Part III.  Landfill Leak Testing


	Now hopefully everyone is convinced that an electrical circuit
can be constructed through water, and that we can see electrical
flow by measuring voltages across the circuit.  The next step is
to build a landfill model to show how this electrical technique
is used to identify leaks in a landfill system.  Figure 2 shows a
typical model set up.

	If you have not already done so, place masking tape along the
edges of the plastic tub.  Use your meter stick to mark off 5 cm
lengths on the tape (start in the lower left corner as the 0
marks).  fill the tub with sand, a little less than half-full.

	Excavate a basin in the sand.  Your landfill basin dimensions
should be roughly 25 by 40 cm.  Try to make the bottom level and
smooth, with steep sides.  Add about 1/4 cup of salt to 2 liters
of water (does not need to be exact).  Gently, and slowly, add
water to the sand to get it saturated. 

	Cut a plastic bag large enough to use as a liner for your
landfill basin.  Check to make sure it does not have any holes in
it first.

	You will need another two liters of salt water to fill up your
lined landfill basin.  Smooth out any large air bubbles,
wrinkles, or bumps in the landfill liner as best you can and then
fill the basin with your salt water solution.

	Use Figure 2 as a location guide, and sink one bare metal wire
into the sand outside of the landfill, and submerge two others
inside the landfill, one for the battery negative and the other
for the meter ground (you'll need to bend the wires so they will
stand upright by themselves - do not poke them through the
liner).  Connect the "+" post of the battery to the wire outside
the landfill, and the "-" post of the battery to the wire inside
the landfill.  Tape the wires to the edge of the plastic tub for
stability.

	Draw in all the boundaries and wire locations on Graph 2.

	Measure the voltage in the landfill (you should see low,
uniform voltage throughout your landfill).  This demonstrates
that there is no equipotential field.

	With the meter probe, puncture a small hole in the plastic
lining, and note on your graph where this hole is (Hint:  since
in this case you can control the location of your hole, you can
optimize your results by:  1, putting the hole far from the
battery input; and 2, making your hole in the center of a grid
cell, so you will have four measurements close to the hole). 

	Set your meter to the "2000m" setting.  You should be able to
measure with the voltmeter that there is now an electropotential
field.  Make sure that you see a gradual increase from the
negative bare wire (in the basin) to the area around the hole.

	Systematically measure voltages at every 5 cm gridpoint, using
the marked tape on the edges of the tubs as a guide.  Try to keep
the probe at about the same depth each time.  Record these values
on Graph 2.  Once you start measuring, work at a steady pace; if
you take too long, you might notice voltage fluctuations from the
battery weakening and some oxidation reactions (bubbling) on the
wire terminals.  Note that in this circuit you will not measure
nearly as high voltages here as you did with the tub of water in
Part II.