Page 1

LAB

Groundwater and Karst Topography OBJECTIVES To learn the difference between porosity and permeability To understand how groundwater flows, its importance as a resource, and risks of contamination

Fresh water lakes 0.009% Saline Lakes and inland seas 0.008% Soil moisture 0.005% Stream channels 0.001% Atmosphere 0.001%

Non-marine hydrosphere

To understand the origin and geologic hazards of karst topography and their relationship to groundwater

Groundwater 0.63%

W

ater from rain, snowmelt, or streams that enters the ground is groundwater. Once in the ground, the water continues to move downward through soil, sediment, and porous rock until it reaches the zone where water fills all spaces or pores in rock and sediment. This zone is the zone of saturation and its top is the water table. The term groundwater refers to any water in the ground, but especially to water in the zone of saturation. Groundwater is the third largest reservoir of water on Earth and the largest reservoir of fresh liquid water (■ Figure 15.1). Only the oceans and ice in icecaps and glaciers contain more water. This makes groundwater an important resource for drinking, irrigation, and industrial purposes. Yet, in many cases, groundwater takes a long time to accumulate underground; thus once people deplete it, groundwater may not recover quickly or easily. After it reaches the water table, groundwater continues to flow in response to gravity at a rate that is slow in comparison to streams — generally meters per day or less for groundwater compared to kilometers per hour for streams. This slow flow is the primary factor making groundwater the limited resource that it is. Groundwater may flow through

Glaciers 2.15%

%

2.8 Oceans 97.2% Hydrosphere

Figure 15.1 Percentages of water in different reservoirs in the hydrosphere (top left), and nonmarine hydrosphere (2.8%, bottom right). At 0.63% groundwater makes up by far the largest quantity of fresh liquid water. Glaciers here include ice caps. From Earth Science Today, by B. Murphy and D. Nance, p. 264. Copyright © 1999 Brooks/Cole. All rights reserved.

pores, cracks, and caves (■ Figure 15.2). As it flows through soluble rocks, groundwater dissolves the rocks, making the openings larger and eventually forming caves. In caves, the flow of water can be as fast as in surface streams, but cave flow is only common in karst regions where limestone or other soluble rocks exist.

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Table 15.1

Pore space

Porosity of Two Sediments (Exercise 1) Sediment Number

2

80

80

Brief description of sediment

(b)

(a)

1

Fractures

Volume of water (ml) Volume of sediment (ml)

(c)

(d)

Porosity (%): vol. water   100 vol. sediment

Figure 15.2 Different types of porosity. (a) Pores between grains of sand or other sizes of well-sorted clasts. (b) Porosity is considerably reduced in poorly-sorted sediment. (c) Soluble rocks such as gypsum or limestone have porosity in solution cavities. (d) If present, porosity in crystalline igneous or metamorphic rocks generally takes the form of fractures. Modified from U.S. News and World Report (18 March 1991): 72–73.

POROSITY

P

orosity is the percentage or proportion of pore spaces in a volume of rock or sediment.

50ml 

ml 

ml.

Enter this number in Table 15.1.

Materials needed: 2 80-ml beakers 2 containers of dry sediment, labeled “dry Sediment 1” and “dry Sediment 2” 2 containers for wet sediment, labeled “wet Sediment 1” and “wet Sediment 2” 50-ml graduated cylinder Water

1. Examine Sediment 1 and enter a brief description in ■ Table 15.1. Measure the porosity of Sediment 1 by placing some of it in a 100-ml beaker up to the 80-ml mark. Tap the beaker gently on the table to pack the sediment down. Add or remove sediment if necessary and repack, measuring carefully. Fill a graduated cylinder with exactly 50 ml of water. Slowly pour water from the cylinder down the side of the beaker into the sediment and measure how

350

much water you need to reach the 80-ml level. As you pour, be sure the water is soaking in, not just running across the surface. Don’t pour too quickly, or you will trap air bubbles in the sediment and not get an accurate measure of the porosity. Read the volume remaining in the graduated cylinder and enter it in the following equation to determine how much water you added to the sediment:

L ab 1 5

Divide this volume of water by volume of sediment and multiply by 100 to get a percentage. Fill the information in Table 15.1. 2. Examine Sediment 2 and enter a brief description in Table 15.1. Measure the porosity of Sediment 2 in the same way as for Sediment 1. Fill this information in Table 15.1.

When you are finished with the sediment, empty the beakers into the wet sediment container for the correct sediment, and clean the beaker for the next person. Do not mix wet and dry sediments or Sediments 1 and 2.

3. How do the porosities of the two sediments compare? Make a numerical comparison.


PERMEABILITY AND FLOW RATE

H

ow fast water travels through rock or sediment depends on (1) the porosity, (2) the size of the pores, (3) how well pores are connected, and (4) the pressure of the water. Permeability, related to the first three, is a measure of the ability of the rock or sediment to allow fluids to move through them. Generally, more porous materials are also more permeable, but the pores must be connected and large enough for the water to flow through rather than cling to the pore surface. Larger pores allow water to flow more readily. The higher the water pressure, the faster the water will flow through permeable material. It is common among sedimentary rocks for groundwater to flow faster parallel to bedding than across the layers.

the bedding. Gravity is pulling water downward. Use this fact in testing the sample. Hold the sample so the bedding is vertical to test how fast the water flows parallel to bedding. Then hold the rock horizontally to test for flow perpendicular to the bedding. Which flows faster?

6. These two characteristics, porosity and permeability, are also important for determining whether or not a rock bed will be a good oil reservoir. Considering the similarities of water and oil, explain why you think this relationship exists.

Materials Needed: Rock samples of coquina and pumice Squirt bottle Water Sink or plastic tray to collect water 4. Examine the coquina limestone and the pumice samples. The solid parts of these rocks have similar densities, yet there is a dramatic difference in the density of the whole rock. a. Which of these rocks has the lower density? b. What gives this rock a lower density than the other has?

