Chapter 32: Managing Water Resources Using ISOTOPES

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on August 21, 2012.

Water is abundant on Earth in all phases – Vapour, Liquid, or Solid and it has potential use to humans.  The water resources include the waters of the oceans, reservoirs, ice caps, freshwater lakes, groundwater, saline lakes and inland seas, rivers, and the atmosphere.  However, according to a report produced by the International Atomic Energy Agency (IAEA), of all the water on Earth, only 2.5 percent is fresh water, the rest is salty water.  Of this freshwater, most is frozen in icecaps, present as soil moisture, or inaccessible in deep underground aquifers, leaving less than 1 percent accessible for use.  This report recognizes the fact that sustainable human development is dependent on the availability of water. It is estimated that more than one third of the global food production is based on irrigation, a significant portion of which may rely of unsustainable groundwater sources. Despite progress in the last two decades to improve access to safe drinking water, some 1.1 billion people today go without it. Areas of water scarcity and stress are increasing, particularly in North Africa and West Asia. In the next two decades, total water demand is expected to increase by 40 per cent. By 2025, two-thirds of the world’s population may live in countries with moderate or severe water shortages.  The graph presented below illustrates the water cycle.

Slide1The water cycle is also known as the hydrologic cycle.  It is a process of circulating water from the land to the sky and back again.  In a simple term, water is evaporated from the Earth’s surface – oceans, rivers and lakes – as a result of the heat generated by the sun.   Plants transpire and lose water to the air out of their leaves.  The water vapor in the air eventually gets cold and changes back into liquid, forming into clouds.  When the clouds meet cool air over land, precipitation is triggered and water falls back to the earth in the form of rain, hail, sleet or snow.  It may fall back in the oceans, rivers, and lakes or may end up on land to start the cycle.  Some of the precipitation soaks into the ground and become part of groundwater.  Most of the water flows downhill as runoff, above ground or underground, eventually returning to the seas as slightly salty water.

Here is an explanation about salty water.  As water flows through rivers, it picks up small amounts of mineral salts from the rocks and soil of the river beds. This very slightly salty water flows into the oceans and seas. The water in the oceans only leaves by evaporating (and the freezing of polar ice), but the salt remains dissolved in the ocean – it does not evaporate. So the remaining water gets saltier and saltier as time passes.

According to the US Geological Survey, here is an explanation about an estimated quantity of where Earth’s water exists: The world’s total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use every day.

As far as continental water surplus and water use is concerned, a special component of the continental water balance is the use and diversion of part of the water surplus by man. The estimated annual 40 000 km3 (266 mm) of runoff from the continents represents the excess water that is left of the 111 000 km3 (746 mm) of rainfall, after evaporation and transpiration by natural vegetation and rain-fed agriculture have taken their share. In principle, this excess water is available for domestic and industrial use and for irrigated agriculture. About 5000 km3 of this water is nowadays withdrawn for these purposes. Irrigated agriculture takes the highest share with 65 percent. It is responsible for more than 50 percent of the world’s food production on 20 percent of the world’s arable land. The high amount of water needed for food production is exemplified by the fact that the production of 103 kg of corn, sufficient to feed six people for one year, requires 300 × 103 kg of water under average climatic conditions. The ratio between water use and dry mass accretion differs between crops and depends obviously on climatic conditions, particularly the potential evaporation. Observations on various types of vegetation give values between 100 and 1000 kg of water per kg dry mass production.Slide2Accordingly, the uneven distribution of water resources in time and space and the irregular spread of the world’s population, restrict the possibilities for increasing the application of this excess water. Most of the water is available in the equatorial tropical areas (The Amazon River, for instance, discharges 20 percent of the world’s total runoff), whereas two-thirds of mankind live outside the tropics. Another negative factor is the variability of the supply in time. Only one-third of the runoff forms a more or less stable supply, while the remaining part occurs in floods. The irregular distribution of water in space and time has led from the earliest civilisations onwards to hydraulic engineering practices, notably the construction of water diversion, transport and storage works, including large-scale drainage and irrigation schemes. These modifications of the regional water balance have certainly resulted in a modification of the regional atmospheric and hydrological circulation and the associated climate.

