Some problematic aspects of piezometric and qualitative monitoring by landfills: the example of Magliano Alpi plant (Cuneo – Piedmont).

Although the alternative techniques (incineration, thermal, composting, recycling, etc..) used all over the industrialized world for managing the municipal solid waste, it seems impossible at present to abandon the traditional landfill, at least as final disposal for the residues of much of these techniques. The current Italian legislation on landfills (D. Lgs. 36/2003) imposes the monitoring of environmental matrices (e.g. groundwaters) by the landfill area, but does not give precise indications regarding its standard procedures. Annex 2 to this Decree defines the objectives for the monitoring of groundwater, leachate, biogas, air quality, climate weather parameters and the status of the body of the landfill, but no reference to how sampling, especially in particular situations. At present, the monitoring specifications are determined by the Public Administration at the authorization issue. This work puts the emphasis on the monitoring of groundwater below a landfill in low permeable deposits as silts and sandy silts not constituting an actual aquifer, but instead an aquitard and frequently considered an aquiclude. These situations are not rare; in fact, many landfills are located above geological formations consisting of fine sediments ensuring good protection to deep aquifer, but, at the same time, because of their low permeability, may become the temporary or suspended groundwater, with a local recharge, which can give problems with respect to the geotechnical stability of the bottom of the landfill and the system of waterproofing. A disposal plant for municipal solid waste, site in Magliano Alpi, (Piedmont – Northwestern Italy) is considered as an example in the note. The plant receives waste from 86 towns (about 90,000 inhabitants), for a total of 31,000 ton/year of waste, divided into 6000 ton/year from waste collection and 25,000 ton/year of unsorted waste. A superficial low permeable hydrogeologic complex and an underlying deeper aquifer have been evidenced by the lithostratigraphic reconstruction. The monitoring piezometers control the groundwater of the upper hydrogeologic complex. Some anomalies encountered in the processed monthly piezometric data (water level raising or lowering in an independent way from one piezometers to another) could be explained by a non continuous horizontal extension of the permeable levels in the hydrogeologic complex at the bottom of the landfill. These levels seems hydraulically unconnected so depleting the general permeability of the layer. The different modalities of charging and emptying of these lens could lead to the different water levels registered in piezometers intercepting different productive levels This hypothesis is supported by the considerable difficulty of purging the piezometers for sampling of control and confirmed by a tentative of pumping test in a sink drain, stopped because of the complete well drainage. In both cases, the static water levels measured before pumping are restored in the long term (from hours to days). Here lies the problem of choosing the layers to consider for a correct monitoring. In fact, the monitoring of a low permeability medium, placed immediately below the landfill, can be considered appropriate to investigate the water pressure below in the system sealing of the landfill, in order to avoid any stress from the soil pore pressure to the waterproofing system. On the contrary, consider an upper saturated low permeable level is not suitable for the monitoring of groundwater quality: the installation of sampling piezometers in that case involves the impossibility to standard methods for sampling groundwater. At the same time, purge wells installed in such layers couldn’t correctly fulfil their function. Both piezometers for water quality control and purge wells to utilize in case of groundwater pollution should be realized in the first aquifer level below the landfill.