5. Now compare the relative permeability of pumice and coquina. With a plastic squeeze bottle, squirt water on the samples as you hold the rocks over a plastic tray or over the sink. Do not allow any water to flow around the edge of either rock. a. Which is more permeable?

b. What makes this rock more permeable than the other?

c. Next check whether the coquina is more permeable parallel or perpendicular to

Experimental Determination of Flow Rate in Sediment When we use groundwater as a resource, an important consideration is how fast it flows to replenish the supply. Low permeability restricts the flow rate; but if no pressure is forcing the water through the permeable material, there will be no flow. Hydraulic head is the height of a column of water above the discharge point or spring. Gravity — the weight of the water from the hydraulic head — creates the pressure that provides the force for the water to flow. The next experiments examine the effects of permeability and hydraulic head on flow rate. These experiments use two columns containing the same sediments for which you measured porosity for Exercises 1 to 3. In one, the sediment is finer grained than in the other. Materials needed 2 columns, one with Sediment 1 in the bottom and the other with Sediment 2 (these are the same sediments used in Exercise 1) 3 beakers 2 funnels (if columns are narrow) Small graduated cylinder Water

7. Read all the instructions before you start this experiment. In some cases, your lab

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b. Now allow one more drop to fall into the graduated cylinder. Read the new volume and subtract the first volume measurement to get an idea of how accurately you are able to measure the volume. If you see no difference, examine the cylinder graduations to determine the smallest increments you are able to measure with your graduated cylinder (commonly 1/2 to 1/4 of the size of marked graduations). Whichever is greater of these would be the minimum error in your

setup may have soft plastic tubing. It is important not to squeeze the tube if you are impatient for the water level to drop so you can start your experiment. By squeezing the tube you would be purposely altering the very thing you are setting out to measure. What characteristics of the sediment would this squeezing change? How?

experiment: To establish the error in your experiments accurately, you would have to repeat the experiment a number of times to see the variation in the results. (However, this is a time-consuming process.)

a. Your lab instructor will assign you two or three measurements to make from the same column. Place a beaker under the appropriate column. Saturate the sediment by adding water until it is flowing (or dripping) out of the column and into the beaker (if other students have already made some measurements, the sediment may already be saturated). Once the sediment is saturated and dripping, fill the column with water slightly above the level assigned. When the level of the water reaches the assigned level, immediately place a graduated cylinder so it will catch every drop of water flowing out of the column during a 30- or 60-s period. At the end of the assigned time, stop measuring into the graduated cylinder by replacing it with the beaker. Enter the volume and time in the appropriate places in ■ Table 15.2.

c. Calculate the flow rate by dividing the volume by the time. Enter these figures in the table on the blackboard, overhead, or computer spreadsheet to share with the rest of the class. (Think of this as the communication stage of the scientific method, see the Appendix.) If more than one group measures the same column and hydraulic head, average all the results together. d. Copy the data collected by other class members from the blackboard/overhead or spreadsheet into Table 15.2. 8. Use the graph paper in ■ Figure 15.3 to graph your results from Table 15.2. If your instructor prefers, use a spreadsheet’s graphing feature supplied by your school.

Table 15.2

Rate of Flow of Water Through Sediment (Exercise 7) Sediment Number

Hydraulic Head Unit of Measure Volume (ml)

1

Time (s) Flow rate (ml/s) Volume (ml)

2

Time (s) Flow rate (ml/s)

352

L ab 1 5

10 cm

20 cm

30 cm

40 cm

50 cm


Figure 15.3 Graph paper for Exercise 8.

a. It is conventional to place the independent variable (hydraulic head in this case) horizontally and the dependent variable (flow rate) vertically. After choosing your scale for each axis, double-check that the spacing between 0 and the first number is equivalent to the spacing for the next two numbers (this is a very common graphing error). Label 0 on each axis of your graph. Label axes and title your graph. Provide an appropriate key. b. Draw a straight line through your data for each sediment. Should the line pass through the origin or not? 9. Determine the equation for each line as follows, or have the spreadsheet graphing feature add a best-fit line to the graph and give you the equation: a. What is the flow rate on the line at 0 hydraulic head? Sediment 1

Sediment 2

b. What is the flow rate on the line at 60-cm hydraulic head? Sediment 1

Sediment 2

c. Next, use the equation for a line given below. The flow rate and hydraulic head are your two variables; the other items are numbers you should substitute into the equation. The final equations should have the form y  mx  b, where y is flow rate, x is hydraulic head, and m (slope) and b (y intercept) are numbers. y  [(flow at 60  flow at 0) / 60]  x  flow at 0 d. What is your equation for Sediment 1?

e. What is your equation for Sediment 2?

10. Think of your equations as hypotheses (Appendix) based on each of the sediments. You

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are hypothesizing that a linear relationship exists between hydraulic head and flow rate as expressed by your equations.

12. Summarize your results: a. How does porosity influence flow rate?

a. Now use the equations to predict the flow rate at 70 cm, if your columns go that high, or 45 cm, if they don’t. What flow rate do you predict?

Sediment 1 Sediment 2 b. Test your hypothesis for either Sediment 1 or Sediment 2. Pour water into the sediment column to set the hydraulic head to the appropriate value and see whether you get the flow rate you predicted. Volume time

s; flow rate

b. How and why does sediment size influence flow rate?

ml; ml/s.

c. How close was your measured flow rate? Subtract your answer from the predicted value.

Is it within the error of the experiment?

c. How and why does hydraulic head influence flow rate?

If not, the next step would be to reevaluate the error in the experiment or reevaluate the hypothesis. 11. Look back at the results in Tables 15.1 and 15.2 and at your graph. a. How do the flow rates at 50-cm hydraulic head compare for the two sediments? d. Is there a mathematical relationship? Yes / No. If so, what is it?

b. Is the difference in the porosity of the two sediments sufficient to explain the difference in their flow rates? Yes / No c. What else could account for the difference in the flow rates?

