{"id":1339,"date":"2026-05-12T21:47:35","date_gmt":"2026-05-12T21:47:35","guid":{"rendered":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/part-1-fundamentals\/"},"modified":"2026-05-13T03:27:37","modified_gmt":"2026-05-13T03:27:37","slug":"part-1-fundamentals","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/part-1-fundamentals\/","title":{"raw":"18.1 Part 1: Fundamentals","rendered":"18.1 Part 1: Fundamentals"},"content":{"raw":"<div id=\"part-1-fundamentals\" class=\"level1\">\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSection 1 contains an overview of the hydrologic cycle and definitions of some of the terms used in the book, including the term aquifer (a subsurface formation that is capable of yielding economically viable volumes of water to a well). This section also discusses systems and natural cycles and helps set the stage for how groundwater systems are studied and understood.\r\n\r\n<\/div>\r\n<div data-custom-style=\"Body Text\">\r\n\r\nSection 2 discusses the distribution of energy in the subsurface and how variations in fluid energy drive the movement of water. The key concepts presented in Section 2 are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>All the components of fluid energy at a point in a groundwater flow system can be collectively quantified by a length \u2013 more specifically, a water level elevation (for a water of uniform density) above a datum \u2013 called head.<\/li>\r\n \t<li>Head varies from point to point within an aquifer system. The variation in head with respect to distance is called the hydraulic gradient.<\/li>\r\n \t<li>Fluid flow in the subsurface is driven by the hydraulic gradient; that is, fluid moves from areas of higher fluid energy (or head) to areas of lower energy.<\/li>\r\n \t<li>Hydraulic gradient exists in three dimensions and does not necessarily mimic or reflect the slope or orientation of the ground surface \u2013 in other words, groundwater does not necessarily flow \u201cdown hill\u201d.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSection 3 describes the properties of the media (i.e., rock and sediments) through which groundwater flows, including quantitative definitions of the basic parameters that affect the occurrence and movement of fluid in groundwater systems. The key concepts in this section are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>Geologic materials contain some fraction of empty (i.e., pore) space. The fraction of pore space by volume is called the porosity.<\/li>\r\n \t<li>The flow of a fluid in a porous media \u2013 either as a velocity in one dimension or a volumetric rate through a cross-sectional area of aquifer \u2013 is linearly proportional to the hydraulic gradient. The constant of proportionality is called permeability.<\/li>\r\n \t<li>Permeability is a function of both the properties of the media (the sizes and shapes of pores as well as the degree of pore connectivity) and properties of the fluid (density and viscosity).<\/li>\r\n \t<li>If the fluid is assumed to be water, then the properties of water are incorporated into the permeability term and it is called hydraulic conductivity.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSection 4 brings together concepts from Sections 2 and 3 to develop a quantitative framework for understanding groundwater systems based on a few general equations. The key concepts are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>Groundwater flow is governed by an empirically derived equation called Darcy\u2019s law, which states that the volumetric discharge of groundwater though a cross-sectional area of aquifer is linearly proportional to the hydraulic gradient with the hydraulic conductivity representing the constant of proportionality.<\/li>\r\n \t<li>For three-dimensional aquifers, groundwater flow is governed by a three-dimensional second-order partial differential equation that describes the divergence of flux and transient changes in storage at each point in a groundwater flow system.<\/li>\r\n \t<li>The divergence of flux is controlled by Darcy\u2019s law, thus it is dependent upon hydraulic gradient and hydraulic conductivity.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSection 5 builds on concepts presented in Section 4 to provide a quantitative overview of the parameters that control storage of water within an aquifer. The key concepts are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>Storage is only relevant to transient conditions when water levels change.<\/li>\r\n \t<li>Storage parameters are quantitative values that relate the change in volume of water in the subsurface to the change in water level and fluid energy. The overall storage parameter for an aquifer is the storativity.