Environmental Engineering Reference
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Secondary porosity in the form of vertical fractures and root fragments combined with
the porosity observed at unconformable contacts may assist in explaining why groundwa-
ter is observed in some locations. Geologic units previously characterized as impermeable
or not having enough groundwater to permit flow may indeed yield measurable water.
The awareness of this possibility may help when evaluating anthropogenic influences on
the geology and hydrogeology of urban areas for groundwater and surface water protec-
tion purposes (Howard and Gerber 1997; Murray et al. 2000).
3.5.7 Water Table and Hydraulic Gradient
The water level in a well penetrating an unconfined aquifer undergoing horizontal flow
will be equal to the level of the water table. When these water levels are joined across
multiple wells in two-dimensional space, a water table plane or potentiometric surface
is defined (Freeze and Cherry 1979). This constantly changing surface is a measure of the
upper physical boundary of the water table. Fluids flow between the differences in pres-
sure or elevations existing at distinct locations. For example, groundwater flows from
areas of high pressure to areas of lower pressure or in hydrologic terms groundwater
flows from areas of higher potential to areas of lower potential. A pressure differential
is called a pressure gradient, whereas the elevation difference between wells is called a
hydraulic gradient. The hydraulic gradient between the water levels in Well 2 and Well 1
depicted in Figure 3.30 is often referred to as hydraulic head. To calculate the hydraulic
head between two wells, take the ratio of the rise or height difference between the two
wells (dH) to the run or horizontal distance between the two wells (dL). This ratio is
shown in Figure 3.31.
3.5.8 Determining the Direction of Groundwater Flow
Calculating the direction of groundwater flow requires at least three data points that
accurately measure the level of groundwater under atmospheric pressure. The verti-
cal accuracy required is usually within 3 mm (0.01 ft). Three wells is the minimum
number necessary to determine the direction of groundwater flow, and if only three
wells are used, their location, pattern, or placement should be in a triangular pattern.
Well B
Well A
h 2
h 1
L
FIGURE 3.30
Hydraulic gradient.
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