Electrical methods for monitoring a site potentially Giorgio Cassiani(1), Rita Deiana(1), Maria C. Caputo(2), Lorenzo De Carlo(2) (1) Department of Geoscience, Università di Padova, Italy, giorgio.cassiani@unipd.it (2) Water Research Institute (IRSA) - CNR, Bari, Italy EGU General Assembly 2011 - Vienna 3-8 April 2011 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. References Abstract 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. Fig. 7: Analysis restricted Conclusions 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. Electrical methods for monitoring a site potentially contaminated by landfill leachate Introduction 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 geo-membrane at the bottom of the landfill. 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. Landfill in construction Old landfill In the old landfill area the limestone lies some meters below the supposedly continuous HDPE 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. Fig. 1: the landfill area in Corigliano d’Otranto. Fig. 2: Detail of electrodes installation along the scarp which divides the old land the new landfill s. Corigliano d’Otranto Geophysical prospecting 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. 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. A B P Fig. 4: Detail of the Mise-a-la-Mass survey. Line 2 Line 1 Line 3 Mise-a-la-Masse results 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). Fig. 6: Voltage distribution caused by current injection between electrodes A and B (see Fig. 3). ERT survey results Fig. 5: Electrical resistivity tomography images along lines 1, 2 and 3 (see Fig.3). Note the strong scale difference between the surveys run within the landfill (Figs 1 and 2) and outside (Fig.3). Line 2 is perfectly placed cutting across the main topographic feature (the scarp). From Line 2 we can conclude that the large, very conductive body representing the landfill waste is much deeper than the expected 20 m waste thickness. This might indicate either (a) a much thicker waste body, or, more likely (b) vertical migration of conductive landfill leachate crossing the supposedly impermeable landfill bottom. The bottom of the conductive body is about 40 m from the top of the old landfill surface, thus well below the bottom of the known quarry that lies at the same level as the new landfill bottom (to the right of the scarp in the figure of Line 2). Note also that the possible leachate plume is more pronounced to the North, towards the scarp, in accordance with the evidence of leachate emergence at the base of the scarp itself. Another conductive feature can be seen in Line 2 below the bottom of the new landfill, possibly representing a clay-silt layer of the many known to be present at the site. 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, both Lines 1 and 2 highlight the presence of a strong heterogeneity induced by the old landfill waste. Line 1 is positioned in such as way as to violate the main hypothesis of lateral homogeneity needed to interpret quantitatively the relevant depths and resistivity values, while