Geoscience_Laboratory_5e_Ch12: I need help with the questions highlighted in…
Groundwater 213 12 Groundwater Topics Topics A. What is the de?nition of groundwater? Why is the composition of geyser deposits variable within Yellowstone National Park? How does the total amount of groundwater on Earth compare with that of surface water? B. How do the dynamics of groundwater in humid regions differ from those in arid regions? C. What are two common problems associated with the water table described here? What is hydraulic gradient, and how does its measurement differ from the way in which geologists measure slope? What is hydraulic conductivity? What is Darcy’s Law, and how does it apply in computing the rate at which groundwater ?ows through a saturated zone? D. How can one map a water table from well data? How can one use such a map to decipher the direction and rate of ?ow of contaminants within the saturated zone? E. What is a potentiometric surface, and what does it have to do with artesian conditions? What does a potentiometric surface have to do with ?owing and non-?owing artesian wells? F. What is a perched water table? What is the purpose of a reservoir liner? G. What is karst topography, and what are its features? How are the direction and rate of ?ow of groundwater in Florida measured from a study of ponds within sinkholes? A. Groundwater de?ned In its broadest de?nition groundwater is all that water that occurs in otherwise open spaces within rocks and sediments. Groundwater that originates from the precipitation of rain and snow—the topic of this exercise—is called meteoric water. of their deposition in ancient seas; and juvenile water, water that was born of magmatic activity. Neither connate water nor juvenile water is a source of In addition to meteoric water, there are two minor sources of subterranean water: connate water (aka sediment water), which is water that was entrapped within sediments at the time Recharge area W a te r Figure 12.1 The vast majority of groundwater is meteoric in origin and is free to move with vagaries of climate. Rates of groundwater ?ow differ with depth, ranging from days to thousands of years to traverse an area the size of a county. ta b l e Days Years Decades Flow lines Centuries Millennia potable water, but connate water can locally be important as a high-salinity environmental contaminant associated with petroleum. Discharge area 214 Groundwater Groundwater—the great dissolver, the great precipitator Groundwater is physically and chemically dynamic. It is constantly on the move, constantly dissolving and/or precipitating a host of rocks and minerals—depending on the chemistry of the water and the chemistry of the rocks and sediments through which it moves (Fig. 12.2). Dissolves some things Precipitates many things Limestone, gypsum, salt (forms caves and landscapes in these rocks) Cave deposits (stalactites, stalagmites, etc.) Few minerals from sandstones, shales, and igneous and metamorphic rocks (rarely forms caves in these rocks) Cements that hold sedimentary rocks together (calcareous, siliceous, ferruginous) Spring and geyser deposits Q12.1 In the southern part of Yellowstone National Park (e.g., the vicinity of Old Faithful), geyserite consists of varicolored siliceous material; whereas in the northern part of the park (e.g., in the vicinity of Mammoth Hot Spring), geyserite consists of snow-white calcareous material. Examine details in Figure 12.3 and try to explain why this difference in the mineral compositions of Yellowstone geyserites. Hint: What goes around, comes around. What water dissolves, water might precipitate. Concretions and geodes Mont. Figure 12.2 Groundwater dissolves rocks and minerals, groundwater precipitates rocks and minerals—depending on the composition of the water and on the composition of the rocks and sediments through which it moves. Yellowstone NP Id. South The variability in the composition of groundwater is illustrated by the variety of geyserites in Yellowstone National Park. (Geyserite is mineral material that is precipitated from groundwater as it emerges from the ground and evaporates, leaving behind elements that were in solution.) Siliceous geyserite Wyo. Tertiary siliceous igneous rocks Calcareous geyserite North Paleozoic limestones Figure 12.3 This is a schematic cross-section showing the variety of rocks in which the ‘plumbing systems’ of Yellowstone geysers occur. Groundwater and geologic wonders—Minerals that grow within cavities in rocks are most commonly precipitated by groundwater (Fig. 12.4A and B). And, petrifaction of trees of Triassic age in the Petri?ed Forest of northern Arizona re?ects the work of groundwater as well (Fig. 12.4C). In the world of geologic wonders, cases documenting the effects of groundwater abound. A B C 1996 LIB Old Faithful TY ER Figure 12.4 A This faceted quartz crystal was precipitated by groundwater within cavities in sandstone of the Ouachita Mountains of west-central Arkansas. B This geode was precipitated by groundwater within cavities in volcanic rocks of Brazil. C. This fossil tree was ‘petri?ed’ by groundwater in the Petri?ed Forest region of Arizona. Groundwater 215 COLUMBIA DAILY TRIBUNE , MARCH 24, 2002 U.N. water report warns of impending shortages The impending global water shortage not only requires that the world develop all available potable water resources in the near future, but we must also do a better job of minimizing the waste of water and guard it against pollution. Crisis will affect 5 billion wo rldwide by 2025. VIENNA, Austria (AP)—Wa people face a critical shortag rning that 2.7 billion 2025, the United Nations ma e of drinkable water by rke Friday with a call for a “blue d World Water Day on rev and tap the seas for new supplie olution” to conserve In fewer than 25 years, abo s. living in areas where it will ut 5 billion people will be to meet all their needs for be difficult or impossible fre looming crisis that overshado sh water, creating “a the Ea rth ’s pop ula tio n,” ws nearly two-thirds of a U.N . rep ort sai d. Groundwater will play a growing role in efforts to provide water for our growing global population, given the fact that it quantitatively competes with other fresh water resources (Fig. 12.5). Water on land 0.2 Oceans 1,350 Glaciers 29 Groundwater 8.4 (x 1,000,000 km 3) Figure 12.5 This is a comparison among the four vast reservoirs of accessible water on Earth. (Each unit is one million cubic kilometers.) Water on land consists of streams, rivers, lakes, and ponds. Q12.2 (A) How many cubic kilometers of water reside within groundwater? (B) How many more times abundant is groundwater than water on land? Groundwater has several advantages over surface water when it comes to providing for municipal needs. Q12.3 Imagine that you are a member of a city council and your town is in need of a new and larger municipal water supply. Discussion has turned to the merits of well water compared to those of surface water. In what ways can you imagine that groundwater might be superior to surface water as concerns the following points? (A) Paying the cost of drilling a well, compared to that of constructing a dam. (B) Contending with the occasional drought in your semiarid region. (C) Minimizing contamination from surface runoff and from the atmosphere. (D) Protecting your water supply against the threat of terrorism. 216 Groundwater B. Anatomy of water tables Saturated and unsaturated zones—Within the subterranean realm of groundwater there are two main zones: A word about arid and semiarid streams (1) The saturated zone (Fig. 12.6) is the zone in which open spaces in sediments and rocks are ?lled with water. The top of the saturated zone is the water table. The slow movement of groundwater—toward streams in humid regions and away from streams in arid regions—is impeded by friction, so water tables are rarely ?at. The shape of a water table in a humid region mimics that of the land surface— i.e., high under hills and low under valleys, where it intersects perennial streams and lakes. Q12.4 Judging from the informa- (2) The unsaturated zone is the zone in which intergranular spaces and fractures are ?lled with air and, at times, ?lms of descending water. If you answered the above question with, “in an arid or semiarid region,” you were correct. Flat-bottomed losing streams like that in Figure 12.6B (Spanish arroyos or barrancos) can be dangerous. There is little rain in arid and semiarid regions, but when rain does come, it is typically torrential. Commonly, the entire annual amount of rainfall arrives in a single afternoon—often creating disastrous ?ash ?oods. In August 2003 this kind of ?ash ?ood swept automobiles and highway dividers from I-35 in Kansas, killing a number of people. A Humid conditions (infiltration is important) Unsaturated zone Wa te table r Gaining stream So il Saturated zone B Arid or semiarid conditions (runoff is important) Aquifer de?ned Losing stream Unsaturated zone tion accompanying Figure 12.6, which kind of stream do you suspect would rise faster (though brie?y) for a given amount of rain—that in a humid region or that in an arid or semiarid region? Ba r e r ock Wa tab ter le Saturated zone Figure 12.6 A In a humid region, water moves (‘seeking its own level’ as it were) in its tendency to develop a horizontal water table, and so the saturated zone feeds a gaining stream. B In an arid or semiarid region, the water table slopes downward from a losing stream, the source of water for the saturated zone. Much of western United States lies within the Great American Desert (Fig. 12.7), a region in which the groundwater situation is like that in Figure 12.6B. In contrast, eastern United States is characterized by conditions shown in Figure 12.6A. Great American Desert Figure 12.7 Because of arid to semiarid climate, approximately one-half of the conterminous 48 states is at risk as concerns the development and management of water resources. Arid Semiarid Before going further in our discussion of groundwater, we need to de?ne the concept of aquifer—a body of sediments or rocks that yields water suf?cient to meet speci?c needs. The saturated zone in Figure 12.6 might or might not be an aquifer. The de?nition of ‘aquifer’ is qualitative; e.g., an aquifer supplying a particular city might cease to be viewed as an aquifer were the population to grow beyond its capacity. Groundwater 217 C. Dynamics of water tables Common problems Hydraulic gradient In Los Angeles County, a 3.5-mile section of I-105 was constructed below ground level in an effort to minimize noise and visual pollution. Caltrans (Calif. Dept. of Transportation) believed the water table to be 30 feet below road level at the time of construction (Fig. 12.8). However, what Caltrans failed to learn was that the water table had been drawn down by over-pumping in the 1950s, and another state agency had recently mandated that the over-pumping cease. (Ref: Calif. State Auditor Geologists describe the magnitude of slope as the vertical angle between slope and the horizontal (Fig. 12.10). rep’t #99113, 1999.) 