Reports published on the subject based on extensive research indicate that the sum of annual evaporation from both ocean and land is balanced with sum of annual precipitation on ocean and land, and it is about 34 times larger than the amount of atmospheric moisture.  Therefore, the residence time of atmospheric moisture is about 11 days. The difference between annual amounts of precipitation and evaporation on land is balanced with the transport of moisture from ocean to land as precipitation, and reversely with the river discharge to ocean.

Because of enormous increases in global population and expansion of agricultural and industrial enterprises together with the growing awareness of the vulnerability of water resources to pollution due to the augmented urbanization, water needs are rapidly escalating and many countries are now engaged on projects to develop water resources.  It is recognized around the world that sustainable use of water resources is a global responsibility.  Through the hydrological cycle, all water on Earth is affected by human activity. With the prospect of growing water scarcity looming ever more real, decisions on where to extract water, how much of it to use and how to manage it must be based on reliable information, if we are to protect these precious water resources for future generations. Slide3

Considering the dynamics, the management of water resources on a global level presents a huge problem in view of the fact that freshwater resources are very often shared by more than one country within a region which calls for a comprehensive strategy to address this challenge at a national as well as an international level with the focus to improve access in those regions lacking water and to improve the efficient use of water in those regions that have water today to ensure that the current water supplies are sustained for future generations.  This strategy acknowledges the key to sustainable management of water resources which is the knowledge needed to make the right decisions.  The good news is that Isotope hydrology is a nuclear technique that uses both stable and radioactive environmental isotopes to trace the movements of water in the hydrological cycle. Isotopes can be used to investigate underground sources of water to determine their source, how they are recharged, whether they are at risk of saltwater intrusion or pollution, and whether they can be used in a sustainable manner.  Stable and radioactive isotopes such as oxygen, hydrogen, carbon, nitrogen and sulfur have proved to be extremely useful tracers of hydrologic pathways, biogeochemical processes, and residence times of waters and solutes.  Isotope hydrology is an important tool being used around the world to provide the information needed to make the right decisions today and for tomorrow.

Here is a brief description on isotopes, stable isotopes, and radioactive isotopes:

1.      ISOTOPES:

Isotopes of a particular element have the same number of protons but a different number of neutrons, resulting in different atomic weights. When light elements such as oxygen, hydrogen, carbon, nitrogen, and sulfur undergo biogeochemical and physical processes, the difference in mass can cause one of the isotopes to be preferentially selected over the other, resulting in a product with a different isotopic ratio than the original. This selection process is called fractionation. An example is evaporation from a stream, where the water vapor will contain more of the lighter oxygen isotope than the stream, which in turn becomes “enriched” in the heavier isotope:Slide4

1.1      Stable ISOTOPES:

Stable isotopes do not change their atomic structure. Hydrogen has two stable isotopes, 1H and 2H (deuterium, D), with the numbers referring to the atomic weight. The most common isotopes of oxygen are 18O and 16O.  Stable isotope data are reported relative to a standard material (ocean water in the case of hydrogen and oxygen isotopes) using the delta (δ) notation.  The delta values represent deviation from the standard in parts per thousand (per mil, ‰). The values of δD and δ18O (and other stable isotopes) measured in water samples can provide information on the processes (such as evaporation) that have affected it and suggest its source; and

1.2      Radioactive ISOTOPES:

Radioactive isotopes undergo spontaneous radioactive decay to form new elements or isotopes, and are used to determine the relative or absolute age of water. Large amounts of tritium, a radioactive isotope of hydrogen (3H), were introduced to the atmosphere by thermonuclear weapons testing from 1952 to 1972, and then entered the subsurface in precipitation. Thus, groundwater samples that contain measurable tritium are considered to be evidence of “post-bomb” recharge.  Tritium is measured in tritium units (TU), in which one TU equals one tritium atom per 1018 1H atoms. Tritium has a half-life of 12.3 years, and its usefulness is declining, as it will become harder to distinguish decayed post-bomb samples from pre-bomb samples. Other radioactive isotopes, such as 14C and 36Cl, have much longer half-lives and are useful for dating groundwater less than 40,000 (14C) to more than one million years old (36Cl).

