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Electrical methods for monitoring a site potentially
contaminated by landfill leachate
Giorgio Cassiani(1), Rita Deiana(1), Maria C. Caputo(2), Lorenzo De Carlo(2)
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(1) Department of Geoscience, Università di Padova, Italy, [email protected]
(2) Water Research Institute (IRSA) - CNR, Bari, Italy
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ERT survey results
Abstract
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The three main ERT lines provided a clear evidence of the effect that the landfill material has on the
electrical resistivity structure of the subsurface. Fig. 5 shows the results. While Line 3 shows a very
regular distribution of electrical resistivity across the entire section,
Lines
1 and 2 highlight the
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presence of a strong heterogeneity induced by the old landfill waste. Line 1 is positioned in such as way
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as to violate the main hypothesis of lateral homogeneity
needed to interpret quantitatively the relevant
depths and resistivity values, while
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resistivity
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(ohm
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Line 2 is perfectly placed cutting
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across the main topographic feature
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(the scarp). From Line 2 we can -20
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conclude that the large, very
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conductive body representing the
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landfill waste is much deeper than
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the expected 20 m waste thickness.
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This might indicate either (a) a much
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thicker waste body, or, more likely
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(b) vertical migration of conductive
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landfill
leachate
crossing
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supposedly impermeable landfill
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bottom. The bottom of the -20
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conductive body is about 40 m from
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Leachate migration ?
the top of the old landfill surface,
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thus well below the bottom of the
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known quarry that lies at the same
resistivity
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(ohm m)
level as the new landfill bottom (to
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the right of the scarp in the figure of
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Line 2). Note also that the possible
resistivity
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leachate plume is more pronounced
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to the North, towards the scarp, in
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accordance with the evidence of -20
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leachate emergence at the base of
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the scarp itself. Another conductive
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feature can be seen in Line 2 below
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the bottom of the new landfill,
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possibly representing a clay-silt
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Fig. 5: Electrical resistivity tomography images along lines 1, 2 and 3 (see
layer of the many known to be Fig.3). Note the strong scale difference between the surveys run within the 0
present at the site.
landfill (Figs 1 and 2) and outside (Fig.3).
In the old landfill area the limestone lies some
meters below the supposedly continuous HDPE Fig. 1: the landfill area in Corigliano d’Otranto.
geo-membrane; if the integrity of the membrane
is not verified, the limestone could represent a
layer of preferential flow of the leachate
preventing the contaminants from reaching the
water table.
Mise-a-la-Masse results
Fig. 2: Detail of electrodes installation along the
scarp which divides the old land the new landfill s.
Geophysical prospecting
Between May and June 2010, 2-D ERT surveys and Mise a La Mass measurements have been
conducted at the site. Particularly, two ERT profiles have been collected inside the old landfill in order to
evaluate the thickness of the waste in the old quarry, while another one has been conducted for
comparison outside the landfill to obtain resistivity data of geological layers not affected by waste
disposal. Resistivity data acquisition was carried out using a 235 m long array of 48 steel electrodes
with 5 m spacing. A Wenner-Schlumberger configuration was used because it represents a good
compromise between resolution and depth of investigation. Full direct and reciprocal data acquisition
was run to ensure a correct error estimation (e.g. Cassiani et al., 2006).
On the other hand, a Mise-a-la-Mass survey was conducted on the old landfill and the surrounding
areas to collect possible further evidence about the integrity of the HDPE membrane within the landfill.
Each profile has been conducted using 48 electrodes, 2 m spaced, considering the current sources A
and B placed inside and outside the landfill, respectively, and remote potential (P) outside the landfill.
The whole area is fairly wide and impervious, and a big effort has been devoted to the acquisition of
geophysical data along the scarp (Fig. 2, 3, 4). Resistivimeter Syscal Switch Pro 48 was used to collect
electrical resistivity profiles, considering for each profile direct and reciprocal measurements.
