Environmental Engineering Reference
In-Depth Information
Table 8.2
Potential evapotranspiration (
ET
p
) for the Charleston,
South Carolina, manufactured gas plant phytoremediation site.
Parameter
Table 8.3
Hydrologic physical properties for the Charleston, South
Carolina, former manufactured gas plant site.
Variable Parameter
Value
Value
Air temperature
70 degrees Fahrenheit (
F)
Q
Groundwater discharge Calculated
6,000 ft
2
Relative humidity
(RH)
72% high: 30% low
A
Area of saturated
aquifer
600 ft by 10 ft aquifer
¼
(557 m
2
)
0.633 KWh/m
2
Solar radiation
i
Hydraulic gradient
(3 ft/30 ft
¼
0.1)
Wind speed
5 mph (8 kph)
K
Hydraulic conductivity 10 ft/day (0.48 m/day)
Precipitation
50 in (127 cm)
n
e
Effective porosity
0.35
Results
ET
p
(at 72% RH)
0.518 in/day (0.548 mm/h)
0.518 in/day/0.25
¼
0.123 in/day (for 6 hours
estimated, then the cross-sectional area,
A
, can be estimated.
The difference in groundwater elevation measured in an
upgradient,
h
1
, and downgradient,
h
2
, well separated by
some distance,
(h) of light)
ET
p
(at 30% RH)
0.588 in./day (0.622 mm/h)
0.588 in./day/0.25
¼
0.147 in./day (for 6 h of
light)
D
l
, gives the head gradient,
i
, where
i
l
. The hydraulic conductivity values from
field or laboratory methods can be used. These physical
properties for the Charleston site are presented in Table
8.3
.
Solving for
Q
yields a flow of groundwater through the
site of given dimension of 44,880 gal/day (169,646 L/day).
If multiplied by the effective porosity of 35%, then
15,708 gal (92,832 L) of groundwater flow through the
10-ft (3 m) thick aquifer each day. When compared to the
calculated
ET
p
of 1,514 gal/day (5,722 L/day), there is
more than enough groundwater entering the site to account
for removal by
ET
p
, and the water table would not be
affected.
If one assumes, however, that the predominant source of
groundwater to plant roots will be in the upper 2-ft (0.6 m)
section of the 10-ft (3 m) saturated thickness, we can recal-
culate the flow of groundwater to be 8,976 gal/day,
(33,929 L/day) and when multiplied by 35% effective poros-
ity, the result is 3,141 gal/day (11,872 L/day), relative to the
1,514 gal/day (5,722 L/day) that can be removed by
ET
p
.
Hence, about 50% of the groundwater discharge through the
area in the upper one-fifth of the aquifer could be removed
by
ET
p
. If a mature poplar tree can remove at least 15 gal/
day/tree (56 L/day/tree), then to achieve this removal rate
would require between 100 and 200 trees in the 18,000 ft
2
(1,672 m
2
) area at a minimum of 10-ft (3 m) on center
spacing.
¼
(
h
1
)
(
h
2
)/
D
Average
ET
p
0.135 in./day (0.342 cm/day)
Average
ET
p
/month
(30 day)
4.05 in./month (10.2 cm/month)
The average
ET
p
of 4.05 in./month (10.2 cm/month) is an
estimate of the total amount of water vapor that potentially
could be removed from the site. The dimension of the area
available for planting was 600 ft by 30 ft, or 18,000 ft
2
(1,672 m
2
). If
ET
p
is applied across this area uniformly, a
daily estimated
ET
p
would be about 1,514 gal/day.
If it is assumed that the planted area does not use precipi-
tation or soil moisture in the unsaturated zone, and only
would use groundwater, how does this daily demand for
water vapor compare to the daily discharge of groundwater
that enters the shallow aquifer? If the aquifer is no more than
10 ft (3 m) thick, and the effective porosity,
n
e
, is 35%, then
the volume of voids can be estimated as (600 ft)(30 ft)(10 ft)
(
n
e
)
63,000 ft
3
(1,783 m
3
), or about 471,240 gal of
groundwater beneath the planted area. If the abstract rate
due to
ET
p
is constant at 1,514 gal/day (5,722 L/day), and no
precipitation or influx of groundwater from upgradient areas
occurs, it would take about 311 day to dewater this aquifer
thickness by evapotranspiration.
However, groundwater flows into the site from
upgradient areas, precipitation does recharge the aquifer,
especially during the wetter summer and fall months, and
plant roots do not remove groundwater from the entire
aquifer thickness. The first two processes result in an
increase in head in the water table, as an increase in ground-
water storage, because for that time, water inflow is greater
than water outflow. As stated in Chap. 4, an increase in head
differential will increase the groundwater-flow rate in that
area, which can be estimated with the Darcy equation. The
Darcy equation can be used as a tool to determine the
specific discharge of groundwater through a unit cross sec-
tion of the water-table aquifer. If the depth of the saturated
thickness along a transect
¼
8.3.2 Groundwater-Level Fluctuation
Monitoring
Groundwater levels have been measured in monitoring wells
at the former MGP site in Charleston, SC, between 1994
(before planting occurred) up to 2010. Between 1994 and the
installation of hybrid poplar trees in late 1998, groundwater
levels across the site reflected a balance between the input
and output of water to shallow groundwater. The input of
water consisted of local recharge and lateral groundwater
through the aquifer can be
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