30 30 ft Unsaturated Saturated Wa t e r t a b l e a t t i m e o f c o n s t r u c t i o n Figure 12.8 Highway engineers recessed a section of I-105 in Los Angeles County in an effort to mitigate noise and visual pollution. At that time, the water table was 30 ft below the highway. Q12.5 So what do you suppose happened when over-pumping of the saturated zone was stopped by that other California state agency? On more than one occasion a gas station in a low topographic setting has allowed the level of gasoline in its storage tanks to become too low (Fig. 12.9). Then came the rains, with runoff making its way into the saturated zone. Figure 12.10. A Brunton compass, with its leveling bubble and sight-adjusted protractor, enables a geologist to measure the vertical angle between slope and the horizontal. But engineers describe the magnitude of slope as the ratio of vertical drop to horizontal distance, aka the percent of grade. Thus, a gradient of 0.05, or 5 percent, designates a vertical drop of 5 feet per 100 feet of horizontal distance. This same convention is used in describing the hydraulic gradient of groundwater (Fig. 12.11). Well #1 h1 h1 h2 Well #2 Wa te r ta b le h2 l Hydraulic gradient = Unsaturated zone Wa te r ta bl e Saturated zone ‘X-Ray view’ of gas storage tanks Figure 12.9 This gas station is very near the water table, which presents a threat to fuel storage tanks. Q12.6 Can you imagine what happened when the water table rose? Hint: Asphalt and concrete are only so strong. h1 h2 l Figure 12.11 h1 is the elevation of the water table in well #1, h2 is the elevation of the water table in well #2, and l (for length) is the horizontal distance between wells. Q12.7 If, for the model in Figure 12.11, h1 were 506 ft, h2 were 497 ft, and l were 150 ft, what would be the hydraulic gradient (in percent) between well #1 and well #2? 218 Groundwater Hydraulic conductivity Darcy’s Law Just as surface water ?ows faster down steeper slopes, groundwater moves faster down steeper hydraulic gradients. But hydraulic gradient is not the only factor affecting the rate of groundwater movement. Equally important is hydraulic conductivity, which is the ease with which sediments or rocks transmit water. Hydraulic conductivity introduces the concepts of porosity and permeability. The most fundamental questions in targeting a prospective groundwater resource are, ‘How much, and how often?’ This volume per time issue is analogous to the discharge of a stream. Porosity is the percentage of a body of sediments or rocks that consists of open spaces, called pores. Porosity determines the amount of water that sediments or rocks can hold. There are many kinds of pores—ranging from pores among sedimented particles, to pores within volcanic rocks, to cavities within soluble rocks, to fractures in any kind of rock (Fig. 12.12). And any of these pores can be ?lled to differing degrees by cements. Permeability is the ability of soil, sediment, or rock to transmit ?uid. Material with low porosity is likely to have low permeability as well, but high porosity does not necessarily mean high permeability. In order for pores to contribute to permeability, they must be (a) interconnected, and (b) not so small that they restrict ?ow. For example, clay commonly has high porosity, but clay grains are so broad in proportion to their microscopic size (i.e., around 0.005 mm) that the molecular force between clay particles and water restricts ?ow. As concerns the potential of sediments and rocks to transmit water—a critical issue in the aquifers—permeability is paramount. Figure 12.12 (White space is open space available to water. Open space along fractures is too thin to illustrate.) Porosity and permeability can result from sedimentation, volcanism, solution, collapse, faulting, and fracturing. Subsequent cementation can reduce the volumes of any of these pores. (Magni?cation is 5–10x.) Pebbles and cobbles Sand Limestone with solution cavities In 1856, Henri Darcy, a French engineer, attempted to determine whether a prospective aquifer could yield water suf?cient for the city of Dijon. Darcy undertook a series of laboratory experiments in which he measured the rate of water ?ow through a variety of sediments in tubes tilted at various angles. Not surprising to us now, Darcy concluded: (1) Groundwater ?ows faster through more permeable rocks. (2) Groundwater ?ows faster where the water table is more steeply inclined. Volcanic rock with vesicles Fragments produced