Environmental Engineering Reference
In-Depth Information
8-ft (2.4 m) deep, 10-in. (25.4 cm) diameter borehole created
with a hollow-stem auger. In each borehole, a slotted 3/4-in.
(1.9 cm) pipe was placed such that 1 ft (0.3 m) remained
above ground, essentially to permit the entry of air to the tree
roots. In each borehole, one 9-ft (2.7 m) long poplar whip, or
pole, was planted such that 8-ft (2.4 m) was below grade in
the sandy backfill amended with compost and slow release
nutrients and 1 ft (0.3 m) was above ground level. No
irrigation system was used. Five wells were installed along
the direction of groundwater flow through the poplar tree
rows, with two wells upgradient, one well within, and two
downgradient from the rows. The wells were screened across
the water table from 5 to 15 ft (1.5-4.5 m) to encompass the
expected range of water-table fluctuations at the site. From
1998 to 1999, groundwater levels were collected using
manual and automated methods.
The depth and distribution of roots was observed in 1998.
Two intersecting trenches were dug almost 9 ft deep adja-
cent to a poplar tree. The exposed roots were counted along
each exposed area. The soil in between the roots was washed
away after pins were driven through the roots along one of
the exposed faces. The LAI was estimated by Ferro et al.
(2001) who, at the end of the growing season, removed and
weighed all the leaves from trees that they had instrumented
earlier with sap-flow sensors. Some leaves were then
measured with a leaf-area meter, and the ratio of leaf area
per unit leaf weight was determined. The leaf area of the tree
so determined was divided by the ground area covered by the
tree to get the LAI . These estimates were then compared to
actual field measurements of these parameters to evaluate
this alternative conceptual framework.
The uptake of groundwater by the trees, V t , was estimated
using Eq. 8.4 . Some of Ferro et al. (2001) results are
reproduced here. Calculations using Eq. 8.4 suggest that
the V t for an individual tree would be approximately 5 gal/
day/tree (18.9 L/day/tree) during the 3rd year from planting
to 18 gal/day/tree (68 L/day/tree) during the 5th year. Sap-
flow measurements were made in the field to provide actual
data for V t to compare to the estimated V t . The estimated ET P
was 5.5 in./month (140 mm/month), and the measured value
was 4.04 in./month (102 mm/month). The estimated V t was
3.8 gal/day/tree (14 L/day/tree), and the measured V t was
2.8 gal/day/tree (10.5 L/day/tree).
One of the assumptions of this conceptual framework
is that V t is derived entirely from groundwater: that is, V t ¼
V gw . During droughts this assumption may be valid. To
account for uptake of any precipitation, however, the V t
can be normalized by the total precipitation measured, V ppt ,
Fig. 8.6 Relation between the average growing season and leaf-area
index, LAI , and time after planting as controls on plant transpiration of
groundwater, V t ; y is a water-use multiplier (Modified from Ferro et al.
2001). One meter is equivalent to 3.2 ft.
8.4.1.1 Case Study: Former Fuel Terminal, Utah
A phytoremediation system was installed at a petroleum-
hydrocarbon contaminated site near Ogden, UT, in 1996.
The 5-acre (20,235 m 2 ) site is a former light petroleum
products terminal, used from the 1950s to 1989. The
phytoremediation system was installed to control the migra-
tion of groundwater to offsite properties, the first hydrologic
goal discussed in Chap. 6, as well as to enhance the degra-
dation of contaminants. At this site, soil remediation of
contaminants was addressed using grasses such as alfalfa
( Medicago sativa ) and fescue ( Festuca spp. ). The USEPA
Superfund Innovative Technology Evaluation (SITE) Pro-
gram, established by the USEPA Office of Solid Waste and
Emergency Response and Office of Research and Develop-
ment, to promote the evaluation of innovative technologies
to remediate Superfund sites, was involved from the
beginning.
The contaminated aquifer is a surficial water-table aqui-
fer. It comprises silty sands, but no tests were performed to
determine the hydraulic conductivity of the aquifer. Because
it is a shallow system, depth to groundwater is about 6 ft
(1.8 m) below land surface. Like many sites with shallow
aquifers, however, this value is not constant, with high
values near 2-3 ft (0.6-0.9 m) below land surface in the
spring to lower values near 7-8 ft (2.1-2.4 m) below ground
surface in the fall. The area is arid, with the ET P often
exceeding precipitation by as much as a factor of 2-10
(Ferro et al. 2001). For example, during the summer months,
ET P can approach 10 in./month (25.4 cm/month) and precip-
itation is less than 1 in./month (2.54 cm/month).
Poplar trees were planted in three rows, each 100 ft
(30.4 m) long, in a location perpendicular to groundwater
flow at the site and at the downgradient edge of a dissolved-
phase plume of petroleum hydrocarbons. Forty hybrid poplar
trees were planted ( Populus deltoides x Populus nigra Impe-
rial Carolina DN 34). Each of the 40 trees was installed in an
V gw ¼
V t
V ppt
(8.5)
Because precipitation, V ppt , was estimated to be 65 gal/
day (245 L/day) and the total water uptake, V t , was estimated
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