After going through the following text and graphics on aquifer and geology, research/discuss major aquifer systems in your own local area (e.g., state, province, county, or city), paying attention to aquifer types, extents, transmissivity/productivity, and the geological processes that produce the different types of aquifers. 

Major Aquifer Systems – Connections to Geology

Definition: An aquifer is a geologic formation, group of formations, or part of a formation that can store and transmit water fast enough to supply reasonable amounts of water to pumping wells and to springs.

We know from our lesson on the water cycle that water infiltrates the land surface almost everywhere, but aquifers do not exist uniformly.  They vary with the lithology (physical makeup) of underlying rocks, the stratigraphy (geometrical relation of sedimentary units), and structure (fractures and folds) of the near- surface geologic units.  Hydrogeologists often refer to hydrostratigraphic units when evaluating groundwater and they use these units to develop a conceptual model of how water moves in subsurface aquifers.  But aquifers may be adjacent to or bounded by aquitards or confining units that are geologic units that have relatively low permeability and transmit water slowly.  Therefore, it is also important to understand properties of non-aquifer materials.

This lesson will focus on understanding the connection between geologic processes and how they are responsible for creating the hydrostratigraphic units that are key to developing realistic and useable models of groundwater flow.   The most common geologic materials known to be aquifers include unconsolidated or semi-consolidated sand and gravel, lithified sandstones, limestone and dolomite, and, to a lesser extent, fractured igneous and metamorphic rock.  We begin an evaluation of modeling groundwater flow and transport of chemicals in groundwater by reviewing the type and locations of the world’s largest aquifers. 

Major aquifers underlie about one third of the earth’s surface (Figure 1) with the lower lying topographic areas, especially basins, being the most common setting for these aquifer systems.  Many aquifers cross national boundaries (Figure 2), a situation that can create conflict if groundwater pumping in one nation creates issues in the adjacent nation.  Similar conflicts also occur internally in some nations where groundwater use in state or local governmental units conflict with expected uses across a local boundary. 

Figure 1 – Generalized map of worldwide distribution of major aquifer systems and “complex but important” aquifers (Ground water and climate change - Scientific Figure on ResearchGate. (https://www.researchgate.net/figure/Simplified-version-of-a-global-groundwater-resources-map9-highlighting-the-locations-of_fig1_258807224 ) [accessed 4 Mar, 2020]

Figure 2 – Global transboundary aquifer map (International Groundwater Resources Assessment Centre (IGRAC) (https://www.un-igrac.org/resource/transboundary-aquifers-world-map-2015) [accessed 3/20/2020]

 Figures 1 and 2 illustrate how important groundwater is on a global scale but does not demonstrate the relation between geology and aquifer properties.  This can be accomplished by moving down scale from global to national. An eanoke us illustrated in Figure 3 for the United States.  This map classifies the aquifers into five lithologic types – unconsolidated and semi-consolidated sand and gravel, sandstone, sandstone and carbonate-rock, carbonate-rock, and igneous and metamorphic-rock aquifers.  Mostly, the aquifers have been named by either the geologic unit of which they are composed, such as the Marshall aquifer which is composed of the Marshall sandstone, or the geographic area under which they lie, such as the Ozark Plateaus aquifer system in the Ozark region of the Midwestern U.S.

Figure 3 – Principal aquifers of the United States, U.S. Geological Survey (https://groundwaterwatch.usgs.gov/compositemap.html)  [accessed 3/12/2020]

By comparing a geologic map with the aquifer map of the United States, it is possible to further demonstrate the geologic and aquifer connections.  The structural relations between rock units and aquifers also becomes apparent (Figure 4).  Although maps in Figures 3 and 4 are at slightly different scales, some of the prominent features of the aquifer map are identifiable on the geologic map.  A noticeable structural feature on this map extends from the Gulf of Mexico into the Canadian Interior Plains following a north-northwesterly trend.  This generally low-lying area is underlain by sedimentary rocks deposited between the Rocky Mountain system to the west, the Appalachian Highlands to the east, and rocks of the Canadian Shield to the northeast.  The most well-known aquifer in this area is the High Plains aquifer whose structure was formed by sediments eroded from the Rocky Mountains which was carried eastward and deposited by a network of streams.  Other large-scale aquifer systems were formed in the valleys of the Basin and Range province of the West, the interlayered basalt flows of the Northwest, the Coastal Plains of the Gulf and Atlantic coasts, and the sand and gravel aquifers formed from continental glaciers that covered the U.S. north of the Missouri and Ohio Rivers and New England west to the Rocky Mountains.  Without going into further detail about other large aquifers, suffice it to say that understanding the hydraulic properties of groundwater bearing rocks and soils begins with geology.

Figure 4 – Geologic map of North America (U.S. Geological Survey (https://www.usgs.gov/media/images/geologic-map-north-america) [accessed 3/12/20]

In North America and Northern Europe, aquifers in glacial deposits are important sources of water.  In the United States, about 41 million people rely on the glacial aquifer system for drinking water, making it the most used aquifer in the Nation.  Thick accumulations of glacial ice covered large parts of the northern hemisphere beginning about 2.6 million years ago.  The ice advanced and retreated during four major intervals, the last of which ended about 10,000 years before present.  Ice as much as a mile thick was effective at eroding large quantities of underlying rock, grinding it up, and re-depositing it 1) by streams fed by melting ice, 2) at the base of the ice mass, or 3) by simply dropping out of the ice as it melted.  The general extent of the ice sheets is illustrated in Figure 5.  In North America, the most southern extent of ice movement is marked by the Missouri and Ohio Rivers that formed along the margin of the ice sheet and carried away large volumes of water and sediment.  Aquifers also exist in the valleys of these and other smaller rivers because of the large sediment load.  So much water was tied up in ice that sea levels dropped by as much as 390 feet, which exposed new shoreline and had an impact on the formation of the coastal aquifer system.  

Figure 5 – Extent of glacial ice during the last glacial epoch (https://iceagenow.com/Ice-Age_Maps.htm) [accessed 3/12/20]

In this lesson, so far we have briefly discussed the importance of lithology and stratigraphy in forming aquifers.  In this discussion, we have only considered the primary pore space of the aquifers.  The igneous- and metamorphic-rock aquifers have very little primary pore space but can provide significant volumes of groundwater if they are fractured by a range of geologic processes.  This is also true, to a lesser degree, for sedimentary rocks and, to an even lesser degree, for unconsolidated aquifers.  A notable and important example of secondary openings in rock aquifers are fractures in rocks containing carbonate minerals that, when cracked by tectonic forces, can be enlarged by mildly acidic solutions of water which dissolves the carbonate minerals and widens these cracks, thus creating very productive aquifers.  In some cases, large cave systems are developed in these rock units.  A map illustrating the location of karst aquifers for the U.S. is shown in Figure 6. 

In the following lessons, more detailed examples of previously mentioned aquifer types will be discussed.  These lessons further illustrate the relations of geology to groundwater storage and flow.

Figure 6 – Karst aquifers in the United States.  Source U.S. Geological Survey (https://water.usgs.gov/ogw/karst/kig2002/jbe_map.html) [accessed 3/15/20]