Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 218

9.2.1 Groundwater Levels

There can be a lag in time between the changes in baro-
metric pressure in the atmosphere relative to a change in
groundwater level, on account of the diffusivity of the unsat-
urated zone. Such a scenario assumes that the well screen is
fully saturated, for no effect will be seen in a well if part of
the well screen is above the water table. A lag time will not
hold true, however, if the groundwater is characterized by a
large fraction of dissolved or trapped separate-phase gas.
In order to observe the anticipated diurnal changes in
groundwater levels due to plant uptake, all the potential
changes that can affect groundwater levels need to be
accounted for. For a change in groundwater level in a well,
D

A diurnal change in groundwater levels observed in areas
where trees tap groundwater provides the most direct line of
evidence of plant and groundwater interaction. Observation
of plant-induced diurnal groundwater-level fluctuations goes
back almost 100 years. As was introduced in Chap. 1, G.E.P.
Smith of the University of Arizona presented a paper in 1922
that showed that daily fluctuations in water levels in wells
placed in a grove of cottonwood and mesquite trees were due
to groundwater withdrawal by these plants (White 1932).
Five years later, O.E. Meinzer (1927) published the first data
that showed the water table in areas that had plants known to
rely on either groundwater or capillary water directly above
often were characterized by fluctuations. White (1932) also
stated that groundwater levels observed to decline during the
day and not at night were caused by the plant-facilitated
uptake of groundwater—the maximum daily drawdown
observed was 0.13 ft (0.039 m). Later, Meyboom (1966)
observed fluctuations in groundwater of about 0.10 ft
(0.03 m) in wells near willow trees on the banks of small
lakes. This groundwater level fluctuation was large enough
in space and time to stop the discharge of groundwater to the
lake during summer and cause lake water to recharge the
aquifer (Meyboom 1966). In Davis and DeWiest (1966), a
method to estimate groundwater use by trees was presented,
which will be discussed in this section in more detail.
As can be seen from these studies, the magnitude of the diurnal
change in groundwater levels attributed to plants can be small,
and therefore, inherently difficult to measure accurately.
Attributing such potentially small groundwater-level
changes to trees often is problematic because other processes
also can affect groundwater levels at a similar scale.
These processes include barometric pressure changes, tidal
loading, and pumpage—these processes may result in
groundwater-level fluctuations that either mimic or obscure
groundwater-level changes caused by plants.
In any case, measurement of groundwater levels, baro-
metric pressure, and tides can be made to determine baro-
metric and tidal efficiencies to show these factors affect the
static groundwater level (Gonthier 2007) and how to remove
these effects to be able to observe the influence of plant-
water uptake. These changes we are hoping to observe are
the barometric pressure-independent water-level changes
and tidal-independent water-level changes. Groundwater-
level changes caused by barometric pressures, however, are
rarely observed in shallow, unconfined groundwater where
planting has occurred. Barometric effects on groundwater
in unconfined aquifers does not occur because the difference
in atmospheric pressure transmitted to the free surface of a
well and the atmospheric pressure transmitted to groundwa-
ter in adjacent aquifer material is negligible (Landmeyer
1996).

W , is defined as the groundwater level at time t+1 minus
the water level at a previous time t , such that

D W ¼ W tþ 1 W t (9.4)

W can be caused by any or all of the following
processes, such as that caused by barometric pressure,

The D

D W b ,

recharge, D W r , pumping, D W p , earth tides, D W g , ocean tides,

D W m , evaporation, D W e , surface-water level changes, D W s ,

transpiration, D W t , and all other possible processes, D W o

(Gonthier 2007). In sum, Eq. 9.4 becomes

D W ¼ D W b þ D W r þ D W p þ D W g þ D W m þ D W e

þ D W s þ D W t þ D W o (9.5)

W ,
groundwater levels should be measured between precipita-
tion events, under conditions of relatively stable barometric
pressure, in areas where no pumping from the site exists,
and tides are under slack conditions, such that

In order to best examine the effect of plants on D

D W r , D W b ,

D W m are near zero. However, because the time

interval for changes in groundwater levels by plants is simi-
lar to the daily changes in barometric pressure experienced
at most sites, these non-plant-induced changes will need
to be determined and removed from the water-level
measurements.
To do this, barometric and tidal efficiencies for each well
will need to be calculated. At sites where barometric or tidal
forces may affect groundwater level, measurements of baro-
metric pressure and tidal height need to be monitored, or
data need to be gathered from nearby existing monitoring
stations. The barometric efficiency,

W p , and D

a b , is calculated as

a b ¼ D W =D B (9.6)

where B is the

barometric change, in equivalent head units. In general,
decreases in barometric pressure result

D W is the groundwater level change and D

in increases in

groundwater levels. The tidal efficiency, a t , is calculated as

a t ¼ D W =D T (9.7)