The COPA+K method: an index-based approach for the karst groundwater vulnerability assessment. The Valseriana springs case (Northern Italy)

Karst aquifers are highly productive from a water supply perspective as well as highly susceptible to pollution. This is primarily due to significant lateral extension, shallow soils, and highly effective recharge points such as sinkholes, sinking streams, and polje. Therefore, attention must be given to the proper water management in these environments and the preservation of the natural state of the water system. Intrinsic vulnerability maps are currently one of the most valuable tools for assessing sensitivity to pollution of an area. Index-based methods allow for its evaluation even in cases where large amounts of data are not available and without requiring intensive computational effort. The main goal of this study was to evaluate and enhance index-based approaches for the assessment of the intrinsic vulnerability, so to consider the peculiar characteristics of the Pre-Alps region and highlight differences within catchments. The study focused on the Nossana and Ponte del Costone Springs in the middle Valseriana (Italy, BG). After applying the classical methods for karst environments (COP and EPIK), the integrative COPA+K method was developed using GIS software. The COPA+K approach exploits the potentialities of the COP method with the addition of two further factors: the influence of the main discontinuities on the water system according to their distance from the spring (A factor), and the development of the karst network (K factor). In comparison to EPIK and COP, COPA+K results showed a differentiation between the two studied catchments more coherent to their know hydrogeological behavior (dominant drainage system Nossana, dispersive system Ponte del Costone). The percentage difference between the most vulnerable areas of Nossana and Ponte del Costone increased from 5.2% to 17.5%. COPA+K also allowed a greater detail in the identification of the most vulnerable areas compared to COP (from 35.6% to 23.6% considering the whole area).