WATER TABLE, GROUNDWATER FLOW, AND WELLS

W

e have been experimenting with saturated sediments in this lab, but remember that the water table is the top of the saturated zone in the ground (â– Figure 15.4). Where the water table in-

354

L ab 1 5


Zone of aeration Capillary fringe Water table Soil zone Infiltration Zone of aeration Capillary fringe

Zone of saturation

Perched water table Water level

Aquiclude

Water table Groundwater (aquifer)

Zone of saturation Figure 15.4

Groundwater in the subsurface is found in a number of forms. These include soil moisture, dampness in the zone of aeration where both water and air occupy the pore spaces, water in the pores in the zone of saturation (which may be perched above an impermeable layer in some places), and water in the capillary fringe that is drawn up by the surface tension of water. The largest quantities are in the zone of saturation. From Earth Science Today, by B. Murphy and D. Nance, p. 267. Copyright © 1999 Brooks/Cole. All rights reserved.

Figure 15.5

Discharge area Stream

Recharge area

Normal Water table

ra H yd Saturated zone Water table during drought

tersects a natural hole or depression in the land surface a lake exists. In stream valleys of humid regions, groundwater may flow out of the ground (discharge) into the stream. Because stream flow is faster than groundwater flow, the stream may draw down the level of the water table to the stream level (■ Figure 15.5). For this reason, it is common in humid regions for the water table to mimic the surface topography, but with lesser slopes. Well water is an important source of water in many regions. A well is a hole drilled into the zone of saturation, below the water table (■ Figure 15.6a). The lining of the well, if it has one, is permeable so water flows into the well from the surrounding rock or sediment. The level of water in the well will correspond to the water table, unless the water is under pressure in a confined aquifer. When water is pumped out of the well, the water level drops; thus the water table around the well also drops,

u l ic g

radient

In humid areas, the recharge of groundwater is generally sufficient to enable the water table to intersect the ground in valleys and the water flows out of the ground (discharge), forming streams. The rate of flow in the streams is faster than in the ground. As a result, the stream draws down the water table, creating a difference in the height of the water table. Areas of higher water level exert a greater pressure, and the difference in pressure causes the groundwater to flow toward the stream.

forming a cone of depression (Figures 15.6b and c). The water level in lakes, swamps, streams, and wells in a region may all indicate the level of the water table (■ Figure 15.7). Knowing the level helps people judge how deep to drill for water when they put in a well. Since the price of drilling a well is usually by the foot, this could be very useful information. An aquifer is a permeable body of rock or sediment below the water table that can sustain a productive water well. Groundwater flows from high pressure to low pressure. The experiments with the sediment columns show that the pressure is higher and water in the sediment flows faster when the water level in the columns is higher. However, in the ground it is not just the pressure but a difference in pressure from one place to another that causes flow. A sloping water table produces a difference in pressure and causes groundwater to flow from high to low areas of the water table. When a cone

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355


Pump

Static water table

Unconfined aquifer

Well bore

(a)

Pump

Cone of depression Water table

Wate Drawdown

r flow

r flow

Wate

Water level in well Well bore

(b)

Septic system

Well

Unsaturated zone Water table

Cone of depression

(c) Figure 15.6 Development of cones of depression and the resulting lower water table. (a) The water table around an unpumped well. (b) A cone of depression forms around a well while it is being pumped. The drawdown creates a hydraulic head causing water to flow toward the well. (c) Diagram showing the three-dimensional form of cones of depression. The cones can overlap, lowering the regional water table. a, b: From Geology and the Environment, 2d ed., by B. W. Pipkin and D. D. Trent, p. 271. Copyright Š 1997 Brooks/Cole. All rights reserved. c: USGS data.

356

L ab 1 5


Figure 15.7

Transpiration

Water on the land such as swamps, streams, and lakes often indicates that the water table is intersecting the ground surface at that location. This information can be useful in determining the level of the water table. From Earth

Rainfall Throughfall Evaporation

Soil water Water table

Surface runoff

Science Today, by B. Murphy and D. Nance, p. 266. Copyright Š 1999 Brooks/Cole. All rights reserved.

Swamp

Groundwater Stream, Lake

of depression forms, the difference in pressure between the well and the surroundings is what brings water to the well.

Interaction between the Water Table and Surface Water

13. Experimental setup: Reduce the water on the outflow end of the stream table to its lowest level. Move all of the sand into one-third of the stream table on the inflow side. Make a level area of sand at the inflow end, but leave a depression for a lake right at the inflow. Make a steep cliff in the sand on the other side. Near one side of the stream table, as shown in â– Figure 15.8, make a stream canyon starting a few inches from the inflow lake and ending in the outflow lake. Dig a series of small to medium holes in the sand, as shown in Figure 15.8. Briefly turn on the water flow of your stream table until the water in the lake at the inflow end is as high as possible without spilling over. Turn on the flow repeatedly, or set the flow to a slow trickle to keep the inflow lake at this level.

D. Pirie

In the next experiment we will observe the interaction of surface and groundwater in a stream table.

Figure 15.8 A photograph of the stream table set up for Exercise 13. You can use a soup spoon to dig out the holes.

14. On a separate sheet of paper, sketch a map view of the configuration of the stream table and all the holes, inflow lake, outflow, and the stream. a. Observe or measure the water level as indicated by your instructor in all the lakes and the stream, including the inflow lake and the outflow lake. If measuring, write the values on your sketch. Try to keep a nearly constant level in the inflow lake while you do this. b. In this experiment the sand represents the ground and the holes and the channel represent lakes and a stream. What hydrological feature in the sand is indicated by the levels of the lakes and the stream?

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c. If you measured the lake levels, draw water-level contours on your sketch map using a contour interval that gives you at least four contours. What surface have you contoured?

d. Is the groundwater flowing? Yes / No How do you know?

f.

How does the stream influence the water table?

