In the central part of the country (internal Dinarides), groundwater accumulates in limestone massifs and emerges as lime wells in the river basins of the Una, Sava, Bosna, Drina and Neretva Rivers. In the south, in the Adriatic Sea catchment, the geology is primarily karst (external Dinarides) and groundwater is mostly found in wells of the Cetina, Neretva and Trebisnjica River Basins. Karst aquifers are very fragile in terms of pollution impacts.