by collapse or faulting Darcy identi?ed the four key variables in groundwater ?ow (or discharge) as… • • • • Discharge (Q) Hydraulic conductivity (K) Hydraulic gradient (h1 – h2 / l) Area (A) (thickness x breadth of the aquifer) …and crafted an algebraic expression (‘Darcy’s Law’) of their relationship: Fractured marble Q = (K) (h1 – h2 / l) (A) Darcy’s Law enables one to calculate the maximum amount of water that an aquifer might yield to an array of wells. Example: Fractured quartzite Fractured shale Q12.8 Hydraulic conductivity of an aquifer is known to be 8 ft/day, and its dimensions are estimated to be 40 ft thick and 18,000 ft wide. Two test wells drilled one mile apart in the direction of ?ow encountered the water table at elevations 5,030 ft and 5,050 ft. Question: How many gallons of water ?ow through the aquifer per day? (To convert ft3 to gal, see page i at the front of this manual.) Groundwater 219 D. Mapping a water table Map the direction of groundwater ?ow within your mapped area Figure 12.23 on Answer Page 230 is a contour map on which 26 water wells have been plotted. Each well site shows the depth to the water table within that well as a negative value in feet below ground level. Procedure: For each well location: (1) Estimate the surface elevation from the proximity of contour lines. Rates of groundwater ?ow— applying Darcy’s Law to your map of the water table (2) Subtract the depth to the water table from surface elevation in order to determine the elevation of the water table. Record that elevation (of the water table) at the well site. Q12.12 What is the difference in (3) After repeating the above two steps for each of the 26 wells, contour the groundwater elevations with a contour interval of 20 feet. (A ‘getting started’ example is framed with a gray rectangle in the lower-left corner of the map.) Q12.9 Draw an arrow between Well A and Well B indicating the direction in which groundwater is likely to be moving. In which direction is the arrow pointing, northeastward or southwestward? Q12.10 (A) At what map coordinates is the difference between the elevation of the ground and the elevation of the water table the greatest? (B) Give the coordinates of a place where you might expect to ?nd a marsh or spring. Q12.11 If contaminants were to ?nd their way into groundwater at Acme Industries, in which well would those contaminants be more likely to appear—the well at the Smith farmhouse, or the well at the Jones farmhouse? elevations of the water table at Well A and Well B? Q12.13 What is the map distance (in feet) between Well A and Well B? Q12.14 What is the hydraulic gradient (h1 – h2 / l) between Well A and Well B? Q12.15 If the hydraulic conductivity (K) of the aquifer is 10 ft/day, and the cross-sectional area (A) of the aquifer is 200 ft x 5,000 ft, what is the rate of ?ow or discharge, (Q), through the aquifer in cubic feet per day? 220 Groundwater E. Artesian conditions and con?ned aquifers ‘Water seeks its own level,’ which explains simple artesian conditions. To illustrate—if we were to add water to the glass tubing in Figure 12.13A, to a particular elevation in conduit A, that water would be driven by hydrostatic pressure to that same elevation in conduits B and C. However, were our tubing (a) ?lled with sand, and (b) open at its downstream end (Fig. 12.13B), water would not rise as high in B and C. Moreover, the farther from the recharge area, the less the hydrostatic potential for lifting water in a conduit. This reduction in potential away from the water’s source describes a sloping potentiometric surface. Notable artesian examples In the real world, an artesian aquifer is more like the situation in Figure 12.13B (rather than 12.13A) in that (a) an artesian aquifer is ?lled with sediments and/or rocks, and (b) water within an artesian aquifer is free to move within that aquifer. Q12.16. In Figure 12.13B, what two things account for less hydrostatic pressure within Conduit C than within Conduit B? Hint: These two things are pretty much spelled out by labels in this ?gure, and one appears in the description of ‘the saturated zone’ (explaining why water tables are rarely ?at) on page 216. Figure 12.14 Fountains at Trafalgar Square are a graphic reminder of the famous London Artesian Basin. A This system is filled with nothing but water and is closed downstream Conduit A Hydrostatic pressure When wells were ?rst drilled in London circa 1900, fountains at Trafalgar Square were ?owing artesian wells (Fig. 12.14). But hydrostatic pressure has since declined, so now water must be pumped to the surface. Conduit B Conduit C Horizontal surface Upward pressures essentially equal Artesian thermal waters at Hot Springs National Park, Arkansas, owe their heat to deep circulation (Fig. 12.15). Slow descent of rainwater is via a large collecting system, with rapid ascent via narrow passageways. (A bent funnel effect.) Thus, heat persists as waters make their quick escape to the surface. Closed B This system is filled with sediment and is open downstream Hot Springs N.P. 60?F 143?F Hydrostatic pressure Pressure reduced Friction impedes flow through sediments Slopin g potenti surfac ometric e Pressure reduced even more 1 mi Open (some pressure relieved) Figure 12.13 A This tubing illustrates the simple principle, ‘water seeks its own level.’ B. This tubing illustrates the variation of this principle that is applicable to the real world of artesian ?ow of groundwater. It’s ?lled with sediment, and it’s open downstream. Figure 12.15 Within folded rocks of the Ouachita Mtns., rainwater enters an artesian aquifer at 60°F, descends to a depth of one mile, and emerges at 143°F (the temperature of hot coffee). Groundwater 221 Details of con?ned aquifers—But ?rst, a look back at uncon?ned aquifers. In Sections B and C (pages 216–218), which deal with the anatomy and dynamics of water tables, aquifers are uncon?ned; i.e., the top of the zone of saturation (i.e., the water table) is free to rise and fall with the vagaries of climate. But in a con?ned aquifer, such is not the case. A con?ned aquifer is one in which water is prevented from rising and falling by relatively impermeable intervals of rock called aquitards (from Latin, retards water). A visual metaphor: Swiss cheese (with its interconnected holes) between slices of dense bread. Aquitards typically consist of shale—the most common and the least permeable of all sedimentary rocks. A con?ned aquifer receives its water in an area where it intersects the land surface, called the recharge area (Fig. 12.16). Where a well is drilled into a con?ned aquifer, water rises toward Recharge area the elevation of the water table in the recharge area, a condition called artesian (recall the glass tubing model on the facing page). Water will not rise quite as high as the water table in the recharge area because (a) friction is associated with water moving through the aquifer, and (b) the water is free to move laterally, thereby reducing hydrostatic pressure. The imaginary level to which water in a group of artesian wells tends to rise is (as de?ned on the facing page) the potentiometric surface (aka the piezometric surface). In Figure 12.16 the potentiometric surface is below ground level in the vicinity of Wells A and B, so they are non-?owing artesian wells. The potentiometric surface is above ground level in the vicinity of Well C, so that well is a ?owing artesian well. Q12.17 On Figure 12.24 on Answer Page 230, label each well with the correct letter as described in the text beside that ?gure. Mapping a potentiometric surface— Figure 12.25 on Answer Page 231 shows six wells (#1–#6) on a ground elevation contour map in the area of a con?ned aquifer. All six wells penetrated the aquifer. The map includes a second set of contours (straight dashed lines) drawn on the aquifer’s potentiometric surface. Flowing artesian wells should occur where the potentiometric surface is higher than ground elevation. Non-?owing artesian wells should occur where the potentiometric surface is lower than ground elevation. Procedure: At every place where a ground elevation contour line crosses a potentiometric contour line of the same value, place a small circle. Then connect the circles with a line. Ground elevations of wells on one side of that line are lower than the potentiometric surface, so the wells should be ?owing artesian wells; whereas ground elevations of wells on the other side of that line are higher than the potentiometric surface, so the wells should be non-?owing artesian wells. Q12.18 (A) Which of the six wells in Well A Well B Water table Aquitard Well Horizontal C Potentiomet ric surface Aquifer Aquitard Figure 12.16 The dotted line is a horizontal projection of the water table in the recharge area. The solid line is the potentiometric surface. Well A is near the recharge area, so the water in it rises almost to the elevation of the water table. Well B is farther away, so friction over a greater distance accounts for the water’s not rising as high as in Well A. The mouth of Well C is below the potentiometric surface, so it is a ?owing artesian well. Figure 12.25 on page 231 should be ?owing artesian wells? (B) Darken the area of the map (like the swatch in the legend) in which the wells should be ?owing artesian wells. P.S. Such a map is useful in assessing land use values. Who wants to purchase a site for a home or a building in a swamp? 222 Groundwater F. Perched water tables Where descending surface water encounters an aquitard, it can accumulate as a local saturated zone with its own local perched water table (Fig. 12.17). This is one of the more common explanations for the occurrence of springs. In areas of ?at-lying sedimentary rocks where relatively impermeable shales alternate with permeable sandstones or limestones, perched water tables (and related springs) are likely. Ephemeral ponds (i.e., short-lived) occur where water is temporarily prevented from sinking into the zone of saturation by relatively impermeable soil or sediments (Fig. 12.19). Such ponds disappear after a time by evaporating and/or sinking into the unsaturated zone. A Pond Water table B Perched water table Water table Spring Aquitard Water table Pond Unsatura ted zone Saturate d zone Figure 12.17 Here, descending water is de?ected by an aquitard to emerge as a spring. The general water table is deeper. Trouble for