PLOTTING THE PETROLEUM PLUME

Linda D. Morse
Department of Geology
College of William and Mary
Williamsburg, VA 23187

Level: Grades 8

Estimated Time Required: Three or four 45 minutes class periods

Anticipated Learning Outcomes

NOTE TO TEACHER: Parts one through four are background information for teachers. Figures 1 - 5 are to be copied for students to use. New vocabulary is in bold letters and has been repeated on the figures.

Background

Part One: Water Table Aquifer

Figure 1 is a cross-section showing the different soil layers and zones in a typical water table aquifer setting. A cross-section is like slicing a layer cake in half and looking at the layers from a side view. A map view is like looking down on the layer cake and reading the "Happy Birthday" slogan.

Where does rainwater go after it hits the ground? Some rainwater may run over the ground surface and flow into a lake or stream. Some of the water may infiltrate into the soils and with gravity slowly migrate deeper until it reaches or recharges water-saturated soils or sediments. The water-saturated soils or sediments are referred to as the water table aquifer and are usually composed of larger particles like sand and gravel or permeable rock. This aquifer is usually the first aquifer encountered at shallow depths in most areas of the United States. Aquifers that are deeper than the water table aquifer may also be composed of sand, gravel and permeable rock. (This exercise will concentrate on a water table aquifer within a sand/gravel layer - not in permeable rock.) The pore spaces between the grains in a water table aquifer are filled with water. The upper surface of the water table aquifer is the water table. The unsaturated zone above the water table may contain moisture between the grains of sediment, but the amount of water is very small.

What keeps groundwater from moving deeper into the soils or sediments? Usually smaller grains of sediment such as silt or clay will confine the water because the pore spaces between the small grains are very small or do not exist. In other words, groundwater sits on top of a clay or silt confining layer.

How does groundwater travel through the ground? Groundwater moves through the pore spaces in soils with gravity. Just as water on the surface of the land flows downhill, water under the ground tends to flow downhill, too. The water table may discharge or empty into streams, rivers, or lakes. Review Figure 1 with the class, then have the students complete Figures 2 and 3. Answers to questions are given on the last page of this activity.


Part Two: Monitoring Wells

Figure 4 shows a cross-section of a monitoring well. What does the well monitor? Monitoring wells are constructed to determine water table elevations and to determine the amount or concentration of different chemical compounds in the groundwater (i.e. pollution). Water samples are retrieved from the well and sent in containers to a laboratory for analysis to determine if groundwater has been contaminated or polluted.

Monitoring wells are very useful in determining if there has been a leak or spill of petroleum fuel oil from fuel tank farms (where fuel oil is stored in huge tanks) or gas stations. The thickness of the petroleum fuel oil can be determined by measuring in inches how much oil is floating on top of the water table.

It is important during the construction of a monitoring well that there is a tight seal of cement at the upper surface around the well pipe. Why? If there have been surface spills of petroleum oil or other contamination, then rainwater could infiltrate around the pipe bringing the petroleum oil down with it. The purpose of the monitoring well is to monitor what is currently in the water table aquifer. If the aquifer is not polluted and the well is not constructed properly (i.e. without the concrete seal), what could happen to the clean aquifer? If could become polluted by infiltration of petroleum oil and rainwater around the well pipe. Review Figure 4 with the class.


Part Three: Creating Your Own Monitoring Well (Teacher Exhibit)

Materials

Procedures

  1. Hacksaw small slits in the lower 8" - 12" section of PVC pipe. This will act as the screen to the monitoring well.
  2. Place the PVC pipe with screen at the bottom in the middle of the bucket.
  3. Pour playground sand around the pipe until it stands alone and until the screen is covered approximately two inches.
  4. Add water to the sand until saturated (like wet beach sand). Let set. (The water should move deeper into the sand.)
  5. Measure the water level with a wooden stick inside the pipe. This represents the water table level. How to measure:
    1. Measure from the lip of the pipe inside to the surface of the water (PIPE). Measure from the lip of the pipe outside to the top surface of the sand or ground surface (STICKUP). Subtract the STICKUP value from the PIPE value [PIPE - STICKUP = WATER LEVEL (below ground surface)].
    2. Assume the ground elevation is 100 ft. To determine the water elevation: [100 FT - WATER LEVEL = WATER ELEVATION]. The second formula is repeated in Part Four.
  6. Pour motor oil on sand around outside of pipe. Add more water. Let stand.
  7. Measure water level again. Check for oily residue on measuring stick.

The water-saturated playground sand represents the water table aquifer. The bottom of the bucket confines the water and therefore acts as a confining layer. Pouring the oil on top of the sand may represent a petroleum spill or possibly petroleum that is migrating down through the unsaturated zone to the water table aquifer. Adding water after the petroleum represents subsequent rain event flushing the oil into the aquifer. Where is the oil found in relation to the water table? Oil and water do not mix and oil is less dense than water; therefore, the oil floats on top of the water table.

