{"id":1123,"date":"2026-05-12T21:46:17","date_gmt":"2026-05-12T21:46:17","guid":{"rendered":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/porosity\/"},"modified":"2026-10-21T15:52:48","modified_gmt":"2026-10-21T15:52:48","slug":"porosity","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/porosity\/","title":{"raw":"3.2 Porosity","rendered":"3.2 Porosity"},"content":{"raw":"
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\r\n\r\nAll geologic materials have some amount of pore space, or empty space, in them. The term porosity (\u03c6) refers to the fraction of the total volume of a rock or sediment that is pore space. More rigorously, it is defined as the volume of the voids divided by the total volume as shown by Equation\u00a0(10)<\/span><\/a>.\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
<\/th>\r\n[latex]\\varphi = \\frac{V_{voids}}{V_{total}}[\/latex]<\/th>\r\n(10)<\/span><\/th>\r\n<\/tr>\r\n<\/thead>\r\n<\/table>\r\n
\r\n\r\nwhere (parameter dimensions are dark green font with mass as M, length as L, time as T):\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n\r\n\r\n
[latex]V_{voids}[\/latex]<\/th>\r\n[latex]=[\/latex]<\/th>\r\nvolume of the voids (L3<\/sup>)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
[latex]V_{total}[\/latex]<\/td>\r\n[latex]=[\/latex]<\/td>\r\ntotal volume of the sample (L3<\/sup>)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\n(The symbol n<\/em> is sometimes used as a variable for porosity.)\r\n\r\n<\/div>\r\n
\r\n\r\nPorosity is a dimensionless quantity and can be expressed as a percentage by multiplying the ratio by 100. We can also define a quantity called the void ratio<\/em> (e<\/em>), which is the volume of voids divided by the volume of solids, as expressed by Equation\u00a0(11)<\/span><\/a>.\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
<\/th>\r\n[latex]e = \\frac{V_{voids}}{V_{solids}}[\/latex]<\/th>\r\n(11)<\/span><\/th>\r\n<\/tr>\r\n<\/thead>\r\n<\/table>\r\n
\r\n\r\nwhere (parameter dimensions are dark green font with mass as M, length as L, time as T):\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
[latex]V_{solids}[\/latex]<\/th>\r\n[latex]=[\/latex]<\/th>\r\nvolume of the solids (L3<\/sup>)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<\/table>\r\n
\r\n\r\nIn nature, geologic materials are almost never completely dry, and the void space will always contain some volume of water. We can think about geologic materials as composed of multiple phases: a solid phase, a water phase, and a gas phase (Figure\u00a010<\/span><\/a>). In turn, the solid phase can be further divided into its individual mineral phases (Figure\u00a011<\/span><\/a>).\r\n\r\n<\/div>\r\n
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\r\n\r\nFigure\u00a010\u00a0\u2013\u00a0<\/strong>Graphic depiction of solids (i.e., minerals) and partially and fully saturated pore space in a rock.<\/span>\r\n\r\n<\/div><\/blockquote>\r\n<\/div>\r\n
\r\n\r\n\"A\r\n
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\r\n\r\nFigure\u00a011\u00a0\u2011\u00a0<\/strong>Phase content (by volume).<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n\r\nEach of the mineral phases in a rock or sediment sample has a volume and mass associated with it. Every fluid and every mineral has a density associated with it.\r\n\r\n<\/div>\r\n
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Density<\/h3>\r\n
\r\n\r\nDensity<\/em> is defined as the ratio of an object's mass to its volume with dimensions of (ML-3<\/sup>). We can therefore express it mathematically as shown by Equation (12)<\/span><\/a>\u00a0.\r\n\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n
<\/th>\r\n[latex]density = \\frac{mass}{volume}[\/latex]<\/th>\r\n(12)<\/span><\/th>\r\n<\/tr>\r\n<\/thead>\r\n<\/table>\r\n
\r\n\r\nOften, we use the symbol [latex]\\rho[\/latex] (i.e., the Greek letter rho<\/em>) for density. Table\u00a02<\/span><\/a> lists densities of some common substances.\r\n\r\n<\/div>\r\n
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\r\n\r\nTable\u00a02\u00a0\u2011\u00a0<\/strong>Densities of some common substances at room temperature and pressure.<\/span>\r\n\r\n<\/div><\/blockquote>\r\n<\/div>\r\n<\/colgroup>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
<\/td>\r\ng\/cm3<\/sup><\/strong><\/td>\r\nkg\/m3<\/sup><\/strong><\/td>\r\n<\/tr>\r\n
Air<\/strong><\/td>\r\n0.0012<\/td>\r\n1.2<\/td>\r\n<\/tr>\r\n
Water<\/strong><\/td>\r\n1.0<\/td>\r\n1,000<\/td>\r\n<\/tr>\r\n
Quartz<\/strong><\/td>\r\n2.65<\/td>\r\n2,650<\/td>\r\n<\/tr>\r\n
Aluminum<\/strong><\/td>\r\n2.70<\/td>\r\n2,700<\/td>\r\n<\/tr>\r\n
Iron<\/strong><\/td>\r\n7.86<\/td>\r\n7,860<\/td>\r\n<\/tr>\r\n
Gold<\/strong><\/td>\r\n19.3<\/td>\r\n19,300<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n\r\nDensity is not a constant; it varies with temperature and pressure. Generally:\r\n\r\n<\/div>\r\n