Isotope hydrology

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Isotope hydrology [1] is a field of geochemistry and hydrology 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. [2] Isotope hydrology applications are highly diverse, and used for informing water-use policy, mapping aquifers, conserving water supplies, assessing sources of water pollution, and increasingly are used in eco-hydrology to study human impacts on all dimensions of the hydrological cycle and ecosystem services.

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Details

Water molecules carry unique isotopic "fingerprints", based in part on differing ratios of the oxygen and hydrogen isotopes that constitute the water molecule. Isotopes are atoms of the same element that have a different number of neutrons 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 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 ages. [3]

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. [4] [5]

Stable isotopes in the water molecule are also useful in tracing the sources (or proportion of sources) of water that plants use. [6] [7] [8]

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. [9]

In Ethiopia, Libya, Chad, Egypt and Sudan, the International Atomic Energy Agency used radioisotope techniques to help local water policy identify and conserve fossil water.

The International Atomic Energy Agency maintains a publicly accessible global network and isotopic database for Earth's rainfall and rivers. [10]

See also

Related Research Articles

<span class="mw-page-title-main">Isotope analysis</span> Analytical technique used to study isotopes

Isotope analysis is the identification of isotopic signature, abundance of certain stable isotopes of chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food web, to reconstruct past environmental and climatic conditions, to investigate human and animal diets, for food authentification, and a variety of other physical, geological, palaeontological and chemical processes. Stable isotope ratios are measured using mass spectrometry, which separates the different isotopes of an element on the basis of their mass-to-charge ratio.

<span class="mw-page-title-main">Water cycle</span> Continuous movement of water on, above and below the surface of the Earth

The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, transpiration, condensation, precipitation, infiltration, surface runoff, and subsurface flow. In doing so, the water goes through different forms: liquid, solid (ice) and vapor. The ocean plays a key role in the water cycle as it is the source of 86% of global evaporation.

<span class="mw-page-title-main">Hydrograph</span>

A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second . Hydrographs often relate change of precipitation to change of discharge over time. It can also refer to a graph showing the volume of water reaching a particular outfall, or location in a sewerage network. Graphs are commonly used in the design of sewerage, more specifically, the design of surface water sewerage systems and combined sewers.

Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water, that is, a particular sample of water whose proportions of different isotopes of hydrogen and oxygen are accurately known. VSMOW is distilled from ocean water and does not contain salt or other impurities. Published and distributed by the Vienna-based International Atomic Energy Agency in 1968, the standard and its essentially identical successor, VSMOW2, continue to be used as a reference material.

Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic fractionations are caused by the effects of the mass of an isotope on atomic or molecular velocities, diffusivities or bond strengths. Mass-independent fractionation processes are less common, occurring mainly in photochemical and spin-forbidden reactions. Observation of mass-independently fractionated materials can therefore be used to trace these types of reactions in nature and in laboratory experiments.

Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of an atmospheric CO2 molecule. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in 13C by 25 per mil (2.5 per cent) relative to most inorganic carbon on Earth.

There are three known stable isotopes of oxygen (8O): 16
O
, 17
O
, and 18
O
.

<span class="mw-page-title-main">Oxygen isotope ratio cycle</span> Cyclical variations in the ratio of the abundance of oxygen

Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to ancient ocean temperature which in turn reflects ancient climate. Cycles in the ratio mirror climate changes in the geological history of Earth.

An isoscape is a geologic map of isotope distribution. It is a spatially explicit prediction of elemental isotope ratios (δ) that is produced by executing process-level models of elemental isotope fractionation or distribution in a geographic information system (GIS).

Oxygen-18 is a natural, stable isotope of oxygen and one of the environmental isotopes.

Runoff is the flow of water across the earth, and is a major component in the hydrological cycle. Runoff that flows over land before reaching a watercourse is referred to as surface runoff or overland flow. Once in a watercourse, runoff is referred to as streamflow, channel runoff, or river runoff. Urban runoff is surface runoff created by urbanization.

Streamflow, or channel runoff, is the flow of water in streams and other channels, and is a major element of the water cycle. It is one runoff component, the movement of water from the land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.

Baseflow is the portion of the streamflow that is sustained between precipitation events, fed to streams by delayed pathways. It should not be confused with groundwater flow. Fair weather flow is also called base flow.

