Abstract
Volcanic and volcano-sedimentary aquifers often result from a complex geological history that induceda heterogeneous distribution of primary and reworked volcanic materials. The different stages,deposition, erosion, and tectonics affect the groundwater through its complex circulation flow. TheTumbaco – Cumbayá – Ilaló aquifer near Quito, Ecuador, exemplifies this context. It is an Andeanconfined volcano-sedimentary aquifer which has been used to supply water to the population in thevalleys near Quito for three decades. In spite of abundant surface water resources, the increase ofpopulation and the industrial development leading to high contamin ation levels of rivers made itnecessary to find new sources of potable water supply. This system is mainly composed of asedimentary series including an aquitard (Cangahua), overhanging two volcanic sedimentary series(Chiche, Guayllabamba) presenting high hydraulic capacities, and a volcanic aquifer cone (Ilaló)virtually dividing the study zone in two. The Chiche-Guayllabamba aquifer is limited by natural barriers(river ravines or volcanoes) in all directions. A large hydrodynamical, geochemical and isotope studywas carried out during four years, 2010 to 2013, complementing some old and partial data survey. Itallowed to propose a conceptual model improving the understanding of the recharge process and flowcirculation, seeking to aid the resource management. Results showed weak piezometric variationsyear round (A40 cm). Geochemical data highlighted a hydraulic continuity between the volcanic andsedimentary aquifers. Stable isotope data indicated the recharge area to be located on the volcanicslopes at local and regional scale. Tritium and radiocarbon gave apparent ages between 400 andmore than 15000 years for the North Chiche formation, and between 11000 and 45000 years for Ilalóvolcano. In the northern part, modern water was found in a spring which could be linked to a fracturedzone allowing direct recharge from the surface by direct infiltration.