5.2 Hydraulics of Flow in Unconfined Aquifers
Groundwater flow in unconfined aquifers obey the same principles as flow in confined aquifers with an added element; the elevation of the top of the saturated zone defines a “water table”, which is the elevation of water that stands in a screened well that is just deep enough to encounter water. For example, the hydraulic head of the water table intersected by the shallow well shown in Figure 21 is 100 m. In addition, consistent with our previous discussions, an equipotential contour connects points of equal hydraulic head, which is measured using wells (field-scale piezometers). The water table is not an equipotential line; it has a variable head because it varies in elevation.
Figure 21 – Equipotential contours in an unconfined aquifer; the contour lines connect points of equal hydraulic head and extend to the water table of the same elevation (Cohen and Cherry, 2020).
Figure 22 shows that the water table can also be represented in map view. Each contour line represents a line of equal elevation of the water table. Note also that the water table gradient increases (steeper water table and more closely spaced contour lines) in the direction of flow because of convergent flow, which decreases the cross-sectional area of flow, as shown previously in Figure 5.
Figure 22 – Potentiometric cross section, groundwater flow direction (blue arrows), and water table contour map in an idealized unconfined aquifer. The contour map represents the topography of the water table and can be used to infer the general direction of flow. The flow geometry beneath the water table is defined by equipotential contours in cross section (Cohen and Cherry, 2020).
Example Problem 5
To what level (elevation) will water rise in the inclined wells?
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Figure 23 shows the vertical head profile measured in nested wells. These data could also be measured in multilevel (multiport) wells, for example. Importantly, the figure shows that although vertical gradients are present, flow also has a horizontal component. That is, the presence of a vertical gradient does not necessarily indicate that flow is completely vertical, only that a component of flow is vertical.
Figure 23 – Example of vertical head profiles in an idealized unconfined aquifer (Cohen and Cherry, 2020).
The potentiometric contours and flow geometry in the unconfined aquifer scenario shown in Figure 23 are representative of a case in which a vertical no-flow boundary is present near the upgradient end of the system (left side). This boundary has the effect of causing significant vertical flow in that region (see Figure 18 for an analogous experimental apparatus and contours). In many instances, a no-flow boundary may not be nearby, and flow in the area of interest is mainly horizontal as shown in Figure 24.
Figure 24 – Mainly horizontal flow in an unconfined aquifer. The potentiometric contours are orthogonal to the aquitard, which has a very low hydraulic conductivity (K) such that it behaves as a no-flow boundary (Cohen and Cherry, 2020).
In practice, horizontal flow in unconfined aquifers is often approximated as purely horizontal, especially when assessed on a sub-regional scale and away from boundary conditions, as shown in Figure 25.
Figure 25 – Water table represented as a planar surface with predominantly horizontal flow throughout the cross section (Cohen and Cherry, 2020).
Example Problem 6
Draw the water table.
Draw the 70 m, 75 m, and 80 m equipotential contours and sketch several flow lines.
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Example Problem 7
a) A piezometer is inserted into an aquifer beneath sediment (low K) of a lake, and water rises to a stable level as shown below. A water table is present but not shown. Is water flowing upward or downward through the sediment? Explain.
b) Draw a schematic representation of the vertical head profile, extending from the water level of the lake to the well screen.
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Example Problem 8
The figure below is a map view of a lake and shoreline. The water table elevation at Well A is 12 m. Assuming the water table is planar (an inclined plane), what is the expected water level elevation in Well B?
a) 12 m
b) 13 m
c) 14 m
d) A value between 12 m and 13 m
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Example Problem 9
Estimate the water level in each well and determine if there is vertical flow through the clay aquitard.
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