{"id":123,"date":"2026-04-10T02:53:08","date_gmt":"2026-04-10T02:53:08","guid":{"rendered":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/ch046-xhtml\/"},"modified":"2026-05-11T22:33:06","modified_gmt":"2026-05-11T22:33:06","slug":"6-2-basic-concepts-and-systematics","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/6-2-basic-concepts-and-systematics\/","title":{"raw":"6.2 Basic Concepts and Systematics","rendered":"6.2 Basic Concepts and Systematics"},"content":{"raw":"
\r\n
\r\n\r\nThe potential for CFCs to function as groundwater tracers was first demonstrated in the 1970s. For example, Thompson and others (1974) injected CFC\u201111 into a sand and gravel aquifer and detected its arrival at monitoring wells in accord with expected groundwater travel times. Similarly, Shultz and others (1976) showed that CFCs were a useful tracer of groundwater recharge from sewage effluent.\r\n\r\n<\/div>\r\n
\r\n\r\nStarting in the early 1980s, CFCs were used as tracers of ocean circulation and dynamics (e.g., Bullister, 1989). New analytical methods for water samples were developed for the simultaneous analysis of multiple CFCs (e.g., Bullister & Weiss, 1988), and the solubilities of CFC-11 and CFC-12 were measured with high precision (Warner & Weiss, 1985). Advances in sample collection procedures and evaluations of the usefulness of CFCs as a groundwater dating tracer followed in the 1990s (Busenberg & Plummer, 1992; Cook et al., 1995; Ekwurzel et al. 1994). Further applications of CFC dating for constraining groundwater flow models (e.g., Reilly et al., 1994), tracing nitrate contamination (B\u00f6hlke & Denver, 1995) and tracing leakage from sinkholes in karst systems (Katz et al., 1995) helped to further refine and articulate the usefulness of the CFC method.\r\n\r\n<\/div>\r\n
\r\n\r\nThe basic concept for CFC dating is like [latex]{SF}_{6}[\/latex] and is shown in Figure 25<\/span><\/a>. Samples are collected such that they do not contact the atmosphere. The aqueous concentration is measured by gas chromatography after the CFCs have been extracted (stripped) from the water sample. The aqueous concentration is then converted into an air concentration using Henry\u2019s Law (i.e., the air concentration that is in equilibrium with the measured aqueous concentration at an assumed temperature, pressure, and salinity at recharge). The equivalent air concentration is then compared to the atmospheric concentration curve to estimate the year the sample was last in contact with the atmosphere (i.e., the recharge year). The tracer age is then the sample collection date minus the recharge year.\r\n\r\n<\/div>\r\n
\r\n\r\n\"\"\r\n\r\n<\/div>\r\n
\r\n
\r\n
\r\n\r\n<\/span>Figure 25\u00a0\u2011<\/strong> General concept of groundwater dating with CFCs. The aqueous concentration of CFCs (above examples are for CFC\u201112) is measured by gas chromatography. Using estimates of the recharge temperature and pressure (i.e., elevation) the aqueous concentration is converted to the air concentration (i.e., the concentration in air that would be in equilibrium with the measured aqueous concentration at the temperature and pressure when the sample was at the water table). Using the atmospheric concentration curve for CFC\u201112, the air concentration is converted into a recharge year. For a water sample with an aqueous concentration of 1.3 pmole\/kg (i.e., picomoles per kg) that recharged at a pressure of 1 atm (i.e., sea level) with a water table temperature of 10 \u00b0C\u2014and assuming no excess air\u2014the equivalent air concentration would be about 250 pptv. The atmosphere had a concentration of 250 pptv in 1976, and the sample was collected in 2015. Thus, the tracer age is 2015 - 1976 = 39 years.\r\n\r\n<\/div><\/blockquote>\r\n<\/div>\r\n
\r\n\r\nThe conversion from an aqueous concentration to an equivalent air concentration (mixing ratio) utilizes Henry\u2019s Law with a coefficient that is temperature dependent. The example solubility curves shown in Figure 25<\/span><\/a> were computed for two recharge temperatures and two recharge pressures (i.e., elevations). In practice, a solubility curve is computed for the exact recharge temperature and pressure conditions for a sample. Also, the measured concentration can include excess air (Section\u00a02.6) that theoretically should be removed from the measured concentration before an equivalent air concentration is computed. However, because CFCs are much more soluble than [latex]{SF}_{6}[\/latex] and light noble gases, this correction is usually minor and is often neglected.\r\n\r\n<\/div>\r\n<\/div>","rendered":"
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\n

