Activity 1 The Water Table Introduction
Within a few hundred meters of the surface, most Earth materials have some porosity and contain some ground¬water. Usually we find a zone, immediately below the surface, that may contain some moisture but that is far from being saturated with water (Figure 1). This is called the zone of aeration and through it water moves downward to a zone that is completely saturated with water, the zone of saturation. The contact between the zone of saturation and the zone of aeration is called the ground-water table, or simply the water table, and can be seen as the level to which water rises in a well. It is the saturated zone that provides water to wells, provided the porosity and permeability are favorable.
Figure 1 is an idealized diagram of the flow of water through a homogeneous, unconsolidated sand with good permeability. Water filtering from the surface into the underground has completely saturated the pores in the sand until a hill of groundwater has built up beneath the high spots in the landscape. The top of this saturated zone is the groundwater table. Figure 1 shows groundwater flowing to stream channels. A ground-water divide separates the water flowing to channels in adjacent valleys. Beneath the surface, ground-water flows. Although water added to the groundwater at the high will flow to the outlet (the stream channel), it does not follow a straight-line path. Rather, it moves in a broad looping path, flowing first downward and that upward toward the stream channel.
FIGURE 1 Ground-water flow in a homogenous body of sand.
Question Set 1 Aspects of the Ground-water Table
i. In Figure 1, what is the relationship of the water table to surface topography?
ii. Considering the cost of drilling and operating a ground-water well, where would you site the least expensive well?
iii. Label on Figure 1 two ground-water divides.
Activity 2 Mapping the Water Table Introduction
The water table is a surface, and like the surface of the ground it can be mapped by drawing contours. Such a map will show the general surface of the ground-water table and delineate its highs and lows. It also gives a picture of the slope of the water table at any point. Because ground water will flow in the direction of the slope, the map will give a general picture of the directions in which the ground water is flowing. Figure 2 is a base map on which are located the positions of surface streams as well as a number of wells. The elevation of the water table in each well is given.
Question Set 2 Contouring the Water Table
i. Contour the water table on Figure 2 using an interval of 50 meters. A small area is contoured to get you started.
FIGURE 2 Base for contouring a water table. Dots Indicate positions of wells; numbers represent elevation of the water table in each well, in meters.
ii. With a pencil differing in color from the one with which you drew the contours, indicate the direction of flow of the groundwater. To do this draw a short arrow, about a half centimeter long, indicating the direction of flow at each well site. A few arrows have been put in to illustrate. The flow will be at right angles to the contours on the water table, which are lines of equal elevation.
iii. With a dashed line indicate the approximate position of groundwater divides.
iv. Into which creek does water flow at the points indicated on Figure 2:
Point W______________
Point X______________
Point Y______________
Point Z______________
Activity 3 Pollution and Ground-Water Flow Introduction
Determining the velocity of ground-water movement is difficult because ground water moves very slowly com¬pared with surface flow, because it lies hidden from view beneath the surface, and because the actual flow paths are curved (Figure 3). It can be important, however, to know the velocity of ground-water flow. For instance, if the ground water becomes polluted at one place, it is useful to know how long it will take the polluted water to reach another area down-flow from the point of pollution. We deal with an example in the question set that follows.
Question Set 4 Pollution Risk and Ground-water Flow
Figure 3 is a map of a contoured water table in weakly cemented sandstone. The map shows a source of pol¬lution, a stream course, and several monitor wells. The factory at X wishes to dispose of a fluid toxic waste. Instead of piping it overland a kilometer to a waste disposal site, the factory managers opt to put it into the underground through an injection well at X, which may be illegal. An injection well is one through which fluids are added to the ground water. Before beginning injection, a network of wells, labeled A through H, is installed to monitor the quality of the ground water. Before injection, all monitored wells had a good quality water. Thirty days after beginning injection, no wells showed any deterioration of water quality. Fifty days after injection began, well C recorded contamination by the factory's waste fluid.
FIGURE 3 A contour map of a water table. The map shows the location of three towns, "R," "S," and "7," as well as the location of an Injection well at "X." Locations of town wells and of monitoring wells are also shown.
i. On the map in Figure 3, indicate the flow directions for the ground water. Remember that the flow lines are at right angles to the contours on the water table. Flow lines are shown in a small area of the map.
ii. What is the apparent speed of the ground water, in meters per day, between the injection well and well C? This is called the apparent speed, since water travels on curved paths as shown in Figure 1 and not straight-line paths as we might calculate it here. In addition to time elapsed, you need to know how far it is between the injection well and well C.
iii. At this rate of flow how long will it take the pollution to travel from the injection point to well D?
iv. Town R draws its water from the well indicated on the map. One of the members of the town board drives a truck for a crew that services the monitoring wells and has learned about the pollution there. What should the board know about the risk of contamination of its town well?
v. News gets around fast. What can you tell Town S about the risk to its well and the reason for your answer?
vi. Town T asks the same question. What do you tell them? And why?