{"id":75,"date":"2026-04-10T02:52:58","date_gmt":"2026-04-10T02:52:58","guid":{"rendered":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/ch016-xhtml\/"},"modified":"2026-05-11T20:27:13","modified_gmt":"2026-05-11T20:27:13","slug":"2-4-total-dissolved-gas-pressure","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/chapter\/2-4-total-dissolved-gas-pressure\/","title":{"raw":"2.4 Total Dissolved Gas Pressure","rendered":"2.4 Total Dissolved Gas Pressure"},"content":{"raw":"
\r\n
\r\n\r\nHenry\u2019s Law gives the relationship between the dissolved concentration of a gas and the partial pressure of the gas in contact with the liquid phase. By knowing the dissolved concentration of the dominant gas species, it is possible to compute the partial pressure of each gas. The sum of these partial pressures is known as the total dissolved gas pressure<\/em> (TDGP<\/em>) as shown in Equation\u00a0(7)<\/span>.\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
<\/th>\r\n[latex]TDGP = \\sum_{i = 1}^{n}{p_{i} = \\sum_{i = 1}^{n}{K_{w,i}c_{i} \\cong K_{{w,N}_{2}}c_{N_{2}}}}{+ \\ K}_{{w,O}_{2}}c_{O_{2}} + K_{w,Ar}c_{Ar}[\/latex]<\/th>\r\n<\/span>(7)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<\/table>\r\n
\r\n\r\nwhere:\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
TDGP<\/em><\/th>\r\n=<\/th>\r\ntotal dissolved gas pressure (ML-1<\/sup>T-2<\/sup>, typically in atm)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
[latex]K_{w,N_{2}}[\/latex]<\/td>\r\n=<\/td>\r\ndimensioned Henry Coefficient for [latex]N_{2}[\/latex] (M2<\/sup>L-4<\/sup>T-2<\/sup>, typically in g\u2011atm\/ccSTP)<\/td>\r\n<\/tr>\r\n
[latex]c_{N_{2}}[\/latex]<\/td>\r\n=<\/td>\r\nconcentration of [latex]N_{2}[\/latex] in water (L3<\/sup>M-1<\/sup>, typically in ccSTP\/g)<\/td>\r\n<\/tr>\r\n
[latex]K_{w,O_{2}}[\/latex]<\/td>\r\n=<\/td>\r\ndimensioned Henry Coefficient for [latex]O_{2} [\/latex](M2<\/sup>L-4<\/sup>T-2<\/sup>, typically in g\u2011atm\/ccSTP)<\/td>\r\n<\/tr>\r\n
[latex]c_{O_{2}}[\/latex]<\/td>\r\n=<\/td>\r\nconcentration of [latex]O_{2}[\/latex] in water (L3<\/sup>M-1<\/sup>, typically in ccSTP\/g)<\/td>\r\n<\/tr>\r\n
[latex]K_{w,Ar}[\/latex]<\/td>\r\n=<\/td>\r\ndimensioned Henry Coefficient for [latex]Ar[\/latex] (M2<\/sup>L-4<\/sup>T-2<\/sup>, typically in g\u2011atm\/ccSTP)<\/td>\r\n<\/tr>\r\n
[latex]c_{Ar}[\/latex]<\/td>\r\n=<\/td>\r\nconcentration of [latex]Ar[\/latex] in water (L3<\/sup>M-1<\/sup>, typically in ccSTP\/g)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\nFor groundwater that equilibrated with the atmosphere, the TDGP<\/em> is essentially the sum of the [latex]N_{2}[\/latex], [latex]O_{2}[\/latex] and [latex]Ar[\/latex] pressures. Because the Henry Coefficient is a function of temperature and salinity, the TDGP<\/em> will change if the water temperature or salinity changes, even if the dissolved concentration stays constant.\r\n\r\n<\/div>\r\n
\r\n\r\nThe TDGP<\/em> is a useful quantity for predicting the formation of bubbles (i.e., the exsolution of dissolved gases). If the TDGP<\/em> is less than the fluid pressure, dissolved gases will remain in solution. If the TDGP<\/em> exceeds the fluid pressure, bubbles can theoretically form; however, bubble nucleation is a complex process that can require activation energy. This is particularly important during sampling as a lowering of the fluid pressure via a pump and delivery at the land surface can result in effervescence if the in\u2011situ pressure is not maintained.\r\n\r\n<\/div>\r\n
\r\n\r\nWhen groundwater that has equilibrated at the water table warms and\/or becomes saltier, the TDGP<\/em> increases because the Henry Coefficient changes, even though the gas concentration does not change. While the increase in fluid pressure is typically much greater than the increase in TDGP<\/em> in recharge areas\u2014areas of downward flowing groundwater\u2014in discharge areas where warmed groundwater approaches the surface, the TDGP<\/em> can exceed the fluid pressure, potentially leading to gas exsolution. When bubbles do form, the concentrations of both the major and minor components in the dissolved phase can be strongly affected. Low solubility gases such as [latex]He[\/latex] and [latex]{SF}_{6}[\/latex] will preferentially partition into the gas phase, leaving the water depleted relative to its original concentration.\r\n\r\n<\/div>\r\n
\r\n\r\nThe TDGP<\/em> can be measured directly using a probe. A TDGP<\/em> probe consists of a length of tubing that is permeable to gases\u2014including water vapor\u2014but not permeable to liquid water (Manning et al., 2003). One end of the tube is connected to a pressure sensor and the other end is plugged. When the tube is placed in water, the gas inside the tube can equilibrate with dissolved gases in the water; the pressure inside the tube will eventually reach the TDGP<\/em>. A TDGP<\/em> probe is particularly useful when collecting samples for dissolved gas analysis. If the fluid pressure is greater than the TDGP<\/em>, the probability of bubble formation is minimal. As described in subsequent sections, the use of sampling containers such as copper tubing that allow the groundwater to be sealed under pressure can effectively eliminate degassing that might otherwise occur. <\/span>Box 1<\/u><\/span><\/a>\u2ba7 provides more details and examples of using a TDGP<\/em> probe for groundwater studies.\r\n\r\n<\/div>\r\n<\/div>","rendered":"
\n
\n