<\/li>\r\n \t<li>Confined aquifers contain an additional component of fluid pressure that causes the water level in a well to rise above the top of the aquifer.<\/li>\r\n \t<li>For confined aquifers, removal of water from the aquifer is associated with changes in fluid pressure that in turn cause change in pore volume. The storage parameter that describes elastic changes in the aquifer is the specific storage, and storativity is the product of the specific storage and the aquifer thickness.<\/li>\r\n \t<li>Unconfined aquifers do not have an additional component of fluid pressure; therefore, water level in a well rises to roughly the same elevation as the water table.<\/li>\r\n \t<li>For unconfined aquifers, removal of water is associated with drainage of pores in the aquifer. The storativity of an unconfined aquifer is thus controlled by the effective porosity of the aquifer.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSection 6 continues to build on the quantitative treatment of groundwater flow systems by introducing the difference between steady-state and transient conditions and presenting basic equations of groundwater flow and methods for solving them. The key concepts are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>Steady state means that the flux of groundwater into a given volume of a groundwater system is equal to flux out of that volume. For this condition, storage does not change, and storage parameters are not relevant to evaluation of the system.<\/li>\r\n \t<li>In transient conditions the flux into a volume of the system is not equal to flux out of the volume. For this condition, storage changes with time and storage parameters are relevant to evaluation of the system.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nThe concepts covered in the first six sections deal with general hydrogeologic situations. Section 7 then introduces a particularly important situation in hydrogeology; specifically, the flow of subsurface fluids to a well. Key concepts are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>A well is a hole constructed for the purpose of accessing fluid in the subsurface.<\/li>\r\n \t<li>Pumping fluid from, or injecting fluid into, a well results in a radial flow field around the well with flow lines either converging on or diverging from the well.<\/li>\r\n \t<li>Changes in head in a confined aquifer due to pumping from, or injecting into, a well are governed by the Theis equation.<\/li>\r\n \t<li>The Theis equation can be used to make predictions about how heads will vary in an aquifer in response to pumping or injection.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSection 8 builds on Section 7 by presenting the fundamentals of aquifer testing. Key concepts are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>The Theis equation, and variations of it, can be used to estimate aquifer parameters by pumping a well at a known rate and solving for the values of permeability and storage needed to produce the observed changes in water levels around the well.<\/li>\r\n \t<li>Aquifer responses to pumping occur in predictable ways that can be used to identify aquifer boundaries and leaky confining units.<\/li>\r\n \t<li>Aquifer testing can also be used to evaluate the future performance of the well and to estimate the sustainability long-term pumping.<\/li>\r\n<\/ul>\r\n<div data-custom-style=\"First Paragraph\">\r\n\r\nSections 9 and 10 expand on the scale of consideration to include study of groundwater flow systems at local, intermediate, and regional scale. Key concepts are as follows.\r\n\r\n<\/div>\r\n<ul>\r\n \t<li>Groundwater flow systems have physical boundaries that affect groundwater flow at various scales.<\/li>\r\n \t<li>Groundwater flow systems are dynamic systems that have areas and mechanisms that allow water to enter the system (i.e., recharge) and natural areas and mechanisms that allow water to leave the system (i.e., discharge).<\/li>\r\n \t<li>Variation of aquifer parameters influence the rate and direction of groundwater flow in aquifers.<\/li>\r\n \t<li>Aquifer studies can be used to develop a conceptual hydrogeological model, which is a summary of the relevant aspects of an aquifer. The conceptual hydrogeological model can be the basis for a quantitative computer-based model.<\/li>\r\n \t<li>Quantitative computer-based models are commonly used to make predictions about aquifer behavior (e.g., changes in water levels or reductions in spring flows in response to pumping from wells). These models are also useful for developing a better understanding of aquifers and for evaluating uncertainty in associated with aquifer characterization.