{"id":89,"date":"2020-11-18T01:44:49","date_gmt":"2020-11-18T01:44:49","guid":{"rendered":"https:\/\/books.gw-project.org\/introduction-to-isotopes-and-environmental-tracers-as-indicators-of-groundwater-flow\/chapter\/stable-isotopes\/"},"modified":"2022-09-20T16:20:15","modified_gmt":"2022-09-20T16:20:15","slug":"stable-isotopes","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/introduction-to-isotopes-and-environmental-tracers-as-indicators-of-groundwater-flow\/chapter\/stable-isotopes\/","title":{"raw":"2.6 Stable Isotopes","rendered":"2.6 Stable Isotopes"},"content":{"raw":"Stable isotopes<\/em> are atoms of the same chemical element (i.e. the same number of protons and electrons) with a different number of neutrons that do not undergo radioactive decay, but they may be present as a result of radioactive decay. Ratios of different stable isotopes of the same element can provide useful indicators of water sources and chemical reactions.\r\n\r\nThe ratios of the less abundant to the most abundant stable isotope of an element (e.g., 13<\/sup>C\/12<\/sup>C, 18<\/sup>O\/16<\/sup>O, 15<\/sup>N\/14<\/sup>N) vary slightly between different materials and vary spatially within the same material. Differences in abundance of the different isotopes are usually caused by small differences in reactivity because of mass differences; lighter isotopes typically being more reactive than heavier isotopes. Natural materials will therefore have different isotopic abundances due to different source materials and different geochemical processes to which they have been exposed. Consequently, changes in isotopic abundance of natural materials can provide information on sources of dissolved substances, and chemical reactions that might have taken place after dissolution. The stable isotope ratios of the water molecule (18<\/sup>O\/16<\/sup>O and 2<\/sup>H\/1<\/sup>H) are particularly useful in hydrology, as they can provide information on condensation and evaporation processes. Manufactured chemicals can have different isotopic ratios due to differences in source materials or manufacturing processes. Fractionation is the name given to processes that affect the relative abundance of light and heavy stable isotopes (light being the isotopic form with fewer neutrons).\r\n\r\nSome of the key stable isotopes that have application to hydrology are listed in Table 2, along with their relative global abundances. The ratio of heavy to light isotopes can be measured with high accuracy and precision, therefore differences between relative abundances at a local level can be used to infer migration of groundwater and geochemical processes occurring along the flow paths. Isotope ratios can be measured either on individual ions of molecules, for example, 15<\/sup>N\/14<\/sup>N and 18<\/sup>O\/16<\/sup>O of NO3<\/sub>-<\/sup>, 34<\/sup>S\/32<\/sup>S or 18<\/sup>O\/16<\/sup>O of SO4<\/sub>2<\/sup>-<\/sup>, 18<\/sup>O\/16<\/sup>O or 2<\/sup>H\/1<\/sup>H of H2<\/sub>O, or on groups of molecules, such as with 13<\/sup>C\/12<\/sup>C of total dissolved inorganic carbon (TDIC). Usually this ratio is expressed relative to a standard using the delta (<\/em>\u03b4<\/em>) notation<\/em> as shown in Equation 4.\r\n\r\n\r\n\r\n
<\/td>\r\n[latex]\\displaystyle \\delta=\\frac{R_{sample}-R_{std}}{R_{std}}\\times1000[\/latex] \u2030<\/td>\r\n(4)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nwhere:\r\n\r\n\r\n\r\n
R<\/em><\/td>\r\n=<\/td>\r\nratio of isotope amount (usually less abundant to most abundant isotope)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nThus, the \u03b4<\/em>-value of a sample which is identical to the standard would be 0 \u2030, positive values indicate a greater proportion of the less abundant isotope than the standard, and negative values indicate a lower proportion of the less abundant isotope (Table 1<\/a>).\r\n\r\nTable <\/strong>2 <\/strong>-<\/strong> Relative global abundances of some commonly used stable isotopes. From Rosman and Taylor (1998).<\/small>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Hydrogen (percent)<\/strong><\/td>\r\nCarbon (percent)<\/strong><\/td>\r\nNitrogen (percent)<\/strong><\/td>\r\n<\/tr>\r\n
1<\/sup>H<\/td>\r\n99.9885<\/td>\r\n12<\/sup>C<\/td>\r\n98.93<\/td>\r\n14<\/sup>N<\/td>\r\n99.632<\/td>\r\n<\/tr>\r\n
2<\/sup>H<\/td>\r\n0.0115<\/td>\r\n13<\/sup>C<\/td>\r\n1.07<\/td>\r\n15<\/sup>N<\/td>\r\n0.368<\/td>\r\n<\/tr>\r\n
Oxygen (percent)<\/strong><\/td>\r\nSulphur (percent)<\/strong><\/td>\r\nStrontium (percent)<\/strong><\/td>\r\n<\/tr>\r\n
16<\/sup>O<\/td>\r\n99.757<\/td>\r\n32<\/sup>S<\/td>\r\n94.93<\/td>\r\n84<\/sup>Sr<\/td>\r\n0.56<\/td>\r\n<\/tr>\r\n
17<\/sup>O<\/td>\r\n0.038<\/td>\r\n33<\/sup>S<\/td>\r\n0.76<\/td>\r\n86<\/sup>Sr<\/td>\r\n9.86<\/td>\r\n<\/tr>\r\n
18<\/sup>O<\/td>\r\n0.205<\/td>\r\n34<\/sup>S<\/td>\r\n4.29<\/td>\r\n87<\/sup>Sr<\/td>\r\n7.00<\/td>\r\n<\/tr>\r\n
