{"id":618,"date":"2019-06-11T14:52:02","date_gmt":"2019-06-11T14:52:02","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/14-1-groundwater-and-aquifers\/"},"modified":"2021-12-08T18:24:46","modified_gmt":"2021-12-08T18:24:46","slug":"14-1-groundwater-and-aquifers","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/14-1-groundwater-and-aquifers\/","title":{"raw":"14.1 Groundwater and Aquifers","rendered":"14.1 Groundwater and Aquifers"},"content":{"raw":"Groundwater is stored in the open spaces within rocks and within unconsolidated sediments. Rocks and sediments near the surface are under less pressure than those at significant depth and therefore tend to have more open space. For this reason, and because it\u2019s expensive to drill deep wells, most of the groundwater that is accessed by individual users is within the first 100 metres of the surface. Some municipal, agricultural, and industrial groundwater users get their water from greater depth, but deeper groundwater tends to be of lower quality than shallow groundwater, so there is a limit as to how deep we can go.\r\n\r\n[pb_glossary id=\"1760\"]Porosity[\/pb_glossary]<\/strong> is the percentage of open space within an unconsolidated sediment or a rock. Primary porosity is represented by the spaces between grains in a sediment or sedimentary rock. Secondary porosity is porosity that has developed after the rock has formed. It can include fracture porosity\u2014space within fractures in any kind of rock. Some volcanic rock has a special type of porosity related to vesicles, and some limestone has extra porosity related to cavities within fossils.\r\n\r\nPorosity is expressed as a percentage calculated from the volume of open space in a rock compared with the total volume of rock. The typical ranges in porosity of a number of different geological materials are shown in Figure 14.1.1. Unconsolidated sediments tend to have higher porosity than consolidated ones because they have no cement, and most have not been strongly compressed. Finer-grained materials (e.g., silt and clay) tend to have greater porosity\u2014some as high as 70%\u2014than coarser materials (e.g., gravel). Primary porosity tends to be higher in well-sorted sediments compared to poorly sorted sediments, where there is a range of smaller particles to fill the spaces made by the larger particles. Glacial till, which has a wide range of grain sizes and is typically formed under compression beneath glacial ice, has relatively low porosity.\r\n\r\n[caption id=\"attachment_614\" align=\"alignright\" width=\"600\"]\"\"<\/a> Figure 14.1.1 Variations in porosity of unconsolidated materials (in red) and rocks (in blue). [Image Description][\/caption]Consolidation and cementation during the process of lithification of unconsolidated sediments into sedimentary rocks reduces primary porosity. Sedimentary rocks generally have porosities in the range of 10% to 30%, some of which may be secondary (fracture) porosity. The grain size, sorting, compaction, and degree of cementation of the rocks all influence primary porosity. For example, poorly sorted and well-cemented sandstone and well-compressed mudstone can have very low porosity. Igneous or metamorphic rocks have the lowest primary porosity because they commonly form at depth and have interlocking crystals. Most of their porosity comes in the form of secondary porosity in fractures. Of the consolidated rocks, well-fractured volcanic rocks and limestone that has cavernous openings produced by dissolution have the highest potential porosity, while intrusive igneous and metamorphic rocks, which formed under great pressure, have the lowest.