Quantity status of groundwater in alluvial aquifers in northern Croatia

The quantity and spatial distribution of groundwater in Croatia is determined by geological composition, climate and hydrological conditions and the hydrogeological characteristics of individual areas. Northern Croatia is part of the southern margin of the Pannonian basin and is dominated by the spacious lowlands of the Sava and Drava rivers, where aquifers of intergranular porosity of Pleistocene and Holocene ages were formed. The lithological composition of the aquifers is dominated by gravel and sand in the western parts of the Drava and Sava basins. Sandy aquifers are prevalent in the central and eastern parts. Groundwater accumulated in these aquifers is the basis of the water supply in northern Croatia, which is the reason why monitoring of its status, both quantitative and qualitative is exceptionally important. Data obtained by long-term monitoring of groundwater levels facilitated assessment of the quantitative status of groundwater. To determine the causes of changes in the hydrodynamic conditions of the aquifers, results of long-term monitoring of precipitation and stages of the Drava and Sava rivers were also used. Linear regression was used to analyse changes in groundwater levels, the Drava and Sava river stages and precipitation quantities. The results generally show that there is a negative groundwater level trend over almost the entire area of the Drava and Sava alluvial aquifer. This is a consequence of deepening of the Drava and Sava river beds and lowering of their stages, together with a decreasing trend of total annual precipitation.