{"id":106,"date":"2026-04-10T02:53:05","date_gmt":"2026-04-10T02:53:05","guid":{"rendered":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/ch035-xhtml\/"},"modified":"2026-05-11T22:16:23","modified_gmt":"2026-05-11T22:16:23","slug":"4-7-summary","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/4-7-summary\/","title":{"raw":"4.7 Summary","rendered":"4.7 Summary"},"content":{"raw":"
\r\n
\r\n\r\nThe tritium\u2011helium\u20113 age\u2011dating method is a robust method for estimating the age of groundwater that was recharged after the mid\u20111900s. Because the method relies on measurement of a parent ([latex]_{}^{3}H[\/latex]) and child product ([latex]_{}^{3}{He}_{trit}[\/latex]) of radioactive decay, [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] age dating is not reliant on a historical record of atmospheric concentrations or activities.\r\n\r\n<\/div>\r\n
\r\n\r\nThe process for determining groundwater tracer age (Figure 15<\/span><\/a>) includes determination of [latex]_{}^{3}{He}_{trit}[\/latex] by first measuring [latex]_{}^{3}{He}[\/latex] in a water sample and then subtracting the [latex]_{}^{3}{He}[\/latex] components from atmospheric and terrigenic reservoirs, including [latex]_{}^{3}{He}[\/latex] from the solubility equilibrium concentration at recharge temperature and elevation, [latex]_{}^{3}{He}[\/latex] from excess air, and [latex]_{}^{3}{He}[\/latex] from terrigenic production. The method is especially sensitive to the choice of noble gas model\u2014particularly regarding excess air\u2014and parameters related to different reservoirs of [latex]_{}^{3}{He}[\/latex] such as the signature of terrigenic [latex]He[\/latex]. Measurement of [latex]_{}^{3}H[\/latex] is also required. For young waters without significant amounts of radiogenic [latex]He[\/latex] and small amounts of excess air, age uncertainties of \u00b12\u00a0years are typical, but large amounts of excess air (e.g., >\u00a010\u00a0ccSTP\/kg) and uncertainty in the appropriate excess air model can lead to uncertainties greater than ten\u00a0years.\r\n\r\n<\/div>\r\n
\r\n\r\nAlthough the basic age equation for the [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] method is straightforward, its application is sometimes complicated by the need to determine a relatively small value ([latex]_{}^{3}{He}_{trit}[\/latex]) by taking the difference of two or more large values ([latex]_{}^{3}{He}_{m}[\/latex], [latex]_{}^{3}{He}_{sol}[\/latex]). Thus, it is important to choose an appropriate noble gas model for the available data. Furthermore, uncertainty in very young groundwater\u2014age less than a few years\u2014can be large on a percentage basis. The [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] age-dating method is well suited for determination of groundwater recharge rates greater than about 30\u00a0mm\u00a0yr-1<\/sup>; lower recharge rates may not allow confinement of produced [latex]_{}^{3}{He}_{trit}[\/latex] in the groundwater. Groundwater tracer ages from samples containing mixtures of young and old groundwater will be biased (sometimes strongly) toward the [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] age of the young groundwater.\r\n\r\n<\/div>\r\n<\/div>","rendered":"
\n
\n

The tritium\u2011helium\u20113 age\u2011dating method is a robust method for estimating the age of groundwater that was recharged after the mid\u20111900s. Because the method relies on measurement of a parent ([latex]_{}^{3}H[\/latex]) and child product ([latex]_{}^{3}{He}_{trit}[\/latex]) of radioactive decay, [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] age dating is not reliant on a historical record of atmospheric concentrations or activities.<\/p>\n<\/div>\n

\n

The process for determining groundwater tracer age (Figure 15<\/span><\/a>) includes determination of [latex]_{}^{3}{He}_{trit}[\/latex] by first measuring [latex]_{}^{3}{He}[\/latex] in a water sample and then subtracting the [latex]_{}^{3}{He}[\/latex] components from atmospheric and terrigenic reservoirs, including [latex]_{}^{3}{He}[\/latex] from the solubility equilibrium concentration at recharge temperature and elevation, [latex]_{}^{3}{He}[\/latex] from excess air, and [latex]_{}^{3}{He}[\/latex] from terrigenic production. The method is especially sensitive to the choice of noble gas model\u2014particularly regarding excess air\u2014and parameters related to different reservoirs of [latex]_{}^{3}{He}[\/latex] such as the signature of terrigenic [latex]He[\/latex]. Measurement of [latex]_{}^{3}H[\/latex] is also required. For young waters without significant amounts of radiogenic [latex]He[\/latex] and small amounts of excess air, age uncertainties of \u00b12\u00a0years are typical, but large amounts of excess air (e.g., >\u00a010\u00a0ccSTP\/kg) and uncertainty in the appropriate excess air model can lead to uncertainties greater than ten\u00a0years.<\/p>\n<\/div>\n

\n

Although the basic age equation for the [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] method is straightforward, its application is sometimes complicated by the need to determine a relatively small value ([latex]_{}^{3}{He}_{trit}[\/latex]) by taking the difference of two or more large values ([latex]_{}^{3}{He}_{m}[\/latex], [latex]_{}^{3}{He}_{sol}[\/latex]). Thus, it is important to choose an appropriate noble gas model for the available data. Furthermore, uncertainty in very young groundwater\u2014age less than a few years\u2014can be large on a percentage basis. The [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] age-dating method is well suited for determination of groundwater recharge rates greater than about 30\u00a0mm\u00a0yr-1<\/sup>; lower recharge rates may not allow confinement of produced [latex]_{}^{3}{He}_{trit}[\/latex] in the groundwater. Groundwater tracer ages from samples containing mixtures of young and old groundwater will be biased (sometimes strongly) toward the [latex]_{}^{3}H[\/latex]\/[latex]_{}^{3}{He}[\/latex] age of the young groundwater.<\/p>\n<\/div>\n<\/div>\n","protected":false},"author":7,"menu_order":21,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-106","chapter","type-chapter","status-publish","hentry"],"part":30,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/106","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":2,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/106\/revisions"}],"predecessor-version":[{"id":410,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/106\/revisions\/410"}],"part":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/parts\/30"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/106\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/media?parent=106"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapter-type?post=106"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/contributor?post=106"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/license?post=106"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}