Isotope hydrology is the use of isotopic and nuclear tools in the study of hydrologic processes in the water cycle.  It was born during the early post-war years through the merging of the experience gathered in the study of the dissemination of radio-nuclides (e.g. radioactive fallout) in the environment with the emerging discipline of isotope geology (and in particular the use of radio-isotopes as “dating” tools) on the one hand, and on the other hand with the knowledge gained on the fractionation of isotopic species as exemplified by the work of H. Urey and his students in the US.  Here is how IAEA defined fractionation in a broad sense – The hydrogen and oxygen isotopes of water varies among reservoirs in time and space because of the fact that fractionation of isotopes occurs in the natural environment.  ISOTOPE fractionation is accompanied by variety of processes, such as, phase change, transportation, diffusion, reduction, oxidation, chemical reaction, and biological metamorphism.

The use of environmental isotopes is growing around the world.  The environmental isotopes are a subset of the isotopes, both stable and radioactive, which are the object of Isotope geochemistry.  The most used environmental isotopes are: deuterium; tritium; carbon-13; carbon-14; nitrogen-15; oxygen-18; silicon-29; and chlorine-36.

The environmental isotopes serve as tracers for water, carbon, nutrient, and solute cycling.  The characteristics of environmental isotopes include: Naturally occurring; found in abundance; principle elements in hydrological, geological, and biological systems; and relatively light elements (Mass Ratio).  The main environmental isotopes are the stable isotopes deuterium (hydrogen-2), carbon-13, oxygen-18, and the radioactive isotopes tritium (hydrogen-3) and carbon-14. Isotopes of hydrogen and oxygen are ideal geochemical tracers of water because their concentrations are usually not subject to change by interaction with the aquifer material. On the other hand, carbon compounds in groundwater may interact with the aquifer material, complicating the interpretation of carbon-14 data.

Environmental isotope hydrology is a relatively new field of investigation based on isotopic variations observed in natural waters.  These isotopic characteristics have been established over a broad space and time scale. They cannot be controlled by man, but can be observed and interpreted to gain valuable regional information on the origin, turnover and transit time of water in the system which often cannot be obtained by other techniques. The cost of such investigations is usually relatively small in comparison with the cost of classical hydrological studies.

The environmental isotopes have been extensively used during the last decades to address key aspects of the water cycle, such as the study of the origin, dynamics and interconnections of for example, groundwater, surface water and the atmosphere.  The use of environmental isotopes as tracers of the water molecule has been extended beyond the classical applications of Isotope Hydrology and today, a growing number of other scientific disciplines, ranging from atmospheric circulation to palaeoclimatology or ecology have incorporated isotopes as powerful tracers to better characterize systems and processes.

Conventional processes used chemicals as tracers for a long time.  The environmental isotopes are being used in many countries around the world to measure the rate of flow of water in canals, closed conduits, and rivers.  Even though these isotopes used on the same principle which applied to chemicals, nevertheless, isotopes are more sensitive to detection and have hence made the measurements more convenient, accurate and cheaper.  For the study of groundwater resources, which is of great importance in arid or semi-arid areas, isotope uses are opening up new possibilities not only for tracing the movement of water underground but also for assessing the amount of water in a particular groundwater body.

