Nebraska Network
for Isotopes in
Precipitation

Justin Smith Morrill
Scholars
Program

Introduction
All matter is composed of atoms. Atoms occur as various nuclides depending on their nuclear structure. Nuclides are composed of protons (+ charge) and neutrons (no charge) in the nucleus, and electrons (- charge) in outer “shells” surrounding the nucleus. For a given nuclide, the number of protons (Z) defines element, and the number of neutrons (N) determines which isotope is present (the word isotope is derived from the Greek words, isos meaning equal, and topos meaning place). For a given nuclide, the sum of its protons and neutrons gives the atomic weight (A), expressed using the notation ^A_ZNu_N. For example, the most common isotope of the element oxygen (O) contains 8 protons and 8 neutrons, and has an atomic weight of 16. Thus, it is expressed as ¹⁶₈O₈. A less common oxygen isotope, containing 8 protons and 10 neutrons, and having an atomic weight of 18, is expressed as ¹⁸₈O₁₀. To simplify the expressions, conventional notation uses only the elemental symbol and atomic weight (e.g ¹⁶O or ¹⁸O).  The number of neutrons can vary, and the number in the nuclide determines which isotope of the nuclide is present, but the range of that variance is limited by the degree of instability resulting from having too few, or too many neutrons present in the nucleus. Isotopes can thus be unstable (radioactive) or stable in nature. Radioactive isotopes will disintegrate spontaneously by some mode of decay to become stable. Stable isotopes do not undergo decay. The decay rate of a given radioactive isotope is expressed in terms of a half-life, which represents the amount of time necessary for ½ the amount of the isotope to decay.

Stable Isotopes in Hydrologic Studies
A number of stable isotopes are used in hydrologic studies, and have been termed “environmental isotopes”. These include oxygen-18 (¹⁸O), deuterium (²H), carbon-13 (¹³C), chlorine-37 (³⁷Cl), sulfur-34 (³⁴S), and nitrogen-15 (¹⁵N), for example. Stable isotopes are typically used to identify the source (origin) of the waters, or their dissolved constituents and any physical, chemical and biological processes which may have impacted the waters since their origin.  Stable oxygen and deuterium isotopes are typically used to trace water movement within the hydrologic cycle.  Thus, know in the isotope composition of precipitation is important and is often viewed as the "starting point" on a parcel of waters journey throughout the water cycle.

Oxygen-18 and Deuterium
Stable isotopes are measured as the ratio of the two most abundant isotopic forms of the element. For oxygen, it is the ratio of ¹⁸O, with an abundance of 0.204%, to ¹⁶O (the more common form), with an abundance of 99.796%. Thus, the ¹⁸O/¹⁶O ratio is about 0.00204. For hydrogen, it is the ratio of ²H (also written as “D”- deuterium), with an abundance of 0.015%, to the more common form, ¹H (protium), with an abundance of 99.985%, giving a ²H/¹H ratio of about 0.000150.

Stable Isotope Notation
Stable isotope concentrations are expressed as the difference between the measured ratio of the sample and the measured ratio of a reference standard. Isotope results are reported as parts per thousand (‰) with respect to the standard using the (d) notation expressed as,

                                    d_sample = { (R_sample - R_standard) / (R_standard) } X 1000

where, R_sample is the ratio of ¹⁸O/¹⁶O or ²H/¹H, for example in the sample and R_standard is the ratio of the international standard for oxygen and hydrogen, VSMOW (Vienna Standard Mean Ocean Water). The analytical precision for d¹⁸O and d²H are 0.2 and 2.0 ‰, respectively. Other stable isotopes will have a different reference standard and different analytical precision, but their ratios can be expressed in the same manner.

When using the delta notation, seawater will have a d¹⁸O value of 0‰. Thus, waters which have negative d¹⁸O (and d²H as well) values (such as is typical with precipitation, or groundwater samples) are said to be depleted relative to seawater, and those with positive d values are said to be enriched. These terms can also be used to describe two isotope compositions relative to each other. For example, a precipitation sample which has a d¹⁸O of -25‰ is said to be depleted relative to a precipitation sample with a d¹⁸O of -11‰. Likewise, a groundwater with a d¹⁸O of -5‰ is said to be enriched relative to a groundwater that has a d¹⁸O of -12‰. Within the literature, the terms heavy and light may also be used interchangeably with enriched and depleted when referring stable isotope d¹⁸O and d²H values. For example, a precipitation sample with a d¹⁸O value of -25‰ is said to be isotopically lighter (or simply light) when compared to a precipitation sample having a d¹⁸O of -8‰. A surface water sample with a d¹⁸O value of -5‰ is considered isotopically heavier (or simply heavy) when compared to a groundwater sample with a d¹⁸O value of -10‰. A simple rule to follow with regard to the terminology is - “the more negative the value...the lighter or more depleted it is, the more positive the value...the heavier or more enriched the sample is said to be."

Oxygen/Hydrogen Isotope Relationships
A plot of d¹⁸O vs d²H can be used to determine examine the seasonal variations in the composition of precipitation (as a function of temperature) and to identify subsequent modification of the waters composition by various processes. The d¹⁸O and d²H values of global precipitation generally plot close to a straight line, called the global meteoric water line, GMWL (Craig, 1961). The equation of this line is d²H = 8 X d¹⁸O + 10. Local meteoric water lines also exist, having slightly different slopes and intercepts than the GMWL, as a result of differences in altitude, local climate and distance from the moisture source. The location of the long-term stable isotope precipitation record is Chicago, Illinois (IAEA, 1992). Recently, local meteoric water lines were determined for Mead (Harvey, 2001) and North Platte (Harvey and Welker, 2000), Nebraska and the Pawnee National Grasslands, Colorado (Harvey, 2005) using archived precipitation data made available by the National Atmospheric Deposition Project (NADP). If groundwater d¹⁸O and d²H values plot near the present precipitation water line for the sampling area, the waters are likely meteoric in origin, that is to say, derived from precipitation without subsequent modification. If they do not plot along this line, they have been impacted by some physical or chemical process prior to recharge, or during the groundwater's journey through the aquifer.
 
Once a water is “formed” as meteoric, a number of processes may alter the water’s signature (the combined d¹⁸O and d²H values) as a result of differences in the degree to which the various isotope participate in physical and chemical processes, or due to the different rates of interaction of each isotope, which result from differences in mass and/or molecular size (referred to as isotopic fractionation). The figure at the right illustrates the effect of such processes on a meteoric water. The process most relevant to hydrological studies is evaporation. As a water evaporates from an open surface (river, lake or irrigated field) its signature moves away from the meteoric water line along a line having a slope of between two and about five (depending on the effect of humidity). This shift off the line occurs due to the difference in the vapor pressures of H₂¹⁸O and ²HHO, which imparts disproportional enrichments in the water phase during evaporation (Clark and Fritz, 1997). For a given water, the greater the degree of evaporation, the farther from the water line the resulting signature will be. Rivers and lakes typically undergo greater evaporation than groundwaters and thus, generally plot off the water line along an evaporation line.

However, if a groundwater plots off the line, along an evaporation line, one of two possibilities can be concluded: (a) the groundwaters have been impacted by evaporation prior to recharge (water reaching the saturated zone), or (b) the groundwater represents a mixture of two “end member” waters; one of meteoric origin (plotting on the water line), and one having undergone evaporation (a river, lake or wetland for example, plotting off the line).


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