\r\n\r\nPorosity is a measure of how much water can be stored in geological materials. Almost all rocks contain some porosity and therefore contain groundwater. Groundwater is found under your feet and everywhere on the planet. Considering that sedimentary rocks and unconsolidated sediments cover about 75% of the continental crust with an average thickness of a few hundred metres, and that they are likely to have around 20% porosity on average, it is easy to see that a huge volume of water can be stored in the ground.\r\n\r\nPorosity is a description of how much space there could be to hold water under the ground, while\u00a0[pb_glossary id=\"1761\"]permeability[\/pb_glossary]<\/strong> describes how those pores are shaped and interconnected. This determines how easy it is for water to flow from one pore to the next. Larger pores mean there is less friction between flowing water and the sides of the pores. Smaller pores mean more friction along pore walls, and also more twists and turns for the water to have to flow-through. A permeable material has a greater number of larger, well-connected pores spaces, whereas an impermeable material has fewer, smaller pores that are poorly connected. Permeability is the most important variable in groundwater. Permeability describes how easily water can flow through the rock or unconsolidated sediment and how easy it will be to extract the water for our purposes. The characteristic of permeability of a geological material is quantified by geoscientists and engineers using a number of different units, but the most common is the [pb_glossary id=\"1762\"]hydraulic conductivity[\/pb_glossary]<\/strong>. The symbol used for hydraulic conductivity is K<\/em>. Although hydraulic conductivity can be expressed in a range of different units, in this book we will always use metres per second.\r\n\r\n[caption id=\"attachment_615\" align=\"alignright\" width=\"600\"]\"\"<\/a>
Figure 14.1.2 Variations in hydraulic conductivity (in metres per second) of unconsolidated materials (in red) and of rocks (in blue). [Image Description][\/caption]As shown on\u00a0 Figure 14.1.2 there is a wide range of permeability in geological materials from 10\u221212<\/sup> metres per second (0.000000000001 metres per second) to around 1 metre per second. Unconsolidated materials are generally more permeable than the corresponding rocks (compare sand with sandstone, for example), and the coarser materials are much more permeable than the finer ones. The least permeable rocks are unfractured intrusive igneous and metamorphic rocks, followed by unfractured mudstone, sandstone, and limestone. The permeability of sandstone can vary widely depending on the degree of sorting and the amount of cement that is present. Fractured igneous and metamorphic rocks, and especially fractured volcanic rocks, can be highly permeable, as can limestone that has been dissolved along fractures and bedding planes to create solutional openings.\r\n
\r\n

Why is clay porous but not permeable?<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\n[caption id=\"attachment_616\" align=\"alignright\" width=\"550\"]\"\"<\/a> Figure 14.1.3 Porosity versus permeability in sand and clay[\/caption]\r\n\r\nBoth sand and clay deposits (and sandstone and mudstone) are quite porous (30% to 50% for sand and 40% to 70% for silt and clay), but while sand can be quite permeable, clay and mudstone are not.\r\n\r\nThe surface of most silicate mineral grains has a slight negative charge due to imperfections in the mineral structure. Water (H2<\/sub>O) is a polar molecule. This means that while it has no overall electrical charge, one side of the molecule has a slight positive charge (the side with the two hydrogens), compared to a slight negative charge on the other side. Water is strongly attracted to all mineral grains and water within that bound water layer (a few microns around each grain) is not able to move and flow along with the rest of the groundwater.