Engineering Geology

Chapter 4
As groundwater moves from areas of higher potential towards areas of lower potential and
as the contours on groundwater contour maps represent lines of equal potential, the direction
of groundwater flow moves from highs to lows at right angles to the contours. Analysis
of conditions revealed by groundwater contours is made in accordance with Darcy’s law.
Accordingly, spacing of contours is dependent on the flow rate, and on aquifer thickness and
permeability. If continuity of flow rate is assumed, then the spacing depends on aquifer thickness
and permeability. Hence, areal changes in contour spacing may be indicative of
changes in aquifer conditions. However, because of the heterogeneity of most aquifers,
changes in gradient must be carefully interpreted in relation to all factors. The shape of the
contours portraying the position of the water table helps to indicate where areas of recharge
and discharge of groundwater occur. Groundwater mounds can result from the downward
seepage of surface water. In an ideal situation, the gradient from the centre of such a recharge
area decreases radially and at a declining rate. An impermeable boundary or change in transmissivity
will affect this pattern.
Depth to water table maps show the depth to water from the ground surface. They are
prepared by overlaying a water table contour map on a topographical map of the same area
and scale, and recording the values at the points where the two types of contours intersect.
Depth to water contours are then interpolated in relation to these points. A map indicating the
depth to the water table also can provide an indication of areas of recharge and discharge.
Both are most likely to occur where the water table approaches the surface.
Water-level-change maps are constructed by plotting the change in the position of the
water table recorded at wells during a given interval of time. The effects of local recharge
or discharge often appear as distinct anomalies on water-level-change maps. For example,
a water-level-change map may indicate that the groundwater levels beneath a river have
remained constant while falling everywhere else. This would suggest an influent relationship
between the river and aquifer. Hence, such maps can help identify the locations where there
are interconnections between surface water and groundwater. These maps also permit an
estimation to be made of the change in groundwater storage that has occurred during the
lapse in time involved.
Assessment of Field Permeability
An initial assessment of the magnitude and variability of the in situ permeability can be
obtained from tests carried out in boreholes as the hole is advanced. By artificially raising the
level of water in the borehole (falling head test) above that in the surrounding ground, the flow
rate from the borehole can be measured. However, in very permeable soils, it may not be
possible to raise the level of water in the borehole. Conversely, the water level in the borehole
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