Geology 103 Planet Earth (QR II), Laboratory Exercises 1 Student Name: Section: Karst Landform: Groundwater Anyone who has viewed Chinese landscape scroll paintings will recognize that the mountains are very different from those of the Alps or the Rockies. An example of such a Chinese scroll painting is given on the right. It shows that mountains often have very steep, nearly vertical sides with cone-shaped tops, and there are columns after columns of this kind of conical hills. For many years, westerners thought that Chinese painters were unable to portray the landscape accurately. It turns out that this is a very unusual geologic formation called karst towers. They have been made famous by the sceneries in Guilin of southwestern China. Shown below is a recent picture of a village in the area, which has become a very popular tourist attraction. Karst topography is produced by groundwater percolating slowly through the interconnected pores or nearly vertical fractures in the subsurface. Dissolution of soluble rocks around these fractures sometimes produced a bowl-shaped depression at the surface, called a sinkhole. Sinkholes are quite common in Florida or southern Illinois. Sometimes, an underground cavity is hollowed out by the solution process forming spectacular caverns such as those of the Mammoth Cave in Kentucky. The karst towers in Guilin or in Chinese scroll paintings represent undissolved materials, commonly located between intersecting sets of fractures. Limestone formations beneath the surface are especially susceptible to groundwater dissolution. Assignment: 1. Given on the last page of this package is a topographic map of an area in southern Florida. The vertical and horizontal grid lines are 1 mile apart. The contour intervals are 10 feet. Recall that hachures of a contour line indicate the direction of downward slope at the surface. Circle at least three contour lines that show a closed depression at the surface with a red pen (or pencil). What type of rock formation would you guess underlying this area?
Geology 103 Planet Earth (QR II), Laboratory Exercises 2 2. Explain why some depressions (i.e., sinkholes) are filled with water to form ponds and lakes and others are dry. (Hint: assume that the water table in this area is the same everywhere; say at about 85 ft elevation.) 3. If you are to drill a water-well at a location marked by the red cross at the lower right hand corner of the map, explain how deep do you need to drill before you can reach the water table? Groundwater Flows: Groundwater flows from areas with a high hydraulic head to areas with lower hydraulic heads. Hydraulic head is a measure of pressure of the groundwater. Under unconfined conditions (i.e., in the absence of an impermeable layer above the groundwater), heights of the water table are a good measure of local hydraulic heads. Note that water table represents a boundary beneath which all pore spaces are filled with water. Groundwater flows so slowly that it does not replenish itself instantaneously as we are accustom to for surface water. To see how groundwater behaves, go to McDonald s in the Illini Union and order a thick shake. Take the top off, insert a straw, and start enjoy your desert. As soon as you suck up the shake, look at the surface of the liquid in the cup. A small depression forms around the straw because the liquid at the end of the straw has been removed, causing the surface to collapse downward. Groundwater behaves in the same way. The depression is often called the cone of depression. The following diagram illustrates how ground water table form a cone of depression around a well. Because of the depression, ground water flows from higher water table towards the well (which has a lower water table), resulting in the drop of water table in the surrounding area. This can become a major problem in highly populated area, where extensive removal of groundwater can produce a large cone of depression causing local wells to run dry. zone of aeration cone of depression water table pores filled with air and water zone of saturation all pores filled with water