Water Table and Potentiometric Surface in a Groundwater Model The column experiment demonstrated that some sediment allows water to flow through it faster than other sediment. Highly permeable materials make good aquifers because they allow faster flow. If the material is less permeable or hardly permeable at all, then it is an aquitard or an aquiclude, respectively. An aquifer may be confined or sandwiched between aquicludes, which may allow the aquifer to become pressurized. Such an aquifer is an artesian aquifer (â– Figure 15.9). The pressure in an artesian aquifer allows the water to rise anywhere the aquifer is intersected by a natural break or a well. The height the water rises to is known as the potentiometric surface (Figure 15.9). It is anal-

e. Is the water table sloping? Yes / No. How do you know?

Hydraulic head Area of recharge for aquifer

Pressurized

Static

Potentiometric surface

Flowing artesian well Non-flowing artesian well

Shale aquicludes

Sandstone aquifer

Figure 15.9 Artesian aquifer. Where aquicludes confine an aquifer, and where the recharge area is at relatively high elevation, the water in the aquifer is under pressure. The potentiometric surface is the level to which the water would rise as a result of this pressure if a well pierced the aquifer. If the ground surface is lower than the potentiometric surface, the water will fountain out of the ground, forming a flowing artesian well. If the ground surface is higher than the potentiometric surface, the well will be a non-flowing artesian well with the water level equal to the potentiometric surface. From Earth Science Today, by B. Murphy and D. Nance, p. 270. Copyright Š 1999 Brooks/Cole. All rights reserved.

358

L ab 1 5


ogous to the water table for an unconfined aquifer. Groundwater flows in response to a difference in pressure caused by a sloping water table. Similarly, water in an artesian aquifer flows from where the potentiometric surface is high — that is, from higher pressure to lower pressure. For the next experiment, use the groundwater model provided, or answer these questions as a set of thought questions. If you use a model, it may be as elaborate as the groundwater model shown in ■ Figure 15.10, or as simple as a clear plastic box with layers of different sized sediment and with clear straws or plastic tubing representing wells. Whatever style model you have, the lowest layer of sediment should have “wells” reaching to it with blue dye, such as in the model pictured. Shallower wells should have green dye. Your model should have drains leading from at least two levels, all of which should start closed. In addition, your model should have a way to

recharge groundwater into the deepest sediment layer as well as the shallower layers. Materials needed: Groundwater model with sediment layers, wells, and other features described above Colored dyes (food coloring works fine) Syringe or pipette Drain or bucket Water

15. Examine your model and the samples of sediment used in it. a. In ■ Table 15.3 list the sediments in order of their level in the model. Indicate which sediment layers, if any, consist of

Lake Overflow drain

Overflow drain

Tank Stream

Figure 15.10 Example of a groundwater model. Your model may have quite a different setup, but should have similar components. It should have layers of sediment with different permeability, deep wells with blue dye, shallow wells with green dye, drains from the upper sediment, and recharge for both upper and lower sediment. In this model, the two blue bottles provide continuous recharge to the areas with sediment. The stream, tank (representing a leaking underground gas or chemical tank), and lake have perforations so that they communicate with the ground water.

Table 15.3

Sediments in the Groundwater Model (Exercise 15) Sediment level

Description

Permeability rank (1  highest)

Aquifer or aquiclude

Uppermost sediment Upper middle Lower middle Lowest sediment

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the same sediment. Rank their permeability, starting with 1 having the highest permeability and 3 or 4 the lowest. How did you decide which are more permeable?

16. Examine the water level in all the wells and in any other features of your model that allow you to see its level, such as the tank (the circle) and any lakes or streams as in our example model in Figure 15.10. a. Where do you think the water table is within the sediment? What is the shape and position of the water table at present?

Which would be the best aquifer, and which would be the best aquiclude?

b. Is the sediment layer between the sediment at the bottom of the green wells and the sediment at the bottom of the blue wells an aquifer or an aquiclude? In our example model, the dark layer would be the sediment layer in question.

c. Carefully sketch a side view of your model and label the sediments on your sketch as either aquifers or aquicludes.

15c sketch area

360

L ab 1 5

b. Is the water table sloping? Is the groundwater flowing? Yes / No. How do you know?

c. Sketch in the position of the water table on your drawing in Exercise 15c, using a color other than green or blue. Next to your sketch, make a key and label this color in the key as “initial water table.” 17. Now open the valve for the drain in the uppermost or upper middle sediment. In our example model, that would be the “stream.” If necessary for your model, add recharge water into your sediment, as suggested by your instructor. Continue to supply recharge


to the same level throughout the experiment. In the model pictured in Figure 15.10, the two bottles at the top recharge the groundwater automatically. Watch the model for a minute or two. Periodically check that the stream is draining properly; sometimes air bubbles in tubing impede the flow. Jiggle or tap the tube to get the flow going again. a. Sketch directly onto your drawing of your model the level of water at each location where you can detect it; use different colors for wells with different color dyes. b. With a green colored pencil, draw in the approximate water table level within the sand between the shallower green-dyed wells. If the water in the drain is in the same aquifer as green-dyed wells, and if you can tell the water level in or near the drain, include that level as you draw the green water table line. d. Label “highest hydraulic head” on your sketch where the green water table is the highest. e. Is the water table higher or lower near the drain (labeled “stream” in our example) compared to the area surrounding the drain? f.

d. Use as straight a line as possible to connect the levels for the blue wells. What surface have you just sketched? (See Figure 15.9, if needed.)

GROUNDWATER FLOW 19. Now study the streaks of green dye in the model. a. Color them in on your sketch. What direction is the groundwater flowing?

b. Summarize the relationship between the direction of groundwater flow and the slope of the water table.

How does the stream influence the water table and why?