\r\n\r\nIn the two diagrams of Figure 14.1.3 the bound water is represented by medium blue lines around each grain, and the water that is not bound, and can flow, is shown in light blue. In the sand, there is still a lot of water that is able to move through the sediment, but in the clay deposit almost all of the water is held tightly to the grains and this reduces the permeability.\r\n\r\n<\/div>\r\n<\/div>\r\nWe have now seen that there is a wide range of porosity in geological materials and an even wider range of permeability. Groundwater exists everywhere there is porosity. However, whether that groundwater is able to flow in significant quantities depends on the permeability. An [pb_glossary id=\"1763\"]aquifer[\/pb_glossary]<\/strong> is defined as a body of rock or unconsolidated sediment that has sufficient permeability to allow water to flow through it. Unconsolidated materials like gravel, sand, and even silt make relatively good aquifers, as do rocks like sandstone. Other rocks can be good aquifers if they are well fractured. An [pb_glossary id=\"1764\"]aquitard[\/pb_glossary]<\/strong> is a body that does not allow transmission of a significant amount of water, such as a clay, a till, or a poorly fractured igneous or metamorphic rock. These are relative terms, not absolute, and are usually defined based on someone\u2019s desire to pump groundwater; what is an aquifer to someone who does not need a lot of water, may be an aquitard to someone else who does. An aquifer that is exposed at the ground surface is called an [pb_glossary id=\"1765\"]unconfined aquifer[\/pb_glossary]<\/strong>. An aquifer where there is a lower permeability material between the aquifer and the ground surface is known as a [pb_glossary id=\"1766\"]confined aquifer[\/pb_glossary]<\/strong>, and the aquitard separating ground surface and the aquifer is known as the [pb_glossary id=\"1767\"]confining layer[\/pb_glossary]<\/strong>.\r\n\r\n[caption id=\"attachment_617\" align=\"alignright\" width=\"600\"]\"\"<\/a> Figure 14.1.4 A cross-section showing materials that might serve as aquifers and confining layers. The relative permeabilities are denoted by hydraulic conductivity (K in metres per second). The pink rock is granite; the other layers are various sedimentary layers. [Image Description][\/caption]Figure 14.1.4 shows a cross-section of a series of rocks and unconsolidated materials, some of which might serve as aquifers and others as aquitards or confining layers. The granite is much less permeable than the other materials, and so is an aquitard in this context. The yellow layer is very permeable and would make an ideal aquifer. The overlying grey layer is a confining layer.\r\n\r\nThe upper buff-coloured layer (K<\/em> = 10\u22122<\/sup> metres per second) does not have a confining layer and is an unconfined aquifer. The yellow layer (K<\/em> = 10-1<\/sup> metres per second) is \u201cconfined\u201d by the confining layer (K<\/em> = 10\u22124<\/sup> metres per second), and is a confined aquifer. The confined aquifer gets most of its water from the upper part of the hill where it is exposed at the surface, and relatively little by seepage through the fine silt layer.\r\n

Image Descriptions<\/h3>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
<\/a>Figure 14.1.1 image description part 1: Percent porosity for solid rocks<\/caption>\r\n
Solid rocks<\/th>\r\nPercent porosity (Unfractured)<\/th>\r\nPercent porosity (Fractured)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Volcanic Rock<\/td>\r\n0% to roughly 20%<\/td>\r\nRoughly 20% to 50%<\/td>\r\n<\/tr>\r\n
Intrusive igneous and metamorphic rocks<\/td>\r\n0% to 5%<\/td>\r\n5% to 10%<\/td>\r\n<\/tr>\r\n
Limestone with cavernous openings<\/td>\r\nN\/A<\/td>\r\n0% to 50%<\/td>\r\n<\/tr>\r\n
Limestone without cavernous openings<\/td>\r\n0% to 20%<\/td>\r\nN\/A<\/td>\r\n<\/tr>\r\n
Mudstone<\/td>\r\n0% to 30%<\/td>\r\nN\/A<\/td>\r\n<\/tr>\r\n
