{"id":1142,"date":"2026-05-12T21:46:21","date_gmt":"2026-05-12T21:46:21","guid":{"rendered":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/aquifers\/"},"modified":"2026-05-13T03:11:14","modified_gmt":"2026-05-13T03:11:14","slug":"aquifers","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/aquifers\/","title":{"raw":"5.2 Aquifers","rendered":"5.2 Aquifers"},"content":{"raw":"<div id=\"aquifers-1\" class=\"level1\">\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nBefore we discuss storage parameters, let\u2019s review what an aquifer is and discuss aquifer conditions and the occurrence of groundwater in nature.\r\n\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nAn aquifer is a geologic unit that can store and transmit enough water to supply a well. The factors that determine if a geologic unit is an aquifer include:\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>the formation must be capable of storing sufficient volumes of water;<\/li>\r\n \t<li>the rock or sediments in the aquifer must have a sufficiently high permeability that flow to a well or spring can be maintained; and<\/li>\r\n \t<li>the aquifer must have sufficient areal extent and saturated thickness to supply water to the well.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nIf the aquifer unit is close to the Earth\u2019s surface, and the sediments that comprise it are relatively permeable, the primary source of recharge to the aquifer will probably be infiltration of groundwater from precipitation that falls directly on the ground. In this case, a water table will form, and the aquifer is called a water\u2011table aquifer or an unconfined aquifer (<a href=\"#Xaa5c47feed27d083ce78d9f7d7461abdad6c347\"><span data-custom-style=\"Hyperlink\">Figure\u00a021<\/span><\/a>). In a water\u2011table aquifer the water table is defined as the point where the pore\u2011water pressure\u00ad and the atmospheric pressure are equal. Under those conditions, water in a well will rise to the point where the water table was encountered during drilling.\r\n\r\n<\/div>\r\n<div class=\"bc-figure figure\" data-custom-style=\"Captioned Figure\"><img style=\"display: block; margin-left: auto; margin-right: auto; ;width: 5.83333in; height: 2.92997in;\" src=\"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-content\/uploads\/sites\/42\/2026\/05\/file23-2.png\" alt=\"\" \/>\r\n<div class=\"bc-figcaption figcaption\">\r\n<div data-custom-style=\"Image Caption\">\r\n<div class=\"figcaption-text\">\r\n\r\n<span id=\"Xaa5c47feed27d083ce78d9f7d7461abdad6c347\" class=\"anchor\"><strong>Figure\u00a021\u00a0\u2011\u00a0<\/strong>Unconfined aquifer.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nIf the regional geology consists of alternating layers of low\u2011 and high\u2011permeability units (e.g., interbedded sand and clay), then the permeable layers that are overlain by the low\u2011permeability units can form confined aquifers (<a href=\"#Xbdd870ff72fe07ce1a49541d44d33204f5160c5\"><span data-custom-style=\"Hyperlink\">Figure\u00a022<\/span><\/a>). Confined aquifers contain an <em>additional component of pressure head<\/em> that causes the water in a well to rise above the top of the aquifer unit. The low\u2011permeability units are called <em>confining layers<\/em>.\r\n\r\n<\/div>\r\n<div class=\"bc-figure figure\" data-custom-style=\"Captioned Figure\"><img style=\"display: block; margin-left: auto; margin-right: auto; ;width: 5.83333in; height: 2.81697in;\" src=\"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-content\/uploads\/sites\/42\/2026\/05\/file24-2.jpg\" alt=\"\" \/>\r\n<div class=\"bc-figcaption figcaption\">\r\n<div data-custom-style=\"Image Caption\">\r\n<div class=\"figcaption-text\">\r\n\r\n<span id=\"Xbdd870ff72fe07ce1a49541d44d33204f5160c5\" class=\"anchor\"><strong>Figure\u00a022\u00a0\u2011\u00a0<\/strong>Interbedded sand and clay units creating both confined and unconfined aquifers. The bent lines on the left indicate recharge to the outcrop of the confined aquifer.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nThis additional component of pressure head (A in <a href=\"#Xbdd870ff72fe07ce1a49541d44d33204f5160c5\"><span data-custom-style=\"Hyperlink\">Figure\u00a022<\/span><\/a>) occurs because water enters at a higher elevation in the recharge area and, even though groundwater loses energy as it flows to the point in the aquifer where the well is completed, the remaining energy in the water is greater than that of the water in the overlying unconfined aquifer.\r\n\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nThis is the same as what we see in the Darcy tube\u2014the reservoir to the left represents the recharge area, and the change in head from one manometer to the other represents the potentiometric (or piezometric) surface (<a href=\"#X8a2419fdd6a5710262f1b8c4f9740ef530a7b24\"><span data-custom-style=\"Hyperlink\">Figure\u00a023<\/span><\/a>).