18. Examine the deeper wells with blue dye. a. How does the level in the blue-dyed wells compare with the wells with green dye?

c. Is there any evidence that groundwater can flow upward? Yes / No. If so, what evidence?

b. Now that the water has run for a while, some of the dye should have spread from the wells. Which sediment layer(s) has(have) blue water? d. What makes this possible?

c. What do we call this(these) layer(s) of sediment? 20. Color in the streaks of blue dye on your drawing (Figure 15.10).

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a. What direction is the groundwater flowing here?

22. Use a syringe or pipette to pump water out of a centrally-located blue-dyed well. a. What happened to the water level in other blue-dyed wells when you did this?

b. Summarize the relationship between the direction of groundwater flow and the slope of the potentiometric surface. (Look carefully to detect the slope in the potentiometric surface.)

b. What happened in the nearby green-dyed

wells? c. What direction is the groundwater flowing when you extract water from the blue-dyed well?

23. Based on what you have learned, what do you think will happen if two families have wells near each other that obtain water from the same aquifer, and one family starts extracting much more water from their well?

Wells Leave the stream outlet on the groundwater model open for the remainder of the experiments. 21. Use a syringe or pipette to pump water out of a centrally-located green-dyed well. a. What happened to the water level in the adjacent green-dyed wells when you did

this? b. What happened to the water level in nearby blue-dyed wells?

c. What direction is the groundwater flowing when you extract water from the well?

d. Identify the cone-shaped drop in the water table around a well.

e. Why did the water in the blue-dyed wells behave differently from the water in the green-dyed wells?

362

L ab 1 5

GROUNDWATER DEPLETION

I

n either confined or unconfined aquifers, excessive extraction can deplete groundwater. Declines in the level of the Ogallala aquifer have become quite serious in some places, as seen in ■Figure 15.11a. If one person depletes the groundwater in their area, the drop in the water table or potentiometric surface may spread to other areas. How does water in an aquifer behave — more like water in a bathtub or more like it is contained in an egg-carton-like configuration (Figure 15.11b)? The bathtub analogy applies if the aquifer acts as a common pool, so a general lowering of the water table occurs when a person pumps the aquifer from one location. For the egg carton model, the aquifer is compartmentalized, so a person pumping in one location does not influence neighboring areas. 24. Study the map in Figure 15.11a. a. What does the blue and lavender coloring indicate?


104°

96°

100°

WYO.

S. DAK. NEBR. NO

RI

ER IV

ER

RI P L AT TE

R VE

VE

R

R EPUBLIC

R IV

SO

ER

UTH

TE AT L P

LAT T

AN

40°

RT HP

KANS.

COLO. A RKA

N SA

S

ER RIV

OKLA. 36°

ER RIV CA

N

AD

IAN

N. MEX

TEX. 0 32°

50

100 mi

(b)

Water-level change, in feet, 1980–1995 Declines 10 – 20 > 40 20 – 40 5 –10 No significant change 5 – 5 Rises 20 – 40 5 –10 > 40 10 – 20 Area of little or no saturated thickness

0 50 100 km (a)

Figure 15.11 (a) Map of the High Plains Aquifer showing changes in the water level between 1980 and 1995 (Exercise 24). The aquifer is also called the Ogallala Aquifer; its extent is outlined in black on the map, and its water-level changes are indicated with different colors. (b) Two models for how aquifers work. (Top) Aquifers may act like bathtubs where draining from one part may drain the whole thing; or (bottom) in the eggcarton model, one person pumping does not affect neighbors’ water levels. Most aquifers are probably between these two extremes. From Geology and the Environment, 4th ed., by B. W. Pipkin and D. D. Trent, p. 218-219. © 2005 Thomson Higher Education.

b. What does the orange and yellow coloring indicate?

e. Does the whole aquifer act like a bathtub? Yes / No f.

Roughly how large are single units or “compartments” of the aquifer?

c. Where is the aquifer most depleted?

d. Where is the aquifer rising?

g. Would the groundwater use by farmers in one part of Kansas influence the levels for people 25 miles away? Yes / No.

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363


GROUNDWATER CONTAMINATION

h. Does the aquifer appear to have uniformly-sized compartments or do they vary in size in different regions?

A

lthough surface water is much more easily contaminated than groundwater because it is more accessible, a number of sources may contaminate groundwater, as illustrated in â– Figure 15.12:

Where do they appear smaller?

Leachate from landfills Salt and oil from roads Infiltration of herbicide Pesticide and fertilizer runoff from agricultural fields Infiltration from polluted rainwater Leakage from sewer lines Normal drainage from septic tanks Industrial chemicals from waste ponds or basins Leakage from buried liquid storage tanks Acid mine drainage Metal contamination from a mining site Leakage from faulty casings (linings) of hazardous waste injection wells or where hazardous waste may leak through fractures or wells, piercing the confining aquicludes

Where do they appear larger?

i.

What problems can you envision for households, farms, and industry in areas colored orange on the map?

Waste lagoon, pond, or basin Mining site

Pumping well

Road salt

Sewer Landfill

Zone of aeration

From Environmental Science, 8th ed., by G. T. Miller, p. 313. Copyright Š 2001 Brooks/Cole. All rights reserved.

364

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Buried gasoline and solvent tanks

Cesspool, septic tank

Figure 15.12 Some sources of contamination of groundwater. The two pumping wells extract groundwater that a number of sources, such as those described in the text, may contaminate.

Hazardous waste injection well

Water pumping well

Unconfined freshwater aquifer dwater Groun

Leakage from faulty casing

Aquicludes Confined freshwater aquifer

fl o w Groundwater Fracture

Aquicludes Confined aquifer

Discharge


Figure 15.13

Landfill

Leachate leaking from an unlined landfill into the groundwater. From Physical Geology: Exploring the Earth, 3d Ed., by J. S. Monroe and R. Wicander, p. 463. Copyright © 1998 Brooks/Cole. All rights reserved.