In geochemistry, paleoclimatology and paleoceanography δ18O or delta-O-18 is a measure of the ratio of stable isotopes oxygen-18 (18O) and oxygen-16 (16O). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, 18O:16O data from corals, foraminifera and ice cores are used as a proxy for temperature.

The Dole effect, named after Malcolm Dole, describes an inequality in the ratio of the heavy isotope 18O to the lighter 16O, measured in the atmosphere and seawater. This ratio is usually denoted δ18O.

<span class="mw-page-title-main">Global meteoric water line</span>

The Global Meteoric Water Line (GMWL) describes the global annual average relationship between hydrogen and oxygen isotope (Oxygen-18 and Deuterium) ratios in natural meteoric waters. The GMWL was first developed in 1961 by Harmon Craig, and has subsequently been widely used to track water masses in environmental geochemistry and hydrogeology.

<span class="mw-page-title-main">Catchment hydrology</span> Hydrology of drainage basins

Catchment hydrology is the study of hydrology in drainage basins. Catchments are areas of land where runoff collects to a specific zone. This movement is caused by water moving from areas of high energy to low energy due to the influence of gravity. Catchments often do not last for long periods of time as the water evaporates, drains into the soil, or is consumed by animals.

<span class="mw-page-title-main">Stable isotope ratio</span> Ratio of two stable isotopes

The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundance of such stable isotopes can be measured experimentally, yielding an isotope ratio that can be used as a research tool. Theoretically, such stable isotopes could include the radiogenic daughter products of radioactive decay, used in radiometric dating. However, the expression stable-isotope ratio is preferably used to refer to isotopes whose relative abundances are affected by isotope fractionation in nature. This field is termed stable isotope geochemistry.

Rayleigh fractionation describes the evolution of a system with multiple phases in which one phase is continuously removed from the system through fractional distillation. It is used in particular to describe isotopic enrichment or depletion as material moves between reservoirs in an equilibrium process. Rayleigh fractionation holds particular importance in hydrology and meteorology as a model for the isotopic differentiation of meteoric water due to condensation.

References

  1. ↑ Gat, Joel (2010). Isotope hydrology: a study of the water cycle. World Scientific.
  2. ↑ Gleeson, Tom. "The global volume and distribution of modern groundwater". Nature. 9 (2): 161.
  3. ↑ Masters, G. & P. Ela. 2008. Global Atmospheric Change. Chapter in: Introduction to Environmental Engineering and Science. 3rd ed. Prentice Hall.
  4. ↑ Kendall and McDonnell, 1998. Isotope Tracers in Catchment Hydrology. Elsevier
  5. ↑ Tetzlaff, Doerthe; Buttle, James; Carey, Sean K.; van Huijgevoort, Marjolein H. J.; Laudon, Hjalmar; McNamara, James P.; Mitchell, Carl P. J.; Spence, Chris; Gabor, Rachel S.; Soulsby, Chris (2015-12-15). "A preliminary assessment of water partitioning and ecohydrological coupling in northern headwaters using stable isotopes and conceptual runoff models: Water Partitioning in Northern Headwaters". Hydrological Processes. 29 (25): 5153–5173. doi:10.1002/hyp.10515. PMC   5012127 . PMID   27656040.
  6. ↑ Evaristo, Jaivime; Jasechko, Scott; McDonnell, Jeffrey J. (2015). "Global separation of plant transpiration from groundwater and streamflow". Nature. 525 (7567): 91–94. Bibcode:2015Natur.525...91E. doi:10.1038/nature14983. PMID   26333467. S2CID   4467297.
  7. ↑ Good, Stephen P.; Noone, David; Bowen, Gabriel (2015-07-10). "Hydrologic connectivity constrains partitioning of global terrestrial water fluxes". Science. 349 (6244): 175–177. Bibcode:2015Sci...349..175G. doi: 10.1126/science.aaa5931 . ISSN   0036-8075. PMID   26160944.
  8. ↑ Langs, Lindsey E.; Petrone, Richard M.; Pomeroy, John W. (2020-12-30). "A δ 18 O and δ 2 H stable water isotope analysis of subalpine forest water sources under seasonal and hydrological stress in the Canadian Rocky Mountains". Hydrological Processes. 34 (26): 5642–5658. Bibcode:2020HyPr...34.5642L. doi:10.1002/hyp.13986. ISSN   0885-6087. S2CID   229410600.
  9. ↑ International Atomic Energy Agency
  10. ↑ "Global Network for Isotopes in Precipitation".
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