Sandstone<\/td>\r\n0% to roughly 10%<\/td>\r\nRoughly 10% to 30%<\/td>\r\n<\/tr>\r\n
Conglomerate<\/td>\r\n10% to 30%<\/td>\r\nN\/A<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Figure 14.1.1 image description part 2: Percent porosity of unconsolidated materials<\/caption>\r\n
Unconsolidated Materials<\/th>\r\nPercent porosity<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Glacial Till<\/td>\r\n11% to 20%<\/td>\r\n<\/tr>\r\n
Silt and clay<\/td>\r\n36% to 70%<\/td>\r\n<\/tr>\r\n
Sand<\/td>\r\n30% to 50%<\/td>\r\n<\/tr>\r\n
Gravel<\/td>\r\n24% to 40%<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[Return to Figure 14.1.1]<\/a>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
<\/a>Figure 14.1.2 image description part 1: Variations in hydraulic conductivity in metres per second in solid rocks.<\/caption>\r\n
Material type<\/th>\r\nHydraulic conductivity (metres per second)<\/th>\r\n<\/tr>\r\n
Limestone with solutional openings<\/td>\r\n10 to the power of negative 5 to 10 to the power of negative 1.2 (10\u22125\u00a0<\/sup>to 10\u22121.2<\/sup>)<\/td>\r\n<\/tr>\r\n
Fractured volcanic rock<\/td>\r\n10 to the power of negative 6 to 10 to the power of negative 1.7 (10\u22126<\/sup> to 10\u22121.7<\/sup>)<\/td>\r\n<\/tr>\r\n
Fractured igneous and metamorphic rock<\/td>\r\n10 to the power of negative 7.5 to 10 to the power of negative 3.5 (10\u22127.5<\/sup> to 10\u22123.5<\/sup>)<\/td>\r\n<\/tr>\r\n
Limestone<\/td>\r\n10 to the power of negative 8.4 to 10 to the power of negative 4.9 (10\u22128.4<\/sup> to 10\u22124.9<\/sup>)<\/td>\r\n<\/tr>\r\n
Sandstone<\/td>\r\n10 to the power of negative 8.9 to 10 to the power of negative 5 (10\u22128.9<\/sup> to 10\u22125<\/sup>)<\/td>\r\n<\/tr>\r\n
Mudstone<\/td>\r\n10 to the power of negative 11.3 to 10 to the power of negative 8 (10\u221211.3<\/sup> to 10\u22128<\/sup>)<\/td>\r\n<\/tr>\r\n
Unfractured igneous and metamorphic rock<\/td>\r\n10 to the power of negative 12 to 10 to the power of negative 8.6 (10\u221212<\/sup> to 10\u22128.6<\/sup>)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Figure 14.1.2 image description part 2: Variations in hydraulic conductivity in metres per second in unconsolidated materials.<\/caption>\r\n
Material Type<\/th>\r\nHydraulic conductivity (metres per second)<\/th>\r\n<\/tr>\r\n
Clay<\/td>\r\n10 to the power of negative 10.6 to 10 to the power of negative 7.7 (10\u221210.6<\/sup> to 10\u22127.7<\/sup>)<\/td>\r\n<\/tr>\r\n
Glacial till<\/td>\r\n10 to the power of negative 10.4 to 10 to the power of negative 5.2 (10\u221210.4<\/sup> to 10\u2212<\/sup>5.2<\/sup>)<\/td>\r\n<\/tr>\r\n
Silt<\/td>\r\n10 to the power of negative 8 to 10 to the power of negative 4.3 (10\u22128 <\/sup>to 10\u22124.3<\/sup>)<\/td>\r\n<\/tr>\r\n
Silty sand<\/td>\r\n10 to the power of negative 6 to 10 to the power of negative 2.7 (10\u22126<\/sup> to 10\u22122.7<\/sup>)<\/td>\r\n<\/tr>\r\n
Sand<\/td>\r\n10 to the power of negative 5.2 to 10 to the power of negative 1.7 (10\u22125.2<\/sup> to 10\u22121.7<\/sup>)<\/td>\r\n<\/tr>\r\n
Gravel<\/td>\r\n10 to the power of negative 2.7 to 10 to the power of negative 0.1 (10\u22122.7<\/sup> to 10\u22120.1<\/sup>)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[Return to Figure 14.1.2]<\/a>\r\n\r\n<\/a>Figure 14.1.4 image description:<\/strong> The relative permeability of various sedimentary layers. Permeability is denoted by hydraulic conductivity (K = metres per second). The buff-coloured, top layer is an unconfined aquifer with K = 10\u22122<\/sup>. The grey-coloured second layer is a confining layer with K = 10\u22124<\/sup>. The yellow-coloured third layer is a confined aquifer with K = 10\u22121<\/sup>. The fourth layer is unlabelled, with K = 10\u22124<\/sup>. The bottom layer is granite, K = 10\u221210<\/sup>. [Return to Figure 14.1.4]<\/a>\r\n

Media Attributions<\/h3>\r\n