Hydrodynamical, geochemical and isotopic preliminary results of Andean groundwater in an active volcanic zone, Mulalo (Ecuador)
Résumé
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). It
is composed of fluviolacustrine deposits, pumice and volcanic ashes that give high water
potential to this formation. The aquifer is partly phreatic but the presence of quaternary
Holocene 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 between
boreholes, wells and springs. The flow direction is from north to south. This groundwater is
used for domestic water supply, irrigation of orchards and greenhouses where roses are
cultivated, which is the main economic activity in this region. To improve the knowledge of
this aquifer and guarantee a good management of groundwater use, a hydrodynamic and
geochemical monitoring was made during the past few years that was completed by two
campaigns of water stable isotopes sampling. Results indicate that the underground phreatic
level 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 mineralized
samples associated with geothermalism or deeper aquifer levels. The chemical type is mainly
HCO3- - Mg2+ (2 3 of samples) or HCO3- - Na+, which is linked with ashes and volcanic
material mineralogy from the Cotopaxi volcano that overhangs the study area, or other
volcanoes 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 at
this altitude according to the isotopic record of Quito. Plotted points are aligned with a very
low slope of 3, which may reveal current or past geothermic processes or a mixing process
between aquifers.