Vulnerability and Risk of Contamination of the Varaždin Aquifer System, NW Croatia

This paper presents the first study that assesses the vulnerability and risk of contamination of groundwater in the Varaždin aquifer system. The alluvial aquifer system is mostly unconfined with favorable hydrogeological features. Out of three wellfields, two still operate at full capacity, while the Varaždin wellfield, once the major source of drinking water, has been abandoned due to high concentrations of nitrates in the groundwater. Index-based methods are employed to assess groundwater vulnerability—two DRASTIC-based methods, standard and P-DRASTIC; two SINTACS-based methods, with normal and severe weighting strings; and the GOD method. Hazard is evaluated according to recommendations from the EU COST 620 action, while the risk intensity of the resource is calculated using the results of vulnerability and hazard assessments. The results reveal that for all vulnerability models, the resulting maps have a similar distribution pattern of vulnerability classes, with the high vulnerability class prevailing. However, notwithstanding the generally high groundwater vulnerability, a moderate resource contamination risk prevails as a consequence of a dominantly low hazard index. The validation of the groundwater vulnerability models demonstrates a weak correlation between the vulnerability indices and mean nitrate concentrations in groundwater. Conversely, a significantly higher correlation coefficient (0.58) is obtained when the groundwater vulnerability index is replaced by the resource risk intensity index, indicating that the results of resource risk intensity assessments are superior to groundwater vulnerability results in predicting the level of groundwater contamination.