Henry\u2019s Law gives the relationship between the dissolved concentration of a gas and the partial pressure of the gas in contact with the liquid phase. By knowing the dissolved concentration of the dominant gas species, it is possible to compute the partial pressure of each gas. The sum of these partial pressures is known as the total dissolved gas pressure<\/em> (TDGP<\/em>) as shown in Equation\u00a0(7)<\/span>.<\/p>\n<\/div>\n\n\n\n\n<\/colgroup>\n\n\n
<\/th>\n[latex]TDGP = \\sum_{i = 1}^{n}{p_{i} = \\sum_{i = 1}^{n}{K_{w,i}c_{i} \\cong K_{{w,N}_{2}}c_{N_{2}}}}{+ \\ K}_{{w,O}_{2}}c_{O_{2}} + K_{w,Ar}c_{Ar}[\/latex]<\/th>\n<\/span>(7)<\/th>\n<\/tr>\n<\/thead>\n<\/table>\n
\n

where:<\/p>\n<\/div>\n\n\n\n\n<\/colgroup>\n\n\n\n\n\n\n\n\n\n
TDGP<\/em><\/th>\n=<\/th>\ntotal dissolved gas pressure (ML-1<\/sup>T-2<\/sup>, typically in atm)<\/th>\n<\/tr>\n<\/thead>\n
[latex]K_{w,N_{2}}[\/latex]<\/td>\n=<\/td>\ndimensioned Henry Coefficient for [latex]N_{2}[\/latex] (M2<\/sup>L-4<\/sup>T-2<\/sup>, typically in g\u2011atm\/ccSTP)<\/td>\n<\/tr>\n
[latex]c_{N_{2}}[\/latex]<\/td>\n=<\/td>\nconcentration of [latex]N_{2}[\/latex] in water (L3<\/sup>M-1<\/sup>, typically in ccSTP\/g)<\/td>\n<\/tr>\n
[latex]K_{w,O_{2}}[\/latex]<\/td>\n=<\/td>\ndimensioned Henry Coefficient for [latex]O_{2}[\/latex](M2<\/sup>L-4<\/sup>T-2<\/sup>, typically in g\u2011atm\/ccSTP)<\/td>\n<\/tr>\n
[latex]c_{O_{2}}[\/latex]<\/td>\n=<\/td>\nconcentration of [latex]O_{2}[\/latex] in water (L3<\/sup>M-1<\/sup>, typically in ccSTP\/g)<\/td>\n<\/tr>\n
[latex]K_{w,Ar}[\/latex]<\/td>\n=<\/td>\ndimensioned Henry Coefficient for [latex]Ar[\/latex] (M2<\/sup>L-4<\/sup>T-2<\/sup>, typically in g\u2011atm\/ccSTP)<\/td>\n<\/tr>\n
[latex]c_{Ar}[\/latex]<\/td>\n=<\/td>\nconcentration of [latex]Ar[\/latex] in water (L3<\/sup>M-1<\/sup>, typically in ccSTP\/g)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\n

