Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 220

in the adjacent aquifer, because a well has 100% porosity,
whereas the aquifer material will be much less porous. A 1-
in. (2.54 cm) water-level change in a well would be a 3-in.
(7.6 cm) change in an aquifer of 30% porosity. The ground-
water-level change measured in a well often is a minimum
representation of that occurring in the adjacent aquifer
sediments. This fact suggests that a lack of change does
not necessarily mean that no groundwater uptake is occur-
ring; it is simply an artifact of using wells as a metric. Also,
if the efficiency of the well screen is lowered by clogging, a
change in the groundwater level caused by plant uptake may
not be measurable. Thus, the condition of the wells used to
measure groundwater-level fluctuations is important.
Because of the influence of well construction on the poten-
tial to discern groundwater-level changes caused by plants,
care must be taken in well construction. For example, the
screened interval should be kept to a minimum length,
because longer screened intervals may provide a preferential
flow path for plants to meet ET demands.
Groundwater-level fluctuations caused by phreatophytes
were used to estimate the specific yield of a riparian aquifer
in the Larned Research Site in the Arkansas River basin in
Kansas by McKay et al. (2004). Wells were placed at
increasing distances from the Arkansas River. The degree
of water-table fluctuation was higher in the riparian ecosys-
tem nearer the river and lower with increasing distance from
the river. The water-table fluctuations were used in conjunc-
tion with measured soil-moisture levels measured by in-situ
neutron probes. They used these data and the Skaggs method
to determine aquifer specific yield (Skaggs et al. 1978). This
method calculates the specific yield by using the difference
in soil-moisture profiles at two different water-table
positions. Using this method, McKay et al. (2004) calculated
specific yield from the water-table decrease caused by
phreatophytes and found that it was similar, or between
0.19 and 0.29 gal/min (0.7-1 L/min), to the values calculated
by an on-site pumping test, that suggested specific yield was
0.16-0.31 gal/min (0.6-1.1 L/min).
This calculation of specific yield was based on an equa-
tion. Simple analytical groundwater models also can be used
to evaluate pump-and-treat relative to phytoremediation as
remedial strategies. By definition, for pump- and-treat to be
effective, there either has to be hydrologic containment or
the contaminated pore volume of groundwater has to be
cleaned up. An aquifer pore volume (PV) is described
by the USEPA as the volume of groundwater within a
contaminated plume. Cleanup is defined when 10-100 pore
volumes of groundwater are removed. The pore volume can
be calculated if the dimensions of the height, width, and
length of a particular aquifer plume can be estimated, as
well as the porosity. The USEPA (U.S. Environmental Pro-
tection Agency 1996) reported that at 24 sites where pump-

and-treat had been started and where the time needed to
pump 20 pore volumes was calculated, the time ranged
from 1 year at a small site (0.3 ha) to 3,015 years at a larger
site (3,100 ha). The average time to achieve 20 pore volumes
at all 24 sites was 274 years at an average site of 189 ha.
There was not a direct relation between site size and time to
remove 20 pore volumes, however, because the extraction
rates at each site differed greatly.
The efficiency of pump-and-treat systems is a function of
the removal, but this, of course, is a function of the aquifer
hydrogeology. Expressed in another way, the removal or
extraction rate, Q e , is a function of the sustainable aquifer
yield, Y s . From what was presented in Chap. 4 regarding
specific yield, S y , the sustainable yield, Y s , is dependent upon
certain constraints that will ultimately impact Q e , such as
hydraulic conductivity, groundwater levels, and effective
porosity. Fortunately, these physical properties can be
assessed and measured and used in simple analytical models
to determine whether or not pump-and-treat will be efficient.
Moreover, as the removal efficiency approaches lower
limits, a case can then be made that groundwater removal
by plants would produce a closer and less expensive match
to the sustainable aquifer yield.
Sustainable aquifer yield is related to the number of
extraction wells used, the sites dimensions, and aquifer
hydrogeology. The NRC (National Research Council 1994)
reviewed 77 pump-and-treat sites and determined that the
average number of extraction wells was 9, with the maxi-
mum number being 15. In addition to the total number of
wells at each site, the distribution in terms of the distance
between wells is important, because each pumped well will
have its own cone-of-depression, or radius of influence,
based on the pumped rate and the aquifer characteristics. If
too few wells are used, it is possible that the cones of
depression will not interact, and groundwater moving to
the wells will pass uncaptured through the aquifer in the
areas where the well's capture zones do not overlap. The
efficiency of containment will decrease as either the distance
between wells is increased or the pumping rate is decreased
or both.
The maximum pumpage rate is ultimately controlled not
so much by the mechanical properties of pumps but by the
maximum sustainable yield of the aquifer. As the sustainable
aquifer yield gets lower than 0.1 gal/min (0.3 L/min), then
a closer well spacing and, hence, increased number of
wells will be required. This scenario represents conditions
under which trees planted on 5-ft (1.5 m) centers for
phytoremediation become a scientifically defensible option
for hydrologic capture.
The frequency of groundwater-level data collection at a
phytoremediation site is an important consideration, as the
data drive the type of approach used. If the goal is to