Isotope hydrology is a field of geochemistry and
hydrology
Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...
that uses naturally occurring stable and radioactive
isotopic techniques to evaluate the age and origins of surface and groundwater and the processes within the atmospheric
hydrologic cycle. Isotope hydrology applications are highly diverse, and used for informing water-use
policy,
mapping
aquifer
An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
s, conserving water supplies, assessing sources of water
pollution
Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...
, investigating surface-groundwater interaction, refining groundwater flow models, and increasingly are used in eco-hydrology to study human impacts on all dimensions of the hydrological cycle and
ecosystem services.
Details
Water molecules carry unique isotopic "fingerprints", based in part on differing ratios of the
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
and
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
isotopes that constitute the water molecule. Isotopes are atoms of the same element that have a different number of
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s in their nuclei.
Air, freshwater and seawater contain mostly oxygen-16 (
16O). Oxygen-18 (
18O) occurs in approximately one oxygen atom in every five hundred and has a slightly higher mass than oxygen-16, as it has two extra neutrons. From a simple energy and bond breakage standpoint this results in a preference for
evaporating the lighter
16O containing water and leaving more of the
18O water behind in the liquid state (called
isotope fractionation). Thus
seawater
Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...
tends to contain more
18O than rain and snow.
Dissolved ions in surface and groundwater water also contain useful isotopes for hydrological investigations. Dissolved species like sulfate and nitrate contain differing ratios of 34-S to 32-S or 15-N to 14-N, and are often diagnostic of pollutant sources. Natural radioisotopes like tritium (3-H) and radiocarbon (
14-C) are also used as natural clocks to determine the residence times of water in aquifers, rivers, and the oceans.
Applications
The most commonly used isotope application in hydrology uses hydrogen and oxygen isotopes to evaluate sources or age of water, ice or snow. Isotopes in ice cores help to reveal conditions of past climate. Higher average global temperature would provide more energy and thus an increase the atmospheric
18O content of rain or snow, so that lower than modern amounts of
18O in groundwater or ice layer imply the water or ice represents a period of cooler climatic eras or even
ice age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and g ...
s.
Another application involves the separation of
groundwater flow and
baseflow from
streamflow in the field of
catchment hydrology (i.e. a method of
hydrograph separation). Since precipitation in each rain or snowfall event has a specific
isotopic signature, and
subsurface water can be identified by well sampling, the composite signature in the stream is an indicator the proportion of the streamflow comes from
overland flow and what portion comes from
subsurface flow.
Stable isotopes in the water molecule are also useful in tracing the sources (or proportion of sources) of water that plants use.
Current use
The isotope hydrology program at the
International Atomic Energy Agency works to aid developing states to create a detailed portrait of Earth's water resources.
In
Ethiopia
Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...
,
Libya
Libya, officially the State of Libya, is a country in the Maghreb region of North Africa. It borders the Mediterranean Sea to the north, Egypt to Egypt–Libya border, the east, Sudan to Libya–Sudan border, the southeast, Chad to Chad–L ...
,
Chad
Chad, officially the Republic of Chad, is a landlocked country at the crossroads of North Africa, North and Central Africa. It is bordered by Libya to Chad–Libya border, the north, Sudan to Chad–Sudan border, the east, the Central Afric ...
,
Egypt
Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...
and
Sudan
Sudan, officially the Republic of the Sudan, is a country in Northeast Africa. It borders the Central African Republic to the southwest, Chad to the west, Libya to the northwest, Egypt to the north, the Red Sea to the east, Eritrea and Ethiopi ...
, the International Atomic Energy Agency used radioisotope techniques to help local water policy identify and conserve
fossil water
Fossil water, fossil groundwater, or paleowater is an ancient body of water that has been contained in some undisturbed space, typically groundwater in an aquifer, for millennia. Other types of fossil water can include subglacial lakes, such as An ...
.
The International Atomic Energy Agency maintains a publicly accessible global network and isotopic database for Earth's rainfall and rivers.
See also
*
Baseflow
*
Hydrograph
*
Water chemistry analysis
References
{{reflist
External links
Isotope Hydrology at the IAEA
Water
Hydrology
Hydraulic engineering