\r\n\r\n<\/div>\r\n<div class=\"bc-figure figure\" data-custom-style=\"Captioned Figure\"><img style=\"display: block; margin-left: auto; margin-right: auto; ;width: 5.0717in; height: 2.84899in;\" src=\"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-content\/uploads\/sites\/42\/2026\/05\/file25-2.jpg\" alt=\"A diagram of a cylindrical object with a straight line Description automatically generated\" \/>\r\n<div class=\"bc-figcaption figcaption\">\r\n<div data-custom-style=\"Image Caption\">\r\n<div class=\"figcaption-text\">\r\n\r\n<span id=\"X8a2419fdd6a5710262f1b8c4f9740ef530a7b24\" class=\"anchor\"><strong>Figure\u00a023\u00a0\u2011\u00a0<\/strong>Darcy tube experiment.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nIn addition to unconfined and confined aquifers, there are also perched aquifers, which are saturated zones above isolated low\u2011permeability units (like clay lenses) as illustrated on the right end of <a href=\"#Xbdd870ff72fe07ce1a49541d44d33204f5160c5\"><span data-custom-style=\"Hyperlink\">Figure\u00a022<\/span><\/a>. These are unconfined aquifers that are not laterally continuous.\r\n\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nThe important concept established here is that there are two types of groundwater conditions. Unconfined conditions exist where the groundwater is not restricted by a confining layer and the pore fluid pressures at the top of saturation are equal to atmospheric pressure. Confined conditions exist where there is a confining layer and the water under the confining layer has an additional component of pressure that causes it to rise in a well to a point above the top of the confined aquifer. This point represents a point on the potentiometric (or piezometric) surface of the confined aquifer.\r\n\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nThe reason we bring this up now is because the storage parameters that characterize an aquifer depend on the aquifer condition. Now we move on to a discussion of storage parameters, focusing first on storage of water in a confined aquifer.\r\n\r\n<\/div>\r\n<\/div>","rendered":"<div id=\"aquifers-1\" class=\"level1\">\n<div data-custom-style=\"First Paragraph\">\n<p>Before we discuss storage parameters, let\u2019s review what an aquifer is and discuss aquifer conditions and the occurrence of groundwater in nature.<\/p>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>An aquifer is a geologic unit that can store and transmit enough water to supply a well. The factors that determine if a geologic unit is an aquifer include:<\/p>\n<\/div>\n<ul>\n<li>the formation must be capable of storing sufficient volumes of water;<\/li>\n<li>the rock or sediments in the aquifer must have a sufficiently high permeability that flow to a well or spring can be maintained; and<\/li>\n<li>the aquifer must have sufficient areal extent and saturated thickness to supply water to the well.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>If the aquifer unit is close to the Earth\u2019s surface, and the sediments that comprise it are relatively permeable, the primary source of recharge to the aquifer will probably be infiltration of groundwater from precipitation that falls directly on the ground. In this case, a water table will form, and the aquifer is called a water\u2011table aquifer or an unconfined aquifer (<a href=\"#Xaa5c47feed27d083ce78d9f7d7461abdad6c347\"><span data-custom-style=\"Hyperlink\">Figure\u00a021<\/span><\/a>). In a water\u2011table aquifer the water table is defined as the point where the pore\u2011water pressure\u00ad and the atmospheric pressure are equal. Under those conditions, water in a well will rise to the point where the water table was encountered during drilling.<\/p>\n<\/div>\n<div class=\"bc-figure figure\" data-custom-style=\"Captioned Figure\"><img decoding=\"async\" style=\"display: block; margin-left: auto; margin-right: auto; ;width: 5.83333in; height: 2.92997in;\" src=\"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-content\/uploads\/sites\/42\/2026\/05\/file23-2.png\" alt=\"\" \/><\/p>\n<div class=\"bc-figcaption figcaption\">\n<div data-custom-style=\"Image Caption\">\n<div class=\"figcaption-text\">\n<p><span id=\"Xaa5c47feed27d083ce78d9f7d7461abdad6c347\" class=\"anchor\"><strong>Figure\u00a021\u00a0\u2011\u00a0<\/strong>Unconfined aquifer.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>If the regional geology consists of alternating layers of low\u2011 and high\u2011permeability units (e.g., interbedded sand and clay), then the permeable layers that are overlain by the low\u2011permeability units can form confined aquifers (<a href=\"#Xbdd870ff72fe07ce1a49541d44d33204f5160c5\"><span data-custom-style=\"Hyperlink\">Figure\u00a022<\/span><\/a>). Confined aquifers contain an <em>additional component of pressure head<\/em> that causes the water in a well to rise above the top of the aquifer unit. The low\u2011permeability units are called <em>confining layers<\/em>.