Page 344 - Laboratory Manual in Physical Geology

Mapping Water Table Topography ■ ( Sinkholes )—surface depressions formed by the
To better understand the topography of the water table collapse of caves or other large underground void
in a region, geologists measure its elevation wherever they spaces.
can find it in wells or where it forms the surfaces of lakes ■ ( Solution valleys )—valley-like depressions formed by
and streams. The elevation data is then contoured to map a linear series of sinkholes or collapse of the roof of a
the water table contour lines ( FIGURE 12.1B ). Since water linear cave.

always flows down the shortest and steepest path it can
find (path of highest hydraulic gradient), a drop of water ■ ( Springs )—places where water flows naturally from
on the water table surface will flow perpendicular to the the ground (from spaces in the bedrock).
slope of the water table contour lines. Geologists use flow ■ ( Disappearing streams )—streams that terminate
lines with arrows to show the paths that water droplets abruptly by seeping into the ground.
will travel from the point where they enter the water table
to the point where they reach a lake, stream, or level water Much of the drainage in karst areas occurs
table surface. Notice how flow lines have been plotted on underground rather than by surface runoff. Rainwater

FIGURES 12.1B and 12.1C . In FIGURE 12.1C , notice how seeps into the ground along fractures in the bedrock

water is being withdrawn (pumped) from a well in an ( FIGURE 12.4 ), whereupon the acidic water dissolves the

unconfined aquifer faster than it can be replenished. This limestone around it. The cracks widen into narrow caves
has caused a cone-shaped depression in the water table (underground cavities large enough for a person to enter),
( cone of depression ) and a change in the regional flow which may eventually widen into huge cave galleries.
of the groundwater. Thus, water table contour maps are Sinkholes develop where the ceilings of these galleries
useful for determining the following: collapse, and lakes or ponds form wherever water fills
the sinkholes. The systems of fractures and caves that

■ Paths of groundwater flow (flow lines on a map), along typically develop in limestones are what make limestones
which hydraulic gradients are normally measured
good aquifers.
■ Where the water comes from for a particular well Eventually, the acidic water that was dissolving lime-

stone becomes so enriched in calcium and bicarbon-

■ Paths (flow lines) that contaminants in groundwater ate that it turns alkaline (the opposite of acid) and may
will likely follow from their source
actually begin precipitating calcite. Caves in karst areas
■ Changes to groundwater flow lines and hydraulic gra- often have stalactites ( FIGURE 12.5 ), icicle-like masses of

dients caused by cones of depression at pumped wells chemical limestone made of calcite that hang from cave
ceilings ( FIGURE 12.5 and FIGURE 6.8 ). They form because

calcite precipitates from water droplets as they drip from
Caves and Karst Topography the cave ceiling. Water dripping onto the cave floor also
The term karst describes a distinctive topography that can precipitate calcite and form more stout stalagmites.
indicates dissolution of underlying soluble rock, gener-

ally limestone ( FIGURE 12.2 ). Limestone is mostly made of

calcite (a carbonate mineral), which dissolves when it reacts
with acidic rainwater and shallow groundwater. ACTIVITY
Rainwater may contain several acids, but the most 12.3 Floridan Limestone Aquifer
common is carbonic acid (H CO ). It forms when water
2 3
(H O) and carbon dioxide (CO ) combine in the atmo-
2 2
sphere (H O + CO = H CO ). All natural rainwater THINK | What is karst topography and how
2 2 2 3
is mildly acidic (pH of 5–6) and soaks into the ground to About It does water flow beneath it?
form mildly acidic groundwater. There, bacteria and other
underground organisms produce carbon dioxide (CO ) OBJECTIVE Construct a water table contour map
2
as a waste product of their respiration (metabolic process and determine the rate and direction of groundwater
whereby they convert food and oxygen into energy, plus movement.
water and carbon dioxide waste). This carbon dioxide
makes the groundwater even more acidic, so it easily dis- PROCEDURES
solves the calcite making up the limestone by this reaction: 1. Before you begin , read the Introduction and
Caves and Karst (above, if you have not already
-1
CaCO + H CO = Ca +2 + 2HCO done so) and the Floridan Aquifer (below). Also,
3
3
3
2
Calcite Carbonic acid Calcium ions Bicarbonate this is what you will need :
dissolved in ions dissolved
groundwater in groundwater ___ calculator
A typical karst topography has these features, which ___ Activity 12.3 Worksheet (p. 325 ) and pencil
are illustrated in FIGURE 12.2 and visible on the US Topo 2. Then follow your instructor’s directions for

orthoimage of the Park City, Kentucky Quadrangle in completing the worksheet.
FIGURE 12.3 .

314 ■ L ABOR ATORY 12