9.3: Aquifers
- Page ID
- 44591
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Geologically, the loose and discontinuous layers of decayed rock debris overlying solid bedrock are termed regolith. Soil, where chemical and physical weathering are the most active, is the uppermost part of the regolith. The regolith is a potential storage medium for water. Above bedrock, which is essentially impermeable to water, the rock is fractured and frequently consists of gravels, sands, and soil particles. As illustrated in Figure 9.2 water can be contained in the pores (interstices) of soil, sand, gravel, and rock (Meinzer, 1923). The substrata containing interstitial water is divided into the unsaturated zone and the zone of saturation. Groundwater that can be successfully extracted for a water supply only occurs in the saturated zone. The boundary between the unsaturated and saturated zones is called the water table. The water table may be at or above the soil surface as in swamps, wetlands, and near lakes and continuously flowing streams.
Permeability or hydraulic conductivity is a measure of the ability of an aquifer to transmit water. The total porosity and permeability of an aquifer depends upon the size and shape of the pores. Table 9.1 provides approximate values for total porosity and relative permeability. The specific yield of an aquifer, the portion of the stored water that can be withdrawn for a water supply, is also given in Table 9.1. Except for clay, porosity is generally a good indicator of the amount that can be withdrawn from an aquifer. Clay, although high in porosity, has a low permeability that limits water flow (Table 9.1). Usually, sand, gravel, and fractured rock are good water-bearing deposits that can be developed as a water supply. Table 9.2 summarizes the ranges of hydraulic conductivity for various aquifer materials.
In some geologic formations, groundwater may be confined under pressure between two impervious layers. This “confined” aquifer may create what is termed an artisan condition. For artesian flow to be possible there must be a pervious stratum that is continuous from a region upslope where water can percolate into the aquifer to a downslope region where the aquifer is confined between upper and lower impervious layers. When a well is installed through the upper impervious layer into the confined aquifer, water will rise up the well to a level depending on the hydrostatic pressure on the aquifer at the well location. If the pressure is high enough, water will flow out of the well under this artesian condition. The more normal condition for both confined and unconfined aquifers is that groundwater must be pumped from the well. Figure 9.3 illustrates the geological conditions that foster these various sources of groundwater. For a more detailed presentation of groundwater, the reader is referred to Freeze and Cherry (1979), Todd (1980), and Sterrett (2007).
| Aquifer Material | Total Porosity (%) | Specific Yield (%) | Relative Permeability |
|---|---|---|---|
| Dense limestone | 5 | 2 | 1 |
| Dense shale | 5 | 2 | 1 |
| Sandstone | 15 | 8 | 700 |
| Gravel | 25 | 22 | 5,000 |
| Sand | 35 | 25 | 800 |
| Clay | 45 | 3 | 1 |
| Aquifer Formation or Material Range of Hydraulic Conductivity (ft/d) | ||
|---|---|---|
| Low High | ||
| Fine to coarse gravel | 101 | 104 |
| Fine to coarse sand | 10-3 | 102 |
| Silt and loess | 10-4 | 100 |
| Glacial til | 10-8 | 100 |
| Karst limestone | 10-2 | 102 |
| Shale | 10-9 | 10-5 |
| Sandstone, well cemented, unjointed | 10-6 | 10-4 |
| Sandstone, friable | 10-4 | 10-1 |
| Unfractured igneous and metamorphic rocks | 10-9 | 10-6 |
Figure 9.3. Cross section of geologic formations illustrating sources of groundwater for water supply. (Modification of image supplied courtesy of Barkmann et al., 2020.)
