Page 96 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 79

Direction of groundwater flow

A (36.2')
36.0'
Water table contours
55'
apart
35.6'

B (35.6')
C (35.4')

FIGURE 3.1
Determination of groundwater gradient and direction.

(d) Connect the points of equal values of elevation (equipotential
lines), which then form the groundwater contours. Here, we con-
nect the elevations of 35.6′ and 36.0′ to form two contour lines.
(e) Draw a line that passes through and is perpendicular to each
equipotential line and mark it as the groundwater flow direction.
(f) Measure the distance between two contour lines, 55 ft in this
example.
(g) Calculate the groundwater gradient from the formula, i = dh/dl:

i = (36.0 − 35.6)/(55) = 0.0073
See Figure 3.1.

Discussion:
The groundwater elevations, especially those of the water-table aqui-
fers, may change with time. Consequently, the groundwater flow
gradient and direction would change. Periodic surveys of the
groundwater elevation may be necessary if fluctuation of the water
table is suspected. Off-site pumping, seasonal change, and recharge
are some of the reasons that may cause the fluctuation of the water-
table elevation.