Environmental Engineering Reference
In-Depth Information
land surface, or 43,560 ft 2 (4,007 m 2 ), is equal to a volume of
27,154 gal (102,642 L) of water. The weight of this water is
more than 113 tons.
A well pumped at a rate equal to that of groundwater flow
with no additional change in groundwater level over time
represents steady-state flow conditions. If, however, the
pumping rate is increased, the groundwater level will decline
over time and represents transient conditions. This subtle
difference is important when it comes to understanding the
source of groundwater removed from a well, or in the case of
a phytoremediation system, the source of groundwater tran-
spired by a plant. Going back to the water budget equation
introduced in Chap. 2, we can state that
In Van Hylckama (1974), the relation between depth to
water table and plant and groundwater use was investigated
further. At a site near Buckeye, Arizona, plastic-lined
evapotranspirometer studies revealed that for planted
Tamarix , the groundwater use was 85 in./year (215 cm/
year) when the depth to water table was set at 5 ft (1.5 m)
below land surface. When the depth to water table increased
to 7 ft (2.1 m), groundwater use decreased to 60 in./year
(152 cm/year); when the depth to water table decreased to
9 ft (2.7 m), groundwater use decreased to less than 40 in./
year (100 cm/year).
4.11 Groundwater Models
Inflow
Outflow
¼ D
S
:
(4.13)
As described previously, the flow of groundwater in porous
media results from a combination of independent and depen-
dent variables. Before Darcy used his innovative column
experiments to look at flow through porous media in order
to solve a surface-water-quality problem, no predictive tool
had been developed for use in assessing the effects of
changes in these variables, such as head gradients, on the
flow of groundwater. Darcy's Law provided hydrologists
with such a tool, which enabled subsequent hydrogeologists
to test various hypotheses without having to perform such
tests in the field. As such, Darcy's Law may be considered
one of the first groundwater-flow models and is still being
widely used every day.
Since 1856, many physical, electrical, analytical, and
numerical models that describe the flow of groundwater
under various aquifer and flow conditions have been devel-
oped. These models were created to answer fundamental
questions about the potential quantity of groundwater avail-
able in a pumped well field. Models that address the effect of
various solutes dissolved in groundwater, such as salt or
petroleum-based or halogenated contaminants have been
developed to address questions regarding groundwater qual-
ity. In either case, it is important that model output be used to
test hypotheses or to refine a conceptual model of the hydro-
logic system under investigation, such as establishing rea-
sonable ranges for particular parameters, rather than
expecting it to be the unique model that will address accu-
rately all current and future questions.
The interactions described previously, such as groundwa-
ter and surface-water interactions, the water-table surface,
recharge, discharge, and evapotranspiration, can be
explained and modeled best by using the concept of a
groundwater system. This system is defined by boundaries
and conditions of groundwater flow or no flow and can be
defined mathematically at each boundary. Examples include
constant flux boundary, specified head, and the magni-
tude of evapotranspiration related directly to the depth to
water table.
The groundwater taken up in a pumped well, or poten-
tially by a tree with roots that reach the capillary fringe and
water table, can come from an increase in groundwater
inflow, a decrease in groundwater outflow, or from a drop
in storage. For a water-table condition where storage is
minimal,
D
S approaches zero such that
Inflow
Outflow
¼
0
:
(4.14)
Thus, groundwater removed can be a result of either an
increase in inflow or a decrease in outflow. This fact can be
useful, as we will see in the next section, in attempting to
apply phytoremediation concepts to reduce the flow of
contaminated groundwater to adjacent surface-water bodies.
Other factors can affect groundwater levels, such as
changes in pressure over the aquifer. One of the first records
of groundwater fluctuations caused by an increase in pres-
sure above the aquifer was in a report of a study in New York
where an increase in the groundwater level in a well located
near a railroad occurred after a train passed. Other causes
include tidal fluctuations, barometric pressure fluctuations,
and even earth tides. Van Hylckama (1968) reported a diur-
nal fluctuation in groundwater levels in plastic-lined tanks
that contained soil, plants, and an artificially controlled
groundwater level. When observed in bare tanks that
contained no plants, the groundwater fluctuation was
correlated to changes in atmospheric fluctuations in baro-
metric pressure acting upon air trapped in the tanks during
filling. Conversely, in tanks that contained transpiring
plants, diurnal groundwater-level fluctuation was not related
to changes in barometric pressure. Van Hylckama (1970)
later reported that the depth to water table was an important
factor in controlling how much groundwater could be used
by plants such as saltcedar, and noted that even small
differences in depth to groundwater, from 4.92 to 6.8 ft
(1.5-2.1 m), for example, can reduce groundwater use even
though the plants remain alive.
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