Importance of evaluating karst features in contamination vulnerability and groundwater protection assessment of carbonate aquifers. The case study of Alta Cadena (Southern Spain)

Karst specific landforms should be especially considered in assessing contamination vulnerability because they can be areas of preferential infiltration (temporary or permanently) bypassing the protective cover. The aim of this paper is to describe the different role of karst features in vulnerability mapping and the need to use specific methods in karst aquifers so that these areas may be distinguished from other carbonate zones. Accurate cartography of karst features and knowledge of their behaviour during periods of recharge are essential for the correct groundwater vulnerability assessment of carbonate aquifers. Three methods of vulnerability mapping were applied to the Alta Cadena pilot site: two generic ones (GOD and DRASTIC) and the COP method, which is specific to karst aquifers, so it takes into account geomorphological mapping in assessing vulnerability. Besides distinguishing between zones where surface runoff flows into a swallow hole and the rest of the area, the COP method allow to assess the surface features (dolines, karrenfields, uvalas, etc) according to the importance of runoff and/or infiltration processes (VIAS et al. 2006a). The pilot aquifer shows globally a High vulnerability where bare Jurassic limestones outcrop. However, vulnerability was Low and Very Low over marly rocks. Karrenfields and areas with infiltration direct via swallow holes present Very High vulnerability whereas dolines present Moderate and Low vulnerabilities because of they are covered by terra rossa. The degree of vulnerability, calculated by the COP method, was congruent with the hydrogeological knowledge of the study area and the results derived from tracer tests. Furthermore, the vulnerability of this aquifer was classified with the GOD and DRASTIC methods as Moderate, but these methods don't permit to distinguish between karst features and rest of aquifer.