The project is earmarked primarily for the two main groundwater resources exploited for the public water supply of the capital city Ljubljana. Ljubljansko polje and Ljubljansko barje are alluvial aquifers, deposited by the river Sava and its tributaries.
Areas of Ljubljansko polje and Barje are from geological point of view tectonic depressions, formed by tectonic subsidence and gradual filling by alluvial and lacustrine sediments. Flat landscape is divided by Golovec, Grajski hrib and Rožnik in the middle. Those elevations represent a basement of the depression, made of Carboniferous and Permian rock, which was uplifted above the surface of the younger sediments. Sinja Gorica, Blatna Brezovica, Vnanje Gorice and other hills on the southern part of Barje are of similar origin. On the southern and western part of Barje the basement consist of permeable Mezozoic limestone and dolomite, while elsewhere the basement is made of above mentioned Carboniferous and Permian schistose mudstone, quartz sandstone and conglomerate, which are practically impermeable for water.
The Ljubljana depression begun to subside in middle Pliocene, about 3 million years ago. Filling with sediments was very intense in Pleistocene – the epoch of glacial and interglacial ages, when Sava river transported material from alpine glaciers to Ljubljana polje. The sediments are composed of well permeable gravel and sand beds with impermeable lenses of conglomerate. Due to the great thickness (which exceeds 100 m in deepest parts) and good permeability, this sandy-gravel aquifer contains significant quantities of groundwater.
The Ljubljana polje aquifer is unconfined, recharged from Sava river and rainfall, percolating through vadose zone. In the northwestern part of Ljubljana polje, area of Roje and Tomačevo, Sava river recharges the aquifer. In the southeastern parts of Ljubljana polje groundwater is drained back to the river. Consequently the groundwater table in western part of Ljubljana polje has direction of hydraulic gradient towards south or southeast and on central parts towards east.
Barje consists of alternating alluvial and lacustrine sediments. Alluvial sediments were brought to Barje by rivers and creeks from southern laying Kriško-Mokriško hills. Gravel of Iška, Borovniščica and Želimeljščica extend far to the north, where it reaches gravel of Gradaščica, Glinščica and Sava that flowed on the southern side of Rožnik till the end of last glacial.
Pleistocene and Holocene alluvial sediments include interbedded lacustrine sediments. Lenses of clay, sand and peat are common. Gravel is often silty or clayey. Surface in central parts of Barje is covered by clayey silt with numerous remains of gastropod. Overall thickness of sediments is up to 170 m.
Due to the heterogeneity of sediments, hydrogeological conditions on Barje are more complicated than on Ljubljana polje. Aquifers of Barje are in dolomite basement and in gravel beds. The last are divided by less permeable layers and therefore contain artesian groundwater.
Holocene aquifers on the surface are recharged directly from rainfall and surface streams, while lower aquifers are fed from permeable limestone and dolomite basement, which collects water from extensive karst catchment southern and western of Barje. Groundwater of Barje discharges to Ljubljanica river and to Ljubljansko polje through narrow between Grajski hrib and Rožnik and Dravlje valley.