construction sites—Perched water tables can cause water problems in basements of homes and buildings. At Saint Louis University, excavation for two buildings breached a perched water table (Fig. 12.18). The only remedy is the installation of sump pumps to lift the water out and away to storm sewers. Lindell Blvd. Saint Louis University Engineering Complex Laclede Ave. Figure 12.18 Perched water tables can cause springs to appear in basements. The symbol for a spring is a small circle with a wiggly tail (see the last item under Rivers, Lakes, and Canals on the Topographic Map Symbols on page 39). Relatively impermeable clay soil Relatively impermeable sediments Figure 12.19 A This ephemeral pond occurs within a sinkhole formed by the collapse of a cave roof. B This ephemeral pond occurs within a depression typical of landscapes carpeted by glacial deposits. Q12.19 In Figures 12.19A and B, two depressions are occupied by water, whereas others are dry. (A) Explain this presence and absence of ponds in these two ?gures. (B) If the two ponds were perennial (i.e., year-round) ponds, because of intersecting the water table, how would the presence or absence of water in the other depressions differ from that which is shown? Groundwater 223 Reservoir liners and the Erin Brockovich case A rancher can build a stock pond (with proper local permits) by simply bulldozing an earthen dam across a stream course. If stream ?ow is considerable, an added feature might be a metal tube or concrete spillway to handle excess water and prevent the water’s topping the dam and washing it away. A second matter, especially common in cases of marginal water supply, is leakage, which can result in pond water descending into the unsaturated zone (Fig. 12.20). Q12.20 Judging from what you A learned from information in Figure 12.19A on facing page 222, how might one seal a leaking stock pond? Hint: We’re talking three steps here, with steps #1 and #2 being the draining and restoring pond water. Pond Unsaturated zone Leakage of pond water through fractures or other permeable flowpaths Water table Saturated zone The Erin Brockovich case (1994) grew out of the undisputed fact that Paci?c Gas and Electric used water containing an alleged carcinogen, chromium-6, to cool pipes that were heated by the compression of gas at its pumping station near Hinkley, California. (Chromium-6 was added to the water to minimize corrosion of the cooling towers.) A group of plaintiffs alleged that chromium-rich water was placed in evaporating ponds that lacked proper reservoir liners. PG&E admitted that there was, in fact, seepage of pond water into the groundwater, but disputed the alleged health effects of such water. Hydrologic and geochemical data marshalled by Brockovich, legal assistant with the Masry law ?rm, which represented the plaintiffs, are not a matter of public record because the case was settled out of court. So any maps and cross-sections that were entered into evidence by either side are unavailable. B Impermeable liner Leakage stopped Figure 12.20 A A pond without an impermeable liner might leak water into the unsaturated zone. B The same pond with an impermeable liner loses water only through evaporation. Such liners are essential in the construction of best land?ll sites. 224 Groundwater G. Karst topography In humid areas where there is an abundance of limestones and/or dolostones (and, more rarely, gypsum and/or salt), the dissolving of these relatively soluble rocks accounts for the development of caves and cave-related features (Fig. 12.21). (The chemistry of dissolution appears on pages 124-125.) Sinkholes commonly develop through the collapse of cave roofs, and disappearing streams descend into underground passages, only to reappear as springs. of Slovenia that is characterized by such features. Karst landscapes are largely solution landscapes. Very little detrital sediment occurs in streams ?owing from such regions. Such a landscape is called karst topography, a name taken from a part A Figure 12.21 (A) Dissolution of limestone occurs within the uppermost part of the saturated zone (just beneath the water table), because it is there that carbonic acid (i.e., rainwater plus atmospheric and soil carbon dioxide) enters the saturated zone. (B) As a valley is deepened by solution, the water table descends, tracking perennial streams with which it intersects. The result is that the cave effectively emerges from the saturated zone, the cave ceases to grow, and, instead, begins to be ?lled with an array of cave deposits (e.g., stalactites, stalagmites, columns, ?owstone, curtains, cave pearls). Unsaturated zone B Disappearing stream Saturated zone Spring Solution valley Sinkholes (collapsed cave roofs) Beware the karst Karst is a proverbial red ?ag in almost every imaginable kind of land develop. In karst terrain, special care should be aken in an effort to comtend with the following.: • Pollution of groundwater, resulting from the rapid ?ow of waste ef?uent from its source to water supplies. • Catastrophic collapse of sinkholes, swallowing buildings and people. • Failure of building foundations, causing irreparable damage to homes and commercial buildings. Of