Hydrology and Physiography of Groundwater and Wetland Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 51

groundwater reservoirs that are useful to humans. The dynamic zone of
transition where both surface water and groundwater influences are found
is referred to as the hyporheic zone . This zone forms a transitional habitat
(ecotone) where there is a change between groundwater and surface water
organisms. A hyporheic zone can be found between groundwater and wet-
lands, streams, or lakes (Gibert et al., 1994).
Much of the water flow from land into the world's oceans is from
rivers. However, some areas, such as the southeastern United States, are
characterized by large discharges of groundwater into marine waters
(Moore, 1996). Likewise, the ecology of streams (Allan, 1995; Jones and
Holmes, 1996; Brunke and Gonser, 1997), wetlands (Mitsch and Gos-
selink, 1993), and lakes (Hagerthey and Kerfoot, 1998) can be influenced
by groundwater (Freckman et al., 1997). Thus, knowledge of groundwa-
ter flows and processes is integral to the study of aquatic systems.
Soil texture and composition determine how rapidly water percolates
into groundwater habitats. Impermeable layers, such as intact layers of
shale or granite, do not allow water to flow deeper. In very fine clays or
those with large amounts of organic material, the rate of percolation can
be very low. In contrast, gravel and sand have relatively rapid water flow
(Table 4.2). Infiltration capacity partially determines the proportion of wa-
ter that flows off the surface and the quantity that enters groundwater or
the aquifer. The rate of which water percolates into an aquifer is referred
to as the rate of recharge .
Infiltration rate can have important practical consequences. For exam-
ple, groundwaters can be contaminated when sewage sludge is disposed of
on cropland if infiltration rates are high enough that contaminants enter
groundwater. Thus, infiltration rate is an important aspect of determining
sewage application levels (Wilson et al., 1996). Once water enters ground-
water, permeability determines the potential rate of flow (hydraulic con-
ductivity) and is variable and dependent on geology. Water will flow slowly
in fine sediments and more rapidly where large channels exist (e.g., in lime-
stone aquifers with channels and unconsolidated sediments with large ma-
terials such as cobble). Hydraulic conductivity is partially dependent on the
Reynolds number (see Chapter 2) because viscosity is high and flow is slow
when Reynolds numbers are small (i.e., when sediment particles are small).
Darcy's law can express the rate at which water moves through aquifers.
This law states that the flow rate in porous materials increases with in-
creased pressure and decreases with longer flow paths. The law is used to

TABLE 4.2 Representative Particle Sizes and Hydraulic Conductivity of
Various Aquifer Materials a

Hydraulic conductivity (m d 1 )

Material Particle size (mm)

Clay 0.004 0.0002

Silt 0.004-0.062 0.08

Coarse sand 0.5-1.0 45

Coarse gravel 16-32 150

a Data from Bowen (1986).