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Exercise 9
Water Resources
James S. Reichard
Georgia Southern University
Student Name _________________
Section _______
In this lab you will:
examine the geology of groundwater resources and the impact that pollution and human
withdrawals can have on natural hydrologic systems.
Background Reading and Needed Supplies
Prior to doing this exercise you should read Chapter 11 on Water Resources in the textbook.
With respect to supplies, you will need a calculator, ruler, and colored markers.
Part I - Groundwater Exploration
In many areas of the world local governments and individual homeowners obtain freshwater
by installing wells in a suitable aquifer. As illustrated in Figure 9.1, installing a well involves first
drilling a hole into the subsurface using a specialized truck. Here a drill bit is attached to a
rotating pipe. As the bit advances into the earth, the drilling operator usually records the type of
material that is forced to come up out of the hole. This results in a written record called a drilling
or well log. The log lists the different types of earth material according to the depth they were
encountered in the hole. By knowing the elevation of the land surface, one can easily determine
the elevation of the contacts between the different types of earth materials. For example,
suppose the driller in Figure 9.1 continued drilling and encountered the contact between the
sand and clay at a depth of 70 feet. By subtracting 70 feet from the land elevation of 575 feet
above sea level (ASL) would tell us that this contact is at 505 feet ASL.
Figure 9.1 – collecting samples as a drill bit advances into the subsurface.
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1) You are to map the subsurface extent of a sand and gravel aquifer located near Perrysville,
Indiana, using actual water-well logs listed below. The logs correspond to the wells numbered
1 through 6 on the USGS topographic map in Figure 9.2.
Well #1
elev. = 608
Well #2
elev. = 568
Well #3
elev. = 540
Well #4
elev. = 550
0-125 clay
125-150 bedrx
0-21 clay
21-154 gravel
0-4 clay
0-12 clay
4-133 gravel 12-34 bedrx
133-134 bedrx
Well #5
elev. = 530
Well #6
elev. = 670
0-80 bedrx
0-106 clay
106-188 bedrx
It is important to note that in this area, glaciers deposited a thick blanket of till (mostly clay) on
top of a pre-existing bedrock surface. During the cyclical glaciations, sand and gravel bodies
were also deposited in the till by melt-water streams. These sand and gravel bodies now serve
as the primary aquifer for the region. To map the aquifer near Perrysville, you will use the above
well data to construct a cross section on the graph paper in Figure 9.3. Carefully follow the stepby-step instructions provided below and use PENCIL only.
a) From the well-log data, plot the surface elevation of each well on Figure 9.3. Then, using a
ruler, draw a straight line between the surface and bottom depth (BD) of each well (i.e.,
where drilling stopped). The lines you drew show where we have data in the subsurface.
b) The topographic map shows that along the cross-section line, the river is about 490 feet
above sea level. Plot this elevation below the point indicated for the river in Figure 9.3.
c) Using a hand-drawn line (NO ruler), show the position of the land surface by connecting
the surface elevation points of the river and wells. Note that your profile should reflect the
presence of the floodplain located on both sides of the river (see map in Figure 9.2).
d) From the drilling data, determine the elevation of the contacts between the different earth
materials in each well. Note that wells 1, 2, 4, and 6 have only one contact, whereas well 3
has two contacts and well 5 has none (i.e., contains all bedrock).
e) Use short horizontal lines (tick marks) on the graph paper in Figure 9.3 to indicate the
elevation of the contacts you determined above.
f) Using a light pencil, record the type of earth material that lies above and below the contacts
in each well.
g) Map the top of the old bedrock surface by drawing a line (by hand) from the contact in
each well that marks the top of the bedrock. Note that all but well 2 were drilled deep
enough to reach the bedrock. Also, the bottom depths (BD) of the wells are NOT contacts,
so do NOT connect your bedrock line to these points.
h) Now you need to draw the remaining contact between the gravel and clay. This is trickier
as both gravel and clay are not present in every well. In some places you’ll have to end the
contact at the land surface, and in others at the top of the bedrock.
i) When finished, check your interpretation with your instructor. After checking, color the
gravel unit yellow, the clay gray, and the bedrock brown.
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Figure 9.2 – USGS topographic map showing the location of water wells near Perrysville, Indiana. Circled wells are used in this exercise.
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Figure 9.3 – Graph paper for constructing a cross section along wells 1 through 6 shown in Figure 9.2.
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2) If you were given the task of installing a large municipal water-supply well along your cross
section, where would the best place be in terms of the geology? Explain why you chose the
location you did.
3) Considering the location of your cross section on the map in Figure 9.2, could there be an
even better location for the new well than the one you described above? Explain where.
Part II – Groundwater Flow
Recall that the water level in unconfined aquifers is referred to as the water table, but in
confined aquifers, which are pressurized, the water level is called the potentiometric surface.
