Isotope hydrology is a field of hydrology

Hydrology

Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

 that uses isotopic

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 dating to estimate the age and origins of water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and of movement within the hydrologic cycle. The techniques are used for water-use policy

Policy

A policy is typically described as a principle or rule to guide decisions and achieve rational outcome. The term is not normally used to denote what is actually done, this is normally referred to as either procedure or protocol...

, map

Map

A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....

ping aquifer

Aquifer

An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

s, conserving water supplies, and controlling pollution

Pollution

Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...

. It replaces or supplements past methods of measuring rain

Rain

Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

, river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 levels and other bodies of water over many decades.

Details

Water molecules carry unique fingerprints, based in part on differing proportions of the oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 and hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 isotopes that constitute all water. Isotopes are forms of the same element that have variable numbers of neutron

Neutron

The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s in their nuclei.

Air, soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 and water contain mostly oxygen 16 ( ¹⁶O). Oxygen 18 ( ¹⁸O) occurs in approximately one oxygen atom in every five hundred and is a bit heavier than oxygen 16, as it has two extra neutrons. From a simple energy standpoint this results in a preference for evaporating

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 the lighter ¹⁶O containing water and leaving more of the ¹⁸O water behind in the liquid state (called fractionation

Fractionation

See also: Fractionated spacecraftFractionation is a separation process in which a certain quantity of a mixture is divided up in a number of smaller quantities in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the...

). Thus seawater

Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

 tends to be richer in ¹⁸O and rain and snow relatively depleted in ¹⁸O.

Carbon 14 dating

Radiocarbon dating

Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...

 is also used as part of isotope hydrology as all natural water contains dissolved carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

.

Applications

One commonly cited application involves the use of stable isotopes to determine the age of ice or snow, which can help indicate the conditions of the climate in the past. Higher average global temperature would provide more energy and thus an increase in atmospheric ¹⁸O water, while lower than normal amounts of ¹⁸O in groundwater or an ice layer would imply that the water or ice represents an evaporation origin during cooler climatic eras or even ice age

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

s.

Another application involves the separation of groundwater flow

Groundwater flow

Groundwater flow is defined as the "...part of streamflow that has infiltrated the ground, has entered the phreatic zone, and has been discharged into a stream channel, via springs or seepage water". In hydrogeology it is measured by the Groundwater flow equation.- See also :*Subsurface...

 and baseflow

Baseflow

Baseflow is the portion of streamflow that comes from "the sum of deep subsurface flow and delayed shallow subsurface flow"...

 from streamflow

Streamflow

Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff...

 in the field of catchment hydrology

Catchment hydrology

Catchment hydrology, is the study of the hydrology regarding catchments.-Water balance:Catchment hydrology is based on the principal of continuity, which is used to perform a water balance on a catchment:I - O = dS/dt,...

 (i.e. a method of hydrograph

Hydrograph

A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, or other channel or conduit carrying flow...

 separation). Since precipitation in each rain or snowfall event has a specific isotopic signature

Isotopic signature

An isotopic signature is a ratio of stable or unstable isotopes of particular elements found in an investigated material...

, and the signatures of subsurface

Subsurface flow

Subsurface flow, in hydrology, is the flow of water beneath earth's surface as part of the water cycle.In the water cycle, when precipitation falls on the earth's land, some of the water flows on the surface forming streams and rivers...

 water can also be identified by well sampling, the composite signature in the stream is an indicator of, at any given time, what portion of the streamflow comes from overland flow and what portion comes from subsurface flow

Subsurface flow

Subsurface flow, in hydrology, is the flow of water beneath earth's surface as part of the water cycle.In the water cycle, when precipitation falls on the earth's land, some of the water flows on the surface forming streams and rivers...

.

Current use

The isotope hydrology program at the International Atomic Energy Agency

International Atomic Energy Agency

The International Atomic Energy Agency is an international organization that seeks to promote the peaceful use of nuclear energy, and to inhibit its use for any military purpose, including nuclear weapons. The IAEA was established as an autonomous organization on 29 July 1957...

 works to aid developing states (including 84 projects in more than 50 countries) and to create a detailed portrait of Earth's water resources.

In Ethiopia

Ethiopia

Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...

, Libya

Libya

Libya is an African country in the Maghreb region of North Africa bordered by the Mediterranean Sea to the north, Egypt to the east, Sudan to the southeast, Chad and Niger to the south, and Algeria and Tunisia to the west....

, Chad

Chad

Chad , officially known as the Republic of Chad, is a landlocked country in Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic to the south, Cameroon and Nigeria to the southwest, and Niger to the west...

, Egypt

Egypt

Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

 and Sudan

Sudan

Sudan , officially the Republic of the Sudan , is a country in North Africa, sometimes considered part of the Middle East politically. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea and Ethiopia to the east, South Sudan to the south, the Central African Republic to the...

, the International Atomic Energy Agency used such techniques to help local water policy deal with fossil water

Fossil water

Fossil water or paleowater is groundwater that has remained sealed in an aquifer for a long period of time. Water can rest underground in "fossil aquifers" for thousands or even millions of years...

.

An arsenic

Arsenic

Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 pollution crisis in Bangladesh

Bangladesh

Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...

 that the World Health Organization

World Health Organization

The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

calls the "largest mass poisoning of a population in history" has been investigated using this technique.