. (Figure insert shows the relationship of 18O and 2H in precipitation worldwide, on which interpretation of hydrological processes are based).