Name:_____________________
Leaky Landfill
Introduction
Humans, and especially Americans, generate a lot of solid waste. Much of this
waste is generated in urban-suburban areas where it is imperative that it is properly
disposed of. The solution is to build sanitary landfills. Solid waste can be transported to
these locations and stored rather than allowing it to accumulate locally and pollute the
whole area. This practice reduces pestilence and disease, leaves the region aesthetically
pleasing, and improves the quality of life for the residents. The problem is the
management of this area of concentrated waste. Because of the high concentration of
dangerous to hazardous substances, leakage can cause severe environmental damage
around the landfill and to the entire groundwater system. To avoid such disasters, a
series of regulations must be strictly enforced in all land fill areas.
There must be a large open buffer zone around the landfill separating it from
residential and business areas. This buffer zone provides an area for monitoring purposes
or to remediate a leak before it becomes a problem for the community. A tough plastic
liner must be placed beneath the landfill to contain liquids (leachate) from spilling into
the groundwater system. A clay cap must cover the landfill to shield it from rain and
runoff water, which will leak chemicals from the waste, and to prevent material from
emerging to the surface. A monitoring system must be established to ensure safety for
the community and local ecosystem. There are many other regulations for landfills, too
numerous to list here.
Leaks and Monitoring
One of the most common problems is that the liner under the landfill may develop
a hole and leak. If a leak develops, gravity drives the leachate and liquid waste through
the hole and into the soil beneath. It than seeps through the soil and into the water table.
There it can be dispersed to communities. Because so much liquid (and eventually solid)
waste can pass through the hole in such short time, it is imperative that a near constant
and highly sensitive monitoring system be established. Even monitoring wells are not
responsive enough. Instead, a geophysical technique called resistivity surveying is
employed. It can detect leaks quickly and locate the problems at minimal cost.
Resistivity
Electricity flows through the ground. It flows more easily through certain
substances than others. The resistance to flow is what a resistivity survey evaluates.
Most rocks are insulators and do not conduct electricity well. Metallic rocks are the
exception, they conduct well. Pure water is also an insulator. Most water, however,
contains many ions and as a result is a good electrical conductor. The higher the ion
content (salt water has a high ion content), the better the conductor. A battery is
connected to the ground to provide the electrical flow to the system. A voltmeter can be
used to measure the potential at each position on the surface of the earth.
The monitoring of the landfill is based on the relative resistivity of the plastic
liner in contrast to the relative conductivity of the wastewater or leachate and
groundwater. The potential across the landfill with no holes in the liner will be relatively
constant. The potential across the landfill with a leak is not evenly distributed. Instead,
the resistance is low around the leak. Therefore, comparison of the electrical field of a
landfill with an intact liner with that of a landfill with a leaky liner determines not only
the presence of a leak but the location of that leak as well. The conductivity will be
higher around the hole and resistivity minimal. The electrical flow will be concentrated
around the hole as well.
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 corner (as instructed).
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,
grouping equal intervals together.
Name:_____________________
Answer the following questions.
1) Compare the two contour maps that you produced:
a) How do the equipotential lines reflect the hole in the liner?
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b) How do the flow lines reflect the holes in the liner?
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c) How do the “voltage potentials” reflect the hole in the liner?
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2) In what instances would the resistivity method be ineffective for determining and
locating leaky landfills?
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