these, pollution of groundwater is the most universal. The movement of groundwater through soil and sediments magically cleanses water of ordinary waste such as that in municipal and home sewage systems—provided that the residence time within the soil and sediments is suf?ciently long (i.e., measured in months and years). Not only does the movement through soil and sediments ?lter solid particles, but slow movement typical of ?ne-grained earth materials provides time for pathogenic bacterial and other troublesome micro-organisms to die. In striking contrast to the tiny passageways in ?ne-grained soil and sediments, solution channels in limestone are so large that through-?ow of liquids is virtually instantaneous. Groundwater 225 Google Earth image of karst topography An example of karst topography occurs at Rock Bridge State Park, Missouri (Fig. 12.22). This area is at the northern margin of the Ozark Plateau, the most spectacular karst region between Mammoth Cave, Kentucky and Carlsbad Caverns, New Mexico. Q 12-21 Examine the Google Earth image in Figure 12.22. Notice the many ponds. At a glance some of the ponds might be mistaken for stock ponds. However, there is good evidence indicating that the large muddy pond at letter ‘A’ in the southwest quarter of the photo is a sinkhole. What is the evidence indicating a sinkhole, rather than a stock pond? Hint: The evidence appears in the relationship of the pond to an adjacent man-made feature. Karst topography Rock Bridge State Park Missouri Ashland, Missouri 7 1/2’ quadrangle N. 38° 52’ 09’’, W. 92° 17’ 10’’ A Figure 12.22 Sinkholes characterize karst topography at Rock Bridge State Park. Some sinkholes are naturally lined with relatively impermeable clay soil (see again Figure 12.19A on page 222), so they can hold water long after the rains that ?lled them. In contrast, other sinkholes are without bene?t of such natural liners, so they fail to retain rainwater for even short periods of time. 226 Groundwater Karst topography on a map—Putnam Hall, Florida, quadrangle The Putnam Hall, Florida quadrangle—on facing page 227—is in a region underlain by limestone of the Florida Aquifer. (The contour interval is10 feet) Countless sinkholes (some with ponds, some without) are marked by bundles of crudely concentric contours with hachures (tick-marks) indicating depressions. Some of these bundles of contours are ‘pinched’ on opposing sides, creating a bird’s eye appearance (e.g., at C-6 and E-3) that suggests solution of the limestone along fractures, aka joints. Q12.22 Do these probable joints appear to be oriented more nearly northeast–southwest, or more nearly northwest–southeast? Q12.23 There appears to be no stream patterns indicating through-?owing surface streams on this map. Why do you suppose that is? Q 12.24 What is the elevation of the surface of (A) Chipco Lake? (B) Mariner Lake? (C) Junior Lake? Q 12.25 Do water levels in these three lakes (as well as others) appear to be governed by the vagaries of spotty rainfall and random surface drainage, or do they appear to mark systematic elevations on a water table? Hint: Notice the elevations of the bottoms of dry sinkholes relative to water levels in the ponds. Q 12.26 (A) What is the elevation of the surface of Grassy Lake? (B) What is the distance (in feet) between the center of Chipco Lake and the center of Grassy Lake? Q 12.27 Blue biodegradable ?uorescent dye was once added to Chipco Lake. Twenty-four hours later the dye appeared in Grassy Lake. What is the approximate velocity of groundwater ?ow between these two lakes in feet per day? Q 12.28 Would you expect to detect that same ?uorescent dye in Mariner Lake? Why, or why not? For a comprehensive short course in groundwater, go to EPA site http://www.epa.gov/seahome/groundwater/src/geo.htm Groundwater 227 228 Groundwater Intensionally Blank Groundwater 229 (Student’s name) (Day) (Lab instructor’s name) ANSWER PAGE (D) 12.1 12.4 12.5 12.2 (A) (B) 12.3 (A) 12.6 (B) (C) 12.7 12.8 (Hour) 230 Groundwater 12.12 12.9 12.10 (A) 12.13 (B) 12.14 12.11 12.15 B C D E F G H I J K L M N O P -10' V W Well B 72 -100' 74 -72' 8 820 800 780 -60' 10 11 -60' 13 -50' -35' -30' 760 12 -25' Acme Industries Smith 740 14 76 Well A -6 0 ' -30' 0 -70' -50' 7 -50' -65' -25' 15 680' (730'-50') 16 -50' 17 -50' 660' (710'-50') 66 720 680 -60' Jones -30' -25' 0 700 20 21 U 0 70 -20' -55' 6 19 T -70' 0 68 4 18 S -30' 0 -60' 3 9 R 0 2 5 Q 0 A 66 1 Ge t t i n g s t a r t e d -40' 0 N 1,000 ft 500 -55' Figure 12.23. A contour map of a humid area. The contour interval is 20 ft. Each dot represents a water well with the depth to the water table indicated in feet with a negative value (e.g., -70’). 