Also recall that groundwater flows in the direction of the hydraulic gradient, which means that
water flows from areas where the hydraulic head (potential energy) is high, to areas where the
head is lower. As illustrated in Figure 9.4, the direction and magnitude of the hydraulic gradient
in the horizontal direction can be found by measuring the height (elevation) of the water in two
different wells. If both the head and distance between two wells are measured using the same
units (e.g., feet), then the computed value for hydraulic gradient will be unitless (i.e., have no
units).
Figure 9.4 – determining the hydraulic gradient in the horizontal direction by measuring the
water level in two wells.
By knowing the hydraulic gradient and the conductivity and porosity of earth materials, one can
determine the actual velocity of groundwater using the following version of Darcy’s Law:
groundwater velocity =
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conductivity
× gradient
porosity
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4) The following table lists the depth to water and top elevation of the three wells shown in
Figure 9.5. From these data, determine the hydraulic head (elevation) in each of the wells.
depth to water
well
casing elevation
above sea level
1A
231 ft
12 ft
1B
231 ft
45 ft
2B
237 ft.
44 ft
hydraulic head
(ft. above sea level)
Figure 9.5 – Cross section showing the depth to water for the wells. Note that the screened
interval of the shallow well is open only to the unconfined aquifer, whereas the two
deep wells are screened in a confined aquifer.
5) Although hydraulic head is determined by measuring the height of the water column in a well,
the head actually represents the amount of potential energy at the BOTTOM where
groundwater enters the screened (open) interval of the casing. To help visual this important
concept, take the head values from the table above and write them next to the
corresponding well screens in Figure 9.5.
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6) Based on the head values you recorded on Figure 9.5, describe whether groundwater within
the confined aquifer is flowing from well 1b to 2b, or from 2b to 1b. Explain how you know.
7) Calculate the magnitude of the hydraulic gradient within the confined aquifer in Figure 9.5.
Your answer will be unitless because both head and distance were measured in the same
units, namely feet (i.e., feet over feet cancel out).
8) Suppose that the hydraulic conductivity of the confined aquifer in Figure 9.5 was determined
to be 0.55 ft/day, and the porosity 0.15 (15%). Use Darcy's Law to calculate the horizontal
groundwater velocity in ft/day within the confined aquifer.
groundwater velocity =
conductivity
× gradient
porosity
9) Use the familiar relationship that velocity equals distance divided by time to determine the
number of years it would take for groundwater to travel between the two deep wells.
10) In terms of the vertical leakage of groundwater across the aquitard in Figure 9.5, describe
whether the vertical flow is upward or downward. Again, explain how you know.
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11) Notice that the separation distance between the unconfined and confined aquifers is 50 feet.
Using this distance and the head values in wells 1a and 1b, compute the hydraulic gradient in
the vertical direction between the two aquifers.
12) Suppose that the hydraulic conductivity of the aquitard in the vertical direction is 0.000030
ft/day and the porosity is 0.25 (25%). Use Darcy's Law to calculate the velocity in ft/day of
water flowing across the aquitard.
groundwater velocity =
conductivity
× gradient
porosity
13) As before, us the relationship that velocity equals distance divided by time, determine the
number of years it would take for groundwater to travel across the aquitard.
14) Based on what you learned in this section, explain why confined aquifers are less likely to
become contaminated compared to unconfined aquifers.
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Part III – Floridan Aquifer System
In this section we will examine how heavy pumping withdrawals from the Floridan aquifer
system in coastal Georgia have led to saltwater intrusion problems. Figure 9.6 illustrates how
this confined aquifer system underlies most of the coastal plain in Georgia, Alabama, and parts
of South Carolina as well as the entire state of Florida. Here the aquifer system is composed of
as much as 2,000 feet of limestone rock, with the more conductive layers serving as the primary
water supply throughout the region. However, saltwater intrusion is occurring in areas of large
groundwater withdrawals. As illustrated in Figure 9.7, saltwater intrusion commonly occurs when
pumping withdrawals create a cone-of-depression that draws saltwater into parts of an aquifer
that once contained only freshwater.
Figure 9.6 – (A) Extent of the Floridan aquifer system; (B) areas of major water-level declines
due to heavy pumping withdrawals (from USGS Professional Paper 1403).
(A)
(B)
Figure 9.7 – Illustration showing how pumping in a confined aquifer creates a cone-ofdepression and allows saltwater to move into the freshwater portion of the aquifer.
