E. García y G
Isotope Hydrology Section, IAEA
A new Tool in Hydrology: Environmental Isotopes
Hydrology as a science has undergone several changes in the tools used to work out its problems. In the last twenty years a number of important results have been achieved in the solution of several hydrological problems by means of nuclear techniques, especially environmental isotope techniques.
Hydrology based on environmental isotopes establishes a really new field of research. It basically uses the observed variations, by natural process, of the isotopic content of water. These isotopic features have been determined by means of several studies carried out in the hydrological cycle. Man cannot modify them, he can only see and interpret these characteristics in order to obtain very useful information when related to the origin, recharge and transit-time of water in a confined region. This information is frequently impossible to obtain by classical methods. On the other hand, it can be said that the cost of these investigations is cheaper than that of traditional hydrology studies.
The isotopes of major interest, from the point of view of hydrology, are on one hand the stable isotopes: deuterium, carbon-13 and oxygen-18, and on the other hand the radioactive isotopes: tritium and carbon-14. Hydrogen and oxygen isotopes constitute the ideal geochemical tracers of water, since their concentrations are not usually altered by interaction with the aquifer itself. As regards carbon compounds in groundwater, the foregoing is different because in that case it could have several interactions with the material of the aquifer. Therefore, interpretation of the obtained data by means of carbon-14 becomes too complicated.
A few years ago, isotope hydrology research turned towards the possibility of using other environmental isotopes like silicum-32 and the 238U/234U ratio, although up to now their applications are very restricted and therefore, they are still to be considered as new hydrology tools.
The stable isotopes Hydrogen and Oxigen-18 in water.
The principal heavy stable isotopic components of water are HD 16 O, H2 16 O and H2 16 O. The variations of the isotopic ratios D/H and 18O/16O in water samples are expressed in terms of per mille difference with respect to the isotopic ratios of mean ocean water, which constitutes the reference standard (SMOW). The data are thus expressed as delta (δ) units defined as follows:
where R refers to the isotopic ratio D/H or18O/16O which is determined by as a mass spectrometer.
The main value of the stable isotope data in a hydrological system is that it is possible to determinate the recharge area of the system when the isotopic variations of precipitation and surface waters are well known.
The most important natural processes that charge the stable isotopic composition of water in nature are evaporation and condensation. The lighter molecules of water, i.e.H216O, are more volatile than the heavy ones, therefore when the atmospheric moisture becomes cool, the heavier molecules are able to condensate more easily, and the residual vapour is depleted in heavy isotopes. This progressive condensation is produced when the air coming from the sea reaches inland or rises to higher altitudes. So, a relationship can be observed between the condensation temperature and the heavier isotopic composition of precipitation: the heavy isotopic content diminishes as the condensation temperature decreases. This due to the fact that the residual vapour requires lower temperatures to become condensated.
The relationship that is observed regarding the temperature shows three important effects. First of all, seasonal variations in the isotopic composition of precipitations; secondly, variations with latitude; and thirdly, variations with altitude. This last effect has special importance in regional hydrological studies, because, for instance, it allows differentiation between the groundwater proceedings from recharge areas located at different heights.
As the precipitations infiltrate downwards, thereby increasing the ground water, the mixture that is produced in the unsaturated zone compensates for the seasonal isotopic variations. Thus, the water of the unsaturated zone has a composition that corresponds to the mean istopic composition is related to that of precipitations which have fallen in the recharge area of the aquifer during the recharge time, and this could be used to determinate the recharge area in relation to the altitude or distance to the sea. It must be borne in mind that all the climatic variations during the recharge time that could have caused an isotopic composition of the original infiltration would differ from the observed at such a time.
On the other hand, it is also possible to verify the recharge of the groundwater aquifers by lateral seepage from surface waters, such as river and lake waters, or by vertical infiltration from permanently or temporally ponded waters, since the isotopic composition of these waters is frequently different from the local recharges. The rivers may collect water from precipitation which has fallen at great altitudes and is, therefore, very depleted in heavy isotopes. Water from lakes or ponds may also be enriched in heavy isotopes through evaporation (the light molecules evaporate faster than heavy ones), and the observed D-18O relationship will be different from the non-evaporated waters in the region.
The radioactive isotope of Hydrogen in water: Tritium.
Tritium from the atmosphere is formed by the action of cosmic radiation and is also released by thermonuclear tests carried out by man. Most of the tritium in the atmosphere rapidly oxidizes to HTO and is incorporated in the hydrological cycle where it constitutes a very useful marker for water that has been in the atmosphere within the past 20 years. The great dilution through H2O results in very low tritium concentrations which can only be measured by means of tritium´s radioactivity, usually after an isotopic enrichment treatment. The mean life of tritium is 12.26 years.