12.16 Water table Aq uife Aqu r Aqu itard itard 12.17 (Label Fig. 12.24.) A—Possibly a ?owing artesian well. B—Possibly a non?owing artesian well. C—A well that might produce from an uncon?ned aquifer. Figure 12.24. Three wells with different hydrologic settings. Groundwater 231 12.18 (A) 12.20 12.19 (A) (B) 12.21 1 A B C D E F G H I J K L M 2 76 800 5 6 820 U V W Well #6 0 82 84 86 88 0 0 0 0 86 0 0 12 T 0 88 11 S 92 0 0 80 860 10 R 90 0 840 9 Q 40 78 Well #5 8 P 780 4 7 O 7 Well #4 3 N 84 0 Well #3 13 14 82 0 15 16 Well #1 80 0 17 18 19 20 21 78 0 76 0 Well #2 74 0 0 1/2 km N Figure 12.25 This is a contour map (with an interval of 20 ft) of an area underlain by a con?ned aquifer. The black dashed lines are Area of flowing artesian conditions drawn on the aquifer’s potentiometric surface, also with a contour interval of 20 feet. 232 Groundwater 12.25 12.22 12.23 12.26 (A) (B) 12.27 12.24 (A) (B) (C) 12.28 Hide preview glg101_r4_Week_5_Groundwater_Worksheet.doc Download Attachment This is an unformatted preview. Please download the attached document for the original format. Groundwater Worksheet | 1 GLG/101 Version 4 Associate Level Material Groundwater Worksheet Answer the lab questions for this week and summarize the lab experience using this form. Carefully read Ch. 12 of Geoscience Laboratory. Complete this week’s lab by filling in your responses to the questions from Geoscience Laboratory. Although you are only required to respond to the questions in this worksheet, you are encouraged to answer others from the text on your own. Questions and charts are from Geoscience Laboratory, 5th ed. (p. 213-228), by T. Freeman, 2009, New York, NY: John Wiley & Sons. Reprinted with permission. Lab Questions: 12.1 Explain the different coloration of the Yellowstone geyserites (in figure 12.3). 12.2A How many cubic kilometers of water reside within groundwater? 8.4 million cubic centimeters 12.2B How many more times abundant is groundwater than water on land? Groundwater/water on land=how many times 8.4/0.2=42 times 12.4 What type of stream would rise faster? The stream in the semiarid or arid region would rise faster. This is because the water table slopes downward from the source of water for the saturated zone. Also, the bottom of the stream is flat which will cause it to rise faster than a sloped bottom. Groundwater Worksheet | 2 GLG/101 Version 4 12.6 What would happen if the water table were to rise at this service station? 12.7 If, for the model in Figure 12.11, h1 were 506 ft, h2 were 497 ft, and l were 150 ft, what would be the hydraulic gradient (in percent) between well #1 and well #2? Hydraulic gradient=h1-h2/l Hydraulic gradient =506-497/150 Hydraulic gradient =9/150 =.06x100=6% 12.10A At what map coordinates is the difference between elevation of the groundwater and elevation of the water table the greatest? 12.11 If contaminants were to find their way into groundwater at Acme Industries, in which well would those contaminants be more likely to appear—the well at the Smith farmhouse, or the well at the Jones farmhouse? At the Jones Farmhouse 12.13 What is the map distance between Well A and Well B in feet? 12.18 Which of the six wells in Figure 12.25 in Ch. 12 should be flowing artesian wells? Wells 4, 5, and 6 should be flowing artesian wells, because there is lower ground elevation 12.19A In Figures 12.19A and B, two depressions are occupied by water, whereas others are dry. Explain this presence and absence of ponds in these two figures. The ponds that contain water have sediments in the bottom which prevent the water from being absorbed into the subsurface. Groundwater Worksheet | 3 GLG/101 Version 4 12.19B In Figures 12.19A and B, two depressions are occupied by water, whereas others are dry. If the two ponds were perennial (i.e., year-round) ponds, because of intersecting the water table, how would the presence or absence of water in the other depressions differ from that which is shown? The ponds which intersect the water table, must contain water I nthe ponds. The same sediments which permitted water in the pond in the previous exa[le wouldn’t permit the water from the water table to penetrate the pond. 12.20 Judging from what you learned from information in Figure 12.19A in Ch. 12, how might one seal a leaking stock pond? Hint: We are talking three steps here, with steps #1 and #2 being the draining and restoring pond water. After the water is cleared you can position a liner at the bottom to prevent water from draining out. 12.21 Examine Figure 12.22. At a glance, several ponds might be mistaken for stock ponds. However, there is evidence indicating that the large pond at coordinates P-5 is surely a sinkhole. What is that evidence? Hint: The evidence appears in the relationship between the pond and a man-made feature. Because the previous highway was straight, the sink hole got some of the highway. Due to this issue, the highway would have to be reconstructed. 12.25 Do water levels in these three lakes (as well as others) appear to be governed by the vagaries of spotty rainfall and random surface drainage, or do they appear to mark systematic elevations on a water table? Hint: Notice the elevations of the bottoms of dry sinkholes relative to the water levels in ponds. The level of water in the pond would be determined by the water table. Lab Summary Groundwater Worksheet | 4 GLG/101 Version 4 Address the following in a 200-300 word summary: ? ? Summarize the general principles and purpose of the lab. Explain how this lab helped you better understand the topics and concepts addressed this week. Describe what you found challenging about this lab. Describe what you found interesting about this lab. Write your summary here: