Conceptual Frameworks for Phytoremediation to Achieve Hydrologic Goals - Introduction to Phytoremediation of Contaminated Groundwater - page 200

Table 8.4 The percentage of rainfall that becomes recharge (%RC), for selected events during 2005, at the manufactured gas plant
phytoremediation site, Charleston, South Carolina. The data shown are for a monitoring well located in the planted area. Greater than 100%
recharge suggests recharge from rainfall was supplemented with additional soil moisture from previous rainfall events. Data given are in feet;
convert to meters by multiplication by 0.3048; n , porosity; D GW, change in groundwater level.

D GW D GW D GW D GW D GW

2005 Rainfall % %

(ft) Level (0.20) (0.30) Rain Rain RC RC

(ft) (n ¼ .2) (n ¼ .3) (n ¼ .2) (n ¼ .3)

2/3 0.08 0.30 0.06 0.09 .06/.08 .09/.08 75 > 100

> 100 > 100

4/2 0.08 0.50 0.10 0.15 .1/.08 .15/.08

5/16 0.13 0.25 0.05 0.07 .05/.13 .07/.13 38 53

5/17 0.08 0.50 0.10 0.15 .10/.08 .15/.08 > 100 > 100

5/30 0.15 0.40 0.08 0.12 .08/.15 .12/.15 53 80

9/28 0.20 0.60 0.12 0.18 .12/.20 .18/.20 60 90

Average 71 87

was calculated for all rainfall events during 2005, using
measured groundwater-level changes and two estimates of
aquifer porosity (Table 8.4 ). The calculated recharge volume
is estimated to be about 326,000 gal (1.2 10 6 L) for 2005.
This amount suggests that recharge was about 75% of annual
rainfall (Table 8.4 ).
The calculated recharge to the water-table aquifer in the
phytoremediation area is not constant but varies with time.
The rather high percentage (71-87%) of precipitation that
became recharge may be due to (1) that fact that the water
table is near land surface in this low-lying area near the
coast; (2) the presence of 3 ft (0.9 m) of topsoil that was
added to the site prior to plant installation; (3) the potential
of increased vertical permeability due to tree roots that have
reached the water table, and; (4) the humid conditions of the
coastal area that characterize the site.

for Charleston, SC, various alternative approaches exist.
This section provides a few examples of these alternative
conceptual frameworks. These examples were selected, in
part, because information about these sites has been
published and can be accessed more readily relative to
other sites where only proprietary or confidential informa-
tion has been generated. Also, the locations of these sites
represent a cross section of climatic conditions from across
the United States.

8.4.1 Water-Use Estimate Framework

One of the disadvantages of using ET P as part of a frame-
work to assess the potential effect of plants on groundwater
is that it does not differentiate the source of the water
removed. The fraction of ET P used by phreatophytes can
be estimated, however, as indicated with the previous exam-
ple, in which the upper 20% of the full aquifer thickness was
assumed to be the source of water removed by ET .
Another approach to estimate the fraction of groundwa-
ter that contributes to ET P is given by Ferro et al. (2000).
The uptake of groundwater by trees, V t , can be estimated
as being a fraction of the ET P for the planted area. The total
volume of water ( V t ; in volume/time) used can be deter-
mined by

8.4 Alternative Conceptual Frameworks

for Groundwater Control

Because of the wide range of site-specific conditions
encountered at sites characterized by groundwater contami-
nation, it may not be practicable to rely on one single
approach that could handle all site conditions. If such an
approach were sought, by definition it would be too vague to
provide much information that would be useful at a particu-
lar site, and it would be hindered by assumptions and have a
high degree of uncertainty. It would be more appropriate to
institute a multiple-line-of-evidence approach at sites to
evaluate the potential for phytoremediation to achieve
hydrologic goals. Evidence to support this conclusion is
exemplified in the list of more than 140 phytoremediation
sites and their phytotechnologies made available at the U.S.
Environmental Protection Agency (USEPA) phytotechnology
project profile at www.clu-in.org .
In addition to the framework of comparing ET P to
groundwater discharge just discussed with the case study

V t ¼ ET P y LAI A (8.4)

where ET P is the potential evapotranspiration (in./day or
mm/day),

y

is a water-use multiplier determined as a fraction
of ET P to represent that percentage of ET that is actually
removed as actual evapotranspiration, ET A , or crop coeffi-
cient, LAI is the leaf-area index, or leaf area per unit ground-
surface area (dimensionless) that is discussed in Chap. 9, and
A is the planted area (in feet or meters) assuming closed
canopy is achieved (Fig. 8.6 ). Two examples are provided
where this approach was applied.