Geology 103 Planet Earth (QR II), Laboratory Exercises 3 Assignment: Given on page 5 is a map showing an area here in Central Illinois. Farmer McDonald has lived in this area for years. Throughout these years, his drinking and irrigation needs are all dependent on a well drilled into the unconfined Mahomet aquifer. A few years ago, the county board brought two new industries into the area: the Grandma s Bottle Water Company and the Sandpiper Quarry. Trouble set in when Farmer McDonald discovered that his well had run dry a year after the companies have moved into the area. Having past Geology 103 at this university many years ago, he suspected that the loss of water was due to the increased water use by the new industries, especially the Grandma s Bottle Water Company since it is closer to his farm. He immediately sued Grandma to stop her from using his water. Grandma argued that she had nothing to do with his problem. The judge, knowing nothing about geology, appointed you as an expert to resolve the problem. Your assignment is to find out where Farmer McDonald s water has gone. So far, you have accumulated a lot of data. A quick survey of the area shows that local topography is flat as a pancake. Ground water table (tabulated with respect to the sea-level) has been mapped both before and after the two companies have moved in. The data are tabulated in the table on page 4. It should also be pointed out that Grandma needs to pump groundwater for her product. Although the Sandpiper Quarry does not need to use the water, it needs to pump water out from the pit so that the quarry can remain dry and operational. a) Based on the tabulated data, plot the contour lines of 10 m, 15 m, and 20 m of the water table before the companies began pumping on map A of page 5. Determine and sketch the general direction of groundwater flows in the region. b) Repeat step (a) and plot the contour lines of 25 m, 30 m, 35 m, 40 m, and 45 m of the water table after the companies began pumping on map B of page 5. c) As has been described earlier, groundwater flows from areas with high water table towards areas of lower water table. Flow lines are always perpendicular to contour line. Draw some flow lines on the contour map that you have constructed for (b). Describe how groundwater flows in this area and determine where Farmer McDonald lost his water. d) Groundwater flow velocity is dependent on the hydraulic gradient, which is equivalent to the slope of the water table. The formula to calculate flow velocities is given below. h v = k x Here, v means the absolute value of velocity v (in other words, direction of the flow velocity is ignored in this formula); k is the hydraulic conductivity, which is a material property dependent on rock type of the aquifer; h is the water table difference; and x is the distance between the two stations where the water table difference was measured.
Geology 103 Planet Earth (QR II), Laboratory Exercises 4 Based on the contour maps that you have constructed and the scale given for the maps, calculate flow velocities between Farmer McDonald and the two companies both before and after the companies began pumping. Let us assume that typical rock type for the aquifer is glacial tills with a hydraulic conductivity of 1 m/day. Height of water table (w.r.t. sea-level) near McDonald s Farm in meters before companies began pumping after companies began pumping station height station height station height station height 1 28 26 40.3 1 18.5 26 21.8 2 51.8 27 46.7 2 13.0 27 18.3 3 47.7 28 51.5 3 18.0 28 17.5 4 42.5 29 46.6 4 23.2 29 21.7 5 40.8 30 43.0 5 25.5 30 25.5 6 43.2 31 40.5 6 26.0 31 18.8 7 44.1 32 44.1 7 26.0 32 27.4 8 43.5 33 48.1 8 24.6 33 22.5 9 47.0 34 51.8 9 24.8 34 19.6 10 34.2 35 52.7 10 26.0 35 16.6 11 47.2 36 49.4 11 18.5 36 20.2 12 41.0 37 53.3 12 24.5 37 13.8 13 42.9 38 52.6 13 19.0 38 16.1 14 46.2 39 47.8 14 26.3 39 23.3 15 46.5 40 44.2 15 22.8 40 26.7 16 46.7 41 44.3 16 25.8 41 24.9 17 34.6 42 45 17 25.4 42 26.6 18 51.1 43 48.1 18 14.2 43 20.5 19 49.1 44 48.7 19 13.8 44 22.0 20 43.2 45 46.2 20 21.6 45 25.4 21 44.9 46 44.8 21 24.5 46 26.1 22 47.4 47 43.5 22 24.2 47 26.3 23 48.0 48 49.1 23 21.6 48 19.8 24 51.0 49 54.0 24 20.9 49 15.0 25 46.5 50 54.8 25 27.0 50 13.9
Geology 103 Planet Earth (QR II), Laboratory Exercises 5 Location Maps of Measurement Stations around McDonald s Farm A. Before the companies began pumping: 4 2 3 11 Sandpiper Quarry 18 19 20 27 28 29 43 42 44 45 12 10 5 13 17 16 22 15 McDonald s 21 Farm 1 23 25 24 26 33 34 31 32 36 35 30 41 40 39 38 37 50 46 49 48 47 Grandma s Bottle Water Co. 6 14 7 9 8 B. After the companies began pumping: 2 3 11 Sandpiper Quarry 4 12 10 18 17 16 22 15 McDonald s 21 Farm 1 23 19 20 25 24 26 33 27 34 28 31 32 29 36 35 30 43 41 40 39 38 42 37 50 44 46 49 45 48 47 Grandma s Bottle Water Co. 5 13 6 14 7 9 8 0 1 2 scale in km
Geology 103 Planet Earth (QR II), Laboratory Exercises 6