The Mise-a-la-Masse survey was run to
cover all accessible areas surrounding
and covering the old landfill. Current was
driven between an electrode (A) inside
the landfill and another electrode (B)
outside to the SW of the quarry complex
(Fig.3).
Evidence of the presence of the HDPE
liner can be seen physically on site along
the SW border of the landfill. This barrier
to electrical current also manifests itself
clearly in the sharp step of electrical
potential along the same boundary.
However, there is little doubt that the
current can circumvent the barrier and
close the circuit. We cannot exclude that
this current flows along the boundaries of
the landfill not easy accessible to the
MaM survey. It is more likely that the
main current allows a deeper pathway
corresponding
to
the
electrically
conductive body evidenced by the ERT
surveys (see Fig. 5, particularly Line 2).
Conclusions
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Fig. 6: Voltage distribution caused by current injection between
electrodes A and B (see Fig. 3).
Fig. 7: Analysis restricted
The use of geophysical methods is particularly useful in characterizing complex structures such as old
landfills. The example we show here confirms that geophysics, and electrical methods in particular, have
the potential to obtain information very hard to be retrieved otherwise. At the Corigliano d’Otranto landfill,
our geophysical surveys confirm that vertical leakage of leachate is likely to take place, and it can
potentially impact on the deeper soil structure and the underlying groundwater resources.
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Line 2
Northing (Gauss-Boaga - meters)
The purposes of the geophysical investigations
are:
• to identify the thickness of the waste in the
subsurface and
• to evaluate the integrity of the HDPE geomembrane at the bottom of the landfill.
A
B
References
P
Fig.3: Electrical Resistivity measurements carried out at
Corigliano site. Red dashed lines represent the main 2D ERT
profiles, while the orange lines indicate the location of electrode
lines used for the Mise-a-la-Mass survey.
Fig. 4: Detail of the Mise-a-la-Mass survey.
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Old landfill
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Landfill in construction
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The test site of Corigliano d’Otranto comprises
an old quarry, authorized to the end of the 80s
for municipal waste disposal. The waste is
thought to be more than 20 m thick, and the
landfill bottom is, supposedly, confined with an
HDPE geo-membrane. In the NW area of the old
quarry a new landfill is currently being built and,
during the digging operations, leachate was
found coming out of the scarp which divides the
old landfill from the new one under construction.
From a geological point of view, the site is
characterized by a thick sequence of fractured
calcarenite(~15 m) overlying vacuolar limestone.
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Introduction
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Corigliano
d’Otranto
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Electrical current methods are particularly used for monitoring sites that are potentially contaminated by
highly conductive aqueous solutions, such as the leachate of the old landfills.
These methods are minimally-invasive and allow for the collection of a large amount of data to obtain
information about wide areas with a good resolution; moreover, they have become popular also because
the electrical resistivity of waste in landfills varies considerably over time because of waste
decomposition and leachate formation. Here we present the results of one such monitoring exercise,
conducted at an old quarry in the municipality of Corigliano d’Otranto, Apulia region (Southern Italy),
utilised some thirty years ago for municipal waste disposal (Fig. 1). Surface electrical resistivity
tomography (ERT) surveys combined with a Mise-a-la-Mass method have been conducted in the study
area, in order to verify whether the leachate is confined by the HDPE membrane within the landfill.
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Cassiani, G., V. Bruno, A. Villa, N. Fusi, A.M. Binley, 2006, A saline tracer test monitored via time-lapse surface electrical
resistivity tomography, Journal of Applied Geophysics, 59, 244-259.
Nimmer, R.E., and J.L. Osiensky. 2002. Using Mise-a-la-masse to Delineate the Migration of a Conductive Tracer in Partially
Saturated Basalt. Environmental Geosciences, Vol. 9, no. 2, pp. 81-87.
EGU General Assembly 2011 - Vienna 3-8 April 2011
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