Groundwater recharge in the NW part of the Varaždin aquifer in Croatia was studied to understand how precipitation, surface water, and groundwater interact. By monitoring water levels, temperature, isotopes, and chloride content over four years, researchers found that the aquifer is recharged by both the Drava River and an accumulation lake. Surface water was identified as the main source of groundwater recharge in the area, contributing between 55% and 100% depending on the proximity to observation wells. This information is crucial for managing and preserving groundwater quality in the region.