Falda

The water is collected from underground springs and wells and from surface – rivers, lakes and even sea.

In Italy, 88% of all water is pumped from underground (50% from the water table and 38% from springs). Only 12% of the water for human use comes from surface water sources.

Water from the water table has the advantage of being naturally filtered through the permeable soil layers. If it is collected correctly from deep enough layers, the water is already safe from the physical, chemical and bacteriological points of view and may therefore be delivered without any treatment or disinfection.

The underground water tables are also well-protected from accidental contamination; when accidents do occur, they can be promptly "spotted" and there is sufficient time left for action because the water table moves very slowly. The drinking water for the City of Milan is collected exclusively from the water table.

The availability of underground water remains almost constant even during drought periods.
The Milan underground sediments are mainly of fluvial and glacial origins: in particular, the size of gravel and sand lithotypes prevailing on surface decreases with depth. These features are reflected in the hydrogeological characteristics, since at the surface there are highly permeable and thick interconnected masses; the permeability decreases with depth and the permeable layers become more and more isolated. This situation causes the presence of unconfined and semi-confined water tables in the more permeable sediments down to the depth of about 100 m (330 ft); these water tables are contained in the aquifer historically exploited by most of the wells and indicated as the “Traditional Aquifer”. It is directly fed by storm water, by losses from watercourses and by irrigation.

Schematically, the Milan underground consists of permeable layers (aquifers) separated by impermeable layers (aquicludes of limestone and clay):

1. first aquifer: a very permeable sediment (gravel and sand with subordinate limestone fractions), down to 30…40 m (100…130 ft) below ground level, containing the water table closest to the surface;
2. second aquifer: medium – high permeability sediment (sand and gravel with fine fractions, conglomerate and sandstone layers) with the thickness ranging from 30…40 m (100…130 ft) to about 100 m (330 ft); the water table in this layer is either confined or free because it often connects with the aquifer above;
3. third aquifer: medium – low permeability sediment (limestone and clay with sand levels); it contains the deepest water table and can be found between 100 m (330 ft) and 200 m (660 ft) or deeper.

aquifer	depth	hydraulic conductivity(m2/s)	qualitative characteristics
1st aquifer	from 0 to 30…40 m (100…130 ft)	10-3/10-4	very vulnerable, may be subject to microbiological and chemical contamination
2nd aquifer	from 40 to 100 m (130…330 ft)	10-4/10-5	peak of chemical contamination may occur, particularly in the zones where it communicates with the overlying water table
3rd aquifer	from 100 to 200 m (330 to 660 ft and more)	10-4/10-6	possible presence of H2S and brackish water deeper than 200 m (660 ft).