Nitrates (NO₃^-) in groundwater are presented as average annual concentrations in selected alluvial and karst-fractured aquifers for the 1993-2004 period. We also presented the frequency of excessive burdening with nitrates between 2002 and 2004 and a comparison with the EU member countries. The Decree on Groundwater Quality Standards (Official Gazette RS No. 100/2005) determined the threshold value of 50 mg/l for nitrates (NO₃^-) to be a standard for the quality of groundwater.

Between 1993 and 2004, average annual concentrations of nitrates exceeded threshold values for groundwater (50 mg/l) in two aquifers in north-east Slovenia (Lower Savinjska dolina valley and the Bolska River valley, and Prekmursko-Mursko polje). In the Lower Savinjska dolina valley and the Bolska River valley, the concentrations were too high in all the years, with the exception of 2002 and 2003. In individual years, the threshold value was exceeded at Prekmursko-Mursko polje as well. From 2002 on, however, the values at this aquifer remained below 50 mg/l. At Krško-Brežiško polje and Ljubljansko polje, average annual concentrations of nitrates were mostly below 30 mg/l, and at karst-fractured aquifers (the karst part of the Ljubljanica River, Trnovsko-Banjška planota and Bay of Trieste-Kvarner) even below 6 mg/l. The majority of aquifers do not indicate characteristic changes, with the exception of Prekmursko-Mursko polje, which shows a moderate lowering of the nitrate concentration in groundwater.

Between 2002 and 2004, we established excessive burdening of groundwater with nitrates (more than 50 mg/l) in the Bolska River valley and Lower Savinjska dolina valley, Prekmursko-Mursko polje and Krško-Brežiško polje. In the Bolska River valley and Lower Savinjska dolina valley, the condition in that period deteriorated (from 45 % of exceeded values in 2002 to 69 % in 2004), at Prekmursko-Mursko polje, the share of values above 50 mg/l was relatively stable (19–21 %), and at Krško-Brežiško polje, the condition improved (from 17 % of exceeded values in 2002 to 8 % in 2004). The aquifers of Ljubljansko polje and the karst-fractured aquifers (Bay of Trieste-Kvarner, the karst part of the Ljubljanica River and Trnovsko-Banjška planota) indicated no excessive burdening of groundwater with nitrates.

Less significant burdening of the karst-fractured aquifer waters in comparison with the alluvial aquifers can be attributed to the different land use, and to a certain degree, to natural conditions. Settlements and intensive agricultural land are located on alluvial aquifers and present the main source of the input of nitrates into groundwater. On the karst-fractured aquifers, however, settlements are scarce and agricultural activities are less intensive. Heavier pollution of the waters in the aquifers in eastern Slovenia is also the consequence of a lower quantity of precipitation and thus reduced dilution of the leached nitrates.

Due to a major difference in the number of studied aquifers, the data for Slovenia are only partially comparable with the data from other European countries. Scandinavian countries and the countries of Eastern Europe as a rule have less nitrates in groundwater than Slovenia, and the situation in the countries of Western, Central and Southern Europe is just the opposite. On average, old EU member countries (EU-15) have more nitrates in groundwater than the new member countries (EU-16-25).

The main source of the input of nitrates into groundwater is point pollution (unregulated livestock manure storage and sewage system) and non-point pollution due to the use of livestock manure and mineral fertilizers. To reduce the burdening of water with nitrates, a stocking density limitation on agricultural land has been in force in Slovenia since 1996, and environmentally friendly ways for the fertilization of agricultural land have been prescribed. Agri-environmental measures, implemented in accordance with the Rural Development Programme, are oriented toward the reduction of water pollution with nitrates. Recipients of payments in this field of work are bound by even stricter measures regarding stocking density on agricultural land than those prescribed by the legislation. The results of stricter regulations and special measures, however, are not yet evident from the state of aquifers. In accordance with the Rural Development Programme, Slovenia started to take measures for the adaptation of agriculture to European standards in 2004 by building new livestock manure storages which will significantly help improve the state of waters. The results of this measure are only expected to show in the years to come.