<\/td>\r\n<\/td>\r\n36<\/sup>S<\/td>\r\n0.02<\/td>\r\n88<\/sup>Sr<\/td>\r\n82.58<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nTwo of the most widely used stable isotope ratios are those of hydrogen and oxygen in the water molecule. Although there are two common stable isotopes of hydrogen and three of oxygen, giving a total of nine isotopically different water molecules, only three are commonly measured in natural waters: 1<\/sup>H1<\/sup>H16<\/sup>O, 1<\/sup>H2<\/sup>H16<\/sup>O and 1<\/sup>H1<\/sup>H18<\/sup>O, with the first being the most abundant form. Variations in 2<\/sup>H\/1<\/sup>H and 18<\/sup>O\/16<\/sup>O ratios in natural waters arise mostly from isotopic fractionation during evaporation and condensation processes, whereby the heavy isotopes of water preferentially remain in the liquid phase during evaporation or pass into the liquid phase during condensation.\r\n\r\nRainfall everywhere in the world is depleted in the heavy isotopes of water relative to seawater through a process known as Rayleigh fractionation<\/em>. (Seawater is used as the standard for \u03b4<\/em> 2<\/sup>H and \u03b4<\/em> 18<\/sup>O measurement, so it has an isotopic composition of \u03b4<\/em> 2<\/sup>H = \u03b4<\/em> 18<\/sup>O = 0 \u2030.) Because lighter isotopes evaporate more readily than heavier isotopes, clouds will contain a greater proportion of light isotopes (and thus have negative values for \u03b4<\/em> 2<\/sup>H and \u03b4<\/em> 18<\/sup>O). The isotopic composition of rainfall will be heavier than that of clouds, but lighter than the seawater which was the original source of the water vapor forming the clouds. As air masses progressively move inland, rainfall becomes progressively lighter (Coplen et al., 2000). Because 2<\/sup>H and 18<\/sup>O are affected in similar ways, \u03b4<\/em> 2<\/sup>H and \u03b4<\/em> 18<\/sup>O values in precipitation are highly correlated. The global meteoric water line<\/em> (GMWL) is an empirical relationship that expresses the average correlation between \u03b4 <\/em>2<\/sup>H and \u03b4 <\/em>18<\/sup>O values in precipitation, and is given by Equation 5 (Craig, 1961).\r\n\r\n\r\n\r\n
<\/td>\r\n\u03b4<\/em>2<\/sup>H = 8 \u03b4<\/em>18<\/sup>O + 10<\/td>\r\n(5)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nOften, however, the stable isotopic composition of local rainfall differs from the global mean, and so it is sometimes useful to define a local meteoric water line<\/em> (LMWL; Figure 7).\r\n\r\n[caption id=\"attachment_113\" align=\"alignnone\" width=\"883\"]\"Graph Figure 7<\/strong> - Monthly rainfall data for Ottawa, Canada, between December 1972 and February 2017, together with the Local Meteoric Water Line (LMWL), calculated as a linear regression on the data. Data from IAEA\/WMO (2018). The global meteoric water line (GMWL), given by Equation 5 is shown for comparison (Cook, 2020).[\/caption]\r\n\r\nIn many cases, the mean \u03b4 <\/em>2<\/sup>H and \u03b4 <\/em>18<\/sup>O composition of groundwater will be similar to the mean amount-weighted \u03b4 <\/em>2<\/sup>H and \u03b4 <\/em>18<\/sup>O composition of precipitation within its recharge area. However, fractionation processes result in variations in isotopic composition of precipitation, and sometimes these variations can be used to identify conditions under which the groundwater recharge occurred (Figure 8). Other factors being equal, the isotopic composition will be more depleted in heavy isotopes during heavy rainstorms than during lighter storm events. So, if groundwater recharge primarily occurs during large rain events, it may cause the mean isotopic composition of groundwater to be more depleted than mean rainfall. Also, rainfall at higher elevations will be more depleted than rainfall at lower elevations, and rainfall during cold climatic periods will be more depleted than rainfall during warmer periods (Ingraham, 1998; Mazor, 2004). Subsequent to a rainfall event, the 18<\/sup>O and 2<\/sup>H composition of water may be further modified by evaporation from the soil or from surface water bodies, and these effects are most notable in more arid climates. Water samples affected by evaporation are typically displaced to the right of the meteoric water line, and often fall on a line of lower slope (Figure 8), while transpiration does not significantly fractionate the stable isotopes. The International Atomic Energy Agency (IAEA) maintains a network of rainfall collection stations around the world, so there is a good database on the \u03b4 <\/em>2<\/sup>H and \u03b4 <\/em>18<\/sup>O composition of precipitation for comparison with groundwater samples.\r\n\r\n[caption id=\"attachment_114\" align=\"alignnone\" width=\"748\"]\"Schematic Figure 8<\/strong> - Schematic representation of the principal processes affecting the 2<\/sup>H and 18<\/sup>O composition of groundwater. The slope of the evaporation line is usually between 4 and 6 for surface water evaporation, and between 2 and 5 for soil evaporation (Barnes and Allison, 1988) (Cook, 2020)[\/caption]\r\n\r\nOther stable isotopes that are widely used in hydrogeology include:\r\n