Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 178

Fig. 7.8 The changing position of the capillary fringe and water table
and effect on salinity, as reported as electrical conductivity (EC). When
the water table was higher, salinity was lower (1983), but increased as
the water table dropped (1992) (Modified from Rural Industries
Research and Development Corporation 2000). One meter is equivalent
to 3.2 ft.

upper soil layers. For example, hard water contains high
concentrations of the cation calcium, which is considered
undesirable to humans for aesthetic reasons. Hard water is
rendered soft in many homes around the world by replacing
the calcium with sodium. With reference to plants, J. Liebig
stated in the early 1840s that the yield of any crop is limited
by the minimum amount of any essential nutrient, which is
called the law of the minimum (Miller 1938). Some infor-
mation regarding the cation-exchange capacity of an area
can be estimated from knowledge of the geologic history of
the area. The geologic conditions of the region ultimately
determine the presence or absence of these minerals.
The bioavailability of these minerals to plant roots
depends on the balance of acids and bases in the soil
profile, or pH, which is the concentration of the activity
of the hydrogen ion. Most plants that can be used for
phytoremediation purposes do not thrive in very acidic
soils or very basic soils, even when cuttings or large plants
are used. The ability of the soil to moderate changes in the
acid or base content, such as occurs during the infiltration of
low pH precipitation, is an indication of the buffering capac-
ity of the soil. The soil pH can be improved by adding

Fig. 7.7 Root density can change with depth over time and, in this
case, affect the distribution of chloride in soil pore water (Modified
from Rural Industries Research and Development Corporation 2000).
One centimeter is equivalent to 0.39 in.

affected by evaporation. In the United States, this process is
called saline seep and is a common problem for western States
like Montana (Jeffery Kuhn, Montana Department of Envi-
ronmental Quality, written commun. 2008).
At a potential phytoremediation site, knowledge of the
cation-exchange capacity of the soil, either under native or
amended conditions, is important for plant health and should
be measured. The cation-exchange capacity is the capacity
for soil to contain an element of net positive charge, or
cation, that can be reversibly replaced with a different ele-
ment, also of net positive charge. This can occur in the