Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 144

groundwater. For example, groundwater, by definition, is
water present in the pores of sediments equal to or greater
than atmospheric pressure as described in Chap. 4. The roots
and root hairs of phreatophytes in contact with groundwater
do not have to overcome water tension present in the capil-
lary fringe. The implication for plants whose roots interact
with the water table is that the concept of wilting point does
not hold true for these roots and water will be available for
uptake on a continual basis, as long as it can be accessed by
the roots and as long as sufficient oxygen is available to
support root respiration.
The selective advantage that phreatophytes have in terms
of adapting to a reduction in soil moisture from decreased
precipitation and drying surface soils and increasing nega-
tive water potentials is their ability to remove water under
less negative tensions than the wilting point. The energy
required to overcome the tension of water is equal to the
osmotic water potential—the roots and the energy to lift the
water through the plants to the atmosphere by the sun. By
comparison, other forms of life in the subsurface, such as
soil and aquifer microorganisms, can tolerate tensions no
less than

soil and water and, therefore, should be determined as part of
site assessment. The bulk density of soil also can be
measured easily. Bulk density refers to the ratio of dry soil
to the total soil volume. In an uncompacted, humus-rich peat
deposit, for example, the bulk density is lower than that of a
similar volume of tightly compacted silty sand.
Conversely, the water that occupies the pore spaces of a
volume of soil can be quantified using the concept of water
content. Water content is determined by subjecting a
weighed, moist, soil sample to oven drying and then
reweighing the sample. The mass lost relative to the wet
mass is the water content. Water content does not indicate
either water availability to plants or the potential for water
flow to occur, as was stated previously.

6.5 Hydrogeologic Assessment

and Characterization

Perhaps the most important hydrogeologic factors that will
determine the success of a phytoremediation project for
hydrologic control at a site characterized by contaminated
groundwater are the thickness of the capillary fringe and
depth to groundwater. Simply put, if the minimum depth of
the water table at its seasonal highest is beyond the reach of
the deepest roots, then alternative remedial strategies may

0.01 MPa (Table 6.2 ).
To summarize for the purposes of site-assessment
activities, the presence of water, or water content in unsatu-
rated soils above the water table, or the water table itself,
does not necessarily guarantee that plants will thrive. This is
because the presence of water does not mean that water can
move through the soil pores into the root hairs. This is
determined by the bioavailability of water, or the water
potential, which indicates whether or not water can be
removed. Because plants derive water from the soil by
osmosis and transport this water to leaves along a vapor-
pressure gradient driven by evaporation, the remaining water
is removed from the soil surface by enough energy to over-
come surface tension (Fig. 6.7 ). At the point where surface
tension is too high for plants to extract additional water
molecules from the soil surface, plant-water uptake stops,
even though water may still be present.

6.4.6 Soil Bulk Density and Water Content

Fig. 6.7 As the water potential becomes more negative as sediments
dry out, it is more difficult for water to enter root hairs. 1 atm is
equivalent to 0.10 MPa.

The concept of bulk density familiar to soil scientists can
provide useful information regarding the relation of plants to

Table 6.2 The water potential of various soil conditions encountered at phytoremediation sites.
Water potential (MPa) Classification Moisture characteristics

30

Unsaturated Hygroscopic water-bound to sediment.

1.5 Unsaturated Wilting point-tension limit at which plants can remove water.

0.03 Unsaturated Field capacity-water left after movement by gravity.

0.99 to 0.001 Capillary Capillary zone and fringe-water that moves due to tension.

> 0 Saturated Water table-water flows to wells.