Environmental Engineering Reference
In-Depth Information
captured by the deep-rooted trees. However, sap-flow
measurements made on 1-year-old trees in 2000 indicated
that tree-water flow was no higher than an average of 1.1 gal/
day/tree (4.3 L/day/tree), an order of magnitude lower than
the values used in the model simulations of 12.5 gal/day/tree
(47.3 L/day/tree).
unplanted area to represent groundwater conditions not
impacted by the trees. Lysimeters also were installed at the
site above the water table near monitoring wells, but were
reported to be problematic (U.S. Environmental Protection
Agency 2000b). Between 2000 and 2001, groundwater level
measurements were made in 21 wells on a continuous basis
with downhole pressure transducers. Because the site was
located near a tidally influenced surface-water body, tidal-
fluctuation data were collected in order to remove their
effect on groundwater levels so that any tree influence on
water levels could be observed. This method is described in
Chap. 9.
In 1996, about 180 hybrid poplar trees (
Populus deltoides
x Populus trichocarpa
) were planted in a 1-acre (4,047 m
2
),
U-shaped area that surrounds the downgradient part of the
pits and is within the contaminant plume. The trees were
planted at a spacing of 10 ft (3 m) from each other. The trees
were planted in excavated holes to which was added plastic
sleeves to encourage downward root growth rather than
lateral root growth. Some trees died within the first few
years, perhaps as a result of the beginning of a 5-year
drought in the eastern United States that started in 1998.
As such, 65 additional trees were planted in 1998, including
hardwood species native to the area, such as tulip trees
(
Liriodendron
8.4.4 Water-Budget Framework
This approach is based on performing a site water budget,
similar to that introduced in Chap. 2. In this case, the sources
of water inflow and outflow to the site are enumerated and
quantified, including the transpiration of water by planted
trees. In essence, the site is viewed as a microcosm of the
hydrologic cycle.
8.4.4.1 Case Study: Aberdeen Proving Ground,
J-Field Superfund Site, Maryland
A phytoremediation demonstration project was implemented
during 1996 at the J-Field Site at the Aberdeen Proving
Ground in Maryland along the Chesapeake Bay. The J-Field
Site consists of two parallel trenches used to burn a variety
of chemicals and wastes between 1940 to the 1970s (U.S.
Environmental Protection Agency 2000b). As a result, a
legacy of contaminants, primarily chlorinated solvents, has
remained in the soils, sediments, and groundwater at the
site. The application of phytoremediation at this site was
designed to (1) remediate the contamination (described in
Chap. 13) and (2) alter the groundwater levels at the site to
stop the movement of groundwater to nearby surface-water
resources. The project was conducted by the U.S. Army, the
USEPA Region III, and the USEPA Environmental
Response Team Center. More information on this site can
be found in Hirsh et al. (2003).
The contaminated shallow aquifer consists of low-
permeability sediments, such as fine sands and clays such
that the hydraulic conductivity varies between 0.3 and 8 ft/day
(0.1-2.4 m/day). Aquifer tests performed at the site indicate
that the specific yield of the shallow aquifer is no greater
than 1 gal/min (3.7 L/min). Groundwater flows radially from
the test pits in higher elevations after recharge and ultimately
discharges in low-lying areas adjacent to freshwater marshes
and a tidally influenced estuary.
Field measurements were made to evaluate the site water
budget using sap-flow meters and a weather station. The
researchers used the meteorological information from the
weather station to determine the overall
ET
P
for the site.
The climate is considered temperate, and precipitation
averages 45 in./year (114 cm/year) distributed evenly
throughout the year (Hughes 1995). The
ET
P
was then com-
pared to the water flow though the trees, as determined
by the sap-flow gage studies. One well was added in an
tulipifera
)
and
silver maple
(
Acer
saccharinum
).
Following the toppling of some trees during storms, it
was concluded that the plastic sleeves placed in the
boreholes before planting were reducing the support strength
of the trees provided by lateral roots, and future use of the
sleeves was not encouraged. Excavation of some trees indi-
cated root depths of at least 7 ft (2 m) by 1998, only 2-years
after planting. Tubing from the surface to the roots was
placed in the backfill in an attempt to introduce atmospheric
oxygen into the subsurface and thus enhance root growth by
removing oxygen as a limiting factor for root respiration. A
drainage system to remove surface-water overland flow from
the areas was also constructed to reduce the amount of
infiltration to the roots from aboveground sources.
The water budget was calculated for the site using mete-
orological data collected between 2000 and 2001 and using
sap-flow data. The equation used was a modification of that
presented as Eq. 2.4
S
0
P
¼
SR
þ
E
þ
SW
þ
T
þ
GR
þ
(8.6)
where
P
is precipitation,
SR
is surface runoff,
E
is evapora-
tion,
SW
is soil water,
T
is transpiration,
GR
is groundwater
recharge, and
S'
is the change in soil water and aquifer
storage. The weather data were used to estimate the
ET
,in
a manner similar to the approach described earlier. Transpi-
ration rates were determined by sap-flow measurements
made in the field. Sap-flow data indicate that, on average,
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