Presentation on theme: "It’ll Go With the Flow Goals: To teach students how to construct a water table elevation contour map. To help students understand how such maps can be."— Presentation transcript:
1 It’ll Go With the FlowGoals: To teach students how to construct a water table elevation contour map. To help students understand how such maps can be used to determine general groundwater flow patterns. To allow students to evaluate a hypothetical landfill site based on th direction of groundwater flow.Subjects: Science, Health, Math, Environmental ScienceDPI Objectives: SC: A1-A3, B3, D1; EH: A3, B3, B4, C1 SS: A1, B3, D3Grades: 7 - 9Activity Source: Groundwater Study Guide, 1990, WI DNR, PUB IE-004(90)RevPresentation created by: Kevin Masarik, Center for Watershed Science and Education.
2 It’ll Go With the Flow….Pleasant Valley is proposing to build a new landfill just outside of the city limits. You have just been hired by a local engineering firm to evaluate the proposed landfill site and give your recommendations to the local officials in Pleasant Valley.Groundwater usually flows in the same direction as the land slopes, often toward a nearby lake or stream. Many factors, such as rate of percolation from the surface and pumping from wells, can influence the direction and rate of groundwater flow, but it is possible to get an idea of how groundwater is moving in a given area by determining the slope or “plane” of the water table. To do this at least three monitoring wells must be installed since three points are necessary to determine a plane. By measuring the static water level or elevation of the water table above sea level, we can estimate how groundwater will flow at a certain location.It is important to consider the direction and rate of groundwater flow when planning land development to avoid potential contamination problems. Using static water level data, you will be asked to determine the general direction and relative rate of groundwater flow in the vicinity of Pleasant Valley. You will then use the information you have collected to evaluate a proposed landfill site based on information showing groundwater flow direction and locations of the city and private wells in the area. Write a paragraph either for or against the location of the proposed landfill site. If you think that locating the landfill at the proposed site is not advisable, suggest two locations that might be better suited for a landfill and give reasons why you think they would be better.
3 Here we see a picture of a landscape with a river running through it Here we see a picture of a landscape with a river running through it. The black lines represent lines of equal elevation, we can also call them contour intervals. Contour lines are used to represent a three dimensional surface on a 2 dimensional surface such as a piece of paper. By learning how to read a contour map, we can get an idea of the what the landscape looks like from the map without actually being there to see it. What else can the contour lines tell us? Well, suppose I drop a ball on the top of the hill where do you think the ball is going to go. We all know that the ball would roll down the hill right, or we can also say the ball will move from areas of high elevation to areas of lower elevation. The blue lines represent where the ball would roll if I dropped it on different areas of the land surface. So we can also use contour lines to represent what direction a ball would move, or also what direction water would runoff. Take a look at the black contour lines. Notice that the blue lines are perpendicular to the contour intervals. If we want to show direction of movement on a contour map we would draw an arrow perpendicular or at right angles to the contour intervals starting with the area where the elevation is high and extending to an area of lower elevation.Impermeable bedrock
4 In the previous picture we were looking at the surface landscape In the previous picture we were looking at the surface landscape. Now lets take a look at what it would look like if we removed all of the unsaturated zone and just looked at the top of the water table. You remember that the water table represents the area below the land surface where all of the pore spaces and all of the cracks in the rock are completely filled with water. Everything below the water table represents groundwater. Just like the surface in the previous figure we can also draw contours representing lines of equal elevation for the water table. By locating areas of high elevation and low elevation we can then get a sense for the direction of groundwater movement. Just like the previous slide we can see that by drawing lines perpendicular or at right angles to the contour lines we can show the direction of groundwater movement. Much like the previous slide the groundwater is moving from areas of high elevation to areas of low elevation which we call discharge areas. In this diagram the discharge area is the river.Groundwater flowImpermeable bedrock
5 7 in.7 in.So how do we make a contour map? A contour map again is representing a three dimensional surface, such as this shape, on a 2-dimensional surface such as a piece of paper. In order to do this we need at least three reference points to represent a plane. Suppose we want to draw in lines representing lines of equal height at intervals of 1 inch how would we begin to go about doing that.0 in.
