Branka Bračič Železnik 1, Barbara Čenčur Curk 2 THE INFLUENCE OF CLIMATE VARIATIONS ON NATURAL BACKGROUND OF NITRATE IN PERCOLATED WATER 1 Public Water Supply Company JP Vodovod-Kanalizacija d.o.o.,Vodovodna cesta 90, 1000 Ljubljana, Slovenia, 2 Faculty of Natural Sciences and Engineering, Aškerčeva 12, 1000 Ljubljana, Slovenia, INTRODUCTION Alluvial aquifer of Ljubljansko polje represents the main source of drinking water for Municipality of Ljubljana. The purpose of our research was to determine the natural background concentrations and stable isotope composition of nitrogen in nitrate in the percolated water at Kleče Lysimeter station. The soil water was collected weekly in a lysimeter in the unsaturated zone at a sampling station located in the unpolluted grassland within the water protection area of the water field Kleče at the northern edge of the urbanized area of Ljubljana city during 4 years ( ). The wider area around the station hasn’t been fertilized or otherwise affected by anthropogenic activities during last few decades. The precipitation water percolates through a 4 m thick profile, consisting of pebble covered by an approx. 20 cm thick soil layer. The concentration of NO 3 -N is monitored. A period of intensive denitrification was observed at the end of the dry and warm 2003 season, while during the following two years, denitrification didn’t play a significant role in the observed system, or was masked by more intensive nitrate leaching from the soil profile. CONCLUSIONS Land use activities exert pressure on water resources and will change according to Climate Change. It is crucial for safeguarding future water supply to anticipate these changes in climate and land use activities and to assess its impacts on water resources. Amount of nitrate in the percolating water of the individual precipitation event was between 0,49 mg to 295 mg NO 3 -N, which is from 0,003 kg NO 3 -N/ha to 1,92 kg NO 3 -N/ha. Nitrate quantity in the percolating water changes during years and depends on the precipitation amount, season, plant uptake and soil processes. Established measurements and monitoring of NO3-N in percolated water from non-fertilised area gives invaluable information about nitrate natural background. Figure 1: The Kleče Lysimeter station was constructed 1985 and is inside the water field protected area on the border of urbanized area. The highway with approximately vehicles per day is on the southern part of the water field. The fields and meadows surrounding the water field all around LYSIMETER LOCATION HYDROMETEOROLOGICAL PARAMETERS OF LJUBLJANA FIELD GEOLOGICAL SETTINGS The upper part of unsaturated zone consists of up to 10 m thick layer of sandy gravel, follow the 5 m thick layer of conglomerate. Under the conglomerate is 5 m thick layer of clay and after more than 30 m thick layer of gravel with conglomerate lenses. The groundwater level is between 20 to 25 m under the surface.. RESULTS Figure 2: The Kleče Lysimeter station Figure 3: The meteorological station Kleče Figure 4: The Lysimeter construction: From the surface to the 60 cm of depth the lysimeter was filled with autochthon soil, then came the 50 cm thick sandy fill, followed by 40 cm of gravel layer and at the end 50 cm of drainage material. Figure 11: The NO 3 -N concentration (mg/l) in the percolated water represents the nitrate natural background level, which comes from air to the soil The climate in Ljubljana is due to the city’s central geographical position in a vast basin surrounded by the pre-Alpine and Karst climatic regions. Typical for the winter months is the so called temperature inversion, a meteorological phenomenom in which cool and moist air stays in the lower air layers. During the summer the weather in Ljubljana is notably under the influence of warm air streams from the Mediterranean so the summers in Ljubljana are relatively hot. Figure 7: The nitrate distribution (mg/l) in the groundwater of Ljubljansko polje Figure 12: The seasonal fluctuation of nitrate concentration (mg/l) and stable isotop composition of nitrate (‰) in the percolated water; highlited is the season of extended denitrification Figure 13: The relation between concentration (mg/l) and stable isotope composition ( ‰) in nitrate in the winter period, when the nitrate eluation from soil is the most intensive Figure 5: Inside the control shaft of lysimeter in Kleče Figure 7: Monthly precipitation (mm/day) in Ljubljana Figure 8: Monthly evapotranspiration (mm/month) in Ljubljana Figure 9: Average daily temperature (  C) in Ljubljana Figure 10: The quantity of percolated water with regard to precipitation Figure 6: Lithological structure of unsatureted zone on Ljubljansko polje Monthly Evapotranspiration