Leaky Landfill
(Teachers Instructions)
Supplies
-Sand saturated with a salt water mixture (~1:10 – 1:5 salt to water ratio)
-Multimeters (volt meters)
-9v battery with positive and negative electrodes
-Gridded transparencies
-Large plastic trays and thick plastic liners
-Leaky landfill worksheet packets
-DVD presentation (Lab #10)
Preparation
Allow sand to soak in salt water solution for several days, creating a homogenous
sand/salt water mixture. Fill the plastic trays ~1/2 full with the saturated sand mixture,
draining excess water, until only a little remains at the top of the sand. Place a thick
plastic liner (no holes) over the tray, also filling with the saturated sand ~1” thick,
making sure that the mixture is in no way in contact, with that in the tray itself. Connect
a 9v battery to the supplied snap with positive and negative electrodes, placing the
negative (black) into one of the corners in the sand (in the tray), and the positive (red)
into one of the corners of the sand in the liner, such that neither will easily fall out of
position. Place a gridded transparency onto the sand in the liner. Place the red electrode
from the multimeter into the corner opposite the positive power wire (within the liner,
same side), leaving the black one free, as students will sequentially move this through the
other numbered holes. Turn on the multimeter, setting it to the single 2, ~at 280° (see
Fig. 1).
Teacher Laboratory Procedure
Students will perform the first voltage trial in the simulated landfill (no holes, as
described above), as in student procedure #1, be sure to inform them not to puncture the
plastic liner, as this will void the entire experiment. Next, with out the students looking,
remove the transparency and slice ~ a 1” hole into the liner, covering it back up with sand
and replacing the transparency in exactly the same position, such that the students cannot
tell where the hole was punctured. Allow students to complete steps 2 and 3 (again,
inform them not to puncture the liner while taking measurements), running a second test
through out the grid, and producing the equipotential contour maps (Fig. 2). Once
students produce the contour maps, allow them to check the results of their resistivity
survey by comparing where they’re contour maps predicted the hole to be, and where it
really is in the liner. Once finished, simply reset for the next class, though don’t leave the
batteries hooked up when not in immediate use, and rinse off saltwater from all metal
electrodes when finished for the day.
*Inform student not to puncture the liner while taking measurements*
Student Laboratory Procedure
A simulated landfill with a liner is placed in a tub full of sand. Electrodes
attached to a 9V battery are planted in the landfill and in the encasing sand, establishing a
current circuit. The sand in both the landfill and surrounding material are saturated with
salt water. A voltmeter is used to test the potential.
1.) Map the potential in a landfill with no leaks. Place the gridded plastic sheet
on the landfill. Stick one probe of the voltmeter in a corner. Stick the other
probe sequentially into each of the 80 holes. Measure and record on the
provided table, the potential (voltage) for each of the holes, multiplying each
number by x1000 before recording. Set the voltmeter at the single 2 (as the
arrow in Fig.1).
2.) After a hole is cut in the landfill, replace the gridded plastic sheet in exactly
the same position as the first task. Perform the same task again on the now
leaky landfill and record your data on the table provided.
3.) Produce a contour map (Fig. 2) of equipotential lines for the data from both
the leaky landfill (test 2) and the leak-free landfill (test 1). Label the contours
in millivolts using 50 mV contour intervals. Produce the maps as topographic
maps on the attached grids.
4.) Draw short straight lines at right angles to the equipotential lines at regular
spacing all around the contour maps. These are electrical flow lines. Their
orientations show which way the current is flowing.
Black Wire
Red Wire
Fig. 1 Set your voltmeter to the
position as shown by the
arrow (one click above
due West).
Fig. 2 Construct a contour map with a
contour interval of 50mV,
drawing lines between each class,
Name:_____________________
grouping equal intervals together.