Zone of aeration

Average water table

Zone of saturation

In general, contaminants will tend to flow with the groundwater, although some float on top and some sink below it. Let’s discuss one of these: leachate from landfills. If rainwater gets into a landfill, it can leach various toxic chemicals out of the landfill material into the water (■ Figure 15.13), forming a solution known as leachate, which you could think of as “garbage juice.” If the leachate leaks out of the landfill, it may enter the aquifer and flow along with the water.

wells, it may be possible to draw a contour map of the water table as in ■ Figures 15.14 and 15.15 and as you may have done for the stream table. The depth of wells and direction of groundwater flow can be determined from such a map. The depth of the water table at any given location is less than the depth needed to drill a well, because a well must penetrate the zone of saturation. To calculate the depth of the water table, first determine the elevation of the land surface at the location. Next, determine the elevation of the water table at the same point, and then subtract from the land elevation. For example, the location marked at xA in Figure 15.15 has a land elevation of 150 ft, and the water table is 104 ft. This makes the depth to the water table 150 ft  104 ft  46 ft.

Using Water Table Contours Where enough information is available about the water table from elevations of lakes, streams, swamps, and

8

5

3

0 62 7 0

58

Figure 15.14

4

60

5

Elevation, feet a.s.l.

1

700 Aerated zone 600

Water ta

500

Saturated zone

ble

0

54

6

2

Contour map of the water table near a city. Numbered dots represent wells, which give information about the water table, allowing a hydrologist to draw the contours (in blue) and determine the direction of groundwater flow. Arrows show the groundwater flow direction, which is perpendicular to the contours. Information about the subsurface geology may also be determined at the time the wells are drilled. Elevations are in feet above sea level. From Geology and the Environment, 2d ed., by B. W. Pipkin and D. D. Trent, p. 276. Copyright © 1997 Brooks/Cole. All rights reserved.

Groundwater and Karst Topography

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xB 90

90

80

80

xB

xD

xD 105

xE 110 120

11

0

11

11

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120

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0

150

115

140 130

130

120

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xA

xC

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110

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xE

10

10

85

10

85

0

0

105

xA

xC

90

150

115

140

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130

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120 130

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Stream

Figure 15.15 Topographic map of an area of karst topography with sinkhole lakes. Contours of the water table surface are heavy dark blue lines. Contour interval  10 ft (Exercise 25).

25. What is the depth to the water table at each of the other lettered locations on the map? xB:





xC:





xD:





xE:





The direction of flow of groundwater is important for a number of reasons. It helps to determine where water in a well comes from and what happens to contaminants and pollution that get into the groundwater (Figure 15.12). Because of gravity and the resulting pressure, water in the ground flows in the direction of the steepest downward slope of the water table, carrying any contamination with it. Flow of groundwater, then, is perpendicular to the water table contours. The arrows in â– Figure 15.16 show the groundwater flow direction.

26. Study Figure 15.16. If a company dumped toxic waste into the lake labeled 115, draw or shade in on the map the path that the toxic waste would take as it traveled through the ground in the groundwater. a. Which, if any, of the Lakes 85 or 105 or locations labeled A, B, C, D, or E would become contaminated?

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L ab 1 5

Figure 15.16 Topographic and water table contour map of the same area as Figure 15.15 with sinkhole lakes. Arrows show the direction of flow of the groundwater (Exercise 26).

b. Would this toxic waste contaminate the part of the stream shown on the map? Yes / No c. If toxic waste were dumped into Lake 105, which, if any, of the locations labeled A, B, C, D, or E would become contaminated? 27. Groundwater flows much more slowly than streams. If the speed of groundwater flow in the area shown by Figure 15.16 is 2 m/d (a fairly high flow rate for groundwater) and the distance from Lake 105 to B is 0.5 km, how long would it take for contaminated water to reach a well at B? Assume that the contamination travels at the same speed as the water. Write out your calculations.

28. Use the map in â– Figure 15.17. a. Draw arrows on the map showing the direction of flow of groundwater. Remember that flow direction is perpendicular to water table contours in the downslope direction. b. If a nearby town decided to make Fenoke Swamp into a landfill, what might happen to the groundwater in this area?


570

form by solution or when the roof of a cave collapses. Karst topography is most common in regions underlain by limestone. The sudden formation of sinkholes is one of the geologic hazards in some karst areas (â– Figure 15.20). Some sinkholes develop suddenly, but some give indications of future sinkhole development. In limestone regions known for sinkholes, watch for the following indicators:

540

0

56

Fenoke Swamp

560

540 540

530

0

53

0

55

0

54

530 0 54

0 54

55

540

0

0

54

55

0

53

0

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0

520

Water pools and new ponds develop. Fences, telephone poles and trees tilt. Buildings develop cracks in floors, walls, and foundations. Vegetation withers. Well water becomes discolored.

0 550

56

0

54

0

54

520

Figure 15.17 Topographic and water table contour map of an area with swamps and sinkholes. Contour interval  10 ft (Exercise 28).

Because limestone is susceptible to dissolution, it may be highly permeable; if the porosity is on the centimeter scale rather than cavernous, the limestone may make an excellent aquifer. However, in many karst regions the limestone has large caves where groundwater flow may be very irregular, which makes finding groundwater a very hit-or-miss proposition.

Dissolution Experiment c. Shade or highlight the area on the map that might eventually have groundwater contaminated by leachate. d. Would the stream become contaminated? Yes / No. e. If so, show the contaminated part of the stream on the map by shading or highlighting this area.

