Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 80

to reach or exceed the ground surface, the well is called a flowing artesian well (Driscoll
1986). Figure 3.14 shows a schematic of an unconfined and a confined aquifer and also
shows artesian and flowing artesian wells. The elevation water can reach in wells drilled
into a confined aquifer is called the piezometric surface and is indicated by the dashed line
in the figure.
Many types of aquifers can exist in certain geological environments where thick
sequences of variable unconsolidated deposits are present. In these cases, the term com-
monly applied is an aquifer system .
In areas of karst topography, groundwater flows through bedrock and initially con-
centrates between bedding layers, fractures, or faults within the sedimentary deposits.
Rock types prone to the development of karst topography contain a high amount of calcite
(CaCO 3 ), which is the primary mineral in limestone. Through time, chemical weathering
along some of the groundwater migration pathways begins to dissolve away the rock and
enlarges the pathway. If this enlargement continues, a cavity or void space called a cavern
is formed. As the cavern grows with continued groundwater migration, it may connect
with other caverns and lead to increased groundwater flow as void space increases. If
water levels drop due to natural groundwater fluctuation or increased rates of groundwa-
ter flow, the caverns fill with air instead of water and become a cave. A cave is a natural
cavity beneath the surface created by groundwater dissolving rock (Burger and Dubertret
1984; Ford and Williams 2007).
Karst formations are cavernous and have high rates of permeability. At many locations
with well-developed karst topography, surface streams disappear and flow underground
through networks of caves only to reappear at the surface sometimes several kilometers
away. Flow rates under these circumstances do not fit into the range of usual groundwa-
ter velocities; they approach those of surface streams. Overall, groundwater flow condi-
tions in karst topography become difficult to evaluate not only because the flow rates vary
widely but also because flow directions can be unpredictable. These flow properties of
karst-resident groundwater are a function of the fracture and dissolution patterns of the
host rock (Ford and Williams 2007). Figure 3.15 is a diagram of karst topography.
Evaluating an aquifer in karst topography requires mapping the fracture pattern of the
host rock. The mapping process begins with tracer testing. A tracer die is placed at an
upgradient location, and the monitoring devices or observation points are set up at several
downgradient locations. The elapsed time between the upgradient introduction of the die
and the first observation of the die at downgradient locations allows for an estimation of the
rate of groundwater flow, general groundwater flow direction, and provides information
on the fracture pattern of the aquifer (Ford and Williams 2007). Figure 3.16 shows fractured
bedrock on the left and a solution enhanced fractured bedrock on the right (Heath 1983).
3.5.2 Heterogeneity and Homogeneity
Heterogeneity and homogeneity are measures of variability used to classify geological
diversity. In geological terms, heterogeneity is a term applied to highly variable or poorly
sorted geologic materials, whereas homogeneity refers to geologic formations without
much variability and characterized as well-sorted (Heath 1983).
Aquifers are typically composed of coarse-grained sediments derived from processes
characterized as high energy. The types of geological processes leading to the formation
of aquifers in unconsolidated sedimentary deposits are fluvial, glacial, lacustrine, beach,
or eolian (USGS 1999). These sediments consist of layered sands and gravels and may
also be cross-bedded (Figure 2.28). As a rule, no two aquifers—or for that matter—no two