Historical Foundation of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 21

Fig. 1.12 One of the earliest
observations of the daily
fluctuation in groundwater over
time from its highest level
( dashed horizontal line ) in a well
installed in a nonirrigated alfalfa
field in 1925. The daily decline
occurred as groundwater was
used by the plants. The
groundwater-level rebound was
sequentially lower each day, as no
precipitation occurred (Modified
from White 1932).

An important aspect of White's work was using labora-
tory approaches at the field scale to determine exactly how
plant removal of groundwater was causing the observed
groundwater-level fluctuations. White designed tanks that
were filled with soil, water, and plants to reproduce the
conditions of the water table in the field, similar to tanks
designed by Lee. Figure 1.15 depicts a diagram similar to
that White used to describe the effect of plant uptake of
groundwater on a simulated water-table surface. At the bot-
tom of each of six experimental tanks, a layer of gravel was
placed and covered by a thicker layer of soil. Water was then
added to the gravel layer through a recharge well in the
center of the tank that was open at the bottom. Water
displaced the air and saturated the highly permeable gravel
and subsequently the less permeable soil above. When these
two layers were completely saturated, with no air spaces, the
addition of new water was stopped. Above the water level in
the saturated soil, water was drawn up farther into the soil by
capillary forces against gravity. This movement created a
zone of water under tension.
White, among others, noted that most plants that relied on
groundwater had roots within the capillary fringe, and some
even had root growth below the water table in fully saturated
sediments. White's experimental tanks contained plants
ranging from grasses to woody plants common to the west-
ern United States, and the results of his experiments
indicated that as the roots took up groundwater from the
capillary fringe to meet transpiration demands, a hydraulic
gradient was established that caused groundwater to move
upward from the capillary fringe. The volume of groundwa-
ter removed from the capillary fringe by plants during the
day induced groundwater to move upward from the saturated

Fig. 1.13 Comparison of the daily fluctuation of the water table in a
field of greasewood ( A , C ) and in a control plot of cleared land ( B , D )
showing the effect of plant and water use on groundwater levels relative
to soil-water evaporation (Modified from White 1932).

groundwater levels recover by induced local convergent
flow, and s is the net fall in groundwater level during the
same 24-h period, in length. The terms r and s are derived
from the water-table fluctuation data generated in a well at a
site over at least a few days.