Environmental Engineering Reference
In-Depth Information
in the saturated porous media. Data collected in the unsatu-
rated zone cannot come from wells, as the flow that occurs
there is in response to gradients in tension or water potential,
not head. Water samples from the unsaturated zone still can
be collected, however, using tensiometers and lysimeters
which rely on vacuum to move water. These methods are
often applied to phytoremediation purposes, but additional
methods have been developed that are more specific to the
root zone and capillary zone activities of plants, and will be
discussed here.
sample collection may not provide a realistic sample of the
actual geochemistry in the aquifer adjacent to the well
screen. The removal of groundwater from the well prior to
sample collection, especially using a rapid manual method
such as a bailer, can cause short-term changes in the hydrau-
lic gradient between the water in the well and that in the
aquifer. In many cases, the head is more rapidly lowered in
the well than in the aquifer and groundwater will cascade
into the well. This high flow rate can affect many of the
physical properties and contaminants present in groundwa-
ter. For example, it could add dissolved oxygen to previously
anoxic groundwater in the well. It also could preferentially
volatilize VOCs.
With low-flow sample collection, peristaltic or submers-
ible or check-valve-type positive-displacement pumps are
run at low flow rates, less than 1 L/min, to minimize the
negative effects of drawdown differences between ground-
water in the well and in the aquifer. The pumped water is
monitored using flow cells where the groundwater geochem-
istry can be monitored in real time to determine when physi-
cal properties such as DO, temperature, and specific
conductance are stable (U.S. Geological Survey, variously
dated). Then samples can be collected using standard
methods with documented confidence that aquifer pore
water is being sampled.
15.2.1 Conventional Well-Sampling Methods
Conventional groundwater monitoring wells were first used
in the early 1900s by O.E. Meinzer and his staff of USGS
hydrogeologists that investigated the water resources of arid
areas of the southwestern United States. They used water
levels in wells to determine the relation between plant distri-
bution and groundwater depth, among other tasks. Diurnal
fluctuations in groundwater levels measured in wells, as
done by G.E.P. Smith, were used to more accurately depict
the interaction between plants and groundwater in natural
environments. Wells were then used to relate the effect of
plants on groundwater hydrology and geochemistry.
At phytoremediation sites characterized by contaminated
groundwater, conventional monitoring wells also can be used
to determine the effectiveness of the interaction between
plants and groundwater. In order to determine the effect of
individual trees on groundwater, wells should be placed as
close to the trunk of the tree in question as is possible. On the
other hand, if the effect of a mass planting of trees is to be
examined, then groups of two or more wells screened at
the same interval can be placed in pairs upgradient and
downgradient of the planting, in order to determine the
removal of groundwater from the aquifer caused by the plants,
as would be evident from lower groundwater levels and,
therefore, flux, in the downgradient pair of wells.
The sampling of monitoring wells to analyze groundwa-
ter for changes in contaminant compounds or other physical
parameters can be accomplished using a wide variety of
methods. These include those that require the removal of
“stagnant” water that has presumably accumulated in the
well bore since the last period of sampling. This can be
accomplished using manual purging methods, such as a
bailer, or automatic methods where a peristaltic or submers-
ible pump is used (U.S. Geological Survey, variously dated).
15.2.3 Diffusion and Dialysis Methods
There often are small-scale differences in contaminant
concentrations across the vertical thickness of the unsatu-
rated and saturated zones due to differences in contaminant
sources and release histories, as well as differences in sedi-
ment characteristics such as hydraulic conductivity. This can
result in sharp concentration gradients over small vertical
and horizontal distances. The same situation occurs with
root distribution within contaminated sediments. As the rhi-
zosphere is a small zone around the roots limited to just a
few millimeters, there has to be close contact of roots with
contaminated water and sediments for degradation to be
observed. Conventional groundwater wells and lysimeters
may sample larger volumes of groundwater and soil than
necessary to fully understand the interaction of roots and
contaminants, especially with VOCs that are not amenable
to accurate collection by vacuum-based lysimeter methods.
In order to sample such small zones, point-samplers based
on diffusion are often used in groundwater investigations.
These samplers, called dialysis samplers, can be used both
within and above the water table (Hesslein 1976). This
approach was used by Jackson et al. (2005) to investigate
the fate of chlorinated solvents in the root zone of a plantation
of poplar trees established at the Aberdeen Proving Ground in
Maryland. They constructed a cylindrical dialysis sampler
15.2.2 Low-Flow Well-Sampling Methods
During low-flow groundwater sampling, it is recognized that
purging a well of presumably stagnant groundwater prior to
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