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Before examining the saltwater intrusion problems with the Floridan aquifer, we need to explore
how a contour map of a water table (unconfined system) or potentiometric surface (confined
system) can be used to construct groundwater flow paths. For example, the map in Figure 9.8A
shows how water-level data from individual wells were used to draw a contour map of the water
table—similar to how the land surface is contoured using elevation data. From this map view
you should be able to visualize the corresponding cross-sectional view (9.8B) showing how the
water table slopes toward the right. Because groundwater flows in the direction where the
hydraulic gradient is the steepest, flow lines are drawn on a map such that they cross the
contour lines at right angles (9.8A). As can be seen in Figure 9.9, if the contours lines
themselves are curved, then the groundwater flow lines must also be curved. Note how this
causes flow lines to diverge or converge, depending on the shape of the contours.
Figure 9.8 – Contour map (A) showing the elevation changes of a sloping water table. The
slope of the water table can also be seen in cross-section (B).
A)
B)
Figure 9.9 – Map view showing how groundwater flow lines maintain a right angle relationship
to water-level contours.
Exercise Questions
The following questions in this section will pertain to the contour maps in Figure 9.10 and
9.11. These maps, located along the Georgia and South Carolina coast, show the figuration of
the potentiometric surface of the Floridan aquifer prior to major groundwater withdrawals (1900)
and after development (1986). These maps also show various landscape features, such as
streams, towns, and county boundaries. It is important to remember that the contours represent
the amount of hydraulic head within the aquifer, NOT the elevation of the landscape.
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15) There are 4 points labeled with an "X" on the pre-development map in Figure 9.10. Using a
red pencil, draw flow lines from each point indicating the path that groundwater would have
taken in the subsurface—put arrows at the end of each line to show flow direction.
Remember that flow lines should cross the contour lines at right angles.
16) Find the black dot representing the town of Pembroke in Figure 9.10. Suppose that in 1900
the people of Pembroke discovered that the groundwater in the Floridan aquifer had become
contaminated from some unknown source. Could the source possibly have been from the
city of Stilson, located about 15 to the northeast? Explain your answer.
17a) Using a blue pencil, draw another flow line from Pembroke to the coast—your arrow should
come close to the point on Hilton Head Island.
b) Determine the curved distance, in feet, between Pembroke and the point on Hilton Head—
here you'll need to convert the map distance to ground distance using the map scale.
c) Based on the head contours from the map and the ground distance you just calculated,
determine the hydraulic gradient in unitless form between Pembroke and Hilton Head.
d) Calculate the pre-development groundwater velocity (ft/day) between Pembroke and Hilton
Head using Darcy's Law. For the conductivity of the Floridan aquifer use 200 ft/day and for
porosity use 0.20 (20%).
groundwater velocity =
conductivity
× gradient
porosity
e) Using the relationship that velocity equals distance divided by time, determine the number
of years it would have taken for groundwater to travel from Pembroke to Hilton Head.
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Figure 9.10 – Potentiometric surface contours (in feet) of the Floridan aquifer in 1900 (USGS Water Supply Paper 2411).
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18) Using a red pencil, draw groundwater flow lines from the seven points labeled with an "X" on
the 1986 post-development map (Figure 9.11). Again, put arrows at the end of each flow line
and be sure that your lines cross the head contours at right angles.
19) Could contaminated water from Pembroke ever reach Hilton Island under the 1986
conditions? Explain your answer.
20) Notice that some of the flow lines going into the cone-of-depression actually originate in the
offshore marine environment. What do these flow lines from the marine environment indicate
might be happening to the aquifer?
21a) What is the lowest elevation shown for the potentiometric surface in the cone-ofdepression (i.e., drawdown cone) on the 1986 map?
b) What was the elevation of the potentiometric surface in this same area under the predevelopment conditions shown on the 1900 map (Figure 9.10)?
c) Determine the number of feet that the potentiometric surface has declined within the coneof-depression between 1900 and 1986.
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Figure 9.11 – Potentiometric surface contours (in feet) of the Floridan aquifer in 1986 (USGS Water Supply Paper 2411)
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22) The major decline in hydraulic head within the Floridan aquifer has indeed allowed saltwater
to intrude into what previously had been freshwater parts of the aquifer. Saltwater today is
found in wells on Hilton Head and Tybee islands.
a) Using a blue pencil, draw a flow line from the point labeled S on Tybee Island to the center
of the cone-of-depression.
b) Determine the actual length of your flow line from point S.
c) Calculate the hydraulic gradient between point S and the center of the drawdown cone.
d) Use Darcy's Law to compute the groundwater velocity. Assume that the aquifer's hydraulic
conductivity is 200 ft/day and porosity is 0.20.
groundwater velocity =
conductivity
× gradient
porosity
e) Using the relationship that velocity equals distance divided by time, determine the number
of years it should take for the saltwater to travel from Tybee Island and the water-supply
wells in the center of the drawdown cone.
23) The salinity of the Floridan aquifer within much of the drawdown cone will eventually
become so high that the groundwater would have to undergo expensive treatment before
being used for municipal or industrial purposes. Describe several steps that could be taken to
minimize the saltwater intrusion problem.
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Ex 9 – Water