Hydrology and Physiography of Groundwater and Wetland Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 52

METHOD 4.1.
Sampling Subsurface Waters
Sampling groundwater and water in the vadose zone is more technically
demanding than sampling streams or lakes. To sample water from the va-
dose zone, lysimeters are used (Fig. 4.6). These samplers have a ceramic
cup on the end that absorbs water from the surrounding soil when the
lysimeter is placed under vacuum (Wilson, 1990).
Wells are generally used to sample groundwaters, but these can cover
only a small part of the habitat. Shallow, temporary wells may be installed
by hand where the water table is close to the surface and there are un-
consolidated sediments. Deeper sampling requires well drilling machinery.
When wells are drilled, samples of the pore water can be collected and sed-
iments can be removed from the drilling apparatus. A split-spoon sample
is commonly used in such cases, in which the drill bit takes a core in its
center as it cuts downward. The drill bit is removed and split, and the core
can be analyzed.
After a well is drilled, a casing is inserted through the length of the
well with slots or screens placed in the region from which water is to be
removed (Schalla and Walters, 1990). The outside of the well is then
packed with sand fine enough to keep the sediments from the aquifer from
entering and plugging the well when water is removed (Fig. 4.6). A prob-
lem with the fine packing materials used at the base of wells is that or-
mathematically describe the flow of groundwater and infiltration through
the vadose zone (Bowen, 1986).
The amount of water that can be held in sediment is given by its poros-
ity, or the volume fraction of pores and/or fractures. Higher flows are of-
ten found in higher porosity sediments because more porous materials tend
to have more channels through which water can pass. For example, gravel
and sand pack with large spaces between the particles for water to flow
through. This packing results in large connected channels. Exceptions to
this relationship exist; high-porosity sediments may have a low hydraulic
conductivity when a large proportion of the pores are dead ends and not
involved in flow. An example of this is carbohydrates excreted by microbes.
These extracellular products have a high proportion of water and many
microscopic pores but allow little if any flow through them because the
pores are small and the Reynolds number precludes flow at such high vis-
cosity. The microbial excretions can lower flow through sediments (Battin
and Sengschmitt, 1999). Porosity may also not be related directly to flow
rates because of uneven distribution of pore sizes and the tortuosity, or av-
erage length of the flow path between two points, which varies as a func-
tion of type of material (Sahimi, 1995).
It can be difficult to determine velocity and direction of groundwater
flow. If the elevation of groundwater at one site is lower than at the sec-