PART A. AQUIFERS & DARCY S LAW - INTRODUCTION

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1 Geol 108 Lab #9 Week of Oct. 29 Nov. 2 GROUNDWATER PART A. AQUIFERS & DARCY S LAW - INTRODUCTION An aquifer is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts to wells. This movement of water through a porous medium can be quantified using what is known as Darcy s Law: Figure 1. Darcy s Law. 1

2 The parameter h/l is known as hydraulic gradient, where h is the height of the water column and l is the distance traveled. K is known as hydraulic conductivity, and is a measure of permeability, or how easily the aquifer transmits water. Q = volumetric flow rate in cm 3 /s h = height of water column in cm A = cross-sectional area of flow tube in cm 2 (A=πr 2 ) l = horizontal distance traveled in cm Porosity for an aquifer is defined as the amount of vacant space in between sediment grains. In general, aquifers that have greater porosity tend to also have a greater permeability. Figure 2. Porosity and permeability. White space is open space available to water. 2

3 PART B. AQUIFER EXPERIMENT Well 3 Well 2 Well 1 Equations: 2 A = π r vel = Q A vel = K Δh Δl ( ) Measurements: radius of aquifer = 1.9 cm cross sectional area of aquifer cm 2 Δl = 18 cm Following the instructions of your TA, you will measure the discharge of each aquifer over a time interval of 10 sec. Record your data here: Aquifer A ( mm) ml/10 sec h = Aquifer B ( mm) ml/10 sec h = Aquifer C ( mm) ml/10 sec h = 3

4 Here is an example: Discharge (also known as Q) = 300 ml/10 seconds or 30 ml/sec (Note: 1 ml = 1 cm 3 ) vel = Q A = 30cm3 /sec 11.34cm 2 = 2.6cm /sec Δh vel = K x Δl where Δh is the difference between the height of water in two outer wells, and Δl is the distance between two outer wells. 2.6 = K x 3.5cm 18cm K = 13.4 cm/sec Calculate K for each of the three aquifers and record your answers in Table 1 (see example above for equations). The value of K for Aquifer D ( mm) has been provided for you. TABLE 1. Aquifer Grain size (mm) K A B C D Which aquifer has the highest hydraulic conductivity? Which has the lowest? Do you notice any relationship between the grain size of each aquifer and their respective K values? If so, what is this relationship, and how does it relate to porosity and permeability? 4

5 PART C. DYNAMICS OF WATER TABLES (reference: Geoscience Laboratory, 2002, 3 rd edition, by Tom Freeman) Case Study. Interstate I-105, Los Angeles In Los Angeles County, a 3.5-mile section of I-105 was constructed below ground level in an effort to minimize noise and sight pollution (for the surrounding neighborhoods). Caltrans (Calif. Dept. of Transportation) believed the water table to be 30 feet below road level at the time of construction. However, Caltrans failed to learn that the water table had been drawn down by overpumping in the 1950 s, and another state agency had recently mandated that the overpumping stop. Question 1. What do you conclude must have happened when overpumping of the saturated zone was stopped? 5

6 PART D. GROUNDWATER MODEL Terminology: Piezometer piezometers are usually installed by researchers studying groundwater in a particular area. Since groundwater flows from high areas to low areas, knowing the height of water in a number of piezometers can allow you to map the direction of flow. Drinking water well drinking water wells are normally regulated by state codes which specify the depth required and the materials used in construction. They are carefully located away from sources of contamination. Concept 1. Height of water table Your TA will add dye to piezometers A, E (blue) and to B, C, D, F, G (green). How can you identify (recognize) the height of the water table? Is the height the same everywhere? Concept 2. Human activities at the land surface. Your TA will add red dye to the leaky landfill (or lagoon, or underground tank), representing pollutants. Once dye is added, drain the outlets. 6

7 Draw the paths of the blue, green, and red dye on your diagram. What happened to the leaky landfill (where did the red dye go)? Give a common, everyday example of where/how this might be occurring. Concept 3. Groundwater can be withdrawn from wells. The pumping wells are used as a source of water for homes, farms, and industries. Notice well 1 and 2 extend to different depths. What happens to the height of the water table when water is pumped out of well 1? What do you notice about the movement of water around piezometer E? What happens to the height of the water table in piezomenter D when water is pumped out of well 2? Is the water that you pumped out of well 1 and/or 2 clear or colored? What does this indicate about the flow of groundwater? What could happen if we continually pump water? 7

8 PART E. WATER WELLS 1. Examples of three different wells are shown below. In the figure, draw the line showing the Potentiometric surface. Mark each well location with the appropriate letter describing the well (assume that water production is from the bottom of each well). A possibly flowing artesian well B non-flowing artesian well C well that might produce water from an unconfined aquifer D dry well 2. What are the differences between unconfined and confined aquifers? 3. What conditions are necessary for an artesian well? 4. Why does a cone of depression form around a well? 5. What will you see when the groundwater table intersects the surface of the land? 8