<\/li>\r\n<\/ul>\r\n<\/div>","rendered":"<div id=\"part-1-fundamentals\" class=\"level1\">\n<div data-custom-style=\"First Paragraph\">\n<p>Section 1 contains an overview of the hydrologic cycle and definitions of some of the terms used in the book, including the term aquifer (a subsurface formation that is capable of yielding economically viable volumes of water to a well). This section also discusses systems and natural cycles and helps set the stage for how groundwater systems are studied and understood.<\/p>\n<\/div>\n<div data-custom-style=\"Body Text\">\n<p>Section 2 discusses the distribution of energy in the subsurface and how variations in fluid energy drive the movement of water. The key concepts presented in Section 2 are as follows.<\/p>\n<\/div>\n<ul>\n<li>All the components of fluid energy at a point in a groundwater flow system can be collectively quantified by a length \u2013 more specifically, a water level elevation (for a water of uniform density) above a datum \u2013 called head.<\/li>\n<li>Head varies from point to point within an aquifer system. The variation in head with respect to distance is called the hydraulic gradient.<\/li>\n<li>Fluid flow in the subsurface is driven by the hydraulic gradient; that is, fluid moves from areas of higher fluid energy (or head) to areas of lower energy.<\/li>\n<li>Hydraulic gradient exists in three dimensions and does not necessarily mimic or reflect the slope or orientation of the ground surface \u2013 in other words, groundwater does not necessarily flow \u201cdown hill\u201d.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>Section 3 describes the properties of the media (i.e., rock and sediments) through which groundwater flows, including quantitative definitions of the basic parameters that affect the occurrence and movement of fluid in groundwater systems. The key concepts in this section are as follows.<\/p>\n<\/div>\n<ul>\n<li>Geologic materials contain some fraction of empty (i.e., pore) space. The fraction of pore space by volume is called the porosity.<\/li>\n<li>The flow of a fluid in a porous media \u2013 either as a velocity in one dimension or a volumetric rate through a cross-sectional area of aquifer \u2013 is linearly proportional to the hydraulic gradient. The constant of proportionality is called permeability.<\/li>\n<li>Permeability is a function of both the properties of the media (the sizes and shapes of pores as well as the degree of pore connectivity) and properties of the fluid (density and viscosity).<\/li>\n<li>If the fluid is assumed to be water, then the properties of water are incorporated into the permeability term and it is called hydraulic conductivity.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>Section 4 brings together concepts from Sections 2 and 3 to develop a quantitative framework for understanding groundwater systems based on a few general equations. The key concepts are as follows.<\/p>\n<\/div>\n<ul>\n<li>Groundwater flow is governed by an empirically derived equation called Darcy\u2019s law, which states that the volumetric discharge of groundwater though a cross-sectional area of aquifer is linearly proportional to the hydraulic gradient with the hydraulic conductivity representing the constant of proportionality.<\/li>\n<li>For three-dimensional aquifers, groundwater flow is governed by a three-dimensional second-order partial differential equation that describes the divergence of flux and transient changes in storage at each point in a groundwater flow system.<\/li>\n<li>The divergence of flux is controlled by Darcy\u2019s law, thus it is dependent upon hydraulic gradient and hydraulic conductivity.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>Section 5 builds on concepts presented in Section 4 to provide a quantitative overview of the parameters that control storage of water within an aquifer. The key concepts are as follows.<\/p>\n<\/div>\n<ul>\n<li>Storage is only relevant to transient conditions when water levels change.<\/li>\n<li>Storage parameters are quantitative values that relate the change in volume of water in the subsurface to the change in water level and fluid energy. The overall storage parameter for an aquifer is the storativity.<\/li>\n<li>Confined aquifers contain an additional component of fluid pressure that causes the water level in a well to rise above the top of the aquifer.<\/li>\n<li>For confined aquifers, removal of water from the aquifer is associated with changes in fluid pressure that in turn cause change in pore volume. The storage parameter that describes elastic changes in the aquifer is the specific storage, and storativity is the product of the specific storage and the aquifer thickness.<\/li>\n<li>Unconfined aquifers do not have an additional component of fluid pressure; therefore, water level in a well rises to roughly the same elevation as the water table.