Part Four: Groundwater Elevations and Product Thickness Contouring

As mentioned in Part Two, monitoring wells can be used to determine water table elevations which, in turn, determine the direction of groundwater flow. Figure 5 represents a map view showing locations of different monitoring wells near a petroleum tank farm. If the ground elevation at a monitoring well is 100 feet and the water level inside the well is 10 feet below the ground surface (see Part 3, How to Measure), then the groundwater or water table elevation at the well location is 90 feet:

Ground Surface Elevation - Measure to Water Table = Water Table Elevation
100 ft - 10 ft = 90 ft

Figure 5 shows eight monitoring well locations with measured elevations (in feet) of the water table and thickness (in inches) of petroleum on top of the water table. Contour lines are drawn to connect points of equal elevation. For example, all long the contour line 110' on the figure, the water table is at an elevation of 110 feet (below the ground surface). Remember from Part One that groundwater generally will flow downhill. Arrows that show the directions of groundwater flow are drawn perpendicular (90) to the groundwater contour lines because groundwater flows from higher elevations to lower elevations; i.e. groundwater flows downhill. (This is an approximation that is generally true. See "Ground Water Flow in a Milk Box" for more discussion on this topic.)

Petroleum product thickness contour lines are constructed in the same manner as elevation contour lines. However, the data used are the thickness (in inches) of product floating on top of the water. Possible contour lines would be: 0 - 1" - 2" - 3", etc. These contours tend to wrap around in concentric circles. Review Figure 5 with the class.

NAME ___________________

Plotting the Petroleum Plume

The figure below is a side view or cross-section of a water table aquifer. Rain water hits the ground and infiltrates into the soil or runs off into a body of water. A confining layer of silt or clay stops the water from moving deeper. The sands and gravels above the confining layer becomes saturated with water because the pore spaces between the grains are filled with water. The water-saturated zone is called the water table aquifer. The upper surface of the water table aquifer is the water table. Above the water table is the unsaturated zone. This zone contains some moisture, but is not saturated like the aquifer. As on the earth's surface, water underground tends to flow downhill.

1. Where does the water table discharge in this figure? ____________________


2. How is a water table aquifer recharged? ____________________________


Figure 1. Water Table Aquifer Cross Section

NAME ___________________

Plotting the Petroleum Plume

This is a cross-section of a petroleum tank farm. There are three steel tanks sitting on the ground, each containing one million gallons of fuel oil. A mound of soil (soil berm) surrounds the tank farm. The containment basin contains any fuel oil which may spill due to overspills while filling the tanks or negligence of workers at the site. The cross-section indicates that the surface spills have infiltrated the soils and are travelling deeper.

3. Continue drawing the petroleum plume as it moves deeper underground. Helpful Hints: Think about oil and water. Do they mix? Where would you find the petroleum oil - above or below the water table?

4. Once the petroleum reaches the water table, where will the oil and water flow and discharge?


5. Your answer to Question 4 is a logical prediction as to where the petroleum plume is flowing. Finish drawing the petroleum plume according to your answer to #4.


Figure 2: Petroleum Oil Tank Farm I

NAME ___________________

Plotting the Petroleum Plume

The tank farm shown here is similar to the tank farm shown in Figure 2, although the underlying units are different. Thin layers of clay pinch in and out throughout the unsaturated zone.

6. Continue drawing the petroleum plume as it infiltrates the soils beneath the tank farm.

7. What will the petroleum oil do as it reaches the clay layers?



8.Finish drawing the plume as it follows the flow of groundwater and discharges.


Figure 3. Petroleum Oil Tank Farm II

NAME ___________________

Plotting the Petroleum Plume

The figure below is a cross-section of a monitoring well. The top of the screen (slotted plastic pipe) of the well is above the water table so that the water and the oil flow into the well. The screen extends down into the aquifer and is seated in the confining layer.

9. If the water table were polluted with six inches of petroleum oil, where would you find the oil in the well and in relation to the water table? Draw the petroleum oil inside of the well and in relation to the water table outside of the well.

10. If the monitoring well were installed so that the top of the screen was two feet below the water table, would you be able to monitor the petroleum pollution?






Figure 4. Cross Section of a Monitoring Well

NAME ___________________

Plotting the Petroleum Plume


This is a map view of four steel petroleum tanks sitting on top of the ground and an office building. The straight parallel lines are contour lines representing the water table elevations (in feet). The arrow crossing the contour lines points the direction in which groundwater flows. Groundwater flows northwest. The heavy dark lines are thickness contours that represent the thickness (0" - 1" - 2", etc.) of the petroleum that is floating on top of the water table.

11. In which direction is the petroleum plume flowing? ______________________

12. What water body will be affected in the future? _________________________

13. Where is the thickest layer of petroleum? ____________________________

14. According to records in the office, there have been no spills of petroleum on the ground surface. If there have been no surface spills, how did the petroleum infiltrate into the water table? _____________________________________________
_____________________________________________________________

15. Which tank do you suspect is leaking? ____________________________

Figure 5. Groundwater Contours, Groundwater Flow Direction, and Petroleum Thickness Contours

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