GROUNDWATER CAUSES EROSION BY SOLUTION

A

s it moves, groundwater may dissolve a soluble rock, creating cavities and caves in limestone, dolomite, marble, rock salt, or gypsum, and resulting in topography called karst. Karst topography is different from stream-dominated topography because much of the high-volume drainage is underground and because solution produces certain unique landforms (â– Figure 15.18). Streams often disappear into closed depressions that lead to networks of caves, or caverns (Figure 15.18), commonly decorated with limestone precipitated from groundwater such as those shown in â–  Figure 15.19. A closed depression in karst regions is a sinkhole (Figure 15.18), which may

In the following experiment you will be able to observe the solubility of limestone or dolomite. Materials needed: Two samples of limestone or dolomite from the same rock Two steep-sided beakers large enough to hold one sample 10% HCl Gloves and tongs Paper towels Ruler Hand lens or microscope

29. As rainwater falls on the ground, it picks up a small amount of acid by dissolving carbon dioxide from the air. This carbonic acid is the same thing that gives soda pop its fizz. Over time, acidic water can dissolve holes in limestone and dolomite just as the dilute hydrochloric acid reacted with the carbonate minerals that you tested in Lab 1. a. Look at the limestone or dolomite samples with a hand lens or microscope and describe or sketch the surfaces. Pay special attention to any pore spaces, voids, or cracks and the cement between

Groundwater and Karst Topography

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Sinkholes Sinking stream

Stream

Water table

Limestone

(a)

Solution valleys

Sinkholes Water table

Sinking stream

(b)

Column Soda straws

Travertine terrace Drip curtain

Stalactite Stalagmite Collapse blocks

Water table

Cave

(c) Figure 15.18 Karst topography and the formation of caves. (a) Groundwater dissolves the limestone, forming a system of openings and passageways. The surface develops sinkholes and surface streams disappear through sinkholes. (b) Groundwater near the water table flows toward surface streams, dissolving more rock, enlarging the openings, and extending the passageways horizontally. Sinkholes at the surface gradually enlarge and coalesce into solution valleys. (c) With uplift and downward erosion and entrenchment of surface streams, the water table drops, leaving interconnected caves above the water table. Once air is present in the caves, features such as stalactites and stalagmites form by the action of dripping groundwater. From Physical Geology: Exploring the Earth, 5th ed., by J. S. Monroe and R. Wicander, Fig. 16.12, p. 482. Š 2005 Thomson Higher Education.

368

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Figure 15.19 Groundwater produces stalactites, stalagmites, and columns in limestone caves.

grains. Accurately measure the dimensions of one sample.

d. Examine the sample with a hand lens or microscope as you did before and record any differences between the two samples. Pay close attention to void spaces and cement between grains.

e. Look at the bottom of the samplesoaking beaker. Where did the particles that you see come from and why did they come loose?

b. Place one of the two samples of limestone or dolomite in a beaker with high sides. Very slowly, pour enough 10% HCl over the surface to allow the sample to sit partially in the solution. This acid is much stronger than carbonic acid in a natural setting, but we want to speed up the process. Record what you see.

Dispose of the liquid as instructed. f.

c. Return to the sample soaking in acid after you have finished this lab, or after it has been soaking for 30 minutes. Using tongs or wearing rubber gloves, pick up the sample and place it in a beaker of

What does the change in the sample indicate about the change in the rock’s porosity?

Frank Kujawa, University of Central Florida

Reed Wicander

water to rinse it. Remove the sample immediately and place it on a paper towel, dry it well, and measure it.

Figure 15.20 This sinkhole formed on May 8 and 9, 1981, in Winter Park, Florida, and had a diameter of 100 m and a depth of 35 m. The white rectangle on the left side of the sinkhole about a third of the way down is a truck trailer. The vertical line across the sinkhole is a sewer pipe. Water in the bottom shows the level of the water table.

Groundwater and Karst Topography

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Karst Topography After an area of limestone is exposed to the action of groundwater, karst topography tends to evolve and change. At first a karst area may be very flat with a high water table. Near the surface of the water table, large openings may form in the rocks under water. These openings are underwater caves, which drain with a drop in the water table. At first a few sinkholes (Figure 15.18) develop by dissolution and by collapse of caves. Where the water table is shallow or where the sinkholes penetrate to the water table, sinkhole lakes form. Sinking streams disappear into underground cave systems. As time passes and the water table drops, more dissolution occurs at deeper levels and individual sinkholes enlarge and merge to form solution valleys. Further erosion and lowering of the water table leads to the development of broad low areas with erosional remnants of low conical hills or high, very dramatic karst towers. In some regions, insoluble rocks may cap limestone, as in the area surrounding Mammoth Cave, Kentucky (â– Figure 15.21). The insoluble rocks make uplands or plateaus with solution valleys containing sinkholes where erosion has cut down to the limestone. Where erosion has mostly removed the insoluble rocks, occasional remnants make knobs or conical hills. 30. Examine the maps of the area near Mammoth Cave, Kentucky, in Figure 15.21 and refer to the diagrams in Figure 15.18 as you answer the following questions. The first map (Figure 15.21a) provides an overview of the area and shows the locations of the maps in Figures 15.21b, c, and d. a. Notice that depressions riddle the northern part of the area in Figure 15.21b. What are these depressions?

b. Use a highlighter, or colored pencil, to emphasize all the streams on the same map. Many of these streams are ephemeral, meaning they do not run all year around, a characteristic typical of regions of karst. c. Where do these streams go?

d. What name do we give this type of stream?

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e. A little farther to the north, in the area shown in Figure 15.21c, the topography changes. What karst features are visible on the map for this area?

f.

The topography continues to change as we move north to the area shown in Figure 15.21d. Use Figure 15.21a to locate the main entrance to Mammoth Cave. What is the minimum length of Mammoth Cave? Hint: Also locate the Frozen Niagara Entrance on the map.

g. Examine the more detailed map in Figure 15.21d. How many caves can you locate on this map? h. The river on the west side of the map is the Green River. How many streams on the map are permanent? how many are

and

ephemeral? i.

How many streams do not make it to Green River?

j.

Study the diagrams in Figure 15.18 and compare them to the area around Mammoth Cave in Figure 15.21d. Does it appear that a stream eroded Eaton Valley? Yes / No. What type of feature is Eaton Valley?

k. How did Eaton Valley form?


