Environmental Engineering Reference
In-Depth Information
other alternatives discussed were natural attenuation and a
permeable reactive barrier (PRB).
An additional problem of using cost comparisons of any
kind as a criterion to select a remedial option is the presence
of many different definitions of what constitutes costs. For
example, there is a need to define gross costs, which may
come encumbered with various indirect charges, such as
overhead or profit, versus net costs, or the actual unit cost
per item. These definitions often are hard to decipher
because their costs may be built into the total costs of the
project.
propose, prepare, install, maintain, analyze, and report dur-
ing a 3-yr period were all covered (see Chap. 8 for site
specifics; Table
10.3
). These costs, however, are site spe-
cific, and relate to hydrogeologic conditions, meteorological
conditions, and plant hydroecological conditions that are
specific to the Forth Worth area. As such, the authors
concluded that these costs are correct within an order-of-
magnitude (U.S. Environmental Protection Agency 2003).
The assumption is that for up to 4 years after tree installation,
a supplemental groundwater control technology, such as
pump-and-treat or an interception trench, would be in place
until the trees reached groundwater; these costs, however,
were not included in their analysis.
As would be expected, a change in any one of the above
factors would result in a different cost per item and, there-
fore, overall cost. More trees may not necessarily increase
costs in a linear fashion because many vendors offer
discounts on unit prices for large-volume orders. If the area
to be planted increased in size, this change would increase
the cost of trees, the cost of the irrigation network, and the
time needed to sample wells.
10.1.3 Use of Phytoremediation
as a Supplemental Remedy
The use of phytoremediation as the sole remedial strategy to
contain groundwater is more of the exception than the rule.
In most cases, phytoremediation will be used in conjunction
with another remedial strategy, such as source removal. In
some cases, performance verification monitoring data col-
lected over time will provide data to support the use of
phytoremediation as the sole remedial strategy, but the lag
time may be on the order of tens of years. Some sites will
have other engineered strategies implemented to control
groundwater, such as trenches (Widdowson et al. 2005a).
Ferro et al. (2005) investigated the cost savings projected
for a pump-and-treat system and phytoremediation system
at a VOC-contaminated site. The goal was to decrease
the number of wells in the existing pump-and-treat system
in order to decrease treatment costs and replace the
removed wells with plants. The total costs to set up the
phytoremediation system were $282,600 (Ferro et al.
2005). The cost savings of using phytoremediation to sup-
plement an equivalent reduction in the pump-and-treat sys-
tem can be determined, assuming that the cost of the
contaminated groundwater is known. In the case offered by
Ferro et al. (2005), the cost was assumed to be $0.05 gal. If
the system was treating groundwater at a rate of 19 gal/min
(71 L/min), and this decreased to 10 gal/min (37 L/min),
then a cost saving would be created. This 9 gal/min (34 L/
min) is made up by the phytosystem, which cost $282,600.
By 2010, the predicted reduction in pump-and-treat costs,
since phytoremediation, would approach $470,000.
Table 10.3
Phytoremediation item, cost, and percentage of total
project cost (Modified from USEPA 2003).
Percent of
total cost (%)
Item
Cost
Site preparation
$42,650
9.1
Characterization
Planting
Irrigation system
Well installation
Permitting and regulations
$55,000
11.8
Capital
$37,833
8.1
Sap flow meter
Water-level recorders
Weather station
Groundwater sampling gear
Fixed costs
$3,783
0.8
Consumables
$19,480
4.2
Soil fertilizers
Trees
Supplies
Labor
$108,000
23.2
Maintenance
Monitoring and sampling
Utilities
$12,900
2.8
Irrigation water
Treatment/disposal
10.1.4 Operation, Maintenance, and Disposal
Issues When Using Plants
0
0
Waste handling
$7,500
1.6
Analytical services
$172,855
37.1
Annual monitoring for 10 years
Operation and Maintenance
One of the most comprehensive reports on the costs
associated with using phytoremediation to hydrologically
control contaminated groundwater was on work performed
at the Air Force Plant 4, Fort Worth, Texas. The costs to
$5,000
1.1
Demobilization
$1,050
0.2
Total
$466,051
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