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Groundwater hydrochemistry of the Quaternary alluvial aquifer in Varaždin region - Croatia

The study area is situated in the north-western part of Croatia. It is quite developed region where agricultural production is among the most important economic branches. Industrial activity plays less important role and is almost completely situated in Varaždin town, which is the central point of the region. The natural quality of groundwater complies with the provisions of the Regulation on health safety of drinking water (OG 47/08), but during the years inadequate land use management took its toll, which became obvious in 1970s when high concentration of nitrates in groundwater was noticed for the first time. There are several polluters – intensive agricultural production together with abundance of poultry farms, as well as lack of sewerage network. Favourable hydrogeological conditions enabled development of three pumping sites – Varaždin (situated in Varaždin town), Bartolovec (situated southeast from Varaždin town) and Vinokovšćak (situated north from Varaždin town). Bartolovec and Vinokošćak pumping sites still operate with full capacity, while the pumping site Varaždin, which was the major drinking water supplier in the past, has recently decreased its capacity as the result of groundwater quality deterioration. The aquifer is composed of gravel and sand with variable shares of silt (Babić, et al., 1978 ; Urumović, 1971 ; Urumović, et al., 1990). It is formed during Pleistocene and Holocene as the result of accumulation processes of the Drava River (Prelogović & Velić, 1988). At utmost north-western area its thickness is less than 5 meters and is gradually increasing in downstream direction reaching its maximum of roughly 105 meters at the eastern part of investigated area. In the central part, near Varaždin town, a tiny aquitard appears dividing the aquifer in two hydrogeological units – the first aquifer and the second aquifer. Groundwater samples were taken from wells in various hydrological conditions. Water samples for chemical analyses were collected and analysed in the Laboratory of the Department of Hydrogeology and Engineering geology of Croatian Geological Survey. On-site analyses included T, pH and EC with WTW probes, as well the alkalinity. Also, periodical chemical analysis made by VARKOM was used for interpretation. According to the chemical composition, groundwater from the Varaždin aquifer belongs to the CaMg-HCO3 hydrochemical type. This is the primary water type which is principally derived from dissolution of carbonate minerals (calcite and dolomite) that compose the aquifer. The pH of the analyzed water samples varies from 6.96 to 7.94 (slightly acid to alkaline). The EC values vary from 596 to 720 S/cm and depend on amount of dissolved solids in water. Nitrate concentrations vary from 3 to 89 mg/l. The highest value was measured in groundwater from the catchment area of Varaždin pumping site. In the most samples from the catchment area of Varaždin pumping site, measured concentrations of nitrate are over the MPC value. On the contrary, they vary from 3 to 38 mg/l in waters from the wells located at the catchment areas of Vinokovšćak and Bartolovec pumping sites. In piezometer PDS-7 (near Varaždin pumping site) concentrations of nitrite are high, while, at the same time, concentrations of nitrate are low. Such phenomena are the consequence of denitrification processes. Generally higher concentrations of nitrite were measured in water samples taken in the vicinity of the Varaždin pumping site, while they are low in other places. Ammonia and orthophosphate concentrations are higher in the water samples from wells located in catchment area of Varaždin pumping site then on the other sampling locations. The reasons for higher values of parameters in the groundwater from the first aquifer in the catchment area of Varaždin pumping site compared to the ones obtained for the water samples in the catchment areas of Bartolovec and Vinokošćak pumping sites area are: the lack of covering layer, thinner aquifer, numerous poultry farms as well as intensive cabbage production. The values of physical, physical-chemical and chemical parameters measured in groundwater from the second aquifer are mostly under MPC levels. The excessive concentrations of nitrates in groundwater from the shallow aquifer at the catchment area of Varaždin pumping site led to gradual decreasing of groundwater abstraction rate and directed the majority of abstraction to the other pumping sites – Bartolovec and Vikonovšćak, where concentrations of nitrate still do not exceed the MPC values. It’s necessary to make improvements in agricultural production, as well as in Varaždin infrastructure, in order to reduce concentrations of contaminants in groundwater and enable sustainable groundwater management and development of the entire region.

alluvial aquifer; hydrochemistry; ammonia; nitrite; nitrate

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