<\/p>\n<\/div>\n<div class=\"bc-figure figure\" data-custom-style=\"Captioned Figure\"><img decoding=\"async\" style=\"display: block; margin-left: auto; margin-right: auto; ;width: 5.83333in; height: 2.81697in;\" src=\"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-content\/uploads\/sites\/42\/2026\/05\/file24-2.jpg\" alt=\"\" \/><\/p>\n<div class=\"bc-figcaption figcaption\">\n<div data-custom-style=\"Image Caption\">\n<div class=\"figcaption-text\">\n<p><span id=\"Xbdd870ff72fe07ce1a49541d44d33204f5160c5\" class=\"anchor\"><strong>Figure\u00a022\u00a0\u2011\u00a0<\/strong>Interbedded sand and clay units creating both confined and unconfined aquifers. The bent lines on the left indicate recharge to the outcrop of the confined aquifer.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>This additional component of pressure head (A in <a href=\"#Xbdd870ff72fe07ce1a49541d44d33204f5160c5\"><span data-custom-style=\"Hyperlink\">Figure\u00a022<\/span><\/a>) occurs because water enters at a higher elevation in the recharge area and, even though groundwater loses energy as it flows to the point in the aquifer where the well is completed, the remaining energy in the water is greater than that of the water in the overlying unconfined aquifer.<\/p>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>This is the same as what we see in the Darcy tube\u2014the reservoir to the left represents the recharge area, and the change in head from one manometer to the other represents the potentiometric (or piezometric) surface (<a href=\"#X8a2419fdd6a5710262f1b8c4f9740ef530a7b24\"><span data-custom-style=\"Hyperlink\">Figure\u00a023<\/span><\/a>).<\/p>\n<\/div>\n<div class=\"bc-figure figure\" data-custom-style=\"Captioned Figure\"><img decoding=\"async\" style=\"display: block; margin-left: auto; margin-right: auto; ;width: 5.0717in; height: 2.84899in;\" src=\"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-content\/uploads\/sites\/42\/2026\/05\/file25-2.jpg\" alt=\"A diagram of a cylindrical object with a straight line Description automatically generated\" \/><\/p>\n<div class=\"bc-figcaption figcaption\">\n<div data-custom-style=\"Image Caption\">\n<div class=\"figcaption-text\">\n<p><span id=\"X8a2419fdd6a5710262f1b8c4f9740ef530a7b24\" class=\"anchor\"><strong>Figure\u00a023\u00a0\u2011\u00a0<\/strong>Darcy tube experiment.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>In addition to unconfined and confined aquifers, there are also perched aquifers, which are saturated zones above isolated low\u2011permeability units (like clay lenses) as illustrated on the right end of <a href=\"#Xbdd870ff72fe07ce1a49541d44d33204f5160c5\"><span data-custom-style=\"Hyperlink\">Figure\u00a022<\/span><\/a>. These are unconfined aquifers that are not laterally continuous.<\/p>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>The important concept established here is that there are two types of groundwater conditions. Unconfined conditions exist where the groundwater is not restricted by a confining layer and the pore fluid pressures at the top of saturation are equal to atmospheric pressure. Confined conditions exist where there is a confining layer and the water under the confining layer has an additional component of pressure that causes it to rise in a well to a point above the top of the confined aquifer. This point represents a point on the potentiometric (or piezometric) surface of the confined aquifer.<\/p>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>The reason we bring this up now is because the storage parameters that characterize an aquifer depend on the aquifer condition. Now we move on to a discussion of storage parameters, focusing first on storage of water in a confined aquifer.<\/p>\n<\/div>\n<\/div>\n","protected":false},"author":8,"menu_order":15,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1142","chapter","type-chapter","status-publish","hentry"],"part":1480,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1142","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\/1142\/revisions"}],"predecessor-version":[{"id":1720,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1142\/revisions\/1720"}],"part":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/parts\/1480"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1142\/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=1142"}],"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=1142"},{"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=1142"},{"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=1142"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}