6 765643251432Well if we had a ruler we could measure the height of the block at 6 in., 5 in., 4 in., 3 in., 2 in., and 1 in and draw a line on the block to represent the one inch contour lines.1
7 765432On a flat piece of paper it would look something like this. The arrow represents the direction that a ball would roll down hill or it could also represent the direction of groundwater movement if this were the water table.1
8 Now that we have some background on what contour lines represent and what a water table map can tell us about groundwater movement, we are going to construct a water table map for Pleasant Valley. To do this we are going to have to use the limited amount of information they give us to construct a water table map and determine the direction of groundwater flow. Because we are mapping the water table, we will first need to locate information regarding the elevation or height of the water table in Pleasant Valley. Because we can’t see underground we will use the wells in Pleasant Valley, the first step is to identify the elevation of the water table.
9 399 ft 19 ft 380 ft Symbol for a well Elevation above sea level Depth to Water Table399 ft19 ftThe circle with the cross is the symbol for a well. Each well gives us enough information to locate the elevation of the water table. The top number is the elevation of the land surface. The bottom number is the depth to the water table. By subtracting the depth to the water table from the elevation of the land surface, we can calculate the elevation of the water table, also called the static water level. So for this example the static water level is 380 ft above sea level. Now use the elevation above sea level and the depth to water table for each well to calculate the static water level for all of the wells on your map.380 ftStatic Water Level
10 44 mm / 4 intervals = 11 mm/interval Calculate the number of 10 ft contour intervals between the two of the wells.400 ft – 360 ft = 40 ft.40 ft/10ft = 4 intervals360380370400390Measure the distance between the two wells.44 mmThe next step we will infer how the water table changes between points where we know the elevation of the water table. To do this we will determine the change in elevation between two points and draw lines of a predetermined equal elevation between wells. In this exercise our contour lines will each represent a 10 ft change in elevation. Let’s do one together using these two wells as an example. You’ll see that the well on the left has a static water level of 400 ft. The well on the right has a static water level of 360 ft. If we want to draw contour intervals to represent a change in elevation of 10 ft, how many intervals will there need to be. To do this we must first determine the change in elevation of the water table between the two wells. So subtract 360 ft from 400 ft and we get 40 ft. So there is an elevation difference of 40 ft between the two wells. If we want to know how many 10 ft intervals we will need to draw to represent this on our map we simply divide 40 ft. which is our total change in elevation by 10 ft. which is our contour interval to give us 4 intervals. We will need to draw four equal intervals between these two wells. Next we will need to determine how big those intervals need to be. So the next step is to measure the distance between the two wells. I come up with a distance of about 44 mm. To calculate how big each interval needs to be, divide the distance between the two wells (44mm) by the number of intervals (4 intervals) to give you the length of each interval needed to represent a 10 ft contour. In this case each interval will need to be 11 mm long. Starting from one of the wells draw a mark along the line every 11 mm. You should now have divided your line into four equal lengths. Make sure to label each contour interval with the elevation that it represents. You’ll need to repeat this process for all of the wells directly adjacent to one another. *It is important to stress that the lines connecting the wells should not cross other lines. Another thing to stress is that the number of intervals and the distance between intervals will be different for each set of wells that they do this for.Calculate the distance between each interval.44 mm / 4 intervals = 11 mm/interval
11 Now that we have calculated the change in elevation of the water table between all of the wells we can begin to draw our water table. To do this we will play a sort of connect the dots to draw in our contour lines. Using a pencil connect all of the lines labeled 410 ft. Once you have connected those, start working your way down and connect all of the lines or wells that are labeled 400 ft. Then 390 ft, 380 ft…so on and so forth. All the lines should be parallel to one another, meaning you will never cross a contour line that has already been drawn. Once all of your contour lines have been drawn, use arrows to draw in the direction that the groundwater will be moving keeping in mind that groundwater flows from areas of high elevation to areas of lower elevation.
12 Discussion QuestionsIn general what direction does groundwater flow in Pleasant Valley?Can we make assumptions about the speed of groundwater movement at certain locations?Would the contour lines change if you had SWL information from more wells? Fewer wells?Based on the information we have is X a good location for the landfill? Why or why not? If why, can you suggest a better site.What is the elevation of Mud River as it passes town?Discussion questions.
13 Discussion QuestionsIn general what direction does groundwater flow in Pleasant Valley?Can we make assumptions about the speed of groundwater movement at certain locations?Would the contour lines change if you had SWL information from more wells? Fewer wells?Based on the information we have is X a good location for the landfill? Why or why not? If why, can you suggest a better site.What is the elevation of Mud River as it passes town?Discussion questions with the answer sheet.