{"id":80,"date":"2026-04-10T02:52:59","date_gmt":"2026-04-10T02:52:59","guid":{"rendered":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/ch019-xhtml\/"},"modified":"2026-05-11T21:48:28","modified_gmt":"2026-05-11T21:48:28","slug":"2-7-noble-gas-thermometry","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/2-7-noble-gas-thermometry\/","title":{"raw":"2.7 Noble Gas Thermometry","rendered":"2.7 Noble Gas Thermometry"},"content":{"raw":"
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\r\n\r\nNoble gas thermometry is an application of the solubility and excess air concepts discussed in this section. The objective is to utilize the temperature dependence of the solubility of atmospherically derived gases to estimate the temperature at the water table when groundwater was last in contact with the atmosphere (i.e., an open system). The general idea is to collect a sample for dissolved gases, analyze the sample for a suite of noble gases (typically [latex]Ne[\/latex], [latex]Ar[\/latex], [latex]Kr[\/latex], and [latex]Xe[\/latex]), select an excess air model, and use the inverse approach to find a temperature and excess air parameters that best fit the suite of noble gas concentrations. [latex]He[\/latex] is not normally used in noble gas thermometry because of subsurface sources. While there are also subsurface sources of [latex]Ar[\/latex], its retention by most minerals makes it suitable for all but very old\u2014and typically deep\u2014groundwater samples.\r\n\r\n<\/div>\r\n
\r\n\r\nNoble gas thermometry has been particularly useful for examining the recharge temperature of old groundwater to help reconstruct past climates (e.g., Stute et al., 1995), and the derived excess air parameters have been used to deduce paleo humidity as large water table fluctuations lead to large amounts of excess air (e.g., Aeschbach\u2011Hertig et al., 2002). Noble gas results are also useful for dating methods such as CFCs when the equilibrium concentration of a tracer is highly sensitive to the recharge temperature, and [latex]{SF}_{6}[\/latex] when the dissolved concentration is sensitive to excess air.\r\n\r\n<\/div>\r\n
\r\n\r\nThe subsurface temperature recorded by noble gas thermometry is related to the mean annual temperature at the recharge location but can also depend on the depth of the water table and the seasonality of recharge. Figure 6<\/span><\/a> shows simulated temperatures in the subsurface for a region in which the mean annual temperature is 10\u00a0\u00b0C but fluctuates seasonally between 0\u00a0\u00b0C and 20\u00a0\u00b0C. The annual temperature fluctuations diminish with depth being about \u00b14\u00a0\u00b0C at a depth of 2\u00a0m and \u00b11\u00a0\u00b0C at 6\u00a0m. At a depth of 10\u00a0m, the annual temperature fluctuations at the surface are nearly completely damped. Thus, in areas where the depth to the water table is greater than about 10\u00a0m, the recharge temperature is likely to be similar to the mean annual temperature\u2014the influence of the geothermal gradient is minimal at this depth\u2014but the recharge temperature could vary significantly when the water table is shallow.\r\n\r\n<\/div>\r\n
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\r\n\r\n<\/span>Figure 6<\/strong>\u00a0\u2011\u00a0Simulated soil temperatures in a temperate climate. The mean annual temperature is 10\u00a0\u00b0C and the air temperature varies from 0 to 20\u00a0\u00b0C. The dampening of the surface temperature with depth is controlled mostly by thermal diffusivity of the surficial material, which in turn is a function of its water content and to a lesser extent its mineralogy. When the water table is more than 10\u00a0m below land surface, the annual temperature variations are nearly completely damped for a typical thermal diffusivity of 0.005\u00a0cm2<\/sup>\/s (redrawn from Aeschbach\u2011Hertig & Solomon, 2013).\r\n\r\n<\/div><\/blockquote>\r\n<\/div>\r\n
\r\n\r\nNoble gas concentrations are a function of four factors: temperature, pressure\u2014which is a function of elevation, salinity, and excess air. In principle, the inverse approach can be used to solve for any of these parameters as long as more noble gas concentrations are available than unknown parameters; that is, the system of equations being inverted needs to be over determined. However, temperature and pressure are correlated, and experience has shown that it is difficult to use the inverse approach to accurately solve for both recharge temperature and pressure (i.e., elevation). In mountainous terrains, where neither recharge temperature nor elevation are typically known, additional information is needed to estimate both parameters. For example, Manning and Solomon (2003) used the relationship between mean annual temperature and elevations, known as the atmospheric lapse rate<\/em>, to help constrain the recharge location of groundwater in a basin\u2011fill aquifer beneath Salt Lake City, Utah, USA.\r\n\r\n<\/div>\r\n<\/div>","rendered":"
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Noble gas thermometry is an application of the solubility and excess air concepts discussed in this section. The objective is to utilize the temperature dependence of the solubility of atmospherically derived gases to estimate the temperature at the water table when groundwater was last in contact with the atmosphere (i.e., an open system). The general idea is to collect a sample for dissolved gases, analyze the sample for a suite of noble gases (typically [latex]Ne[\/latex], [latex]Ar[\/latex], [latex]Kr[\/latex], and [latex]Xe[\/latex]), select an excess air model, and use the inverse approach to find a temperature and excess air parameters that best fit the suite of noble gas concentrations. [latex]He[\/latex] is not normally used in noble gas thermometry because of subsurface sources. While there are also subsurface sources of [latex]Ar[\/latex], its retention by most minerals makes it suitable for all but very old\u2014and typically deep\u2014groundwater samples.<\/p>\n<\/div>\n

