Hydrology and Physiography of Groundwater and Wetland Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 53

ganisms larger than those able to pass though the packing material cannot
be sampled. Thus, without specially designed wells (e.g., like those in Fig.
4.6 but without screens or fine packing materials), groundwater ecologists
may miss significant components of the groundwater fauna.
Material is packed in the hole outside the casing above the slotted por-
tion to form a seal. Otherwise, water could move vertically into the aquifer
from the surface or between aquifer layers and contaminate the sample
from the desired depth. Bentonite is commonly used for this sealing be-
cause it is relatively chemically inert and swells when wetted.
Once a well is installed, it must be developed. Developing entails re-
moval of a large volume of water and sediments in the water to ensure that
the well flows clearly and supplies water representative of the aquifer. The
well must be sampled regularly, with several well volumes removed in each
sampling, so water will not become stagnant. During sampling, several
volumes of water in the casing must be removed before the actual sample
is collected to ensure that the water sampled is from the aquifer outside
the well.
Various pumps and bailers are available for sampling groundwater.
The type of analysis to be performed on the samples collected should be
known before selecting the system. For trace metal analysis, pumps with-
out metal parts that may contaminate samples are used. When organic ma-
terials are to be analyzed, pumps that do not use oil are essential because
the oil may contaminate the water samples.
ond, and the two are hydraulically connected, then it is assumed that wa-
ter is flowing from the higher to the lower site. The difference in elevation
between the two sites is known as hydraulic head . Releasing a tracer at the
upper site and monitoring its appearance at the lower site can indicate wa-
ter velocity directly.
Groundwater habitats can be divided into a variety of subhabitats (Fig.
4.4). For example, aquifers can flow through regions of continuous homo-
geneous substrata (even distribution of permeable substrata such as sand,
clay, or gravel) with little obstruction. Other aquifers can occur where many
alternate flow pathways exist because heterogeneous distribution of imper-
meable materials in the subsurface results in variable flow patterns and di-
rections (e.g., aquifers with large rocks embedded in fine sediments or with
patches of low-hydraulic-conductivity material interspersed among high-
hydraulic-conductivity materials). An aquifer between two impermeable
layers is confined. Complicated groundwater flow patterns make determin-
ing the fate and source of waters difficult. Heterogeneous flow patterns are
of concern when considering groundwater contamination problems because
such patterns interfere with assessing the extent of the problem and attempts
to clean up contaminants. Methods are available for sampling groundwater
(Method 4.1), but the number of samples that can be collected is limited
relative to those from surface water habitats.