In arid and semiarid regions, a significant component of recharge to sedimentary basin aquifers occurs along mountain front zones (i.e. mountain front recharge and mountain blocks recharge). Central Chile is characterized by a north-south Central Depression (~400 m a.s.l) filled by Quaternary alluvial deposits and bordered to the east by fractured volcano-sedimentary rocks of the Andes (2000-4000 m a.s.l). At 33°S, mean annual precipitation varies with the orographic gradient from 200 mm.yr-1 in Central Depression to 550 mm.yr-1 in the Andes. Considering the geomorphological and hydroclimate features, the Western Andean Front is a high interesting area for the recharge of adjacent sedimentary basin aquifers.
However, in Chile, mountain front zones are often considered as impermeable boundaries and not used in aquifer modelling. Thus, generally the main aquifer recharge is associated to river infiltration. We suggest that this simplification do not provide a properly assessment of Central Chile groundwater resources. This has a great relevance in water budget, especially considering the prolonged drought with severe withdrawals in rivers and dams occurring since 2010. Because water is a strategic challenge for socio-economical activities, the assessment of groundwater recharge contribution from the Western Andean Front is mandatory for improve the understanding of aquifers functioning and to national groundwater management plans.
Using hydrogeochemical approach in the Aconcagua basin (32°50’S), where several springs outflowing at different elevations, we provide the evidences of groundwater recharge within the Andes. Water from springs, boreholes and precipitation were sampled for major, minor, trace elements and stable isotopes (δD-δ18O) analyses. Electrical conductivity (EC), temperature and pH were measured in situ.
Preliminary results show that EC ranges between 14 to 1188 µS/cm, temperatures are between 11.5 to 27°C and pH vary from 5.7 to 8.5. Waters are mostly HCO3--Ca+2 types. Most of major ions have a negative correlation with the altitude, unlike the temperature and pH. Furthermore, Ca+2 vs HCO3- plot indicates that the main process which controls the chemistry along a same groundwater flow path is the anorthite weathering in presence of CO2. By other hand, stables isotopes indicate that evaporation is affecting the spring’s waters while not the borehole’s waters.
Spring geochemistry provides information about hydrogeological processes in the Western Andean Front. The increase of ions concentrations from the highest springs in the mountains to the boreholes water in the basin, and the same process controlling the chemistry, suggest that the Andes are connected with the basin. Therefore, first results indicate that the Andes in Central Chile have an important role in the recharge of Central Depression sedimentary aquifers.
This work is funded by FONDECYT 1170569, and a contribution to the CEGA FONDAP/CONICYT 15090013 and to PCI ITAL170012. Taucare’s PhD studies are funded by CONICYT-Beca Doctorado Nacional 21160325.