The tritium content of natural waters is expressed in tritium units (T.U). one tritium unit corresponds to a concentration of 1 tritium atom per 1018 hydrogen atoms.
Cosmic radiation establishes a concentration of about 10 T.U. in temperate-zone continental meteoric waters. This was the concentration observed before 1953, in northern hemisphere precipitation. After 1953, the tritium content of precipitation increased as a result of thermonuclear testing. Values of up to 10,000 T.U. were reached in the northern hemisphere in 1963 following the extensive testing period in the last two years. From that year, the tritium content has decreased as a consequence of the moratorium established for the explosion of thermonuclear devices in the atmosphere. In precipitation, the tritium content presents several considerable seasonal and geographical variations depending on the area where great amounts of tritium are released and also on then transport mechanisms from stratosphere to groundwater.
In the hydrological studies, tritium measurements give the information about the transit time or else the renewal time of water in a given system. In a confined groundwater system, a relationship can be established between the tritium concentration and the amount of the isotope that has been deposited but, of course, having in mind the possible recharge effects and dispersive mixture.
From a qualitative point of view, concentrations below 3 T.U. in some continental regions in the northern hemisphere, indicate that the water recharge took place before thermonuclear tritium was spread, which means before 1953; concentrations up to 20 T.U. indicate a much faster water flow in the system. In a open hydrological system, surface or underground, a water mixture of different ages is the major component and the concentrations of tritium are interpreted in function of the permanent mean time.
The carbon isotopes in water – carbon14 and carbon13.
Carabon-14, like tritium, is the result of the interaction between cosmic radiation and atmosphere, also produced by radiation released from thermonuclear testing. Carbon oxidizes and forms carbon dioxide that is mixed with the atmospheric carbon dioxide entering the global carbon cycle. Because of its long half-life (5730 years), it is useful for studying groundwater systems with very long transit times. Generally speaking, carbon-14 produced by nuclear testing is not interesting from the hydrological point of view, because tritium is more effective as a tracer in recent waters due to tis short half-life.
The use of carbon-14 for groundwater dating is based on the fact that carbon dioxide, which is fround in the soil, is of biological origin and is produced through the respiration of the plant roots and plant decay and, hence, contains carbon-14 derived by plants form the atmosphere- this biogenic CO2 dissolves in infiltrating water and is carried down the ground reservoir. Its carbon-14 content decreases through radioactive decay and the fraction remaining from the original amount indicates the period of time since it was removed from the soil zone where its 14C content was taken up, in other words, the time since infiltration.
Carbon-14 is measured relative to the total carbon content of the sample, so we must consider the origin of both the carbon-14 and the stable carbon of sample. It is important to bear in mind that not the entire stable carbon content of groundwater carbonate is of the same origin as carbon-14. Infiltrating water, containing carbon dioxide dissolved from the soil zone, dissolves carbonate minerals in the soil. Nevertheless, the carbon originating from limestone, in general, contains no carbon-14 because of its old age, therefore, the water that reaches the water table contains dissolved carbon with carbon-14 content lower than that present in the soil biogenic CO2. The most difficult problem in determining the age of water by means of carbon-14 is the evaluation of the dilution of soil CO2 which originally contained 100% of carbon-14 modern with carbon-14-free carbonate, in order to estimate the initial concentration in recharge water which reached the water table considered.
Experience shows that, regarding the infiltrated recent water before thermonuclear test, the content of modern carbon-14 is approximately 85%. In order to get a more accurate estimation of this value it is possible to study the carbon-13 content from the ground water carbonate sample. It must be borne in mind that carbon-13 is a stable isotope. Since the biogenic carbon dioxide has a much lower carbon-13 content than limestone, it could possible to calculate the respective ratios of the biogenic carbon and the limestone produce reckoning from the carbon-13 content of the carbonate groundwater.
The carbon-14 method can be used for waters younger than 30,000 years. It is generally applied to study the water movement in confined aquifers. Where recharge occurs only in an out-crop area and the water chemistry and isotopic composition of dissolved carbon species are relatively uniform, the age differences in space are not affected by the uncertainties which affect the absolute age determination of water. Then it is possible to determine the flow velocity of water by determining the age differences between two sampling points at know distance, i.e., knowing the relative age of the flow. This allows to estimate the mean permeability of the region. Carbon-14 measurements, together of water with tritium measurements, may also give information on mixing processes of water ages within a given aquifer.