N

B

Frozen Niagara Entrance

Courtesy: USGS

C

D

km mi

1 0.5

2 1

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4 2.5

5 3

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7 4.5

8 5

M

*G

(a) Boxes on this small-scale map outline the locations for the maps in b, c, and d. Figure 15.21 Topographic maps of the area around and to the south of Mammoth Cave, Kentucky. (a) USGS Mammoth Cave Quad and USGS Park City Quad 1:100,000 series. (b, c) USGS Park City Quad 1:24,000 series. (d) USGS Mammoth Cave Quad 1:24,000 series.

Groundwater and Karst Topography

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Courtesy: USGS

(b) Region east of Rocky Hill, Kentucky, about 6 miles south of Mammoth Cave.

Courtesy: USGS

N

M 0

0.7 0

0.4

1.4 0.8

2.1 1.2

2.8 1.6

(c) The Knobs, Kentucky, in a region about 4 miles south of Mammoth Cave. Figure 15.21 (Continued) Topographic maps of the area around and to the south of Mammoth Cave, Kentucky.

*G

3.5 km 2 mi

M  3.428 G  0.565


Courtesy: USGS

N

M 0

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2.1

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1.6

(d) Mammoth Cave, Kentucky, and vicinity.

*G

3.5 km 2 mi

M  3.428 G  0.565

Figure 15.21 (Continued) Topographic maps of the area around and to the south of Mammoth Cave, Kentucky.

31. Study the geologic map in â– Figure 15.22 of the same general area around and south of Mammoth Cave. â–  Figure 15.23 shows a stylized and exaggerated geologic cross section of the area from southeast to northwest.

c. Explain why these features form in these rocks.

a. What specific rock type typical of karst topography is present in the area?

b. For each of the following features list the dominant specific rock type forming the feature; you may need to refer back to Figure 15.21 and Exercise 30 for some of the features: Sinkholes: ______________________ Sinking streams: _________________ Solution valleys: _________________ Conical hills: ____________________ Plateaus: _______________________ Caves: _________________________

Groundwater and Karst Topography

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Courtesy: University of Kentucky, Kentucky Geological Survey

Figure 15.22 Geologic map of the area near and south of Mammoth Cave, Kentucky. Used with permission of the Kentucky Geological Survey, University of Kentucky, Lexington, KY. http://www.uky.edu.KGS

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SE

NW Chester Upland Conical hill

Mainly insoluble rocks

Elevation (feet)

Limestone-floored solution valley with sinkholes

1,000 Green River

Pennyroyal Plateau

800 Sinkholes

Girkin Formation

springs

Ste. Genevieve Limestone St. Louis Limestone

Mammoth Cave

600 400 200

Average dip of rocks 1/3 to 1/2 degree

Impure limestone

Approximately 8x vertical exaggeration Figure 15.23 Geologic cross section of Mammoth Cave area. Figure 15.21b is similar to the area southeast of the conical hill; The Knobs in Figure 15.21c correspond approximately to the location of the conical hill; and Figure 15.21d corresponds to the Mammoth Cave and solution valley part of the cross section. Note that the vertical exaggeration increases readability of detail but also results in an exaggerated slope.

d. The rocks here are dipping very gently to the northwest at about 1/3º to 1/2º, as shown in Figure 15.23. In what way does the dip influence the changes seen in the topography from the maps in Figure 15.21b to c to d?

e. Over time, what has happened to the water table in the area near Mammoth Cave? Use the steps shown in Figure 15.18 to help you infer this. Describe at least three pieces of evidence to support your answer.

Another important region of karst topography occurs in Florida.

32. Examine the map of the area near Lake Wales, Florida (■ Figure 15.24). a. What are depressions in this area?

Groundwater and Karst Topography

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N

27° 54

27° 54

27° 5330

27° 5330

27° 53

27° 53

27° 5230N 81° 35

81° 34

Scale: 1:24,000 Contour interval: 5 ft.

Figure 15.24 Lake Wales, Florida, USGS Topographic Map. Scale 1:24,000; contour interval  5 ft (Exercise 32).

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b. What is the term for the features forming the lakes?

c. What geologic hazard is likely in this area as a result of this type of topography?

d. Why do some depressions have water in them while others don’t?

e. What is the range of levels of the water table in this area? f.

Mark an H on the map in the location with the highest water table.

g. What rock type do you think occurs in the Lake Wales area? 33. Use the map of the area near Interlachen, Florida, in â– Figure 15.25 to answer the following questions. a. What evidence suggests this is a region of karst topography?

Imagine you own land at the red X between Junior and Church Lakes near Interlachen. You want to drill a well on your land. Your sister also wants to drill a well on her land at the X marked 98 southeast of Interlachen. To aid in determining the depth of the wells and, therefore, the cost of drilling, use the contours of the water table on the map of Interlachen in Figure 15.25 and answer the following questions: b. For your well site, what are the land elevation, the groundwater elevation, and the difference? Land:

0 0

0.4 0.3

0.8 0.6

1.2 0.9

1.6 1.2

2 km 1.5 mi

Figure 15.25 Interlachen, Florida, USGS 15 minute Topographic Map. Scale 1:48,000; topographic and water-table contour interval  10 ft. Blue lines are water-table contours (Exercise 33).

Water:

Difference:

Groundwater and Karst Topography

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c. Elevations at your sister’s? Land:

Water:

Difference: d. How deep must your well be to reach the water table? e. Your sister’s? f.

378

Draw arrows on the map of Interlachen in Figure 15.25 showing the direction of flow of the groundwater.

L ab 1 5

g. If the people of Interlachen dumped toxic chemicals into Jewel Lake, would either you or your sister expect to eventually have chemical contamination in your well? h. If you go fishing in Gum Creek, can you expect your fish to be free of contamination for a long time to come, or would the toxic chemicals eventually kill them?

Groundwater and Karst Topography  

Подпочвени води и карстова топография

Groundwater and Karst Topography  

Подпочвени води и карстова топография

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