engleski

Variations of nitrate concentrations in ground and surface waters of Varaždin alluvial system

The TRANITAL project (Origin, fate and TRAnsport modelling of NItrate in the Varaždin ALluvial aquifer) financed by Croatian Science Foundation (HRZZ) represents interdisciplinary research on origin, fate and distribution of nitrate in the alluvial aquifer system. Within the project groundwater samples are taken from 10 observation wells and surface waters are taken from the stream Plitvica, the Drava River, two gravel pit Šijanec and Varaždin Lake (accumulation). Prior to sampling, the electrical conductivity (EC), pH, temperature (T) and dissolved oxygen content (O2) were determined. Nutrients concentrations (nitrate, ammonium, nitrite, orthopshate), total and dissolved organic and inorganic carbon, total nitrogen, base cations and ions, trace elements and stable isotopes are measured. The monitoring started in June 2017 and will finish in June 2019. In this paper preliminary results are showed. The aquifer of the research area is composed of gravel and sand with variable portions of silt and it is formed during Pleistocene and Holocene as the result of accumulation processes of the Drava River. A semipermeable layer, mainly composed of silt and clay, splits the aquifer into two layers – upper and lower aquifer. The recharge of the upper aquifer occurs by means of precipitation infiltration and percolation of surface water. The general groundwater flow direction is NW-SE and is parallel to the Drava River. The covering layer of the aquifer is not continuously developed. In the research area, it rarely exceeds two meters, while often it completely disappears. Such conditions are favourable if they are considered from the aspect of aquifer recharge, but at the same time tiny covering layer composed of clay, silt and sand particles mixed with various content of organic matter, make the aquifer quite vulnerable. In few observation well, especially in well PDS-5 large variations of nitrate concentrations were observed from 200 to 68 mg/L and in the gavel pit Šijance from 0.6 to 60 mg/L. In the catchment area of the observation well PDS-5 intensive agricultural production of vegetables is present and intensive irrigation during late spring and summer time when the highest concentrations were measured. At the gravel pit Šijanec during summer period alga bloom were observed and at that time measured concentrations were very low. During colder-rainy period, nitrate concentrations decrease at the observation well PDS-5 and increase in the gravel pit, and at both observation places concentrations reached almost the same concentration. Different hydrological, geochemical and biological processes are effecting variations in nitrate concentrations in observed waters.

groundwater, surface water, nitrates, variations

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