{"id":1299,"date":"2026-05-12T21:47:25","date_gmt":"2026-05-12T21:47:25","guid":{"rendered":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/review-of-elastic-response-in-a-confined-aquifer\/"},"modified":"2026-05-13T03:22:55","modified_gmt":"2026-05-13T03:22:55","slug":"review-of-elastic-response-in-a-confined-aquifer","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/review-of-elastic-response-in-a-confined-aquifer\/","title":{"raw":"15.2 Review of Elastic Response in a Confined Aquifer","rendered":"15.2 Review of Elastic Response in a Confined Aquifer"},"content":{"raw":"
\r\n
\r\n\r\nConsider a confined aquifer that consists of a layer of sand overlain by a layer of clay that acts as a confining layer. If a well is installed in this confined aquifer and water is removed from it via a pump, the heads around the well will lower in such a way that the well creates a cone of depression<\/em> in the potentiometric surface (Figure\u00a0119<\/span><\/a>). The more water that is removed from the well, the larger the cone of depression. The example in Figure\u00a0119<\/span><\/a> shows that, even though pumping has removed a volume of water and lowered the water level in the well and in the surrounding aquifer, the saturated thickness of the aquifer has not changed\u2014that is, the aquifer is still saturated up to the base of the confining layer.\r\n\r\n<\/div>\r\n
\"\"\r\n
\r\n
\r\n
\r\n\r\nFigure\u00a0119\u00a0\u2011\u00a0<\/strong>Pumping well in a confined aquifer.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n\r\nWhat is happening has to do with the additional fluid pressure, or excess fluid pressure<\/em>, in the confined aquifer (Figure\u00a0120<\/span><\/a>). The pressure head in the aquifer right next to the well screen is related to the column of water above that point in the well. In an unconfined aquifer, the pressure head is equal to the thickness of the saturated zone above the well screen. In a confined aquifer, it is equal to the thickness of the saturated zone plus the additional pressure that is reflected in the elevation of the potentiometric surface above the base of the confining layer.\r\n\r\n<\/div>\r\n
\"\"\r\n
\r\n
\r\n
\r\n\r\nFigure\u00a0120\u00a0\u2011\u00a0<\/strong>Excess fluid pressure in a confined aquifer.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n\r\nTo put it into other words, in a confined aquifer, there is some additional pressure in the pressure head component, which is represented by the length of the column of water in the well above the top of the aquifer. If there was no additional pressure\u2014as in an unconfined aquifer\u2014the water level in the well would rise to the water table. It is the extra pressure in the confined aquifer that is pushing the aquifer matrix apart and creating extra space for the water in the aquifer, and when that pressure is relieved, the grains relax back to their original positions (Figure\u00a0121<\/span><\/a>). Decreasing the fluid pressure in the aquifer causes the potentiometric surface to decline and the individual pore spaces to decrease. The change in volume of each individual pore is miniscule but can add up to a significant change in volume of the aquifer.\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\nFigure\u00a0121\u00a0\u2013\u00a0<\/strong>Reduction of pore fluid pressure causes grains to move closer together such that the confined aquifer thickness decreases.<\/span>\r\n\r\n<\/div><\/blockquote>\r\n<\/div>\r\n
\r\n\r\nWhat we are talking about here is a strain<\/em>\u2014that is, change in volume or change in length\u2014resulting from a stress<\/em>\u2014some force or pressure applied in three dimensions. The stresses in an aquifer are due to the weight of the column of rock and water above a given point, and the strains that we observe are consolidation of aquifer materials, which can often result in subsidence of the ground surface. Before we talk about strain\u2014that is, consolidation\u2014let\u2019s talk about the states of stress in the subsurface.\r\n\r\n<\/div>\r\n<\/div>","rendered":"
\n
\n

Consider a confined aquifer that consists of a layer of sand overlain by a layer of clay that acts as a confining layer. If a well is installed in this confined aquifer and water is removed from it via a pump, the heads around the well will lower in such a way that the well creates a cone of depression<\/em> in the potentiometric surface (Figure\u00a0119<\/span><\/a>). The more water that is removed from the well, the larger the cone of depression. The example in Figure\u00a0119<\/span><\/a> shows that, even though pumping has removed a volume of water and lowered the water level in the well and in the surrounding aquifer, the saturated thickness of the aquifer has not changed\u2014that is, the aquifer is still saturated up to the base of the confining layer.<\/p>\n<\/div>\n

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

Figure\u00a0119\u00a0\u2011\u00a0<\/strong>Pumping well in a confined aquifer.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

\n

What is happening has to do with the additional fluid pressure, or excess fluid pressure<\/em>, in the confined aquifer (Figure\u00a0120<\/span><\/a>). The pressure head in the aquifer right next to the well screen is related to the column of water above that point in the well. In an unconfined aquifer, the pressure head is equal to the thickness of the saturated zone above the well screen. In a confined aquifer, it is equal to the thickness of the saturated zone plus the additional pressure that is reflected in the elevation of the potentiometric surface above the base of the confining layer.<\/p>\n<\/div>\n

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

Figure\u00a0120\u00a0\u2011\u00a0<\/strong>Excess fluid pressure in a confined aquifer.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

\n

To put it into other words, in a confined aquifer, there is some additional pressure in the pressure head component, which is represented by the length of the column of water in the well above the top of the aquifer. If there was no additional pressure\u2014as in an unconfined aquifer\u2014the water level in the well would rise to the water table. It is the extra pressure in the confined aquifer that is pushing the aquifer matrix apart and creating extra space for the water in the aquifer, and when that pressure is relieved, the grains relax back to their original positions (Figure\u00a0121<\/span><\/a>). Decreasing the fluid pressure in the aquifer causes the potentiometric surface to decline and the individual pore spaces to decrease. The change in volume of each individual pore is miniscule but can add up to a significant change in volume of the aquifer.<\/p>\n<\/div>\n

\n

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

\n
\n
\n

Figure\u00a0121\u00a0\u2013\u00a0<\/strong>Reduction of pore fluid pressure causes grains to move closer together such that the confined aquifer thickness decreases.<\/span><\/p>\n<\/div>\n<\/blockquote>\n<\/div>\n

\n

What we are talking about here is a strain<\/em>\u2014that is, change in volume or change in length\u2014resulting from a stress<\/em>\u2014some force or pressure applied in three dimensions. The stresses in an aquifer are due to the weight of the column of rock and water above a given point, and the strains that we observe are consolidation of aquifer materials, which can often result in subsidence of the ground surface. Before we talk about strain\u2014that is, consolidation\u2014let\u2019s talk about the states of stress in the subsurface.<\/p>\n<\/div>\n<\/div>\n","protected":false},"author":8,"menu_order":64,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1299","chapter","type-chapter","status-publish","hentry"],"part":1510,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1299","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/wp\/v2\/users\/8"}],"version-history":[{"count":2,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1299\/revisions"}],"predecessor-version":[{"id":1769,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1299\/revisions\/1769"}],"part":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/parts\/1510"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1299\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/wp\/v2\/media?parent=1299"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapter-type?post=1299"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/wp\/v2\/contributor?post=1299"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/wp\/v2\/license?post=1299"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}