Sandstone-Rock Aquifers
Sandstone-rock aquifers are close corollaries of the sand column that Henry Darcy used in his 1856 experiments with porous media flow. The resulting Darcy equation generally holds up well for sandstone aquifers, the major difference being that sandstones usually have lower porosity and permeability than sand because some of the pore space is filled with the lithifying material that creates rock rather than unconsolidated sand.
Figure 1 – Schematic showing grain size, pore space, and pore space from fractures (Meinzer, 1923).
- Well sorted sand or sandstone with impervious particles
- Poorly sorted sand or sandstone
- Well sorted sand or sandstone with porous particles
- Well sorted sand or sandstone with porosity reduced by lithification
- Rock weathered by solution
- Rock porous by fracturing
Discussion Questions:
After going through the following text and graphics, answer the following questions about sandstone aquifers and their relation to water resources:
1. Briefly summarize the underlying geologic processes that create sandstone aquifers found around the world
2. Study a sandstone aquifer in your country/state/province/local area. Briefly summarize its geomorphology / geologic origins and hydraulic characteristics (estimated porosity, storage, hydraulic conductivity)
3. Comment on the differences in hydraulic properties of a sandstone whose grains are porous compared to a sandstone whose grains are impervious
4. In what ways are sandstones whose grains are composed of calcite similar to chemically derived limestone?
Sandstones are composed of sand-sized particles that have been lithified to form solid rock. They are generally more permeable than rocks with smaller grain sizes such as siltstones and shales, and they are generally more porous than siltstones depending on roundness and grain sorting. Porosities generally range from 0.05 to 0.3, specific yields range from 0.03 to 0.15 , and hydraulic conductivities may range from about 0.001 to 10 feet per day.(Freeze and Cherry, 1979, and Rivera, 2014). Fractures and bedding plane partings can increase hydraulic conductivity substantially. Unlike some carbonate-rock aquifers, sandstone generally conforms to porous media flow assumptions unless highly fractured. Rock characteristics which have the potential to influence groundwater quantity, movement, and quality are: 1) porosity, 2) mineralogical composition, 3) bedding, and 4) faults, fractures, and joints.
Figure 2 – Sandstone-rock aquifers at or near land surface in the United States. Dark line indicates the limit of continental glaciation. (https://water.usgs.gov/ogw/aquiferbasics/sandstone.html) [accessed 3/27/20]
In the United States, sandstones comprise fifteen principal aquifers mostly near the land surface in the Middle Western portion of the country. A number of these regional aquifers are overlain by unconsolidated materials from continental glaciation (Figure 2). Freshwater in these sandstones is mostly confined by overlying deposits. Sandstones are also important aquifers in many parts of the world. One of the most interesting aquifers is the Nubian Sandstone Aquifer System that contains “fossil” water. Located in the eastern Sahara Desert, it underlies four countries in north-eastern Africa (Sudan, Chad, Libya, and Egypt). Because of iron and other minerals in the rock matrix of the sandstone and because the water has remained out of circulation for thousands of years, much of the extracted water is brackish to saline.
In the southwestern United States, the Navajo Sandstone, where buried deeply enough, is part of an important aquifer system that supplies water to ranches and small communities (Figure 3). Most sandstones were deposited in water in ocean environments, but the Navajo Sandstone is of eolian origin with distinct cross bedded layers from intersecting dunes that were formed during the Jurrasic era when this area was much drier than now. Because of its wind-blown origin, the grains are relatively uniform in size but fine-grained. Horizontal hydraulic conductivities range from 0.01 to about 30 feet per day and average about 2 or 3 feet per day. Storage coefficients are about 0.002 but with saturated thickness of 500 to 700 feet, wells can produce significant quantities of water. (Heilweil and others, 2002).
Figure 3 – Cross bedded Navajo Sandstone in western United States (photo from Norman Grannemann)
Most sandstones are not as uniform as the Navajo Sandstone. More typically, they consist of layers of consolidated sand interspersed with finer-grained rocks such as siltstone and shale (Figure 4). Because of the variability of grain size and cementation, the hydraulic properties of these aquifers can range considerably. Often, wells drilled into an aquifer such as in Figure 4, will be open to both coarse- and fine-grained rock units. As a result, most water is produced from the bedding plane partings and from the coarse-grained parts of the well bore. For a person modelling groundwater flow in a rock unit such as this, average or mean hydraulic properties will be required as it will not be possible to simulate flow in each bed of rock.
Figure 4 – Sandstone, siltstone, shale layers are common in many detrital rocks because of variations in wave energy during deposition. (https://www.geologyin.com/2015/09/how-to-identify-transgressive-and.html) [accessed 3/27/20].
References:
Freeze, R.A. and Cherry, J.A., 1979, Groundwater, Prentice-Hall, 604 p.
Heilweil, V.M., Watt, D.E., Solomaon, D.K., and Goddard, K.E., 2002, The Navajo Aquifer System of Southwestern Utah, Geological Society of America, Rocky Mountain Section Annual Meeting, 130 p.
Meinzer, O.E., 1923, The occurrence of groundwater in the United States, U.S. Geological Survey Water-Supply Paper 489.
Rivera, A., Editor, 2014, Canada’s Groundwater Resources, Library and Archives of Canada, 803 p.