f Environmental assessment of municipal waste dump sites with electrical resistivity and induced polarization multielectrode methods

A high resolution electrical resistivity and induced polarization survey was carried out inside and around a modern municipal waste dump site located in the Venice province, eastern Italy. The waste dump area is adjacent to two parallel rivers (Brenta and Bacchiglione), so that part of the measurement arrays had to be layed out directly on fluvial argins. The complex aquifer system was only partially known before the survey, although many boreholes were drilled in the past, before and after the construction of the dump site. However, the geology of the site is characterized by alternate thick clay and silt aquitards that should have guaranteed the protection of deeper aquifers. The most important things to assess with geophysical methods were the presence of higher permeability layers or lenses and the potential leakage from the waste area towards the above mentioned rivers or the adjacent farm land. The instrumentation used is a time domain resistivity/IP meter that allows for automatic SP compensation and digital stacking, with 1200 W maximum power and programmable IP windows. Special multi-core cables were constructed to ease the operations with up to 64 electrodes. The electrodes were made with standard copper-coated iron rods, that seemed to produce very low spurious polarization effects (although a cross-check with non polarizable porous pots is not easy to perform in multielectrode configuration). Different array configurations (Wenner, Dipole-Dipole) and different electrode spacings (2 to 5 meters) were tested, trying to optimize the results in terms of noise reduction and spatial resolution obtained. The data sets were interpreted using different 2D resistivity/IP inversion software, especially to test the influence of different IP modeling strategies on the final results. An example of a Resistivity / IP reconstructed section over the center part of the landfill is shown in the figure below: the solid waste mass appeared to be more resistive (25 -> 50 Ohm*m) than background soil (clay, pit and silty sand, ranging from 8 to 15 Ohm*m) and highly polarizable (over 20 mV/V). The IP results enabled to distinguish between different media having high conductivity, as clays or silty sands saturated with leachate originated by the waste. In fact, in the left part of the section a low resistivity layer ( 8 -> 15 meters depth, resistivity 4-5 Ohm*m) corresponds to a high chargeability zone, indicating a potential leakage from the bottom of the landfill (this older part was not protected with HDPE coating). From the center to the right of the section (the newer part of the landfill, with HDPE installed below 8 meters of waste) the chargeability decreases rapidly with depth to the low values encountered outside the area as well. In this case the conductive layer can be explained with the presence of a clay aquitard, indicated by the borings.