Environmental Engineering Reference
In-Depth Information
individual trees were moving water, some of which was
groundwater, at a rate of 6.8 gal/day/tree (26 L/day/tree)
during the growing season. This rate was used to estimate
that the entire planted area may be using 1.058 gal/day
(4,000 L/day) in 1999. Because closed-canopy conditions
were not yet reached at this site, and these conditions may
take longer given the 10-ft (3 m) spacing between trees, it
may take as long as 30 years before the transpiration rates
increase enough to double the total water flow through trees
of 2,010 gal/day (7,600 L/day). Even so, these higher rates of
groundwater removal would be about one fifth of the total
groundwater flow through the area that discharges to adja-
cent surface-water systems.
Site researchers state that transpiration by the plants is a
significant sink for groundwater during the summer growing
season (Hirsh et al. 2003). In 1999, groundwater-level mon-
itoring indicated a depression in the water-table surface
beneath the planted area. The decrease in groundwater levels
was observed to be about 5.4 in. (12 cm) during the summer
growing season. The groundwater-level data were
normalized for nonplant-induced changes, by correcting for
tidal and barometric efficiencies. Daily fluctuations in the
groundwater table in wells in the planted areas were
measured to be near 1.8 in. (4.5 cm). During the summer,
groundwater levels in the planted area become lower than
even the adjacent marsh surface-water levels, and surface
water would then have the potential to recharge the aquifer.
This situation would reverse during the winter, when precip-
itation would be greater and
ET
would decrease. Interest-
ingly, the authors suggest that groundwater flow is induced
upward to the trees during the summer, but, unfortunately,
data to support this assertion are not provided.
the chlorinated solvents PCE and TCE and their degrada-
tion byproducts.
The hydrogeology in the area comprises unconsolidated
Pleistocene glacial deposits. These deposits range from sands
to gravels, and silts to clays. Although the landfill proper
comprises fill and debris, the native geology also is heteroge-
neous, with interbedded sands, gravels, and alluvial and fluvial
deposits. The depth to groundwater at the site is about 10-15 ft
(3-4.5 m), which approaches the maximum limit for efficient
groundwater interaction. There is an unconfined aquifer, a
discontinuous confining unit, and an underlying intermediate
aquifer. The water table is affected by tidal fluctuations, which
need to be monitored in order to determine the effect, if any,
that the trees may have on shallow groundwater. The ground-
water-flow rate in the shallow contaminated aquifer is about
29-83 ft/year (8.8-25 m/year). The site has wet winters and
dry summers. Although the Pacific Northwest is known for its
precipitation, the site actually receives a modest amount of
about 30 in./year (76 cm/year), with most falling between
October and March.
The shallow groundwater beneath the landfill and in the
direction of local groundwater flow to adjacent surface water
in Dogfish Bay has been documented. The site is being
evaluated by members of the Naval Facilities Engineering
Field Activity Northwest (EFA NW), the Washington
Department of Ecology, USEPA Region 10, the USGS, the
Suquamish Tribe, the local Regional Advisory Board, and
the faculty and staff of the Universities of Washington and
South Carolina. There are domestic drinking-water wells
downgradient from the landfill, although in deeper aquifers.
The goals of the phytoremediation planting are to affect
the site hydrology and to decrease contaminant levels.
Phytoremediation is stipulated as the remedial action in the
Record of Decision (ROD) for the site.
The plantings occurred in two separate areas of the for-
mer landfill (Rohrer et al. 2000). Prior to plant installation,
intensive activity was required to prepare the site for trees.
The asphalt cap was removed. Landfill material was exposed
during site preparation, of which some was left behind, but a
large amount (24 tons of debris) was disposed of offsite.
Clean fill was added to replace that removed to depths of
between 1 and 2 ft (0.3-0.6 m) at the north planting and
between 2 and 3 ft (0.6-0.9 m) at the south planting, for a
total of about 3,100 cubic yards. To compensate for fill soil
chemistry, lime and urea were added and turned into the fill
with a chisel plow.
After soil preparation was completed, hybrid poplars
were installed as 8-in. (20 cm) hardwood cuttings in April
1999. Irrigation was necessary after installation due to the
depth to water table being near the maximum extent of
efficient phytoremediation. The irrigation system was a
drip type installed at 2-ft (0.6 m) intervals, with a maximum
flow rate of 10 gal/min (27.8 L/min). The irrigation was
8.4.5 Plant-and-Monitor Framework
This approach involves the installation of plants and moni-
toring of wells for groundwater levels and contaminants in a
manner similar to that of other remedial strategies.
8.4.5.1 Case Study: Landfill, Washington
A phytoremediation system was installed as part of overall
remedial actions in early 1999 at a former landfill located
at the Naval Undersea Warfare Center (NUWC), Division
Keyport, Washington, about 11 water miles (17.6 km) from
Seattle in central Puget Sound. The total landfill area is
about 9 acres (36,423 m
2
), and it is about 10 ft (3 m) above
sea level. It used to be marshland connected to tidal flats
but was filled in by the U.S. Navy. The landfill was
operated between 1930 and 1973. It accepted domestic
and industrial wastes generated by Naval activities. The
landfill is unlined, but part of the landfill is covered by
asphalt. The predominant groundwater contaminants are
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