Development of a Hydrogeological Conceptual Model of the Varazdin Alluvial Aquifer

The Varazdin aquifer represents the main source of water for public supply, agricultural, and industrial purposes in the Varazdin County in NW Croatia. In the last decades, this area has experienced contamination of groundwater with nitrates. This study describes the conceptualization of the Varazdin aquifer for the purpose of developing numerical model of groundwater flow and nitrate transport. Within the study, three important elements are defined: aquifer geometry, recharge from precipitation, and other boundary conditions. 3D aquifer model revealed that Varazdin aquifer consist of three layers: upper aquifer, semipermeable interlayer, and lower aquifer. The Wetspass-M model was used for the assessment of spatial and temporal distribution of water balance components for the period 2008-2017. Results of the model indicate that the average annual precipitation is distributed as 34% groundwater recharge, 21% surface runoff, and 45% actual evapotranspiration. The maps of equipotential lines show the behavior of the aquifer system and define boundary conditions, i.e., recharge and discharge areas of the aquifer: an inflow boundary from Drava River and accumulation lake Varazdin on the northwest and north, no flow boundary on the west and south, and an outflow boundary on the east.