<\/li>\n<li>For unconfined aquifers, removal of water is associated with drainage of pores in the aquifer. The storativity of an unconfined aquifer is thus controlled by the effective porosity of the aquifer.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>Section 6 continues to build on the quantitative treatment of groundwater flow systems by introducing the difference between steady-state and transient conditions and presenting basic equations of groundwater flow and methods for solving them. The key concepts are as follows.<\/p>\n<\/div>\n<ul>\n<li>Steady state means that the flux of groundwater into a given volume of a groundwater system is equal to flux out of that volume. For this condition, storage does not change, and storage parameters are not relevant to evaluation of the system.<\/li>\n<li>In transient conditions the flux into a volume of the system is not equal to flux out of the volume. For this condition, storage changes with time and storage parameters are relevant to evaluation of the system.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>The concepts covered in the first six sections deal with general hydrogeologic situations. Section 7 then introduces a particularly important situation in hydrogeology; specifically, the flow of subsurface fluids to a well. Key concepts are as follows.<\/p>\n<\/div>\n<ul>\n<li>A well is a hole constructed for the purpose of accessing fluid in the subsurface.<\/li>\n<li>Pumping fluid from, or injecting fluid into, a well results in a radial flow field around the well with flow lines either converging on or diverging from the well.<\/li>\n<li>Changes in head in a confined aquifer due to pumping from, or injecting into, a well are governed by the Theis equation.<\/li>\n<li>The Theis equation can be used to make predictions about how heads will vary in an aquifer in response to pumping or injection.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>Section 8 builds on Section 7 by presenting the fundamentals of aquifer testing. Key concepts are as follows.<\/p>\n<\/div>\n<ul>\n<li>The Theis equation, and variations of it, can be used to estimate aquifer parameters by pumping a well at a known rate and solving for the values of permeability and storage needed to produce the observed changes in water levels around the well.<\/li>\n<li>Aquifer responses to pumping occur in predictable ways that can be used to identify aquifer boundaries and leaky confining units.<\/li>\n<li>Aquifer testing can also be used to evaluate the future performance of the well and to estimate the sustainability long-term pumping.<\/li>\n<\/ul>\n<div data-custom-style=\"First Paragraph\">\n<p>Sections 9 and 10 expand on the scale of consideration to include study of groundwater flow systems at local, intermediate, and regional scale. Key concepts are as follows.<\/p>\n<\/div>\n<ul>\n<li>Groundwater flow systems have physical boundaries that affect groundwater flow at various scales.<\/li>\n<li>Groundwater flow systems are dynamic systems that have areas and mechanisms that allow water to enter the system (i.e., recharge) and natural areas and mechanisms that allow water to leave the system (i.e., discharge).<\/li>\n<li>Variation of aquifer parameters influence the rate and direction of groundwater flow in aquifers.<\/li>\n<li>Aquifer studies can be used to develop a conceptual hydrogeological model, which is a summary of the relevant aspects of an aquifer. The conceptual hydrogeological model can be the basis for a quantitative computer-based model.<\/li>\n<li>Quantitative computer-based models are commonly used to make predictions about aquifer behavior (e.g., changes in water levels or reductions in spring flows in response to pumping from wells). These models are also useful for developing a better understanding of aquifers and for evaluating uncertainty in associated with aquifer characterization.<\/li>\n<\/ul>\n<\/div>\n","protected":false},"author":8,"menu_order":82,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1339","chapter","type-chapter","status-publish","hentry"],"part":1518,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1339","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\/1339\/revisions"}],"predecessor-version":[{"id":1787,"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1339\/revisions\/1787"}],"part":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/parts\/1518"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/wp-json\/pressbooks\/v2\/chapters\/1339\/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=1339"}],"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=1339"},{"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=1339"},{"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=1339"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}