Geology Reference
In-Depth Information
EXERCISE
12
Groundwater Hydrology
INTRODUCTION
Groundwater is an important component of the
hydrologic cycle. It feeds lakes, rivers, wetlands, and
reservoirs; it supplies water for domestic, municipal,
agricultural, and heating and cooling systems.
Groundwater resources at a site vary with natural and
artificial recharge and discharge conditions. Because
we dispose of wastes improperly or mishandle
materials on the land surface, we pollute some
groundwater reservoirs. For resource planning and
waste management, it is essential that we understand
the quantity, quality, and movement of water in
bedrock and regolith or surficial aquifers. This exercise
is an introduction to the basics of groundwater
hydrology (hydrogeology) and to interpretation of the
subsurface with geologic cross sections.
available for transmitting water. (In some coarse-grained
materials effective porosity approximates specific yield
and gravity drainage.) Effective porosity is difficult to
measure and is often approximated from total porosity
and lab test data. Effective porosity is a factor in
velocity of groundwater flow and is expressed as a
percentage (Table 12.1).
The movement of water through a rock is also
controlled by its permeability or hydraulic conductiv-
ity. The term hydraulic conductivity (K) is used in hydro-
geology to describe the ease with which water can
move through a formation and is often measured in
units of length/time. Both fine grained and poorly
sorted materials have low K values. Values for K are
obtained in the lab and in the field. Representative val-
ues for different rock and unconsolidated materials are
given in Table 12.1 in ft/day or ft d -1 . Sometimes val-
ues for K are given in m/d, m/s, or gal per day/ft 2 .
Some materials, such as clay and silt, may have a high
porosity and hold much water; however, they have
low effective porosity and low hydraulic conductivity
because the openings are very small or not connected.
Such units are aquitards because they retard the flow
of water. Aquifers that have high hydraulic conductiv-
ity provide large quantities of water to wells.
In some aquifers groundwater occurs under
water table or unconfined conditions (Figure 12.2). In
this case the water table is the boundary between the
zones of aeration and saturation. Where the water table
intersects the land surface, springs, seeps, streams, and
lakes are formed. The position of the water table can
be determined by measuring the depth to water in a
well tapping an unconfined aquifer.
Layers of low permeability confine many aquifers,
and water in them is stored under pressure (Figure 12.2).
When a well is drilled into such a confined or artesian
aquifer, water rises in the well to some level above the
base of the confining bed. In some cases the well may
even flow at land surface. The water level (also known
as the potentiometric, piezometric, or water-pressure surface)
represents the artesian pressure in the confined
aquifer.
PART A. GROUNDWATER
Part of the water that reaches the land in the form of
precipitation infiltrates to become groundwater. Ground-
water occurs in openings in rocks and unconsolidated
materials (Figure 12.1) and moves under the influence
of gravity or pressure. An aquifer or groundwater reser-
voir is a water-saturated geologic unit composed of
rock or unconsolidated materials that yields water to
wells or springs. Generally, unconsolidated materials
such as sand and gravel have more spaces than solid
rock; the openings are due to incomplete cementation
of the grains or to fracturing or partial solution of the
rock. Openings in igneous and metamorphic rocks are
generally due to fractures and joints. The ratio of the
open spaces relative to the rock or regolith volume is
called porosity (n), which is expressed as a percentage
(Table 12.1). Porosity is a storage factor.
Not all the pores of a rock or regolith are available
for flow of water. Some water does not flow because of
molecular forces, surface tension, and dead-end pores.
Effective porosity (n e ) is the amount of pore space that is
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