The correct identification of recharge zones and understanding groundwater flow through fractured and karstic media are some of the main issues in hydrogeology and of utmost importance in managing groundwater resources. The Aburrá Valley in Colombia is an excellent example of a promising aquifer system for future water supply with recharge zones and hydrogeological units (HGU) still not well understood. One of those HGUs, the Medellín Dunite, is an ultramafic igneous rock with significant secondary porosity due to the high grade of fracturing and the presence of a pseudokarst system formed by the dissolution of magnesium-rich minerals. In this study, we aimed for the first approximation of this unit’s hydrogeologic characteristics by performing a hydro-structural analysis based on a systematic collection of structural data in rock outcrops and karstic landforms available secondary information. The main objective was to calculate the hydraulic weight of discontinuities, and to determine the associated preferential groundwater flow directions, in order to improve the understanding of the relations between the analyzed HGU and the other units in the Valley. The results showed that the predominant directions of groundwater flow are WNW and ENE, which imply that lateral flows from the Dunite not only recharge the alluvial aquifers of the Aburrá Valley Aquifer System (AVAS) but could also be recharging the San Nicolás Valley Aquifer System (SNVAS), another rapidly developing and populating region. This study’s findings show the importance of studying the Medellín Dunite and implementing management and protection measures for this aquifer unit.

KEYWORDS: recharge zones, fractured rocks, field techniques, hydro-structural characterization of rock masses, Colombia