\n

Noble gas thermometry has been particularly useful for examining the recharge temperature of old groundwater to help reconstruct past climates (e.g., Stute et al., 1995), and the derived excess air parameters have been used to deduce paleo humidity as large water table fluctuations lead to large amounts of excess air (e.g., Aeschbach\u2011Hertig et al., 2002). Noble gas results are also useful for dating methods such as CFCs when the equilibrium concentration of a tracer is highly sensitive to the recharge temperature, and [latex]{SF}_{6}[\/latex] when the dissolved concentration is sensitive to excess air.<\/p>\n<\/div>\n

\n

The subsurface temperature recorded by noble gas thermometry is related to the mean annual temperature at the recharge location but can also depend on the depth of the water table and the seasonality of recharge. Figure 6<\/span><\/a> shows simulated temperatures in the subsurface for a region in which the mean annual temperature is 10\u00a0\u00b0C but fluctuates seasonally between 0\u00a0\u00b0C and 20\u00a0\u00b0C. The annual temperature fluctuations diminish with depth being about \u00b14\u00a0\u00b0C at a depth of 2\u00a0m and \u00b11\u00a0\u00b0C at 6\u00a0m. At a depth of 10\u00a0m, the annual temperature fluctuations at the surface are nearly completely damped. Thus, in areas where the depth to the water table is greater than about 10\u00a0m, the recharge temperature is likely to be similar to the mean annual temperature\u2014the influence of the geothermal gradient is minimal at this depth\u2014but the recharge temperature could vary significantly when the water table is shallow.<\/p>\n<\/div>\n

\n

\"\"<\/p>\n<\/div>\n

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<\/span>Figure 6<\/strong>\u00a0\u2011\u00a0Simulated soil temperatures in a temperate climate. The mean annual temperature is 10\u00a0\u00b0C and the air temperature varies from 0 to 20\u00a0\u00b0C. The dampening of the surface temperature with depth is controlled mostly by thermal diffusivity of the surficial material, which in turn is a function of its water content and to a lesser extent its mineralogy. When the water table is more than 10\u00a0m below land surface, the annual temperature variations are nearly completely damped for a typical thermal diffusivity of 0.005\u00a0cm2<\/sup>\/s (redrawn from Aeschbach\u2011Hertig & Solomon, 2013).<\/p>\n<\/div>\n<\/blockquote>\n<\/div>\n

\n

Noble gas concentrations are a function of four factors: temperature, pressure\u2014which is a function of elevation, salinity, and excess air. In principle, the inverse approach can be used to solve for any of these parameters as long as more noble gas concentrations are available than unknown parameters; that is, the system of equations being inverted needs to be over determined. However, temperature and pressure are correlated, and experience has shown that it is difficult to use the inverse approach to accurately solve for both recharge temperature and pressure (i.e., elevation). In mountainous terrains, where neither recharge temperature nor elevation are typically known, additional information is needed to estimate both parameters. For example, Manning and Solomon (2003) used the relationship between mean annual temperature and elevations, known as the atmospheric lapse rate<\/em>, to help constrain the recharge location of groundwater in a basin\u2011fill aquifer beneath Salt Lake City, Utah, USA.<\/p>\n<\/div>\n<\/div>\n","protected":false},"author":7,"menu_order":7,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-80","chapter","type-chapter","status-publish","hentry"],"part":26,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/80","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/users\/7"}],"version-history":[{"count":3,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/80\/revisions"}],"predecessor-version":[{"id":390,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/80\/revisions\/390"}],"part":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/parts\/26"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/80\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/media?parent=80"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapter-type?post=80"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/contributor?post=80"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/license?post=80"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}