The METON site in the city of Milan

Site history

The METON site is localized in the south east of the city of Milan. In this factory, where in 1960 were employed about 2500 workers, was born the Fluor’s chemical, with the production of Teflon and other materials.

The industrial plant, which covers a surface of 660.000 m², produced, from 1914 till the beginning of the eighties, inorganic chemicals such as phosphoric acid, sulfuric acid, detergents, bleaching agents and hydrogen peroxide. In the sixteen it also started producing pesticides and herbicides, mainly thiocarbamate and phosphoric ester.

Remediation actions

l) The remediation actions started in 1993 with the removal of about 250.000 m³ of industrial wastes and contaminated soils by heavy metals and pesticides which were placed in a new controlled landfill in the south part of the area.

2) Inside the area there was an old quarrel (called Bistoletti Pit– 25.000 m²) filled with constructional debris and others wastes. The Municipality of Milan authorized a containment action (cover in HDPE) with monitoring and drainage wells.

3) In June 1999, has been started the Soil Venting, an in situ remediation technology, in the north west part of the area, consisting of soil vapor extraction from 14 wells deep 4,5 m. The goal of this method is to decrease chlorinated hydrocarbons (Tetrachloroethylene, 1,1,1 Trichloroethane, 1,1,1 Trichloroethane) concentrations in the superficial soil. Air sparging in 14 small wells deep 5.5 – 7 m, at the top of a 1 meter clay layer, has been started too. In the same period was executed a recovery well, deep 30 m, at a distance of about 100 m at down gradient.

Up to now the maximum concentration of total chlorinated hydrocarbons found in monitoring wells is about 16.000 mg/l. In the south east part of the area there’s also a residual contamination of Prothoate (about 1-5 mg/1). Both the contaminations are recognizable also in the monitoring wells located outside the METON site in the southern RADAELLI area (a dismissed foundry).

Hydrogeology of Milan plain

The Milan plain subsoil is characterized by plio-pleistocen sediments with a remarkable water reserve resource. The upper sediments, of alluvial and river-glacier origin, are gravel and sand, with few layers of silt and clay whereas the deeper sediments, of sea origin, are mainly silt and clay. This kind of deposition had been caused by the changing of the sedimentation mechanisms in the end of Pleistocene. The Alps’ rise in fact promoted the sea regression and the deposition of sediments of delta-lagoon origin, often characterized by a thin granulometry alternated with coarse materials. After that on Quaternary, the glaciation began and the rivers, flowing out their southern borders, deposited in predominance gravel and sand. The increase of clays and silts occur not only with the depth but also from the north to the south of Milan plain because the quaternary rivers far from the glacier border lost theirs transport energy and deposited sediments more and more thin.

Fig. 2 : hydrogeological N-S section trough the city of Milan

In the Milan plain you can distinguish three main aquifers:

•  
• The "Traditional Aquifer" (TR): called in this way because its groundwater is used for public and industrial water supply since the beginning of XX century. The TR is a phreatic aquifer with a good transmissivity (from 5·10^-2 to 1·10^-3 m²/s) and permeability (from 5·10^-3 to 1·10^-4 m/s). Its thickness range from 60 to 120 m. The sediments which characterize this aquifer are the quaternary ones: gravel and sand mainly, although, moving from north to south, the presence of clay-silt layers increases. From about the middle of Milan city to the south, the clay-silt layers allow you to distinguish between a TRa and a TRb aquifer: the first one is a shallow unconfined aquifer whereas the second is an half-confined one, because the separation layer is a discontinuous aquitard, 5-10 m thick, made of clayey-silt.

•  
• The "Continental Aquifer" (C): it is located into the Pleistocene sediments of delta-lagoon origin. The separation from the upper aquifer, made by continuous and thick layers of clay, TR is very good. The C aquifer represents an important resource for the quality of its waters but its low permeability and transmissivity limit the exploitation. At a regional scale the C is considered a unique aquifer but actually it is made by the alternating of permeable layers and clay layers. The C aquifer become thicker from north to south and its base has a slope of about 1.4 %, greater than the plane surface one. The groundwater resource in the C aquifer is named 2° Falda and flows from north to south.
•  
• The "Sea Aquifer" (M): it is the deeper aquifer, in Milan you can find its top between - 230 and - 300 m from the topographic surface. It is exploited by a little number of wells, so there are few informations about its hydrogeological parameters.

Site hydrogeology

In the METON area we have about 40 startigraphy of weels/piezometers and about 30 statigraphy of geognostic boreholes. Every environmental consulting who worked in the area have found the same hydrogeological structure:

Layer 1: thickness 0.6-1.1 m

Layer 2: thickness 0.6-3.2 m

Layer 3: thickness 2.8-7.0 m

Layer 4: thickness 6.0-10.0 m

Layer 5: thickness 40-50 m

Layer 6: thickness 5.0-10.0 m

Clay lens with a good continuity in the space. It’s considered the base of the TRa aquifer and separate this one from the TRb.