Here are some environmental isotope techniques/applications which are derived from an IAEA report which was published by B. R. Rayne who is Head, ISOTOPE Hydrology Section, in the IAEA’s Division of Research and Laboratories:

2.      ENVIRONMENTAL ISOTOPES TECHNIQUES:

2.1       Tracking Rainfall:

In arid regions precipitation is subject to irregularities in terms of amount, intensity and location. For example, there may be no precipitation for many years in some places. To assess the potential groundwater resources, the rate at which the groundwater is replenished has to be estimated. Because of the small amounts of precipitation in arid regions, estimates of the rate of replenishment, using standard hydrological methods are subject to considerable error and it may not even be possible to tell whether the groundwater is actually being recharged.

In the 1950s and early 1960s large amounts of tritium were injected into the atmosphere by nuclear bomb tests.  As a result, precipitation has been labelled so that if a sample of groundwater contains a significant amount of tritium that is unequivocal proof that the water has been recharged during the last two or three decades.

An example of this is provided by a collaborative study between the Food and Agriculture Organization (FAO) and IAEA in Qatar. The mean annual precipitation is about 80 mm in the northern part of Qatar and about 50 mm in the south.  The spatial and temporal occurrences of rainfall are variable. It was believed that short, intense downpours recharge the groundwater system. However, such storms occur quite randomly. Therefore it was important to demonstrate the occurrence of recharge by an independent method.Slide5Isotope analysis revealed that more than 60 percent of the groundwater samples contained significant amounts of tritium and thus that these waters had been recharged since 1952. There was more tritium in the northern part of the peninsula where the amount of precipitation is higher. It was estimated that about 8 per cent of the annual precipitation since 1952 has been effective in replenishing the groundwater reserves.

2.2       Carbon-14 Dating of Water:

The relatively short half-life of tritium, 12.43 years, means that it is only of use in relatively fast-flowing groundwater systems or, as outlined above, in studies of recharge. In 1962 carbon-14 dating was first applied in an attempt to measure the age of groundwater in the western desert of Egypt. The carbon-14 technique is more difficult to apply because carbon is not part of the water molecule. It occurs in groundwater in the dissolved inorganic carbon species, and chemical changes generally tend to dilute the carbon-14 and thus give erroneously high ages. However, corrections can be made using chemical and carbon-13 (a stable isotope of carbon) data, to provide good estimates of the time water was recharged. Even though a groundwater system may not be recharged under the present climatic conditions, it is important in the assessment of a groundwater resource to have information on past recharge conditions. Research is being carried out on the dating possibilities of other radioisotopes, such as silicon-32, argon-39, and chlorine-36; but carbon-14 remains a very useful routine hydrological tool.

Infiltration of precipitation may not be the only source of recharge to a groundwater system. Recharge may also come from a river. Losses to groundwater may be indicated by differences in flow-rates over a certain reach of the river and by the gradient of groundwater levels in the vicinity of the river. However, neither approach provides a proof of actual transfer of water from the river to the groundwater system. Stable isotope data provide a unique tool to answer this question. Most of the water flowing in a big river will come from higher elevations and so will contain less of the heavy isotopes deuterium and oxygen-18 than local rainfall. Thus each potential source of recharge is characterized by a different isotopic composition, so that the contribution of river-water to the groundwater can be clearly demonstrated. The IAEA has worked on this type of problem in Ecuador, Mexico, Republic of Korea, and Sudan.Slide6The investigation in Sudan was near Khartoum where the Blue Nile and White Nile rivers merge. The two rivers are distinctly different in isotopic composition which enabled the extent of infiltration from both rivers below their confluence to the groundwater to be studied.  The isotope data did not provide any evidence of significant recharge by infiltration of recent precipitation. Carbon-14 data demonstrated that groundwater flowing to the area from the east was recharged some tens of thousands of years ago and is therefore not a renewable source of supply. The stable isotopic composition of this ancient groundwater is markedly different from that of the Blue Nile and thus provides an excellent indicator of the mixing with river-water.