The potential for CFCs to function as groundwater tracers was first demonstrated in the 1970s. For example, Thompson and others (1974) injected CFC\u201111 into a sand and gravel aquifer and detected its arrival at monitoring wells in accord with expected groundwater travel times. Similarly, Shultz and others (1976) showed that CFCs were a useful tracer of groundwater recharge from sewage effluent.<\/p>\n<\/div>\n

\n

Starting in the early 1980s, CFCs were used as tracers of ocean circulation and dynamics (e.g., Bullister, 1989). New analytical methods for water samples were developed for the simultaneous analysis of multiple CFCs (e.g., Bullister & Weiss, 1988), and the solubilities of CFC-11 and CFC-12 were measured with high precision (Warner & Weiss, 1985). Advances in sample collection procedures and evaluations of the usefulness of CFCs as a groundwater dating tracer followed in the 1990s (Busenberg & Plummer, 1992; Cook et al., 1995; Ekwurzel et al. 1994). Further applications of CFC dating for constraining groundwater flow models (e.g., Reilly et al., 1994), tracing nitrate contamination (B\u00f6hlke & Denver, 1995) and tracing leakage from sinkholes in karst systems (Katz et al., 1995) helped to further refine and articulate the usefulness of the CFC method.<\/p>\n<\/div>\n

\n

The basic concept for CFC dating is like [latex]{SF}_{6}[\/latex] and is shown in Figure 25<\/span><\/a>. Samples are collected such that they do not contact the atmosphere. The aqueous concentration is measured by gas chromatography after the CFCs have been extracted (stripped) from the water sample. The aqueous concentration is then converted into an air concentration using Henry\u2019s Law (i.e., the air concentration that is in equilibrium with the measured aqueous concentration at an assumed temperature, pressure, and salinity at recharge). The equivalent air concentration is then compared to the atmospheric concentration curve to estimate the year the sample was last in contact with the atmosphere (i.e., the recharge year). The tracer age is then the sample collection date minus the recharge year.<\/p>\n<\/div>\n

\n

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

\n
\n
\n

<\/span>Figure 25\u00a0\u2011<\/strong> General concept of groundwater dating with CFCs. The aqueous concentration of CFCs (above examples are for CFC\u201112) is measured by gas chromatography. Using estimates of the recharge temperature and pressure (i.e., elevation) the aqueous concentration is converted to the air concentration (i.e., the concentration in air that would be in equilibrium with the measured aqueous concentration at the temperature and pressure when the sample was at the water table). Using the atmospheric concentration curve for CFC\u201112, the air concentration is converted into a recharge year. For a water sample with an aqueous concentration of 1.3 pmole\/kg (i.e., picomoles per kg) that recharged at a pressure of 1 atm (i.e., sea level) with a water table temperature of 10 \u00b0C\u2014and assuming no excess air\u2014the equivalent air concentration would be about 250 pptv. The atmosphere had a concentration of 250 pptv in 1976, and the sample was collected in 2015. Thus, the tracer age is 2015 – 1976 = 39 years.<\/p>\n<\/div>\n<\/blockquote>\n<\/div>\n

\n

The conversion from an aqueous concentration to an equivalent air concentration (mixing ratio) utilizes Henry\u2019s Law with a coefficient that is temperature dependent. The example solubility curves shown in Figure 25<\/span><\/a> were computed for two recharge temperatures and two recharge pressures (i.e., elevations). In practice, a solubility curve is computed for the exact recharge temperature and pressure conditions for a sample. Also, the measured concentration can include excess air (Section\u00a02.6) that theoretically should be removed from the measured concentration before an equivalent air concentration is computed. However, because CFCs are much more soluble than [latex]{SF}_{6}[\/latex] and light noble gases, this correction is usually minor and is often neglected.<\/p>\n<\/div>\n<\/div>\n","protected":false},"author":7,"menu_order":30,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-123","chapter","type-chapter","status-publish","hentry"],"part":35,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/123","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\/123\/revisions"}],"predecessor-version":[{"id":422,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/123\/revisions\/422"}],"part":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/parts\/35"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/123\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/media?parent=123"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapter-type?post=123"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/contributor?post=123"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/license?post=123"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}