For groundwater that equilibrated with the atmosphere, the TDGP<\/em> is essentially the sum of the [latex]N_{2}[\/latex], [latex]O_{2}[\/latex] and [latex]Ar[\/latex] pressures. Because the Henry Coefficient is a function of temperature and salinity, the TDGP<\/em> will change if the water temperature or salinity changes, even if the dissolved concentration stays constant.<\/p>\n<\/div>\n

\n

The TDGP<\/em> is a useful quantity for predicting the formation of bubbles (i.e., the exsolution of dissolved gases). If the TDGP<\/em> is less than the fluid pressure, dissolved gases will remain in solution. If the TDGP<\/em> exceeds the fluid pressure, bubbles can theoretically form; however, bubble nucleation is a complex process that can require activation energy. This is particularly important during sampling as a lowering of the fluid pressure via a pump and delivery at the land surface can result in effervescence if the in\u2011situ pressure is not maintained.<\/p>\n<\/div>\n

\n

When groundwater that has equilibrated at the water table warms and\/or becomes saltier, the TDGP<\/em> increases because the Henry Coefficient changes, even though the gas concentration does not change. While the increase in fluid pressure is typically much greater than the increase in TDGP<\/em> in recharge areas\u2014areas of downward flowing groundwater\u2014in discharge areas where warmed groundwater approaches the surface, the TDGP<\/em> can exceed the fluid pressure, potentially leading to gas exsolution. When bubbles do form, the concentrations of both the major and minor components in the dissolved phase can be strongly affected. Low solubility gases such as [latex]He[\/latex] and [latex]{SF}_{6}[\/latex] will preferentially partition into the gas phase, leaving the water depleted relative to its original concentration.<\/p>\n<\/div>\n

\n

The TDGP<\/em> can be measured directly using a probe. A TDGP<\/em> probe consists of a length of tubing that is permeable to gases\u2014including water vapor\u2014but not permeable to liquid water (Manning et al., 2003). One end of the tube is connected to a pressure sensor and the other end is plugged. When the tube is placed in water, the gas inside the tube can equilibrate with dissolved gases in the water; the pressure inside the tube will eventually reach the TDGP<\/em>. A TDGP<\/em> probe is particularly useful when collecting samples for dissolved gas analysis. If the fluid pressure is greater than the TDGP<\/em>, the probability of bubble formation is minimal. As described in subsequent sections, the use of sampling containers such as copper tubing that allow the groundwater to be sealed under pressure can effectively eliminate degassing that might otherwise occur. <\/span>Box 1<\/u><\/span><\/a>\u2ba7 provides more details and examples of using a TDGP<\/em> probe for groundwater studies.<\/p>\n<\/div>\n<\/div>\n","protected":false},"author":7,"menu_order":4,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-75","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\/75","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":5,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/75\/revisions"}],"predecessor-version":[{"id":380,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapters\/75\/revisions\/380"}],"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\/75\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/media?parent=75"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/pressbooks\/v2\/chapter-type?post=75"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/contributor?post=75"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/age-dating-young-groundwater\/wp-json\/wp\/v2\/license?post=75"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}