This type of problem may also arise within the context of water quality in industrialized areas. An example is an investigation of groundwater in the neighbourhood of Seoul in the Republic of Korea. The study is part of an RCA Isotope Hydrology Programme financed by Australia. The first phase of the project focused on the metropolitan area and the isotope data showed that infiltrated river-water is the dominant source of recharge to groundwater. In contrast, the groundwater in the valley downstream of Seoul is recharged by infiltration of precipitation. The work is currently being extended to cover a larger area and also to study the origin of groundwater in deeper crystalline rocks.

2.3       Waste Disposal:

The application of isotope techniques to the hydrology of crystalline rocks has recently gathered momentum.  On the one hand this has been due to the fact that, in many areas, water in fractured crystalline rocks is the only source of supply. However, in general these rocks are unable to transmit significant volumes of groundwater.  In the extreme situation of essentially no groundwater flow, these rocks are of interest as potential host rocks for storage of radioactive waste. In conjunction with other hydrological and conventional methods, one may expect an increased use of isotope techniques not only in the assessment of potential sites for storage of radioactive waste, but also for the disposal of municipal wastes.

Groundwater often occurs in distinct horizons which are separated the one from the other by impermeable layers. One of the water-bearing horizons may contain water of inferior quality, such as high salinity, and it is important to know whether this poorer quality water is leaking to the other water-bearing horizon, or aquifer, which is being used as a supply of water.Slide7One part of the study in Qatar cited earlier dealt with this type of problem. The inferior quality of water from a number of wells in the south-western part of the country was proved to be due to the leakage of brackish water from an underlying deeper aquifer. However, for some wells it was shown that sea-water also contributed to the salinity problem.

2.4       Alternative Energy:

The global concern for conservation of energy and development of alternative energy sources has prompted exploration for geothermal energy resources.Slide8Stable isotopes can help provide information on the origin of the geothermal waters. However, the key question in the assessment of any system is the temperature of the fluid. This will determine whether the fluid can be used to produce electricity or whether it is suitable only for domestic and agricultural space heating. Isotope geothermometers may be used to estimate temperatures at depth. The principle depends upon the fact that the distribution of isotopes among the phases and components of a geothermal system is a function of temperature. Thus from measurements of a given isotope in two components an estimate of the temperature may be derived. An example of an isotope geothermometer is the sulphate-water exchange reaction in which the oxygen-18 distribution is measured. This geothermometer has been used in a number of different geothermal fields in different parts of the world. The Agency has already been involved in the use of isotope techniques in geothermal exploration in, for example, Costa Rica, India, Italy, Mexico, and Thailand.

3.      CONCLUSIONS:

Isotope techniques are now an established tool for the hydrologist.  Some of the techniques can provide unique information which is unobtainable by other methods. Although much work remains to be done on the refinement of existing methods and development and proving of new techniques, there remains a need for continuing effort to transfer the knowledge of already established techniques to the developing countries.

Resources:

  1. IAEA – Managing Water Resources Using ISOTOPE Hydrology;
  2. The Water Cycle;
  3. USGS – The Water Cycle;
  4. IAEA Environmental ISOTOPES in the Hydrological Cycle – Principles and Applications;
  5. Tracking Groundwater Sources with Environmental isotopes;
  6. Wikipedia – ISOTOPE;
  7. Wikipedia – Stable ISOTOPE;
  8. Wikipedia – ISOTOPE Geochemistry;
  9. Wikipedia – Deuterium;
  10. Wikipedia – Tritium;
  11. Wikipedia – Carbon-13;
  12. Wikipedia – Carbon-14;
  13. Wikipedia – Nitrogen-15;
  14. Wikipedia – Oxygen-18;
  15. Wikipedia – isotopes of Silicon-29;
  16. Wikipedia – Chlorine-36;
  17. ISOTOPE Hydrology Techniques – Practical Tools to Solve Water Problems;
  18. Environmental ISOTOPES in Hydrology by UTEP (Woocay);
  19. IAEA Environmental ISOTOPE Hydrology; and
  20. IAEA Use of ISOTOPEs in Hydrology.

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