Poster Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Integrated geophysical, hydrogeological and environmental tracer assessment to improve groundwater-surface water systems conceptualisation and sustainable use in the Aconcagua River Valley, Chile (526)

Matthias Raiber 1 , Rodrigo Rojas 1 , Igor Aguirre Araneda 2 , Axel Suckow 3 , Matías Luna 4 , Felipe Cerda 2 , Jorge Martinez 1 , Cornelia Wilske 3
  1. CSIRO Land and Water, Dutton Park, QLD, Australia
  2. Fundación CSIRO Chile, Santiago, Chile
  3. CSIRO Land and Water, Adelaide, SA, Australia
  4. University of Chile, Santiago, Chile

The Aconcagua River Valley is located in the Valparaiso Region approximately 100 km north of Santiago. The catchment is geologically very dynamic due to its location at the foothills of the Andes, with topographic gradients of several thousand meters from the mountain ranges to adjacent valleys.

Intensification of agricultural activities during the recent decades and climate change (predicted increases in temperature and decreases in river discharges) warrants a thorough characterisation of groundwater and surface water dynamics. Furthermore, a detailed understanding of the system is required to ensure long-term sustainability of the water resources and to support the selection of potential sites for future groundwater resource developments, particularly from deeper aquifers that are not currently exploited.

During this study, multiple independent lines of evidence were assessed and integrated to identify potential sites for groundwater development. The results of various geophysical surveys (TEM, gravity and seismic from both historic and newly-acquired data) were integrated with historical geological data to develop a 3D geological model using SKUA 3D geological modelling software package (Paradigm®). Groundwater samples were collected across the region (approximately 11000 km2) and then analysed for hydrochemistry and environmental tracers (e.g. δ2H and δ18O, 3H, 14C, CFCs, SF6 and noble gases (He, Ne, Ar, Kr, Xe)).

The geophysical data and the newly-developed geological model allowed the refinement of sub-surface architecture, adding more constraints to the thickness and extent of valley sub-divisions and the geometric relationships of the alluvial aquifers with adjacent mountains. Furthermore, it allowed the differentiation of stratigraphic sections within the alluvium with expected distinct hydraulic properties and robust identification of the aquifer basement in selected areas.

Stable isotope data confirmed an altitude effect for some groundwater samples. In others, a shift towards more negative values in δ18O may indicate infiltration of river water originally fed by higher altitude precipitation. All water samples, including those obtained locally from depths of over 100 m, contained measurable concentrations of nitrate, a likely effect of intensive agricultural practices in the region. These tracers may suggest high recharge rates of modern groundwater within these alluvial systems, which will be independently confirmed using the groundwater age tracers and noble gases.

This integration of various datasets and investigation techniques allowed the characterisation of major hydrogeological processes, including recharge, residence time and inferences on SW-GW interactions. It also allowed the proposition of areas for future groundwater development based on the physical and hydraulic properties of the alluvium.