Environmental Engineering Reference
In-Depth Information
T Kb
=
(3.7)
where
T is the transmissivity
K the hydraulic conductivity
b the aquifer thickness
The aquifer parameters of transmissivity (T) and storage coefficient (S) are variables that
can dictate the shape of the cone of depression. The storage coefficient is the volume of
water that a permeable unit will absorb or expel from storage per unit surface area per unit
change in head. For a confined aquifer, the expansion of the cone of depression in response
to depressurizing (pumping) is very small, but the compression pressures can be signifi-
cant. This permits the cone of depression to expand and deepen rapidly when pumped,
so a lower storage coefficient (S) value will create a deeper and wider cone than a higher
storage coefficient. An aquifer with a low transmissivity will develop a deep and narrow
cone of depression, and an aquifer with a high transmissivity will develop a shallow and
wide cone of depression.
3.6 Surface Water-Groundwater Interaction
Surface water and groundwater are fundamentally connected, and their interaction occurs
everywhere in all watersheds. It is difficult to separate the two because they “feed” off of
each other and are interconnected through the water cycle.
3.6.1 Stream Interaction with Groundwater
Groundwater accounts for most and often all of baseflow in rivers and streams. Baseflow
is defined as groundwater seepage into a stream channel (Freeze and Cherry 1979). This
influx of groundwater is very important to streams and rivers because it provides flow
during periods of low precipitation or drought and helps sustain aquatic life. On hill-
slopes, many streams originate as groundwater as shown in Figure 3.35 (Winter et al. 1998).
When groundwater flows toward and discharges to a surface stream, the stream is
termed a gaining stream as depicted in Figure 3.36. When a stream loses water to ground-
water, the stream is termed a losing stream (Figure 3.37).
Location of start
of flow of stream
Flowing (gaining) stream
Water table
Saturated zone
FIGURE 3.35
Beginning of stream flow from groundwater.
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