Abstract
Mulaló aquifer is located 80 km south of Quito, between the Royal and Western Cordilleras.The Upper Pleistocene Latacunga formation contains the aquifer (maximum depth 250 m). Itis composed of fluviolacustrine deposits, pumice and volcanic ashes that give high waterpotential to this formation. The aquifer is partly phreatic but the presence of quaternaryHolocene deposits (ash and lahar) in the central area, possibly makes the aquifer semiconfined. The study area covers about 200 km2 and concerns the north part of aquifer.Annual precipitation is around 900 mm. There are about 50 water points betweenboreholes, wells and springs. The flow direction is from north to south. This groundwater isused for domestic water supply, irrigation of orchards and greenhouses where roses arecultivated, which is the main economic activity in this region. To improve the knowledge ofthis aquifer and guarantee a good management of groundwater use, a hydrodynamic andgeochemical monitoring was made during the past few years that was completed by twocampaigns of water stable isotopes sampling. Results indicate that the underground phreaticlevel lies between 2-3 m and 26 m, and fluctuates weakly year round, less than 0.5 m.Electrical conductivity was found between 100 and 4500 IS cm, with the more mineralizedsamples associated with geothermalism or deeper aquifer levels. The chemical type is mainlyHCO3- - Mg2+ (2 3 of samples) or HCO3- - Na+, which is linked with ashes and volcanicmaterial mineralogy from the Cotopaxi volcano that overhangs the study area, or othervolcanoes around. Isotopic composition shows values from -10.5 to -12.8‰ (18O) and -84 to-91‰ (2H). The most depleted values reflect the isotope content of annual precipitation atthis altitude according to the isotopic record of Quito. Plotted points are aligned with a verylow slope of 3, which may reveal current or past geothermic processes or a mixing processbetween aquifers.